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LABORATORY EVALUATION OF MICROWAVE HEATED ASPHALT PAVEMENT MATERIALS

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Laboratory evaluation o f m icrowave heated asphalt pavem ent
m aterials
Al-Ohaly, Abdulaziz Abdulrahm an, Ph.D.
University of Washington, 1987
UMI
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Ann Arbor, MI 48106
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Laboratory Evaluation of Microwave Heated Asphalt Pavement
Materials
by
ABDULAZE A. AL-OHALY
A dissertation submitted in partial fullfillment
of the requirements for the degree of
Doctor of Philosophy
University of Washington
1987
Approved by
(Chairpe
Program Authorized
to Offer D egree__
Date
e)
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University of Washington
Abstract
Abdulaziz Abdulrahman Al-Ohaly
Laboratory Evaluation of Microwave Heated Asphalt
Pavement Materials
Chairperson of Supervisory Committee: Professor Ronald L. Terrel
Department of Civil Engineering
The potential use of microwave energy to heat asphalt mixtures and pavements has
begun attracting attention. Microwave heating is rapid, deep and uniform. With
microwaves, heat is generated by the treated material under the excitation of an alternating
electromagnetic field caused by the passing microwaves. Some materials such as water
heat very well with microwaves, while others such as Teflon do not Asphalt cement is
similar to Teflon, but many aggregates seem to possess favorable microwave heating
properties.
Though the material is the heat-generating element in microwave heating, only
very limited information about the interaction of pavement materials with microwave
energy is available in the existing literature. Rather, the emphasis, so far, has been put on
the equipment side.
This thesis, however, focuses on pavement materials and their interaction with
microwave energy as a heating method.
The interaction between asphalt pavement
materials and the applied microwave energy has been evaluated in two phases. First, the
effect of microwaves on some properties of virgin and recycled mixtures has been
investigated. Potential benefits to adhesion and water stripping resistance of asphalt film
to aggregate are promising but need further investigation. Secondly, the effect of several
mixture variables on microwave heating efficiency has also been studied. Research has
shown that some of the variables studied have a pronounced effect on microwave heating,
while others have minimal or no effect at all on the heating efficiency.
In this study, a home type microwave oven was used as the source of microwave
energy. Special compacting and heating molds made of Teflon were used to facilitate
heating in the oven. In order to approximate the Held application of microwaves, molds
containing specimens were wrapped with aluminum paper around their outer perimeter.
Enhancement of microwave heating efficiency was demonstrated to be achievable
by adding selected chemicals such as carbon black.
TABLE OF CONTENTS
Page
C hapter One: Introduction
Objectives...............................................................................................................5
Research Approach................................................................................................6
C hapter Two: Background
I.
History of Microwave Heating in Paving............................................................... 9
II.
Safety of Pavement Microwave Heaters...................................................... 14
III. Economics of Microwave Heating........................................................................ 14
IV. Microwave Heating Concepts...............................................................................16
V.
Dielectric Properties of Pavement Materials................................................ 24
C hapter Three:
Effect of Microwave Heating on Asphalt M ixture
P roperties
I.
Introduction...........................................................................................................31
II.
Experiment............................................................................................................34
A. Preparation of Mixtures and Specimens.......................................................35
1. Virgin Plain Mixtures..............................................................................35
2. Virgin Mixtures with Polar Anti-stripping Agents................................. 40
3. Recycled Mixtures..................................................................................44
B. Testing........................................................................................................ 42
III. Test Results and Evaluation......................................................................... 52
A. Virgin Plain Mixtures............................................................................ 52
B. Mixtures with Anti-stripping Agent.......................................................63
C. Recycled Mixtures................................................................................ 73
Table of Contents (cont.)
C hapter Four:
Effect of M ixture Variables on Microwave Heating
E fficiency
I.
Introduction........................................................................................................... 86
II.
Materials and Equipment.......................................................................................88
A. Materials...................................................................................................... 88
B. Equipment....................................................................................................88
C. Use of Teflon Molds................................................................................... 88
D. Arrangement of Specimens in the Microwave Oven.................................. 90
III. T estin g .................................................................................................................98
■ A. Preparation of Specimens........................................................................... 98
B. Testing Procedure....................................................................................... 98
IV. Asphalt Mixture Factors...................................................................................... 100
A. Effect of Asphalt Content.................................................................100
B . Effect of Aggregate Gradation...................................................................104
C. Effect of Compaction Effort (Density).......................................................104
D. Effect of the Initial Temperature of the Specimen..................................... I l l
V.
Testing
C hapter Five:
Problem s.......................................................................................... 115
Effect of W ater Content on the Efficiency of Microwave
Heating of Pavement M aterials
I.
Introduction
.................................................................................................. 120
II.
Experiment...........................................................................................................122
A. Use of Plain Aggregate............................................................................. 122
B. Testing....................................................................................................... 124
C . Water Content Range............................................
124
Table of Contents (cont.)
C hapter Five:
Effect of W ater Content on the Efficiency of Microwave
Heating of Pavement M aterials (cont.)
D. Testing Procedureds................................................................................ 125
III. Test Results and Evaluation........................................................................125
IV. Water-Material Combinations and Limits........................................................... 133
C hapter Six: Microwave Heating Enhancers for Pavement M aterials
I.
Introduction......................................................................................................... 137
II.
Identifying Microwave Heating Improvers (Additives)......................................139
HI.
Use of Carbon Black in Asphalt Mixtures.......................................................... 141
IV.
Experiment...........................................................................................................144
A. Materials.....................................................................................................145
B. Equipment..................................................................................................145
C. Preparation of Specimens......................................................................... 145
D. Test Procedure........................................................................................... 146
V.
Test Results and Evaluation........................................................................146
C hapter Seven:
Sum m ary, Conclusions and Recommendations
I.
Summary............................................................................................................. 153
II.
C o n c lu s io n s ....................................................................................................154
III. R e c o m m e n d a tio n s........................................................................................ 156
IV. Needed Research................................................................................................. 157
R eferences
160
LIST OF FIGURES
Number
page
1.1
Comparison of heating time for bridge deck at 3 in. depth..................................2
1.2
Thermal gradient characteristics of bridge deck heating methods........................ 3
2.1
Effect of energy concentrating aluminum foil implanted 5 in. in asphalt
pavem ent ................................................................................................ 11
2.2
The central plant system used for heating RAP with microwave energy
and fluidized bed......................................................................................... 13
2.3
Electromagnetic spectrum................................................................................... 17
2.4
Effect of applied alternating electromagnetic field on dipoles orientation.......... 19
2.5
Types of polarization......................................................................................... 21
2.6
Comparative heating of aggregates in a microwave oven of 2450 MHz
and 600 watt on 300 gm. samples at room moisture................................. 27
2.7
Comparative heating in a 500 watt 2450 MHz microwave oven for two
minutes for a range of materials used in pavements....................................28
3.1
Mechanisms of asphalt adhesion improvement by microwave energy
treatment..............
33
3.2
Steps of preparing conventionally heated virgin mixtures.................................39
3.3
Steps of preparing microwaves heated virgin mixtures.....................................41
3.4
Steps of preparing conventionally heated plus microwaves zapped virgin
mixtures...................................................................................................... 43
3.5
Change in temperature of mixtures with Tallow Tetramine after varying
microwave zapping time..............................................................................45
3.6
Steps of preparing conventionally heated recycled mixtures....................... 48
3.7
Steps of preparing microwave heated recycled mixtures................................... 50
3.8
Steps of preparing conventionally heated plus microwaves zapped
recycled mixtures....................................................................................... 51
3.9
Steps of preparing conventionally plus microwaves heated recycled
mixtures...................................................................................................... 53
3.10
Effect of heating method on resilient modulus of virgin mixtures.................... 55
List of Figures (cont.)
N um ber
page
3.11
Effect of heating method on resilient modulus of virgin mixtures after
Lottman's conditioning............................................................................... 56
3.12
Effect of heating method on resilient modulus of virgin mixtures
(averages)................................................................................................... 57
3.13
Effect of heating method on the percent of retained resilient modulus of
virgin mixtures after Lottman's conditioning............................................. 59
3.14
Effect of heating method on the percent of retained resilient modulus of
virgin mixtures after Lottman's conditioning (averages)............................60
3.15
Effect of heating method on split tensile strength of virgin mixtures after
Lottman's conditioning...............................................................................61
3.16
Effect of heating method on split tensile strength of virgin mixtures after
Lottman's conditioning (averages)..............................................................62
3.17
Effect of heating method on resilient modulus of virgin mixtures with
Tallow Tetramine........................................................................................ 66
3.18
Effect of heating method on resilient modulus of virgin mixtures with
Tallow Tetramine after Lottman's conditioning.................................. 67
3.19
Effect of heating method on resilient modulus of virgin mixtures with
Tallow Tetramine (averages).......................................................................68
3.20
Effect of heating method on the percent of retained resilient modulus of
virgin mixtures with Tallow Tetramine after Lottman's conditioning.........69
3.21
Effect of heating method on the percent of retained resilient modulus of
virgin mixtures with Tallow Tetramine after Lottman's conditioning
(averages)................................................................................................... 70
3.22
Effect of heating method on split tensile strength of virgin mixtures with
Tallow Tetramine after Lottman's conditioning.................................. 71
3.23
Effect of heating method on split tensile strength of virgin mixtures with
Tallow Tetramine after Lottman’s conditioning (averages)................... 72
3.24
Effect of heating method on resilient modulus of recycled mixtures
3.25
Effect of heating method on resilient modulus of recycled mixtures after
Lottman's conditioning............................................................................... 77
3.26
Effect of heating method on resilient modulus of recycled mixtures
(averages)................................................................................................... 78
76
List of Figures (cont.)
N um ber
3.27
3.28
page
Effect of heating method on the percent of retained resilient modulus of
recycled mixtures after Lottman's conditioning
Effect of hearing method on percent retained resilient modulus of
recycled mixtures after Lottman's conditioning (averages)
79
..........80
3.29
Effect of heating method on split tensile strength of recycled mixtures
after Lottman’s conditioning.......................................................................82
3.30
Effect of heating method on split tensile strength of recycled mixtures
after Lottman's conditioning (averages)..................................................... 83
4.1
Specimen arrangement in microwave oven........................................................ 92
4.2
Positions where temperature was measured within plain aggregate
specimens................................................................................................... 94
4.3
Contours of equal temperature following heating of plain aggregate in a
microwave oven for one heating condition................................................96
4.4
Horizontal profile of temperature at midheight of one size aggregate
samples....................................................................................................... 97
4.5
Positions where temperature was measured within compacted mixture
specimens................................................................................................... 99
4.6
Effect of asphalt content on microwave heating of compacted asphalt
mixture specim ens.............................................................................. 102
4.7
Effect of asphalt content on microwave penetration into compacted
specimens................................................................................................. 103
4.8
Aggregate gradations used in microwave heating tests.....................................105
4.9
Effect of aggregate gradation on microwave heating of compacted
asphalt mixture specimens................................................................ 107
4.10
Effect of single size aggregate particle on microwave heating of
aggregate only...........................................................................................108
4.11
Effect of aggregate gradation on microwave penetration into specimens
4.12
Effect of compaction level on microwave heating of compacted
specimens................................................................................................. 113
4.13
Effect of compaction on microwave penetration into compacted
specimens................................................................................................. 114
110
List of Figures (cont.)
N um ber
4.14
Effect of initial temperature of specimen on microwave heating of
compacted specimens
page
117
4.15
Effect of initial temperature of specimen on microwave penetration into
specim en..................................................................................................118
5.1
Effect of moisture content on microwave heating of sand............................... 128
5.2
Effect of moisture content on microwave heating of coarse aggregate
5.3
Effect of moisture content and exposure time on microwave heating of
sand........................................................................................................... 130
5.4
Effect of moisture content and exposure time on microwave heating of
coarse aggregate........................................................................................ 131
5.5
Time (energy)-temperature relationship for heating wet aggregate - case
1 and 2 ....................................................................................................... 135
5.6
Time (energy)-temperature relationship for heating wet aggregate - case
3 .................................................................................................................136
6.1
Temperature of microwave heated rubbers...................................................... 142
6.2
Effect of carbon black content on microwave heating of rubber......................143
6.3
Effect of adding carbon black to asphalt mixtures on microwave heating
of mixture.................................................................................................. 148
6.4
Effect of carbon black content on the change of the temperature at the
core of microwave heated specimens (averages)....................................... 149
6.5
Effect of carbon black addition to asphalt mixtures on microwave
penetration in compacted specimens......................................................... 150
6.6
Effect of carbon black content on difference between the higher
temperature of the core and that of the bottom of microwave-heated
specimens (distance between core and bottom = 1.5 in.).................... 151
129
LIST OF TABLES
Table
Page
2.1
Comparison of asphalt pavement recycling costs by conventional a
microwave heating methods.................................................................15
2.2
Frequency bands for industrial, scientific, and medical (ISM)
equipment...................
18
2.3
Dielectric properties of asphalt pavement materials.............................................25
2.4
Chemical compositions of some aggregates in relation to microwave
heating efficiency..........................................................................................29
3.1
Summary of the experiment design ...................................................................36
3.2
Gradation of aggregate used in mixtures............................................................. 37
3.3
Temperature of hot virgin asphalt mixtures after 2 minuteszapping in
the microwave oven......................................................................................42
3.4
Temperature of recycled loose samples before and after microwave
heating...........................................................................................................49
3.5
Summary of test results on virgin mixtures................................................54
3.6
Results of the statistical significance test on the virgin mixtures data................64
3.7
Summary of test results on virgin mixtures with Tallow Tetramine...................65
3.8
Results of the statistical significance test on the data of mixtures with
Tallow Tetramine..........................................................................................74
3.9
Summary of test results on recycled mixtures.....................................................75
3.10
Results of the statistical significance test on the recycled mixtures data............. 85
4.1
Mixture variables that might affect microwave heating of asphalt
pavement mixtures as related to type and form of the material.................... 87
4.2
Summary of the experiment design..................................................................... 89
4.3
Temperature of specimens with different insulation systems............................. 91
4.4
Average temperatures of plain aggregate samples heated in the
microwave oven for 5 minutes.................................................................... 95
4.5
Effect of asphalt cement content on microwave heating and penetration
of asphalt mixtures......................................................................................101
List of Tables (cont.)
Table
Page
4.6
Effect of aggregate gradation on microwave heating and penetration in
asphalt mixtures.......................................................................................... 106
4.7
Effect of aggregate particle size on microwave heating and penetration
in plain aggregate samples................................................................ 109
4.8
Effect of compaction effort on microwave heating and penetration in
asphalt
m ixtures................................................................................. 112
4.9
Effect of initial temperature of specimen on microwave heating and
penetration in asphalt mixtures.................................................................. 116
5.1
Effect of moisture content on the dielectric properties of some
construction materials................................................................................. 121
5.2
Gradation of coarse aggregate and sand.................................................. 123
5.3
Results of microwave heating of sand at different moisture
contents....................................................................................................... 126
5.4
Results of microwave heating of coarse aggregate at different moisture
contents....................................................................................................... 127
6.1
Microwaves heating times of various chemical compounds..............................138
6.2
Examples of materials of high dielectric properties........................................... 140
6.3
Effect of carbon black addition on microwave heating of asphalt
pavement mixtures...................................................................................... 147
x
ACKNOWLEDGEMENT
Highest praise and thanks to Almighty Allah, the Sustainer and the Cherisher of the
Universe, who bestowed upon me countless bounties, one of which is the completion of
this research.
The financial support of King Faisal University, in the Eastern province of Saudi
Arabia through the author's graduate studies in the United States is gratefully
acknowledged.
The author finds himself in a great debt to professor Ronald L. Terrel, Chairman of
the supervisory committee, who suggested the topic, for his inspiration, careful guidance,
and valuable advice and insight through the course of this research. The assistance of the
other members of the supervisory committee, professors Graham Allan, Jim Hinze, Don
Janssen, Joe P. Mahoney, and G. Scott Rutherford, also gratefully recognized.
Many thanks are due to the Islamic community of Seattle for its moral support and
services, especially brother Mohammed A1 Sharif and brother Khalid Blankinship.
The author also wishes to express his humble gratitude and respect to his beloved
parents, brothers, sisters and in laws for their sincere, continuous prayers, love and
encouragement
Finally, many thanks to my wife, Hanan, who has been caring, sacrificing and
helping in many unseen ways. Thanks to my son Fawaz and my daughter Alhanoof for
their love and patience.
To my fa th e r :
Abdulrahman Noser Al-Ohaly
CHAPTER ONE
IN TRO D U CTIO N
The use of microwave energy in heating and drying pavement materials has already
been conceptualized as many as 20 years ago [1].
In the last 5 years, however,
commercial applications have become of interest to private parties and government
agencies in the USA. The Federal Highway Administration (FHWA) and some State
Departments of Transportation (DOTS) [2], the U.S. Air Force [3] and the Department of
Energy [4] are some of the agencies that have sponsored early work on microwave heating
applications in asphalt and portland cement concrete pavements. Prototype equipment has
been built and several field trials have been carried out in USA and Europe to demonstrate
the feasibility of microwave heating of pavement beds. A system that utilizes microwave
energy to heat reclaimed materials for recycling is already in operation in Texas.
Development of more efficient and adaptable equipment for the production of various
pavement materials and for road construction and maintenance is in progress.
Microwave heating possesses certain qualities that have made it a very attractive
heating alternative, which should encourage advancement of microwave energy adaptation
to pavements. Microwave heating of asphalt pavements is very rapid. In one instance, it
took only 10 minutes to heat a 3 in. thick bridge deck from 70 F to 185 F with a 4 kw/ft2
microwave system, while it took an infrared system of 1 kw/ft2 two hours, as shown in
Fig. 1.1 [5]. This fact has made microwave energy a potential heating candidate for
asphalt pavement runways with the Air Force [6], Another valuable advantage of
microwave heating is that it heats with a smoother thermal gradient through the depth of
the pavement layer as illustrated in Fig. 1.2. It would then be possible to heat pavements
at depth up to 5 in. without the risk of overheating the surface and burning the asphalt
cement One of the concerns about the use of microwave heating is that when it is applied
over road beds, energy will be wasted because the heating will be deeper than needed.
2
200
185 F
Microwave at
4 kw/ft2
(single pass)
Temperature (F)
150
Infrared at
1 kw/ft 2
(single pass)
Blanket at
100
n
2
h
50
0
1
2
3
4
Heating Time (hours)
Figure 1.1 Comparison of heating time for bridge decks at 3 in. depth [5].
3
Temperature (F)
350
300
Infrared heating, 1 kw/ft
after 2 hours
250
Electric blanket heating, 0.1 kw/ft
after 8 hours
200
150
100
Microwave (915 MHz), 4 kw/ft
after 10 minutes
0
1
4
2
3
Depth Below Surface (In.)
5
6
Figure 1.2 Thermal gradient characteristics of bridge deck heating methods [5].
Another concern is that microwave absorption characteristics vary with the aggregate
used. Most aggregates respond very well to microwaves and only a 14% variation in the
microwaves' absorption of these types has been reported [5]. Consumption of large
quantities of electrical energy and the relative inefficiency of converting to microwave has
been one of the major drawbacks of this approach to heating. Developments in technology
and research, some of which are reported here, have produced some options that should
reduce the impact of these disadvantages.
In recognition of the advantages of microwave heating, some applications have been
tried and many others have been proposed. Although the original concept was aimed
towards heating pavement in depth for recycling, other applications and appropriate
equipment were foreseen and patented [7,8,9,10]. Applications that have been tested
include recycling, longitudinal and transverse crack repair, pothole patching and bridge
deck repair. Other applications such as rut repair and curing of liquid asphalt mixtures in
place have been suggested [5,11,12].
The complementing element in microwave heating is the material that is to be heated.
In microwave heating, microwaves excite
treated material to generate its own heat In fact,
if there is no microwave absorbant object in the path of the microwaves, no heat can be
generated. Materials do not absorb microwaves equally and thus do not heat equally. A
material such as water heats very well when subjected to microwaves due to its high
dielectric properties, while others like Teflon heat very little. Although asphalt cement is
passive in the presence of microwaves, many types of aggregates, fortunately, respond
positively to them. When an asphalt mixture is exposed to microwaves, aggregates
generate the heat that would soften the asphalt cement Both road beds and loose materials
have been heated successfully with microwaves. Heating of reclaimed asphalt pavement
(RAP) chunks might be considered if the pavement breaking cost plus the additional
microwave heating is less than, the milling cost. Loosening the chunks by deep
microwave heating may preserve the original gradation, and probably will reduce
pollution. Production of fresh mixtures utilizing microwave energy, although not
accomplished yet, is possible if asphalt cement is heated conventionally then is mixed with
microwave heated aggregate, since asphalt cement does not heat by microwaves due to its
poor dielectric properties. Conventionally-produced virgin mixture might be post-treated
with microwaves, if proven to be of any benefit since aggregate is already mixed with
asphalt cement Heating of plain aggregate has not been targeted, but the addition of plain
aggregate in order to modify gradation of RAP was expected [13]. Also, a number of
laboratories have used microwave ovens for rapid determination of moisture content of
soils and aggregates [14].
Despite of the vitality of a material’s presence in the path of microwaves to microwave
heating, only a little work has been done on pavement materials in relation to microwave
heating. Some of the reported work was in the form of the temperature of pavement
materials before and after microwave heating [5,15,16]. Limited data are available on the
effect of microwave heating on properties of asphalt mixtures [17,18]. These studies
were primarily designed to benefit from the microwave oven's faster heating in laboratory
for quality control and mix design purposes.
The shortage in basic information that describes the material interaction with
microwaves has inspired the author to tackle some of those aspects. The research reported
herein addresses three issues in the microwaves-materials relationship: 1) the effect of
microwave heating on properties of asphalt mixtures, 2) the effect of changing mixtures
variables such as asphalt content, aggregate gradation...etc., on microwave heating
efficiency of these materials, and 3) the enhancement of microwave heating efficiency of
these mixtures via the addition of selected chemicals.
O bjectives
The overall objective of this research is to describe the extent of asphalt pavement
materials compatibility and interaction with microwave energy as a heating method. The
hope is that the results will assist in optimizing the use of microwave energy in production
and recycling of asphalt pavement materials as well as in construction and maintenance of
pavements. The specific objectives a re :
1.
To examine the effect of microwave energy on asphalt mixture quality.
2.
To examine the effect of some mixture variables on microwave heating efficiency
of pavements.
3.
To evaluate the possibility of enhancing the heating process of asphalt pavement
materials with microwave energy via the addition of selected chemicals.
Research A pproach
This research is a study on the interaction of laboratory prepared mixtures with
microwave energy as a heating method. Microwave heating was accomplished by using a
kitchen type microwave oven. This effort included the following tasks:
I.
Effect of Microwave Energy on Asphalt Mixture Quality
Properties that were used to evaluate microwaves' effect on asphalt mixtures
are: 1) resilient modulus, 2) resistance to water stripping, and 3) tensile strength.
Three mixtures were the subjects of this task. They were:
1) Virgin asphalt mixtures,
2) Virgin mixtures with polar anti-stripping additives, and
3) Recycled asphalt mixtures.
For each mixture, samples were prepared with different heating procedures
that included microwave energy at one time or another. For control, conventional
heating in a convection oven was also used.
II.
Effect of Mixture Variables on Microwave Heating
The effect of mixture variables on microwave heating efficiency was
evaluated by the change in the temperature of compacted specimens after heating.
The major activities of this task were:
1) The arrangement of sample in the microwave oven.
2) The effect of the asphalt mixture variables on microwave heating of
mixtures. Four variables were changed while all other variables were
fixed.' These variables were:
- asphalt content.
- aggregate gradation.
- compaction effort
- initial temperature of fabricated specimens,
m.
Effect of Moisture Content on Microwave Heating
Plain samples of aggregate, coarse and sand, at different moisture contents
were heated in the microwave oven. The effect of water content was evaluated by
measuring the temperature of heated samples. The different activities of this task
were:
1) preparation of testing materials at different water contents.
2) testing, i.e., heating and temperature measuring.
3) defining water content limits of aggregate for microwave heating.
IV. Microwave Heating Enhancers
The effectiveness of the addition of carbon black on microwave heating of
asphalt mixtures was evaluated by measuring the temperature of compacted
specimens after microwave heating. The major activities of this task were:
1) identifying the enhancer.
2) testing and evaluation.
V.
Summary. Conclusions and Recommendations
Finally, summary and conclusions in addition to recommendations and needed future
research are discussed.
CHAPTER TWO
BACKGROUND
I. H istory, of M icrowave H eating in Paving
In the USA, the use of microwave energy as a heating method for pavements was
started by a contract of the FHWA and five States' Departments of Transportation with
the Syracuse University Research Corporation (SURC), presently known as Syracuse
Research Corporation (SRC), in 1969 [2], The system developed by SURC was
designed for rapid patching of portland cement concrete pavements with polymer concrete.
Heat generated by the aggregate matrix subjected to microwaves was supposed to initiate
polymerization reaction. The specially designed premixed concrete (aggregate plus
chemicals) was cured with microwaves after placing it in potholes and on bridge decks.
Some potholes were painted with aluminum powder premixed with the resins, which was
used to allow chemicals to penetrate adjacent concrete. The addition of powdered
aluminum was to reflect microwaves back into the patching mix in order to reduce heating
time. The effectiveness of aluminum addition was not measured because of the
unavailability of sensitive measuring devices. The resulting polymer concrete bonded
very well to old materials with a satisfactory quality for airport and road uses [3,19,20].
For field trials a mobile truck-mounted microwave power generator was used. The
applicator consisted of 8 X 2.5 kw radiating units, for a total of 20 kw, with a frequency
of 2450 megahertz (MHz) covering an area of 8.5 square feet (29 in X43 in). Elaborate
safety features were included in the design of the system. Microwave applicators were
surrounded by a metallic enclosure with a lock system that was part of the on-off
switching mechanism. Also, a remote controlled switching arrangement was used. The
measured power density (leakage) was 0.5 mW/cm2 at 5 cm distance from the enclosure.
10
In the fall of 1971 and 1973 microwave energy was used to seal transverse cracks
in asphalt pavement roads in Canada [15]. One of the cracks was 1/2 inch wide. A 3 in.
strip of the pavement surface including the crack was heated to 150° C (302° F), then
reworked and compacted. The 1/2 in. crack reopened in the winter months. This failure
was attributed to the fact that the crack was located over discontinuous underlying concrete
slabs. The microwave equipment that was used consisted of a 2.5 kw microwave power
unit of 2450 MHz frequency. Microwaves were applied to the asphalt surface by a simple
horn in contact with the road surface. The measured leakage from the applicator was less
than 1 mW/cm2 at a distance of 3 feet
Morris Jeppson and the Microdry Corporation of San Ramon, California, jointly
designed and built a large 100 KW and 915 MHz microwave asphalt pavement heater to
demonstrate the feasibility of microwave heating of asphalt pavements and materials [21].
Microdry also built and owned a demonstration 20 KW pothole patching machine that
was capable of heating 40 in. diameter potholes. The machine was used in Capitola,
California for pothole patching and longitudinal crack repair [22]. In Microdry systems,
applicators were enclosed by a curtain made of closely-spaced short vertical metal chains.
The progress and attention to microwave energy applications in pavement heating was
supported by a U.S. Department of Energy contract with Mr. Jeppson to investigate
microwave methods and equipment for paving [4,23]. A higher power system, up to 400
kw, which may cost more than $250,000, is being studied [24]. Jeppson showed that
energy that might be wasted into base material can be trapped and concentrated into the
surface layer [25]. The experiment included placing aluminum foil at a 5 in. depth within
the surface layer. The foil was capable of reflecting energy to heat desired layer faster, as
shown in Fig. 2.1. As a result of this simple idea, a geotextile fabric laminated with a film
of aluminum was developed for this purpose[26].
11
500
Temperature (F)
400
300
200
100
0
0
5
10
Depth (in.)
Figure 2.1 Effect of energy concentrating aluminum foil implanted 5 in.
in asphalt pavement (2 min. heating with 80 kw pothole
applicator) [25].
12
CYCLEAN Inc. of Georgetown, Texas, has developed a microwave recycling
system that is capable of heating reclaimed asphalt pavement materials as well as new
aggregate [13]. One of the significant advantages of this system, as the name implies, is
the very low pollution level. Gaseous and particulate matter emitted into the air are
claimed to be similar to those emitted from a hot mix in a haul truck. The heating of
materials to job temperature is done in two steps. First, the materials are preheated to
about 250° F in a fluidized bed warm-air oven using a conventional burner. Second,
materials were carried on a conveyor belt into a microwave heating tunnel to finish heating
of materials to 300° F. The configuration of the system is shown in Fig. 2.2. The system
can be mounted on a series of highway trailers which makes mobilization of the system
easy and fast. Electricity for the entire system is supplied by a diesel electric generator.
The microwave tunnel has a power of 200 kw allowing a present maximum production
rate of 200 tons per hour. Units that will be capable of processing 400 tons per hour are
being developed. The present system was used in the summer of 1986 to produce a 100%
recycled asphalt concrete mix for a test project in the city of Austin, Texas. The produced
mix met the city's Hveem limits after the addition of a recycling agent.
In France, a 24 k w , 2450 MHz mobile microwave heating prototype machine was
built and used to conduct cold longitudinal joint and transverse crack repair in different
highways and airports in 1985 [16]. Microwave heating was followed by compaction
only, which did seal cracks and joints along the depth of the pavement. Core samples that
were sawed immediately after repair showed excellent sealing. Failures and lack of
durability of repair in some joints and cracks were blamed on negligence in cleaning them
of dirt in some cases and on defects of the base layer in other cases. The French system
consisted of a heavy truck that is used to transport a minitractor carrying the microwave
generator as well as applicators. Electricity is supplied by a generator that is mounted on
the heavy truck.
100 FEET
-200 FEETBAGHOUSE
HEATER
GENSET
-3- 1 MICROWAVE ■ + ( REJUVENATING^!
m j-O A D IN G
SCREEN
controlt—
L_j
OVERSIZE
PROCESSOR
TRUCK
LOAO OUT
HOPPER
HOUSE
|
I
I
STOCKPILE
Figure 2.2 The central plant system used for heating RAP with microwave
energy and fluidized bed [13].
14
n.
Safety of Pavem ent M icrowave H eaters
Although there are no standards that regulate leakage from microwave heating
machines for paving, the microwave equipment manufacturers have taken the initiative of
conforming to the requirements for home microwave ovens [27]. The Food and Drug
Administration requires that power density at any point 5 centimeters or more from the
external surface of a new microwave oven shall not exceed 1 milliwatt per square
centimeter (1 mW/cm2) [28]. Through the use of some of the leakage control devices that
have been mentioned above, the industry has been successful in meeting these standards
in spite of a power level that is 125 times the power of home ovens.
III. Economics of M icrowave H eating
The economics of microwave heating have been evaluated primarily by comparing
them to hot in place recycling methods. Comparisons similar to that is shown in Table 2.1
obviously favor microwave heating. Hauling costs between site and central plant, fleet
trucks and batch plant are eliminated, resulting in a tremendous savings. Other savings
can be realized when the addition of virgin aggregate and, consequently, extra binder are
avoided. A witness to this assumption is the 100% recycling approach by CYCLEAN that
offers contractors up to 40% saving on production of recycled mixtures [13]. A National
Bureau of Standards report [29] that evaluated a field test of the first generation of
microwave pavement heaters concluded that about 40% to 50% energy savings could be
obtained over competing recycling methods. The uniform, deep, fast heating of over-theroad machines could shorten construction time, thus resulting in savings in labor cost and
traffic interruption cost and time. The anticipated lower air pollution rates should result in
minimizing pollution control costs.
Machines are currently expensive. The prototype machine that was built by
Microdry cost about $250,000, while some proposed systems for over-the-road heating
15
Table 2.1
Comparison of asphalt pavement recycling costs per ton by
conventional and microwave heating methods [25].
Item
Price FOB
Hot Mix Plant/ton
Haul Cost
15 mile@ 50 g/ton-mile
RAP Charge
Paver
Compaction
10% Corrective Agg. Delivered
1% Asphalt
Rejuvenator
Cold Mill
Diesel Generator Fuel
Magnetrons
Operators
Maintenance
Equipment Write-Off
(over 4000 hrs.)
Total Operating &
Equipment Costs
All New
50/50 Hot
Recycling
$22.00
$18.00
$7.50
$ 7.50
$ 1.00
$ 0.50
$0.50
—
$0.50
$0.50
—
—
—
—
—
—
$ 1.00
—
—
—
--
—
—
—
—
—
““
$31.50
Assumptions
- Paving contractor is separate from hot mix
plant operator- so profit is included in the
FOB price of the plant operator.
- Paving contractor profit must be added in
all examples.
- Assume conventional recycling must use
50% RAP plus 50% virgin materials.
- Assume hot mix temperature at plant is 270
F and mix temperature for in-place MW
recycling is 250 F.
$27.50
Microwave
Recycling
—
—
$0.50
$ 0.50
$ 1.44
$ 1.60
$ 1.00
$ 1.00
$ 1.42
$0.50
$ 1.00
$0.50
$4.23
$13.69
16
could cost over $1,200,000 [26], The prototype machine had an overall efficiency of
17% in converting diesel fuel into pavement heating [5]. The fuel utilization efficiency
was increased from 17% to over 28% when hot exhaust gases from diesel engines were
injected into material that had been heated. Innovation and demand should help in
improving efficiency and bringing prices of equipment down.
IV. M icrowave H eating Concent
Microwaves are part of the electromagnetic spectrum, as are radio and TV waves
as shown in Fig. 2.3. However, microwaves are characterized by their small wave length
which is in the magnitude of inches compared to radio waves that are measured in miles.
Microwaves are radiated outward from a central source (a magnetron or a klystron)
millions of times per second and travel at the speed of light carrying small bundles of
energy called photons [30], The Federal Communication Commission (FCC) has
designated frequency bands for Industrial, Scientific, and Medical uses (ISM) as shown in
Table 2.2 [31]. Microwave frequencies that have been developed and used are 915 MHz
and 2450 MHz. The corresponding wave lengths for these frequencies are 32 cm (about
12.5 in.) and 12 cm (about 5 in.).
.... . ^
When microwaves pass through a material, the material is subjected to an
alternating electromagnetic field that is changing millions of times per second. If the
material is electrically neutral, that is it has no electric charge, microwaves will pass
through it like it is not there. Carbon tetrachloride, benzene, paraffin wax and carbon
dioxide are examples of microwave transparent materials. However when a material is not
electrically neutral, its dipolar molecules, which carry a pair of closely spaced charges
equal in magnitude but opposite in sign, tend to act like microscopic magnets in the
presence of microwaves and attempt to line up (polarize) with the field as shown in Fig,
2.4. Maximum polarization occurs when all dipoles align with the applied field.
i
I
-10
20
Figure 2.3 Electromagnetic spectrum.
Gamma Rays &
X-Rays
MO 17
Ultraviolet
Visible”
■MO15
Infrared
m o 13
Microwaves
-10 10
— Dielectric ■
Radiowaves
-
10
*
-105
18
Table 2.2 Frequency bands for ISM equipment [31].
ISM frequency
6.16
13.56
27.12
40.68
915
2450
5,800
24,125
61.25
122.50
245.00
MHz
MHZ
MHz
MHz
MHz
MHz
MHz
MHz
GHZ
GHz •
GHz
Tolerance
± 15.0
± 7.0
± 163.0
± 20.0
+ 13.0
+ 50.0
± 75,0
± 125.0
± 250.0
± 500.0
± 1.0
KHz
KHz
KHz
KHz
M Hz
MHz
MHz
MHz
MHz
MHz
GHz
19
*i
ir
Figure 2.4 Effect of applied alternating electromagnetic field on
dipoles orientation.
20
Polarization is not restricted to dipolar molecules since any relative displacement of
positive and negative charges within the material is considered as a form of polarization.
Other forms of polarization include electronic, atomic and in the case of two adjoining
materials interfacial polarization as illustrated in Fig. 2.5 [32]. The total polarization is the
sum of all these forms of polarization. Molecules usually fail to keep up with the rapid
change in the direction of the field because of some forces that restrict their movement,
such as viscosity or solidity of the surrounding medium and the effect of simultaneous
movements of molecules. In trying to overcome these forces, microwave energy is
converted to heat The time (microsecond) during which molecules lag behind the field
is known as the dielectric relaxation time (x).
Materials differ in their response to microwave energy. Some, such as water and
aggregate, heat very well, while others, like Teflon and asphalt, exhibit poor response.
The penetration of microwaves will be infinite in perfectly transparent substances, zero in
reflective materials such as metals, and a finite value in other absorptive materials. The
efficiency of a material in absorbing microwave energy, which affects the rise of
temperature and penetration of microwaves into the material, may be described by its
dielectric properties. The variables that are of interest are the dielectric constant of the
material (e '), the dielectric loss factor (e") and the dissipation factor or loss tangent of the
material (tan 8). The dielectric constant (e1) influences the amount of energy that can be
stored in the material in the form of electric field. The dielectric loss factor (e") indicates
how much of that energy a material can dissipate in the form of heat. The loss tangent is
equal to e'/e". The loss tangent has its maximum value (high energy dissipation) when
the angular frequency {co=2 ji x microwave frequency (f)} equals the reciprocal of
relaxation time (x). The dielectric constant and loss tangent values have been tabulated for
many materials at different frequencies and temperatures [32,33].
The power absorbed by the material, and thus the rate of temperature rise,
increases with frequency (f), while microwave penetration into the heated substance
21
No Field
Field A pplied
No Field
Field A pplied
£ 3 E 2 ie 3 ^
E3C 3Q 3C 3
G 3E3G 3E3
a. Electronic polarization
©■nofr-o ©-nr~r~r~o
b. Atomic polarization
c. Dipole polarization
+ -
+
- + -
- + - + - +
+ + + - -
- + + -
+
+ -
+- + -+ -
+- + - - +
- + - + - +
- + + - - -
d. Interfacial polarization
Figure 2.5 Types of polarization.
22
increases as frequency decreases. For example waves of 2450 MHz heat faster than 915
MHz waves, but waves of 915 MHz penetrate deeper than those of 2450 MHz.
The following are formulas that describe the relationship between dielectric
properties of the material, and microwave characteristics with power absorption,
penetration and rise in temperature.
The power absorbed in watts/cm3 by the material is calculated from the following
equation [34]:
or
where
P = 55.6 x 10-14 f e’ tan 8 E02
(1)
P = 55.6 x 10-14 f e" E02
(2)
f
= frequency in cycle/sec
e'
= dielectric constant of the material
E0
= field strength in volt/cm
tan 8
= loss tangent of the material
e"
= dielectric loss factor of the material
Tube suppliers or handbooks can supply the typical field strength for this
relationship.
The temperature change rate at the surface of a load subjected to microwaves
exclusive of any heat loss by conduction, convection and radiation or any phase change in
the material heated, °C per second can be calculated from the following equation:
AT/dt = 13.288 x 10-14Eo2 fe ' tan 8
(3)
cp
where:
AT/dt
= rate of temperature rise with time °C/s
23
f
= frequency in cycle/sec
e’
= dielectric constant of the material
E0
= field strength in volt/cm
tan 8
= loss tangent of the material
c
= specific heat of the material cal/g. °C
P
= density of the material gm/cm3
The effect of dielectric properties of the material and microwave frequency on
microwaves penetration in the material can be calculated from the attenuation of the electric
field E in the material which may be expressed as
E = E0 e_kx
(4)
where (k) is the attenuation constant and (x) is the distance in the material at which
E is to be determined and E0 is the field strength at the surface.
For vertical waves
application
7t V e'r tan 8
k
where
X
e’r
S
(5)
wave length in cm.
relative dielectric constant of material which
equals the dielectric constant of the material
over the dielectric constant of vacuum, £'/£&
If all variables are known, theoretically, it is possible to calculate the field strength
at any depth from equation (4), then substitute with calculated values in equation (1) or
(2), then (3) to predict change of temperature at that depth after correcting for losses.
24
V. Dielectric Properties of Pavement M aterials
The importance of dielectric properties of materials for microwave heating is clear
from the previous section.
The properties of interest are e', tan 8 and e". These
properties are functions of temperature, frequency and pressure [32].
The dielectric properties of asphalt cement at ISM frequencies are very low. In
fact asphalt cement is assumed to be transparent to microwaves.
Aggregates also have low dielectric properties at high frequencies. The lattice
forces in solids tend to restrict the orientation of polar molecules [35]. Dielectric
properties of some types of aggregates, soils , rocks and minerals are available
[5,16,32,36]. Dielectric properties of asphalt cement and some types of aggregate are
listed in Table 2.3.
Jeppson [5], however, attributed the success of microwave heating of asphalt
pavement materials to the low specific heat of aggregate. He showed that the energy
required to raise the temperature of asphalt pavement materials is 1/5 the energy required
to raise the temperature of an equal weight of water by the same temperature margin as
follows:
AE = m . c . AT
where AE is absorbed energy, m is mass in grams, c is the specific heat of the
material in cal/gm. °C and AT is rise in temperature °C.
For water and pavement materials:
AEw
AEp
m c w AT
m c p AT
cw
cp
specific heat of water =1.0
specific heat of aggregate = 0.2 and
1 _5
1
25
Table 2.3 Dielectric properties of asphalt pavement materials.
Temp.
°C
freq.
MHz
e'
tan 8
10'4
Ref.
Water
25
25
300
3000
77.5
76.7
160
1570
[32]
[32]
Asphalt cement
AC 60/70 Esso
AC 40/50 Shell
AC 180/220
26
20
20
20
3000
2450
2450
2450
2.5
2.43
2.52
2.45
11
—
[32]
[16]
[16]
[16]
Aggregate -Diorite
20
2450
5.6-7
178 - 357
[16]
Asphalt concrete
with diorite
with limestone
withquarzite
20
20
20
2450
2450
2450
5.8
6.7
4.0
344
149
62
[16]
[16]
[16]
Material
—
—
26
specific heat of asphalt = 0.3 (ignored because asphalt
content <10% of mix weight)
However, the ability of water to absorb and convert microwave energy into heat is
higher, by a great margin, than that of aggregate as indicated by their dielectric properties.
Another factor that might have contributed to the reported favored response of
aggregate to microwave energy was the presence of moisture in tested aggregates. It was
found in the current research that a negligible amount of moisture as low as 0.5%, in the
pores of aggregate was enough to boost microwave heating of aggregate in a microwave
oven. Details about this point are presented in Chapter 5.
Fig. 2.6 and 2.7 show temperatures of plain aggregate samples and pavement
mixtures that were heated by microwaves as given in references [5 and 16].
Microwave absorption characteristics of asphalt mixtures vary with the aggregate
type used. It is clear that quartzite, which is almost entirely quartz (Si02), is the aggregate
that can be least efficiently heated with microwaves, This should be of no surprise, since
quartz is an electrically neutral mineral where all four oxygen ions of the unit silica
tetrahedron are shared, thus satisfying all charges. The presence of quartz in aggregate
could lower its microwave heating efficiency, and as the amount of quartz increases the
efficiency decreases. By listing chemical compositions of some aggregates that are similar
to some of those in Fig. 2.6, the expected relationship between quartz content and
microwave heating of aggregate is supported, as shown in Table 2.4. In addition to low
quartz content, aggregates which heated fast with microwaves contained high percentages
of metallic minerals such as augite, hornblende and plagioclase. Aggregate that heated well
with microwaves also had high specific gravity. As a rule of thumb, aggregate heating
efficiency increases inversely with its content of quartz, but in direct proportion with its
metallic components such as aluminum, iron, magnesium..etc., and hence with its specific
gravity due to their high metallic contents. For example igneous rocks that contain less
than 45% quartz would be a good source of a better microwave heating aggregate.
27
diorite
gabbro
amphibolite
150
granite
schist
GT100
sandstone
limestone
75
water
quartzite
50
25
Heating Time (min.)
Figure 2.6 Comparative heating of aggregates in a microwave oven 2 450 MHz
600 W on 300 gr. samples at room moisture [12].
28
— 300° F
Sand - Incline, NV
---------------
Crushed road rock - Truckee, CA
250° F
Fresh asphalt mix - Washoe County, CA
Glass rock-Lake County, CA -------------
7
River bed gravel - Carmel, C A ------------200° F
Old concrete - Monterey, C A --------------
— 150° F
Water
— 100° F
Pyrex
Quartz
Pure Asphalt
t
—
50° F
—
0° F
Initial Temperature
Figure 2.7 Comparative heating in a 500 watt 2450 MHz microwave oven for
two minutes for a range of materials used in pavements [4].
Table 2.4
Chemical composition of some aggregates in relation to
microwave heating efficiency.
Agg. type
MW Heating
SiC>2 (%) Metallic Mineral (%)* Specific
rank ( Fig. 2.6) [Ref. 37]
[Ref. 37]
Gravity [Ref. 37]
Diorite
1
8
Gabbro
2
Amphibolite
3
3
Granite
4
30
Schist
5
36
Limestone
6
—
—
2.66
Sandstone
7
79
—
2.54
Quartzite
8
> 84
—
2.69
—
H(27), P(30)
2.92
P(44), A(28), H(9)
2.96
H(70), P(8)
3.02
P(B)
2.65
2.85
* where (A): Augite, (H): Hornblende, and (P): Plagioclase.
30
Most aggregates are heterogeneous, the presence of several secondary chemical
compositions is very common, and their type may vary from one source to another.
Therefore, deviation from these sets of rule of thumb may occur.
CHAPTER THREE
EFFECT OF MICROW AVE HEATING ON ASPHALT MIXTURE
PRO PERTIES
I. In tro d u c tio n
The concern about the effect of microwave heating on the quality of asphalt
mixtures, whether they are fresh or recycled, arises from a number of factors that are
associated with microwave heating. First, microwave heating is fast, all the material being
heated simultaneously. Second, the treated material is forced to generate its own heat
from within. Finally, the heating is a result of arid is accompanied by polarization of
molecules.
Rapid heating of materials should cut exposure time of asphalt cement to high
temperature. Also, with simultaneous heating, overheating of asphalt surfaces in order to
heat up the whole pavement layer is avoided. In recycling, superheating virgin aggregate
to heat reclaimed material without overheating asphalt cement may not be necessary, since
the heating comes from coated aggregate and the fear of burning the asphalt is reduced.
Thus, asphalt cement is not expected to age as much when microwave heating is used as
with conventional heating methods. In two separate studies [17,18], asphalt mixtures
heated in microwave ovens gave lower Marshall stabilities than mixtures heated
conventionally. Asphalt cement extracted from microwave heated mixtures had higher
penetration [17]. Thus, the design of recycled mixtures should change if microwave
heating is adopted. At least, the amount of asphalt rejuvenator would be decreased, since
increase in viscosity due to reheating of RAP will be minimized because of rapid heating
of microwaves.
When pavement material is exposed to microwave energy, aggregate will be
generating and transferring heat to the asphalt cement. As long as microwaves are
32
applied, heat generation will continue. The added asphalt cement, though it will absorb
some of the heat, it will not stop the temperature from rising in the mix. The environment,
i.e., the air temperature, around the self-heating material is cooler than the material itself.
In recycled mixtures, asphalt cement film on aggregate particles might be melted,
redeposited and even impregnate permeable voids in the particles due to the continuous
heating as shown in Fig. 3.1 (a). Thus, chances for improved adhesion of asphalt
cement to aggregate and resistance to water-stripping action might be better.
»
Polarization and alignment of polar molecules and charges on aggregate surface
and compatible binder are also expected to improve adhesion. While polarization would
be responsible for orienting dipolar molecules within one material, interfacial polarization
would be responsible for bringing opposite charges on adjoining surfaces to accumulate
along the interface as illustrated in Fig. 3.1 (b) and (c). Randomness in orientation of
polar molecules in asphalt cement also might be reduced for higher cohesion and shear
resistance, if enough energy is available to overcome the asphalt cement viscosity.
The use of polar anti-stripping agents to promote adhesion of asphalt to aggregate
could benefit from the polarization effect of microwaves. Positively-charged (cationic)
emulsifiers migrate to and get adsorbed by the aggregate surface, lowering its affinity to
water and increasing its affinity to o il. This preferential modification of aggregate surface
charge favors asphalt cement over water, resulting in a stronger adhesion and waterstripping resistance. The degree of success of these agents depends on the concentration
of the surfactant used, the efficiency of migration and the force or strength of adsorbing
bond. The addition of these materials to asphalt cement in hot mixing is believed to be
inefficient [38]. The migration of agent to the aggregate interface is hindered by the
increase in asphalt viscosity upon cooling. In typical hot mix, unless the mix is kept hot
for a long time (12 hours), only approximately 30-40 percent of the original concentration
of anti-stripping agent is performing in the proper manner [38], The microwave energy
could be used to speed up the migration of agents by forced polarization action as shown
33
B e f o r e MW
A f te r MW
Asphalt Film
Asphalt Film
Aggregate
*.**>:*, Aggregat0
a) Heating and melting effect
Asphalt Film
Asphalt Film
<3>
-g
a 1
Aggregate
a
Aggregate
b) Molecular orientation effect
Asphalt Film
Asphalt Film
•
*
ill Aflsre9ate lllli
l i l Aggregate
•I*!*!*!*
c) Polarization effect
Asphalt with cationic
surfactant
Aggregate
j
| | | | | |
Asphalt with cationic
surfactant
|1
| | | | Aggregate'
d) Enhancem ent of polar additives migration
Figure 3.1 Mechanisms of asphalt adhesion improvement with microwave
energy treatment.
34
in Fig. 3.1 (d). The least microwaves can do is to reduce the randomness in the
orientation of charged molecules at the interface of the aggregate surface between agent
and aggregate on one side and agent and asphalt cement on the other.
The viscosity of
the asphalt at which polarization is optimum is a key factor in taking advantage of this
phenomenon. The benefit from microwave polarization may not be attainable if it
becomes necessary to heat asphalt cement to unacceptable mixing temperatures. At this
time, it is not known if there is an asphalt cement which has a viscosity that would permit
polarization at or below mixing temperatures. If no such asphalt exists, it might be
possible to modify an asphalt chemically for this purpose. Interfacial polarization may
not be as strong at microwave frequencies as it is at radio frequencies. That little effect in
addition to dipolar polarization could cause a favorable result. On the other hand, if
poorly compatible aggregate and binder are treated with microwave energy, mismatching
and a double layer may occur, resulting in a weaker bonding, or complete debonding.
In summary, fast heating of microwaves in conjunction with its polarization effect
would reduce asphalt cement aging and would improve the bonding of asphalt to
aggregate, as well as its resistance to water action. Furthermore, the increase in the
polarity of the binder would increase its ability to polarize under the applied field,
yielding stronger bonding.
II. E x p erim e n t
The relative effect of microwave heating on asphalt aging, bonding and, thus, on
the stiffness of the mixture was evaluated by the diametral resilient modulus test as
described by Schmidt [39]. In conjunction with the resilient modulus test, Lottman's
procedure to predict moisture damage to asphalt mixtures [40] was used to assess
microwave-heated mixtures resistance to water action.
In order to address the issues that have been discussed above, three groups of
mixtures were tested. Recycled mixtures were used to evaluate the asphalt aging issue.
35
Mixtures with added polar anti-stripping agent were used to examine the polarization effect
of microwave heating. Virgin mixtures were used to investigate microwave heating on
fresh mixtures, which is necessary if this method of heating is to be used in their
production.
Several heating methods were used in preparing specimens from the three groups
of mixtures.
Conventional heating, microwave heating alone, and the latter in
combination with conventional heating were the three basic heating methods. These
were based on present heating systems that incorporate microwave heating such as the
CYCLEAN system [13], in addition to potential applications. The experiment design is
summarized in Table 3.1.
t
A. P reparation of M ixtures and Specimens
1. Virgin Plain Mixtures:
a. Materials:
These mixtures were prepared from the same sources of materials used earlier.
The aggregate gradation that was used conformed to the Washington State Department of
Transportation (WSDOT) class B aggregate as shown in Table 3.2. The asphalt cement
content was 5.5% of the total weight. Each sample was prepared by adding 70 gm. of
asphalt to 1200 gm. of aggregate.
b. Methods of Mixture Preparation :
Mixtures were prepared in several different ways that represented current and
expected use of microwave energy in treating pavement materials. These methods are as
follows:
36
Table 3.1
Summary of the experiment design.
Heating method/Mix.
Plain
Tallow Tetramine
Convection oven
(A&B)[1&2]
(A&B)[1&2]
Microwave oven
(A)[l&2]
(A)[l&2]
(M)[l&2]
—
(M)[l&2]
—
Recycled
(M&R)[1&2]
Convection plus MW zapping for
0.5 min.
2
min.
(M)[l&2]
(M)[l&2]
(M)[l&2]
5
min.
—
(M)[l&2]
—
—
—
(M)[l&2]
Convection plus MW
supplemental heating
A: Plain aggregate.
M: Asphalt mixture.
1: Resilient modulus test
B: Asphalt cement
R: Rejuvenator.
2: Split tensile strength.
37
Table 3.2 Gradation of aggregate used in mixtures.
Sieve Size
5/8"
1/2
3/8"
1/4
#10
#40
#80
#200
WSDOT Class B
% passing
100
90-100
75-90
55-75
32-48
11-24
6-15
3-7
Used Gradation
% passing
100
100
82.5
64
39.5
17
9
5
38
i) Conventional Oven (CC): Plain aggregate was heated overnight at 330° F in a
conventional oven. Hot asphalt of 300° F was added, then hand mixed until
uniform coating was achieved. The mixture was placed in a Marshall mold and
compacted with 50 blows of a Marshall mechanical compactor on each face. The
temperature of the mixture before compaction ranged from 287 to 290° F. This
mixture was used as a control.
The procedure of preparation is illustrated
graphically in Fig. 3.2.
ii) Microwave Heating (MW): Since aggregate is the component of an asphalt mixture
that would heat when subjected to microwaves, only plain aggregate was heated in
the microwave oven. Asphalt cement was heated in a conventional oven to 300° F
since its dielectric properties were not suitable for microwave heating . Aggregate
was kept at 75° F until microwave treatment. Moisture content of aggregate ranged
from 0.3% to 0.5%. Aggregate was heated in a ceramic plate on top of a turntable in
the microwave oven for 7 minutes. This was followed immediately by 3 minutes
heating in the microwave oven without the turntable, in order to get the center of the
sample as hot as its edges. Preliminary trials showed that the use of the turntable
had resulted in heating the edges more than the center of the sample. The sample
came out of the microwave oven after this sequence of treatment at a temperature
above 330° F at most points, but well above 330° F in some places. Before adding
the hot asphalt the aggregate sample was placed in a hot bowl on a hot plate and left
to equalize, until the maximum temperature observed was 330° F. Hand mixing of
dry hot aggregate was repeated 3 to 4 times to enhance temperature uniformity
throughout the mixture. It took the aggregate mixture from 6.5 to 7 minutes to cool
down to the desired temperature. Temperature ranged between 314° to 330° F in the
aggregate mixture before the adding of asphalt cement. Cooler points were
expected to be at the surface and at the edges of the bowl. The minimum
Plain aggregate in
conv. oven
330 F
Hand mixing
AC in conv.
oven
300 F
Figure 3.2 Steps of preparing conventionally heated virgin mixtures.
Compaction
40
temperature of the asphalt mixture before compaction was 275° F. Marshall
specimens were prepared with 50 blows on each face using a Marshall mechanical
compactor. Fig. 3.3 shows this method of preparation.
iii) Conventional Oven Plus "Zapping" in Microwave Oven (CZMW): Both aggregate
mix and asphalt cement were heated in a conventional oven to 330° F and 300° F
respectively. After hand mixing of asphalt and aggregate, the mixture was then
treated in the microwave oven using a turntable for 2 minutes. The temperature of
the mixture dropped due to the cooling effect of the microwave oven stirring fan and
the lag of microwave heating during those two minutes as shown in Table 3.3. The
objective of zapping was to induce molecular orientation if possible, rather than to
heat the mixture. Therefore, temperature after zapping was assumed satisfactory,
especially since the mixture's temperatures were close to those of other treatments
prior to compaction. Mixtures were then hand remixed and compacted. This
method of preparation is shown in Fig. 3.4.
2. Virgin Mixtures with Polar Anti-stripping Agents:
a. Materials:
These mixtures were similar to the plain mixtures in A. The only difference was
that a 0.5 % of a polar cationic surfactant was added to the asphalt cement A fatty amine
anti-stripping agent of the commercial name Tallow Tetramine from Exxon (Tomah) was
used.
b. Preparation of the Binder:
In preparing the binder, asphalt cement was heated to 300° F in a metal can. Anti­
stripping agent was added, then mixed with a mechanical mixture for 3 minutes, covered
Plain agg. in
MW oven
for 10 min.
300 - > 300 F
Hand mixing
330 F
Hand mixing
AC in conv.
oven
Figure 3.3 Steps of preparing microwave heated virgin mixtures.
Compaction
42
Table 3.3
Temperature of hot virgin asphalt mixtures after 2 minutes zapping in
the microwave oven.
Sample no.
Temp, of hot mix
Max. Temp, after MW zapping
1
300
220 *
2
306
285
3
295
287
* Power level was 120 watts (20% of maximum power used with other samples).
5
Plain agg. in
conv. oven.
330 F
\
f
AC in conv.
oven
Hand mixing
300 F Zapping 2
min. in MW
oven
=280 F
Hand mixing
300 F
Figure 3.4 Steps of preparing conventional heated plus microwaves zapped virgin mixtures.
Compaction
44
and stored cool. When the mixture was to be prepared, asphalt was heated to 300° F,
then mixed for 1/2 minute before adding to hot aggregate.
c. Methods of Mixture Preparation:
Mixtures were prepared in a similar fashion to that used for virgin plain mixtures.
Emphasis was put on microwave zapping.
Three different zapping times were tried; 0.5
min. (CZMW(0.5)-T ), 2 min. (CZMW(2)-T) and 5 min. (CZMW(5)-T). Additionally,
conventionally (CC-T) and microwave heated (MW-T) mixtures were prepared. The
change in the temperature of mixtures after the various zapping times is shown in Fig.
3.5.
3. Recycled Mixtures:
a. Materials:
Mixtures were prepared from artificially aged materials that were similar to those
used with virgin mixtures. However, asphalt cement was reduced to only 4% of the total
weight Rejuvenator (Pester RA-275) was added to aged materials in the amount of 2% of
total weight.
b. Artificial Aging of Asphalt Mixtures:
In order to eliminate as much variability as possible in recycled mixtures, it was
decided to age materials in the laboratory. To further insure tight control on materials,
batches of 1200 gm aggregate of identical gradation as in Table 3.2 were mixed with 4%
asphalt cement before aging. All individual batches were heated together in a forced draft
oven at 220° F for 27 hours. Matrecon Inc. of California performed viscosity and
penetration tests on virgin asphalt cement samples and on extracted asphalt cement from
the artificially aged samples. Test results showed that the absolute viscosity at 140 F
45
47.
Change in Initial Temperature (F)
50
40
30
20
8.5
10
:0.5
-10
-20
-13.5
12.5
Microwave Zapping Time (min.)
Figure 3.5 Change in temperature of mixture with Tallow Tetramine after varying
microwave zapping time.
46
(ASTM D-2171) had increased from 1170 to 14,800 poise, and the penetration at 77 F
(ASTM D-5) had decreased from 85 to 25.
c. Methods of Mixture Preparation:
Preparation of mixtures involved four basic steps: 1) heating RAP to about 300° F
before the addition of rejuvenator, 2) addition of rejuvenator and mixing, 3) curing at 270280° F for 20 minutes in conventional oven, and 4) remixing and compaction. Three
heating methods involving microwave energy treatment plus one control representing THE
conventional heating method were used. Specimens were also fabricated from unaged '
materials and aged materials before the addition of rejuvenator. The heating methods and
preparation procedures used were:
i) Virgin Mixture (VM): Aggregate and unaged asphalt cement were heated in the
conventional oven then hand mixed and compacted. Asphalt cement amounted to
4% of the total weight
ii) Aged (RAP) Mixture (AM): Virgin mixture (VM) samples produced as in 1 above
were artificially aged according to the procedures described earlier to produce these
mixtures. After cooling overnight, aged samples were heated at 300 F for 2 hours in
the conventional oven, then remixed and compacted. No rejuvenator was added and
binder content was only 4% of total weight
iii) Conventional Recycle (CC-R): Three aged material samples were heated for 2 hours
in a conventional oven at 300° F. Rejuvenator in the amount of 2% of mixture
weight was added and hand mixed until a uniform dispersion of the rejuvenator was
observed. The product mix was then left to cure in an oven at 275-280 F for 20
minutes. After the curing period, the material was remixed and compacted with a
47
Marshall mechanical compactor with 50 blows on each face and left to cool at room
temperature. This method of heating is shown in Fig. 3.6.
iv) Microwave Recycle (MW-R): Three aged material samples at room temperature of
75° F were heated in the microwave oven for a total of 10 minutes ( 7 minutes on
turntable plus 3 minutes without the turntable). The use of the turntable caused the
edges to heat more than the center, as has been mentioned earlier. The temperatures
of these materials after microwave heating are shown in Table 3.4. Mixtures were
then hand mixed to enhance temperature equalization before the addition of the 2%
rejuvenator. The product mix was then left to cure in an oven of 275-280 F for 20
minutes. After the curing period, the material was remixed and compacted with a
Marshall mechanical compactor with 50 blows on each face and left to cool at room
temperature. Fig. 3.7 shows this preparation procedure schematically.
v) Conventional Plus Zapping Recycle (CZMW-R): Loose aged materials were heated
in a conventional oven of 300 F for 2 hours. The rejuvenator was then added and
hand mixed with hot materials. That was followed by 2 minutes heating (zapping)
in the microwave oven. Temperature of the sample before and after zapping are
shown in Table 3.4. Zapped materials were left to cure in an oven of 275-280 F for
18 minutes. After the curing period, the material was remixed and compacted with a
Marshall mechanical compactor with 50 blows on each face and left to cool at room
temperature. The preparation procedures of zapped materials is shown in Fig. 3.8.
vi) Conventional Plus Microwave Oven Supplemental Recycle (CSMW-R): Three aged
material samples were left in a conventional oven of 250 F for 2 hours before they
were heated in microwave oven for 4 minutes on a rotating table. The temperature of
the loose material samples before and after microwave heating are shown in Table
3.4. After microwave heating, the materials were remixed by hand, and the 2%
RAP in
conv. oven
for 2 hrs.
300 F
Hand mixing
Curing 20 min. 280 F
conv. oven
Hand mixing
Rejuv, in
conv. oven
280 F
Figure 3.6 Steps of preparing conventionally heated recycled mixtures.
Compaction
49
Table 3.4
Sample no.
Temperature of recycled loose samples before and after microwave
heating.
MW-R
CZMW-R
CSMW-R
1
72.5 ~ > 245-375
275 — > 300
250 —> 275
2
72.5 —> 240-370
280 —> 245-300
250 —> 305
3
72.5 --> 230-340*
2 7 0 —> 243-283
250—> 300
* One spot smoked at 480 F
RAP in
MW oven
for 10 min..
=300 F
Hand mixing
Curing 20 min. 280 F
conv. oven
Hand mixing
Rejuv. in conv.
oven
280 F
Figure 3.7 Steps of preparing microwave heated recycled mixtures.
Compaction
RAP in
conv. oven
2 hrs.
300 F
Hand
mixing
Rejuv. in
conv. oven
Zapping 2
min. in MW
oven
Curing 18
min. in
conv. oven
Hand
mixing
280 F
Figure 3.8 Steps of preparing conventionally heated plus microwave zapped recycled mixtures.
Compaction
52
rejuvenator was added and mixed. These materials were then stored iri the
conventional oven of 275-280 F for the curing period of 20 minutes . After the
curing period, the material was remixed and compacted with a Marshall mechanical
compactor with 50 blows on each face and left to cool at room temperature. Fig. 3.9
shows this preparation procedure schematically.
B. T esting
Since the resilient modulus test is non-destructive, each sample was used for MR
determination dry, MR after water conditioning and finally, failure by split tensile
strength.
The resilient modulus test was conducted at about 74 F before and after Lottman
/
conditioning. Applied load was 100 lb. at a frequency of 20 cycles per minute and
loading duration of 0.1 second. Each specimen was tested twice by rotating it 90 degrees
and by taking the average of 5 readings after 50 conditioning pulses. Prior to dry testing,
specimens were stored in a temperature control chamber overnight. In the case of tests
after moisture conditioning, specimens were placed in a water bath at 74 F for at least 2
hours. A split tension test was also performed at 74 F with loading rate of 2 in./min.
III. Test Results and Evaluation
A. Virgin Mixtures
Tests results for all virgin mixtures are summarized in Table 3.5. The resilient
moduli (MR) before and after the one cycle of freeze and thaw conditioning are shown in
Fig. 3.10 and Fig. 3.11 respectively. A comparison between the averages of the resilient
modulus of the two conditions for all mixtures is shown in Fig. 3.12. The percent MR
retained after conditioning is plotted against method of preparation for all specimens in
RAP in conv.
oven 2 hrs.
250 F
RAP in MW
oven 4 min.
=290 F
mixing
Rejuv. in
conv. oven
Curing 20
min. in
conv. oven
280 F
Hand
mixng
280 F
Figure 3.9 Steps of preparing conventionally plus microwaves heated recycled mixtures.
Compaction
Table 3.5 Summary of test results on virgin mixtures.
Density
p/cf
Voids
%
Mr Dry
1,000 psi
CC1
CC2
CC3
148.2
148.2
148.2
3.71
3.70
3.74
239.26
227.67
227.91
MW1
MW2
MW3
147.5
148.1
149.0
3.98
3.67
2.99
248.92
234.95
278.16
CZMW1
CZMW2
CZMW3
148.0
148.2
148.5
3.79
3.40
3.80
265.27
259.00
263.6
Specimen
no.
Test temp. (F)
Loading rate
Loading duration
73°
20 pulse/min
0.1 sec.
Avg.
1,000 psi
MRCond.
1,000 psi
231.61
207.98
185.55
197.32
254.01
169.19
169.08
206.96
262.62
211.04
213.63
234.72
Avg
1,000 psi
%M r
Retained
196.95
86.9
81.5
86.5
181.74
67.9
71.9
74.4
219.8
79.5
82.4
89.0
73°
20 pulse/min
0.1 sec.
CC:
Conventional oven heating
CZMW: Conventional heating plus zapping in microwave oven
MW: Microwave oven heating
Avg. % Mr
Retained
st
psi
Avg. St
psi
85
73.4
99.03
106.4
92.9
71.5
70.5
82.88
89,5
81
83.7
111.5
111.1
119.4
73°
2 in/min
114
55
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
300
280
(jsdooo'l.)
260
240
220
200
CC
MW
CZMW
Heating Method
Figure 3.10 Effect of heating method on resilient modulus
of virgin asphalt mixtures.
56
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
240
220
8.
200
cc
s
180
160
CC
MW
CZMW
Heating Method
Figure 3.11 Effect of heating method on resilient modulus of virgin
asphalt mixtures after Lottman conditioning.
57
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
Dry
j
|
After Lottman
300
250 "
mm
-+■
+
MW
Heating Method
Figure 3.12
CZMW
Effect of heating method on resilient modulus of
asphalt mixtures (averages).
58
Fig. 3.13 and for average values in Fig. 3.14. The results of the diametral split tensile
strength (St) test after conditioning are shown in Fig.3.15. Average values of St are
plotted on Fig. 3.16.
Results show that CZMW mixtures had a higher MR before and after conditioning
and higher St than conventionally prepared mixtures (CC). Zapped mixtures were the
only material that was entirely treated with microwaves. The apparent increase in MR is
probably due to the improved coating and adhesion that was caused by heat generated
from aggregate particles coated with asphalt cement. It is possible that heat from zapping
caused the asphalt to flow and satisfy aligned charges on the aggregate particles' surface
and even to flow into permeable voids. Although the temperatures of mixtures did not
change significantly, as shown in Table 3.3, aging of the asphalt cement may have been
hastened by the extra heating. CZMW mixtures were also remixed after zapping as shown
in Fig. 3.4, which may have contributed to the apparent increase in the stiffness of the
mixture. If microwave zapping is to increase the stiffness of the mixture, it should be
considered whether an early strength gain is desired or unwanted. Tender mixes might be
zapped with microwaves to stiffen them. The failure surface after the split tension test
showed no signs of stripping and compared very closely to conventional mixtures . The
percent MR retained was the same as for conventional mixture values.
Mixtures prepared from microwave heated aggregate (MW) show the second
highest averaged dry MR. When one look at actual data, however, they appear to be
widely scattered. One of the points is high, causing the mean value to be to the high side.
However, after conditioning, MR suffered about 29 % loss, which made MR after
conditioning is even lower than that of conventionally prepared mixtures. The lack of
uniform heating of aggregate with microwaves and the presence of aggregate particles that
were not sufficiently hot when hot asphalt was added and mixed did not facilitate good
coating and adhesion. Steps that were taken to insure uniform temperature through the
59
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
100
95
90
g
85
CC
2
80
75
l
CC
70
65
60
CC
MW
CZMW
Heating Method
Figure 3.13
Effect of heating method on the percent of retained resilient
modulus of virgin mixtures after Lottman conditioning.
60
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
1 0 0 -95 ■■
90
CC
MW
CZMW
Heating Method
Figure 3.14 Effect of heating method on the percent of retained resilient
modulus of virgin mixtures after Lottman conditioning (averages).
61
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping in microwave oven
I
120
110
100
I
co
90
80
70
60
50
CC
Figure 3.15
MW
CZMW
Heating Method
Effect of heating method on split tensile strength
of virgin mixtures after Lottman conditioning.
Conventional oven heating
MW:
Microwave oven heating
CZMW:
Conventional heating plus zapping In microwave oven
st (psi)
CC:
t :.I.J
CC
MW
CZMW
Heating Method
Figure 3.16 Effect of heating method on split tensile strength of
virgin mixtures after Lottman conditioning (averages).
63
aggregate mix probably were not enough. The scattering of points in Fig. 3.10 indicates
that uniform heating was not achieved. The examination of the failure surface after the
split tension test revealed severe stripping action. In addition to probable defect in
coating, the alignment of charges on the aggregate surface which were not satisfied by
asphalt cement may have increased the affinity of aggregate to water.
The split tensile test results correspond very closely with MR results after
conditioning for all mixtures as shown in Fig. 3.15 and Fig. 3.16.
The results of a statistical analysis of the data by Wilcoxon's rank-sum test [41] at
a = 0.05 are summarized in Table 3.6.
B. Mixtures with Anti-stripping Agent:
Tests results for all mixtures are summarized in Table 3.7. The resilient modulus
(M r )
before and after the one cycle of freeze and thaw conditioning are shown in Fig.
3.17 and Fig. 3.18 respectively. A comparison between the averages of resilient
modulus of the two conditions for all mixtures is shown in Fig. 3.19. The percent
M
r
retained after conditioning is plotted against the preparation method for all specimens in
Fig. 3.20 and for average values in Fig. 3.21. The results of the diametral split tensile
strength (S[) test after conditioning are shown in Fig. 3.22. Average values of St are
plotted on Fig, 3.23.
Test results show that mixtures made with microwave-heated aggregate (MW-T)
had a higher dry M
r
.
This is believed to be due to the improved orientation of aggregate
surface charges. Again, the lack of uniform microwave heating is considered to be the
major reason for the greater loss of strength after water conditioning. Yet, M
r
and St after
the freeze-thaw cycle are slightly higher than conventional mixing (CC-T). If percent MR
retained is the criterion used with which to evaluate mixtures, CC-T mixtures would have
an advantage over MW-T mixtures. If the value of MR is used to judge mixtures, then
64
Table 3.6 Results of the statistical significance test on virgin mixture data.
MW vs. CC
CZMW vs. CC
CZMW vs. MW
M r dry
SH
SH
NS
M r conditioned
NS
SH
SH
Retained Mr (%)
SL
NS
SH
St
SL
SH
SH
Test /Heating method
SH: Significant - Higher
SL: Significant • Lower
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW: Conventional heating plus zapping in microwave oven
NS: Not significant
Table 3.7 Summary of test results on virgin mixtures with Tallow Tetramine.
Specimen
no.
Density
p/cf
Voids
%
Mr Dry
Avg. M r Cond.
1,000 psi 1,000 psi 1,000 psi
188.23
87.00
89.70
98.50
st
Avg. % M r
Retained
psi
91.70
99.70
106.20
110.30
105.40
82.60
114.10
110.00
116.40
113.50
103.20
105.10
114.90
110.00
115.70
132.20
205.20
249.05
237.50
219.30
235.30
211.30
183.90
188.00
194.40
84.80
77.40
85.70
5.32
4.50
201.80
229.20
215.50
203.30
241.80
222.50
100.70
105JO
145.25
147.13
147.16
3.79
4.18
4.33
215.40
222.80
225.20
221.10
209.90
222.20
233.00
221.70
97.40
99.70
103.50
100.30
114.20
117.00
116.00
147.77
148.44
3.94
3.48
264.20
258.80
261.50
260.50
256.78
258.64
98.60
99.20
98.90
132.70
131.60
146.70
146.84
147.51
4.45
4.23
3.97
214.50
195.70
205.50
MW-T1
MW-T2
MW-T3
147.66
147.28
146.99
3.93
4.24
4.31
CZMW(03)-T1
CZMW(0.5)-T2
145.62
146.95
CZMW(2)-T1
CZMW(2)-T2
CZMW(2)-T3
CZMW(5)-T1
CZMW(5)-T2
CC-T:
MW-T:
CZMW-T(n):
%M r
Retained
186.60
175.60
202.50
CC-T 1
CC-T 2
CC-T 3
Test Temp. °F
Loading Rate
Load Duration
Avg.
1,000 psi
73°
20 pulse/min
0.1 Sec.
73°
20 pulse/min
0.1 Sec.
Conventional oven heating
Microwave oven heating
Conventional heating plus zapping in microwave oven. Number in parentheses indicates u pping time.
Avg. St
psi
73°
2 in/min
66
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
270
260
250
&
240
C.
230
2
220
210 *
i
200
190
CC
MW
CZMW(0.5)
Heating method
CZMW(2)
CZMW(5)
Figure 3.17 Effect of heating method on resilient modulus of virgin
mixtures with Tallow Tetramine.
67
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
270
260
*
250
4 ►
240
&
<
230
4 ►
220
(E
2
►
210
4
200 4
190
°
►
4
*
180
< *
170
CC
:
►
>
MW
CZMW(0.5)
Heating method
CZMW(2)
CZMW(5)
Figure 3.18 Effect of heating method on resilient modulus of virgin mixtures
with Tallow Tetramine after Lottman conditioning.
68
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
300
after Lottman
250
200 ■■
8.
§
150-
(E 100-2
50-.
fill
0CC
MW
CZMW(0.5)
CZMW(2)
Heating method
CZMW<5)
Figure 3.19 Effect of heating method on resilient modulus of virgin
mixtures with Tallow Tetramine (averages).
69
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
110-
_
.
.
_
10S .
C
S'S"
i>
<*
100.
*►
cc
t
i:
CC g c ,
i 90<
■i
<
IS 85
_
«
,f
so ■
i►
75CC
MW
CZMW(0.5)
Heating method
CZMW(2)
CZMW(5)
Figure 3.20 Effect of heating method on the percent of retained resilient modulus of
virgin mixtures with Tallow Tetramine after Lottman conditioning.
70
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping In microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
120t
103
100
2
100
■-
60 -
«2 40 ■■
MW
CZMW(0.5)
CZMW(2)
Heating method
CZMW(5)
Figure 3.21 Effect of heating method on the percent of retained resilient modulus of
virgin mixtures with Tallow Tetramine after Lottman conditioning
(averages).
71
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping in microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
135
130
125
120
115
co
110
'
105'
100
,
95
CC
2 MW
CZMW(0.5)
CZMW(2)
Heating method
CZMW(5)
Figure 3.22 Effect of heating method on split tensile strength of virgin
mixtures with Tallow Tetramine after Lottman conditioning.
72
CC:
Conventional oven heating
MW:
Microwave oven heating
CZMW (0.5): Conventional heating plus zapping In microwave oven for 0.5 minutes
CZMW (2):
Conventional heating plus zapping in microwave oven for 2 minutes
CZMW (5):
Conventional heating plus zapping in microwave oven for 5 minutes
140 T
114
120
00
S
co
132
105
-
80
60
40
2 0 --
CC
MW
CZMW(0.5)
CZMW(2)
Heating method
CZMW(5)
Figure 3.23 Effect of heating method on split tensile strength of virgin mixtures
with Tallow Tetramine after Lottman conditioning (averages).
73
MW-T mixtures are a little bit better. Exposing MW-T mixtures to more than one cycle of
freeze and thaw is needed to determine the extent of loss of strength of such mixtures.
In zapped mixtures (CZMW-T), MR before and after freeze and thaw cycle,
percent of MR retained and St are higher than those for CC-T mixtures. In this case the
entire mixture, including anti-stripping agent that had been incorporated in asphalt cement,
was exposed to microwaves.
The test values were in direct proportion to zapping time.
The increase in test values of CZMW-T mixtures is expected to have been caused by the
mechanisms described earlier. First, microwave heating might have facilitated uniform
coating. Second, the polarization effect of microwaves improved polar molecule
orientation and then increased rate of anti-stripping migration toward the aggregate
interface. Finally, the extra heating and mixing of these materials aged and stiffened them
more. However the extra heating did not seem to have great impact on stiffness at short
zapping times. For example, in the case of zapping for 0.5 minute, the temperature
actually decreased and resulted in greater air voids. Yet all test values i.e., MR before and
after the freeze and thaw cycle, percent of MR retained and St are higher than those for
CC-T mixtures. At a longer zapping time of 5 minutes, where temperature increased by
an average of 41 F the extra heating effect might be more significant. The data in Table
3.7 were statistically analyzed by Wilcoxon's rank-sum method at a = 0.05, and are
summarized in Table 3.8.
C. Recycled Mixtures
Tests results for all mixtures are summarized in Table 3.9. The resilient modulus
(Mr ) before and after the one cycle of freeze and thaw conditioning are shown in Fig.
3.24 and Fig. 3.25 respectively. A comparison between the averages of resilient
modulus of the two conditions for all mixtures is shown in Fig. 3.26. The percent of MR
retained after conditioning is plotted against the method of mixture preparation for all
specimens in Fig. 3.27 and for average values in Fig. 3.28. The results of the diametral
Table 3.8
Results of the statistical significance test for the data of mixtures with Tallow Tetramine.
Heating method/Test
Mr dry
Mr conditioned
Retained Mr(%)
St
MW-T vs. CC-T
SH
NS
SL
SH
CZMW-T(0.5) vs. CC-T
NS
SH
SH
NS
CZMW-T(2) vs. CC-T
SH
SH
SH
SH
CZMW-T(5) vs. CC-T
SH
SH
SH
SH
CZMW-T(5) vs. CZMW-T(0.5)
SH
SH
SH
SH
CZMW-T(5) vs. CZMW-T(2)
SH
SH
NS
SH
CZMW-T(2) vs. CZMW-T(0,5)
NS
NS
NS
NS
SL: Significant - Higher
CC-T:
MW-T:
CZMW-T(n):
SL: Significant - Lower
NS: Not significant
Conventional oven heating
Microwave oven heating
Zapped mixtures. Number in parentheses indicates zapping time.
Table 3.9 Summary of test results on recycled mixtures.
Specimen
no.
Density
p/cf
Voids
%
Mr Dry
1,000 psi
Avg.
1,000 psi
266.30
103.10
91.80
1032.50
660.30
750.60
772.90
639.00
579.30
603.60
600.30
582.00
588.20
587.00
580.50
639.93
775.69
656.00
675.50
596.00
479.20
597.20
591.90
VM1
VM2
142.99
142.47
8.49
8.79
264.30
268.30
AMI
AM2
AM3
140.67
139.70
140.10
10.00
10.30
10.18
1018.50
1072.70
1006.30
CC-R1
CC-R2
CC-R3
144.66
143.99
145.18
4.80
5.10
4.56
664.30
665.50
587.30
MW-R1
MW-R2
MW-R3
145.60
146.00
145.48
4.30
4.00
4.30
595.50
616.50
534.10
CZMW-R1
CZMW-R2
CZMW-R3
145.70
144.90
145.50
4.10
4.78
4.26
694.30
645.70
579.79
CSMW-R1
CSMW-R2
CSMW-R3
145.28
145.10
144.95
4,27
4.42
4.27
533.50
665.20
589.30
Test Temp. (F)
Loading Rate
Loading Duration
74°
20 pulse/min
0.1 sec.
VM: Virgin materials.
AM: Aged materials.
CZMW-R: Conventional plus microwave zapping recycle.
Mr Cond.
1,000 psi
74°
20 pulse/min
0.1 sec.
%M r
Retained
Avg. % Mr
Retained
St
psi
Avg. St
psi
97.50
39.00
34.20
36.60
44.20
42.10
43.20
727.93
64.80
69.90
76.80
70.50
143.30
153.30
137.30
144.63
594.40
87.20
90.70
102.20
93.02
195.50
190.10
197.20
194.27
585.23
98.70
95.20
108.60
100.56
206.70
205.90
207.20
206.60
702.40
111.70
101.60
116.50
109.76
223.50
208.70
207.10
213.10
556.10
89.80
89.80
100.30
93.30
181.60
203.10
208.80
197.80
Avg.
1,000 psi
74°
2 in/min.
CC-R: Conventional recycle.
MW-R: Microwave recycle.
CSMW-R: Conventional plus microwave supplemental recycle.
76
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R:
Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
1100
I
1000
(1,000 psi)
900
800
700
600
2
500
400
300
200
VM
AM
CC-R
MW-R
CZMW-R
Heating Method
Figure 3.24 Effect of heating method on resilient modulus of
recycled mixtures.
CSMW-R
77
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R:
Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
800
700
600
500
400
300
200
■
100.
t
VM
AM
COR
MW-R
CZMW-R
CSMW-R
Heating Method
Figure 3.25 Effect of heating method on resilient modulus of
recycled mixtures after Lottman conditioning.
78
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R:
Conventional plus microwave zapping recycle
CSMW-R:
Conventional plus microwave supplemental recycle
1200
HI Dry
Mr (1,000 psi)
1000
□
after Lottman
800
600
400
200
VM
AM
CC-R
MW-R
CZMW-R
CSMW-R
Heating method
Figure 3.26 Effect of heating method on resilient modulus of recycled
mixtures (averages).
79
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R: Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
120
f
110
100
^
Retained M
DC
90
80
70
60
50
40
30
VM
AM
CC-R
MW-R
Heating Method
CZMW-R
CSMW-R
Figure 3.27 Effect of heating method on the percent of retained resilient
modulus of recycled mixtures after Lottman conditioning.
80
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R: Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
120
t
110
101
Retained M r
(%)
1 0 0 --
96
93
8060 -•
40.20
37
-■
VM
AM
CC-R
MW-R
CZMW-R
CSMW-R
Heating Method
Figure 3.28 Effect of heating method on the percent of retained resilient modulus
of recycled mixtures after Lottman conditioning (averages).
81
split tensile strength (St) test after conditioning are shown in Fig. 3.29. Average values of
St are plotted on Fig. 3.30.
Results show that there was no major difference among dry MR of all recycled
mixtures of the different heating methods. Mixtures that were prepared with microwaveheated reclaimed material before the addition of the rejuvenator MW-R and CSMW-R
show slightly lower MR> which might have resulted from less aging of the asphalt cement
due to fast heating with microwaves, as reported by others [17,18]. The aging might even
have been reduced if the loose sample was covered during microwave heating to protect it
from the air circulated by the exhaust fan. The other two mixtures, CC-R and CZMW-R,
were exposed to 300 F for two hours before the addition and mixing of the rejuvenator, to
which the higher MR might be attributed.
Unlike the fresh mixtures behavior, dry MR for both CC-R and CZMW-R
mixture are almost identical. In this case the two minutes zapping time were deducted in
the curing period after zapping. The curing time for zapped material was only 18 minutes
compared to 20 minutes for all other mixtures. The mixture probably was so aged that a
short period of microwave zapping did not make a significant difference. It is true that
zapping time was deducted from curing time but the temperature of the mix essentially did
not increase with zapping. However, MR of CZMW-R mixtures after Lottman's
conditioning exhibited a slight increase for all samples tested. Although the number of
samples may not be enough to make an inference about this behavior, the CZMW-R
mixture was the only material that was exposed to microwave energy after the addition and
mixing with the rejuvenator. It is possible that some structuring or polarization of the
asphalt cement and the rejuvenator had taken place and that in the presence of water and
exposure to a temperature of 140 F for 24 hours, aging or strengthening resulted. If that
is the case, then the selection of a rejuvenator that is compatible with microwave
treatment may become an issue and ought to be of concern. Split tensile strength is
82
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R:
Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
240
220
200
st (psi)
180
160
140
120
100
80
60
40
VM
AM
CC-R
MW-R
Heating Method
CZMW-R
CSMW-R
Figure 3.29 Effect of heating method on split tensile strength of recycled
mixtures after Lottman conditioning.
83
VM:
Virgin materials
AM:
Aged materials
CC-R:
Conventional recycle
MW-R:
Microwave recycle
CZMW-R:
Conventional plus microwave zapping recycle
CSMW-R: Conventional plus microwave supplemental recycle
2 5 0 -r
st (psi)
200
194
■ •
198
• ■
50 ■■
0
213
145
150 ■•
100
207
43
■-
VM
AM
CC-R
MW-R
CZMW-R
CSMW-R
Heating Method
Figure 3.30 Effect of heating method on split tensile strength of recycled
mixtures after Lottman conditioning (averages).
84
shown to be the highest for CZMW-R, which is higher by about 10% than CC-R
mixtures, while that of MW mixtures is higher only by 7%.
Examining of samples after split tensile strength reveals that none of the recycled
mixtures suffered any stripping.
The results of the statistical analysis of the data by Wilcoxon's rank-sum test at
a = 0.05 are shown in Table 3.10.
85
Table 3.10
Results of the statistical significance test on recycled mixture data.
Heating method/Test Mr dry
Mr conditioned
Retained Mr (%)
St
MW-R vs. CC-R
NS
NS
NS
NS
CZMW-R vs. CC-R
NS
SH
NS
SH
CSMW-R vs. CC
NS
SH
NS
SH
SH
SH
NS
SH
s
CZMW-R vs. MW-R
SH: Significant - Higher
CC-R:
MW-R:
CZMW-R:
CSMW-R:
SL: Significant - Lower
NS: Not significant
Conventional oven heating recycle.
Microwave oven heating recycle.
Conventional heating plus microwave zapping recycle.
Conventional heating plus microwave supplemental recycle
CHAPTER FOUR
EFFECT OF MIXTURE VARIABLES ON MICROWAVE HEATING
EFFICIENCY
I. Introduction
Heating of pavement materials with microwave energy results from the excitation
o f these materials by transmitted waves so that they generate their own heat. But asphalt
mixtures and pavements vary from one project to another due to variation in sources of
materials, subgrade condition, environment, traffic,.etc. Mixtures may differ in asphalt
content, moisture content, aggregate gradation, type of aggregate, etc. Since microwave
heating is dependent on the material to be heated, these variations are expected to affect
absorption and trapping of microwaves, and thus the heating efficiency of asphalt
mixtures. This assumption has led to investigating how microwave heating efficiency
would vary if some of the asphalt pavement mixture variables were changed. The term
"mixture variables" refers to physical quantities (values) that are generally used by
pavement engineers to describe these materials, such as asphalt cement content, aggregate
gradation, density of the mix, moisture content, etc. Microwave heating efficiency will be
judged by 1) the rise in temperature of materials treated with microwaves for a fixed
treatment period, and by 2) microwave penetration into fabricated compacted specimens.
There are many variables that might influence the heating efficiency of pavement
materials, as listed in Table 4.1. When work in this research started, the fast, deep
uniform heating of pavement surface layers with microwaves was being highly praised,
especially when compared to other methods of in-place heating of roads, such as by infra­
red heaters [5,22,27,29]. This has persuaded the author to concentrate efforts on
compacted asphalt pavement mixtures in this portion of the work. Variables that were
87
Table 4.1
Mixture variables that might affect microwave heating of asphalt pavement
mixtures as related to type and form of the material.
Form \Material
Loose
Plain aggregate
DType
2) Gradation
3) Agg. Spec. Grav.
3) Moisture Content
5) Initial Temperature
Asphalt mixture
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
Agg.Type.
Agg. Gradation
Specific Gravity
Moisture Content
Initial Temper.
Asphalt Content
Asphalt Type
Age of material
Presence and type of rejuv.
Presence of chemical additive
in surface and under lying layers.
Compacted
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
Density (% air voids)
Initial temperature
Agg. type
Agg. gradation
Moist, content
Thickness of layer
Type of material at the
bottom of the layer.
Asphalt content
Asphalt type
Age of mix
Presence and type of rejuv.
Presence of chemical
additive in surface and
underlying layer
88
studied here are 1) asphalt cement content, 2) aggregate gradation, 3) compaction effort,
and 4) initial temperature of specimen. Table 4.2 summarizes the experiment design.
II. Materials and Equipment
A. Materials
- Asphalt: Asphalt cement AR-4000 W (Chevron).
- Aggregate: Steilacoom Valley granite glacial aggregate ( Lakeside construction,
Fremont Asphalt Plant- Seattle).
B.
Equipment
The following equipment was used in preparing and testing specimens:
- A Marshall mechanical compactor.
- Teflon molds that were 3 in. high, 4 in. diameter and had a wall thickness of 1/2
inch. These Molds had three holes on the side for temperature probe insertion.
- A microwave oven that was of 2450 MHz frequency, with a maximum power of
600 watt and a stirring fan to spread waves in the cavity.
-
A temperature recorder that displayed it in (F) and (C) and was suitable for type-K
sensors.
- A metal probe (sensor) type- K (1 ft. long x 1/8 in. diameter).
C.
Use of Teflon Molds
The decision to use Teflon molds to heat materials in the microwave oven cavity
was based on their low dielectric properties where e' is less than 2 and tan 5 is also less
than 2 at microwave frequencies [32]. Unlike steel molds, they would not interfere with
electromagnetic fields inside the oven. Teflon containers had already been used in a
study on the rapid dissolution of ores, slags and furnace products caused by heating
with a microwave oven [42]. Specimens that needed to be heated in the microwave
89
Table 4.2
Summaiy of the experiment design.
Variable/Values
1
2
3
Asphalt content (%)
4.5
5.5
6.5
Aggregate gradation
Dense
Open
Sand
Compaction effort
None
Light
Medium
Initial temper. (F)
32
67
112
4
Heavy
90
oven were compacted in Teflon molds from hot mixtures. Since Teflon was not as stiff
as steel, some expanding and rebonding of mold's wall and the specimen during
compaction may have occurred, and densities might have been less than those of steel
mold compacted specimens. The Teflon molds were not as durable as steel. Chipping
of the inside wall from mixture compaction occurred but did not affect specimen surface
integrity or texture. Since all specimens in question were compacted in Teflon molds
and since designing a mix was not an issue at that time, these problems were not of a
great concern. The thickness of a mold's wall (1/2 in.) was selected to be less than 1/4
of a wave length of microwaves in the oven (about 4.5 in.) to prevent the passage of
waves within the wall when mold was wrapped with aluminum foil to approximate plain
wave application.
D. Arrangement of Specimen in the Microwave Oven
Microwaves in the oven cavity were expected to bounce around inside the cavity
which would result in the heating of the specimens along their depth from all sides and
would not represent the proposed vertical application of microwaves on pavement beds.
Monitoring the temperature gradient along the depth of the specimen, then, would have
been meaningless. In order to approximate the field's vertical application, the Teflon
molds were wrapped with aluminum foil around their outer circumference. Complete
wrapping of objects by aluminum foil stops heating completely [43]. The use of
aluminum foil in this fashion had been done before and reduced heating [44]. Jeppson
[25] demonstrated the ability of aluminum foil to reflect applied microwaves. In order to
reduce the effect of waves reflected from the bottom of the oven up into the specimen, the
mold containing the specimen was placed on top of a similar mold filled with loose
aggregate to absorb the reflected waves.. The use of an aluminum-foil plate under
aggregate would reflect waves upward. Table 4.3 shows the temperature of similar
asphalt mixture specimens heated in the microwave oven with the different arrangements
attempted. The arrangement finally adopted is shown in Fig. 4.1. Although the decrease
91
Table 4.3 Temperature of specimens with different insulation systems.
System Description
No Alum, wrap
& no Agg.
No Agg.
underneath
Agg. & Alum,
underneath
Initial Temp. (F)
67
67
66
Specimen Height (in)
2.34
2.34
2.30
Weight of Spec, in air (gm)
1027.1
1032.6
1039.0
HeatingTime (min.)
10
10
10
Depth where Temp, was
measured at the center (in)
0.84
0.84
0.80
Temperature recorded (F)
388*
364*
331**
* one specimen.
** highest of 3 specimens tested.
92
Stirring Fan
10:15
Oven Cavity
Compacted Asphalt Mix
Specimen
Aluminum Foil Wrap
FR
Teflon Mold
Air
*
•
♦
4
Plain Aggregate
Figure 4.1 Specimen arrangement in microwave oven.
93
in the specimen's temperature is not indicative of success in simulating plain microwave
application, it is indicative of success in shielding the specimen from side heating and in
reducing the effect of waves reflected from the bottom of the oven. This arrangement was
tested by heating loose one-sized aggregate samples in the microwave oven, then
measuring the resulting temperatures. Temperatures were recorded at nine different
points in the sample as shown in Fig. 4.2. Test results show that temperature was the
highest at the core and the lowest at the edges of the sample, as shown in Table 4.4, Fig.
4.3 and Fig. 4.4. The non-uniform temperature was due to heat loss at the surface,
which was augmented by the exhaust fan in the oven, to the different angles at which
microwaves entered the specimen, and to the concentration effect of the aluminum foil.
The loss of heat was permitted because it will be encountered in the field. The
penetration angles are characteristic of the microwave oven dimensions. The angles of
microwave incident produced in a microwave oven are not suitable for vertical
application to be simulated. Technology was capable of providing needed equipment, but
time and resources were not available. The comparative nature of this study, where all
materials had to be subjected to the same treatments and conditions, made contending
with an available system easier. The concentrating effect of the aluminum foil was
tolerated because it was believed to mostly affect the temperatures of the horizontal planes
and because the foil provided the desired thermal gradient along the depth of the
specimen. It was obvious that the shielding effect was greater than concentrating effect,
since the temperature of the specimen without aluminum foil (388 F) was higher than that
of one with aluminum foil (364 F).
94
2/3"
13
12
2/3" H
t
11
T*
23
co
lM m § , 21
+
I
a
33
32
M
ill
Ci
Plane of nodal
locations
Probe
Figure 4.2 Positions where temperature was measured within plain aggregate specimens.
95
Table 4.4 Average temperatures of plain aggregate samples heated in the microwave
oven for 5 minutes.
Aggregate Size
Weight of Sample (gm)
Initial Temp. (F)
point **
11
12
13
21
22
23
31
32
33
no. 4
no. 8
no. 16
879.5
849.2
840.2
852.9
69
69
68
68
121
132
143
138
162
169
124
133
134
126
143
149
130
156
160
108
124
127
107*
127*
134*
109*
131*
135*
101*
108*
107*
108
119
124
105
116
124
99
101
106
no. 30
** All points are an average of three tests except those with one (*), which are the
average of two tests. See Fig. 4.2 for location of points. Temperatures were
measured in the order shown in the point column for all samples.
96
’ 4"'
h
2 - 1/2
Specimen
160F
15 0140
130
Figure 4.3 Contours of equal temperature following heating of plain
aggregate in a microwave oven, for one heating condition
(initial temperature was 70F).
97
170
no.4
160
Temperature (F)
150
, 140
no. 8
no. 16
130
120
no. 30
100
0.0
0.2
0.4
0.6
0.8
1.2
1.0
Distance from Specimen's Center (in.)
1.4
Figure 4.4. Horizontal profile of temperature at midheight of one size aggregate,
samples.
98
m . T estin g
A.
Preparation of Specimens
Specimens were prepared according to Marshall procedures (ASTM D-1559).
However, the molds that were used to compact specimens were made of Teflon to
facilitate heating in the microwave oven with minimum interference with electromagnetic
fields in the oven cavity. Specimens produced were left to cool down to room temperature
overnight before heating in the oven. Fabricated specimens were in triplicate unless
otherwise mentioned.
B.
Testing Procedures
Test procedures were as follows:
1.
Mixture preparation and specimen fabrication was done according to the
Marshall method.
2.
Specimen was left to cool to room temperature ( for Teflon molds,
specimens were left to cool overnight).
3.
Height and diameter of specimen were measured while specimen still in the
mold using micrometer and vernier scale.
4.
Mold was wrapped with an aluminum foil around the outer circumference.
5.
Before heating in the oven, the top of the specimen was covered with a
plastic wrap and held tight with a rubber band. In many cases, it was
melted by heat from specimens.
6.
Specimen was heated in microwave oven for 10 minutes.
7.
Immediately, a probe was inserted into the specimen through the side holes
to the required positions shown in Fig. 4.5. When stabilized, temperature
was recorded.
8.
Specimen was allowed to cool, then was extruded and weighed in the air.
99
.
H
1 in
.
1 in
H
0- 0.5 in
T
X - 1.0 in
Sample
*
I
1.0 in
0.5 in
A
Figure 4.5
Positions where temperature was measured in compacted
specimens.
100
IV. A sphalt M ixture Factors
The effect of four variables of asphalt mixture on microwave heating were examined.
The variables were: asphalt content, compaction effort, aggregate gradation, and the
initial temperature of fabricated specimens. Each variable was changed while all other
variables were kept fixed.
A. Effect of Asphalt content
Three levels of asphalt content were used to examine the effect of asphalt content
on microwave heating of asphalt pavement mixtures. They were 4.5,5.5, and 6.5 percent
of total weight. Table 4.5 summarizes the test results. Results show that mixtures of lower
asphalt content heat better than mixtures with higher asphalt content in the range of asphalt
contents used. Fig. 4.6 shows that the average temperature of specimens of 4.5 and 5.5
percent asphalt content at their core are equal, while the average temperature at the core of
specimens made from a mixture with 6.5 percent asphalt content was less by 23 F.
However, the points plotted in Fig. 4.7, which shows temperature averages that were
measured 1 in. from the mold's wall, confirm this observation. This result can be
attributed to the fact that the addition of asphalt, which is of lower dielectric properties
than aggregate, has lowered the overall dielectric properties of the mix, and, as the amount
of asphalt increases, the overall dielectric properties decrease. Also, since asphalt cement
is almost transparent to microwaves, and the amount of energy generating aggregate heat
was kept constant, as the amount of asphalt increases, the overall temperature of the
mixture is expected to decrease. Temperature gradient patterns along the specimen's depth
are not very different for the three asphalt contents. Asphalt content did not and should not
have a major effect on microwave penetration into specimens, since it is transparent to
them (see Fig. 4.7). The accuracy of surface temperatures were the least, due to rapid
t
heat loss to air.
Table 4.5
AC
%
4.5
5.5
6.5
Effect of asphalt cement content on microwave heating and penetration in asphalt mixtures after
10 minutes of heating.
Wt.
(gm)
Ht.
(in)
1
2
3
1033
1039
1029
2.25
2.30
2.31
66
66
67
261
277
262
290
308
282
318
331
313
285
284
277
Avg.
1034
2.28
66
267
293
321
282
1
2
3
1054
1036
1047
2.25
2.27
2.25
68
66
66
269
268
273
278
298
286
314
332
318
264
280
266
Avg.
1046
2.26
66
270
287
321
270
1
2
3
1066
1053
1049
2.25
2.25
2.25
68
68
68
259
270
259
260
255
275
292
300
302
249
248
253
Avg.
1056
2.25
68
263
263
298
250
Spec.
no.
Init. Temp.
(F)
Final Temperature (F) at points as in Fig. 4.5
1
2
3
4
102
Temperture of Specimen's Core (F)
340
330
O
Averages
♦
Actual Values
310
300
290
4.5
5.0
5.5
6.0
Asphalt Content {% of total weight)
6.5
Figure 4.6. Effect of asphalt content on microwave heating of compacted
asphalt mixture specimens.
103
Asphalt Content
300
4.5%
290
280
Li.
2
5.5%
270
*
6.5%
250 240 ■■
230
220
0.0
0.5
1.0
1.5
2.0
Depth - in.
Figure 4.7. Effect of asphalt content on microwave penetration into compacted
specimens (temp, measured T'from center).
104
B.
Effect of Aggregate Gradation
Three gradation matrixes of aggregate were used here, dense-graded, open-graded
and sand as shown in Fig. 4.8. Mixtures contained the same asphalt content of 4.5
percent of total weight
The temperature was notably higher in specimens of mixtures made with densegraded aggregate, followed by open-graded and finally sand as shown in Table 4.6 and
Fig. 4.9 and Fig. 4.10. Voids in mineral aggregate (VMA), which in this case represent
the volume of non-heat-generating material (asphalt cement plus air), are expected to be
the major factor behind these differences. Particle size might have also contributed to this
behavior. The effect of aggregate particle size on microwave heating was examined by
> *•
measuring the temperature of one-sized aggregate samples that were heated in the
microwave oven for 5 minutes. Aggregate particle sizes tested were; 4.76 mm (no.4),
2.38 mm (no. 8), 1.19 mm (no. 16), and 0.71 mm (no. 30). Tests results which are
displayed in Table 4.7 and Fig. 4.11 illustrate that the temperature of a heated sample at
any particular point increased as the particle size increased. In this group of tests the
volume of air voids was not constant, therefore the differences were probably caused by
the volume of air voids as well as aggregate size.
C.
Effect of Compaction Effort ( Density)
The density of asphalt mixtures ( air voids) was expected to have a great influence
on microwave heating of these mixtures and on penetration of waves in mixtures. The
ability of the material to trap microwave energy increases with its density since more
absorbant material per unit volume is subjected to microwaves.
Total Percent Passing
100
SAND
60
DENSE
GRADED
OPEN
GRADED
200
100
50
30
15
8
4
3/8"
Sieve Number
Figure 4.8 Aggregate gradations used in microwave heating tests.
3/4"
1- 1/ 2 "
Square Opening
Table 4.6
Effect of aggregate gradation on microwave heating and penetration in asphalt mixtures
after 10 minutes of heating.
Final Temperature (F) at points as in Fig. 4.5
4
1
2
3
Agg.
Grad.
Spec.
no.
Wt.
(gm)
Ht.
(in)
Init. Temp.
(F)
D.G
1
2
3
1033
1039
1029
2.25
2.30
2.31
66
66
67
261
277
262
290
308
282
318
331
313
285
284
277
Avg.
1034
2.28
66
267
293
321
282
1
2
3
1009
1040
1036
2.50
2.53
2.54
66
66
66
227
200
220
235
204
240
271
244
266
214
197
219
Avg.
1028
2.52
66
216
226
260
210
1
2
3
1019
1023
1035
2.5
2.49
2.53
68
68
68
200
215
210
190
194
202
215
222
220
168
168
178
Avg.
1026
2.51
68
208
195
219
171
O.G
Sand
o\
107
340
£
320
£
8 300
♦
Actual
O
Average
(O
c
Q
>
E 280
co 260
I
"o
£
i 240
I 220
.V
200
Sand
O.G.
D.G
Aggregate Gradation
Figure 4.9 Effect of aggregate gradation on microwave heating of compacted
specimens (temperature measured I 1from center).
108
Aggregate gradation
300 T
_
D.G.
280 ■■
Temperature (F)
260 240 ■■
•.G.
200
-
SAND
180 ■■
160
0.0
0.5
1.0
1.5
Depth - in.
2.0
2.5
Figure 4.10 Effect of aggregate gradation on microwave penetration into
specimens (temperature measured 1' from center).
109
Table 4.7 Effect of aggregate particle size on microwave heating and penetration
in plain aggregate samples.
Agg. Size
Depth from Surface
(in)
Temperature at Center of Spec.
OF)
No. 4
0.5
1.5
2.5
143
169
134
No. 8
0.5
1.5
2.5
149
160
127
No. 16
0.5
1.5
2.5
134
135
107
No. 30
0.5
1.5
2.5
108
124
106
110
180
160
Temperature (F)
140
120
100
Temp, measured at Point 23 (see Fig. 4.2)
80
60
40
20
4
16
8
30
Sieve Size
Figure 4.11 Effect of single size aggregate particle on microwave
heating of aggregate only (initial temperature was 69F).
Ill
A stuffy on some agricultural and industrial particulate materials has shown that
dielectric properties of materials tested increased with the increase of density [45,46].
The increase in dielectric properties means faster heating of the material. Penetration, on
the other hand, obviously is expected to be less in denser material. In order to examine
whether such a relationship exists for asphalt mixtures, the temperature of a constant
mass of a loose asphalt mixture was measured at four depths along the center line after
heating in the microwave oven for 10 minutes. After cooling overnight, that mass was
then compacted lightly (5 blows) with the Marshall hammer. Then the heating ,
temperature-measuring cycle was repeated. This process was repeated for the same mass
for medium (5+15=20 blows) and heavy (5+15+30=50 blows) compaction. It was
found that the temperature of specimens increased with their densification level as shown
in Table 4.8 and Fig. 4.12. A low level of compaction had a very pronounced effect on
heating. Fig. 4.13 shows that loose and light compaction curves are flatter than those of
heavy and medium compaction indicating more .uniform heating due to easier penetration
and less trapping of microwaves by the mixtures.
The densification level of a compacted specimen was represented by the number
of blows applied on specimens because the probe used to measure temperatures resulted
in destroying specimens before extrusion.
D. Effect of the Initial T em perature of the Specimen
It was anticipated that the final temperature of cooler specimens that were heated
by microwaves would not be as high as specimens of higher initial temperature since the
microwave energy input had been kept constant. However, differences may arise from
the fact that the dielectric constant of pavement materials, especially aggregate, will
decrease with the rise in temperature. At the microwave frequencies, changes can be
assumed to be negligible [32,36]. However, it was of interest to determine the relative
response of specimens at different initial temperatures which would be useful in designing
Table 4.8
Compact.
Effort
None
Goose)
Low
(5 blows)
Med.
(20 blows)
Heavy
(50 blows)
Effect of compaction effort on microwave heating and penetration in asphalt mixtures after
10 minutes of heating.
Spec.
no.
Wt.
(gm)
Ht.
(in)
Init. Temp.
(F)
Final Temperature (F) of points at depth of
0.5"
1.0"
1.5"
2.0"
1
2
626
626
2.8
2.8
67
67
186
188
200
195
211
200
206
200
Avg.
626
2.8
67
187
197
206
203
1
2
626
626
1.69
1.81
65
65
335
337
365
372
352
359
Avg.
626
1.75
65
336
369
356
1
2
626
626
1.56
1.50
67
67
354
325
377
340
337
324
Avg.
626
1.53
67
340
359
331
-
1
2
626
626
1.44
1.44
66
68
390
397
406
417
380*
387*
-
Avg.
626
1.44
67
394
412
384*
-
* Temperature was measured near the bottom of the specimen.
-
-
-
113
450
♦
c
Actual
350
o
OAverages
o
Temperature(F)
400
♦
300
250
200
150
A
-------------- 1------ ------------- 1------------------ 1------------------ 1
Loose
Light
Medium
Compaction Level
Heavy
Figure 4.12 Effect of compaction level on microwave heating of compacted
specimens (temperature measured 1 in. below surface).
114
Compaction Level
Heavy
Temperature (F)
Medium
Light
250 ■■
200
Loose
■■
150
0.4
H
-f-
0.6
0.8
1.0
1.2
1.4
Depth (in.)
1.6
1.8
2.0
Figure 4.13 Effect of compaction on microwave penetration into
compacted specimens (temp, measured at the center).
115
future experiments relating initial temperature of pavement (atmospheric temperature) to
microwave application time. Three temperatures representing the four seasons of the year
and different geographical areas were selected: 32,66, and 112 F. Specimens were kept
in the compacting Teflon molds sealed in a plastic bag inside a temperature-controlled
chamber for 24 hours before testing according to described procedure. Table 4.9 and Fig.
4.14 show that the final temperature of microwave-heated specimens, as expected,
increased as their initial temperatures increased. However, and more importantly, the
changes in temperature (AT) of specimens for the different initial temperatures were not as
significant as the differences in initial and final temperature, as Fig. 4.14 illustrates. The
maximum, difference in (AT) was 20 F, suggesting that initial temperature has a small
effect on microwave energy absorption of pavement materials. The pattern of microwave
penetration was found to be similar for all temperatures, and the gradient curves were
nearly parallel, as shown in Fig. 4.15.
The presence of moisture in a frozen pavement is a variable that is worthy of
investigation, since presence of water, especially as ice, should reduce the efficiency of
pavement heating significantly. Ice dielectric properties are very low compared to water.
The values of e’ and tan 5 for ice at 3000 MHz are 3.2 and 9 x 10-4 (at -1 2 C), while
for water at the same frequency they are 76.7 and 1570 x 1(H (at 25 C) respectively
[32]. It should be possible to take advantage of studies on microwave heating systems
for the thawing of frozen soils and other outdoor applications to facilitate late season
construction [47].
V. T esting Problem s
There were some difficulties that were experienced during the course of this part
of the work which may have affected the accuracy of results obtained. Those difficulties
are listed here to assist in planning and conducting future research:
Table 4.9
Init.
Temp.
32
66
112
Effect of initial temperature on microwave heating and penetration in asphalt mixtures after
10 minutes of heating.
Wt.
(gm)
Ht.
(in)
Final Temperature (F) at points as in Fig. 4.5
1
2
3
4
1
2
1026
1042
2.24
2.29
244
243
266
275
306
291
244
240
Avg.
1034
2.27
244
270
298
242
1
2
3
1033
1039
1029
2.25
2.30
2.31
261
277
262
290
308
282
318
331
313
285
284
277
Avg.
1034
2.28
267
293
321
282
1
2
1032
1037
2.27
2.28
345
326
374
364
396
381
328
343
Avg.
1035
2.27
335
369
388
335
Spec.
no.
117
380-Final Temperature (F)
T 360
Temperatures
°
Average
•
Actual
340
Final Temperature
360
■■ 320
340 ■■
■■ 300
320
■■ 280
Temperature Change
300
■■ 260
280
240
30
40
50
60
70
80
90
Initial Temperature {F)
100
110
Figure 4.14 Effect of initial temperature of specimen on microwave heating of
compacted asphalt mixtures.
120
Temperature Change (F)
400
118
380 r
Initial Temp.
112 °F
360
Temperature (F)
340
320 ■■
300 ■■
66 °F
280
260 240 ■■
220
0.0
0.4
1.2
0.8
Depth (In.)
1.6
2.0
Figure 4.15 Effect of initial temperature of specimen on microwave penetration into
specimens (temperatures were measured 1" from center).
119
1. It was very difficult to insert the probe more than 1 in. parallel to the surface
from the lowest side hole. It was even more difficult to do this with opengraded aggregate. Specimens made with sand mixtures were the easiest to
insert the probe into. This is why temperature gradient was measured closer
to the wall of the mold ( mid-point between wall and center i.e., 1 in. from
wall) rather than along the center of the specimens.
2. Measuring temperature at any point in specimens took about one minute. This
meant an average of 4 minutes during which heat transfer and loss between the
layers and air took place especially at the surface of the specimen.
3. It was not possible to accurately calculate the percent of air voids of compacted
asphalt specimens by saturated surface dry procedures because the probe
disturbed the heated specimens.
4. Since it was not possible to have a constant height of specimen and since
temperatures were measured through the side holes, it was not possible to
measure temperature for all specimens at the same points and hence the bottom
of the specimens acted as the datum.
5. Accuracy of measured temperatures would be the least for surface or near
surface temperatures. Side holes through which the probe was inserted to
measure temperatures were fixed but specimen height varied; thus, sufficient
material cover over the probe was not always available. Temperatures were
measured top to bottom in all specimens.
CHAPTER FIVE
EFFECT OF WATER CONTENT ON TEH EFFICIENCY OF
MICROWAVE HEATING OF PAVEMENT MATERIALS
I. In lto d u c tto n
In microwave cooking, water is considered to be the principal heat-generating
agent. Its interaction with microwaves has been extensively studied and its dielectric
properties at various temperatures and frequencies are readily available. Dielectric
properties of water were shown to be high and thus heat easily in microwaves. It is
anticipated that water present in pavement materials would have a significant impact on
microwave heating of these materials. A number of researchers have demonstrated that
presence of moisture in soils, rocks, concrete and bricks boosts their dielectric properties
[32,48,49,50]. Examples of those materials are listed in Table 5.1. The improvement in
dielectric properties of materials implies that their absorption of microwave energy would
be greater, and consequently, they would heat faster but would reduce heating at depth
due to the high absorption. When microwave energy was used for rapid patching of
Portland cement concrete with polymer concrete, it was found that the addition of water to
dry aggregate reduced the radiation time [2]. The report stated that, in one instance, the
addition of 0.5 percent of water to one patch reduced the heating time by 8 minutes (from
45 minutes to 37 minutes), and when 1.0 percent of water was added, the time required to
heat the patch to the desired temperature was cut to 25 minutes compared to the 45 minutes
for dry aggregate patches. Jeppson [5] believes that the presence of water in pavement
materials would substantially increase the energy requirement for heating these materials
above 212 F. He demonstrated in a hypothetical example that a 4% moisture in reclaimed
asphalt pavement material would increase the required microwave energy to heat the
material to 300 F by 128% compared to dry material.
121
Table 5.1
Material
Sandy Soil
Effect of moisture content on the dielectric properties of some
construction materials.
wc %
e'
0 .0
2.55
4.50
3.8
16.8
Loamy Soil
0 .0
2 .2
13.77
Clay Soil
0
20.09
PC Concrete
20 .0
2.47
3.50
20 .0 0
2.38
20 .0 0
0 .0
4
0.5
8
tan 5
0.01
Ref.
[32]
0.03
0.03
0.0065
0.06
0.16
[32]
0 .2
[32]
0.52
0.05
0.1875
[49]
122
It was felt that the presence of water, which has high dielectric properties, would
cause pavement material to heat more readily with microwave energy. However, if water
and material become two separate identities, as they were treated by Jeppson in his
analysis, heating will be sequential. Water, which is more lossy than pavement material,
will consume initial energy input for evaporation, which will raise the temperature of the
mix only to the boiling temperature of water of 212 F, before the material is actually
activated by microwaves to generate heat In a sense the water shields the materials from
microwaves.
In Chapter Five, an attempt has been made to describe how the effect of water on
microwave heating of pavement material varies in the range of water content discussed
above. Also, Chapter Five has attempted to define limits of water content that determine
when the presence of moisture is beneficial or detrimental to the efficiency of microwave
heating of pavement materials, using plain aggregate as a testing material.
II. Experim ent
A. Use of Plain Aggregate
The decision to use plain aggregate for this part of the study was based on several
factors. First, it is the part of a pavement mixture that would absorb water in permeable
voids and/or attract it to its surface, causing displacement of asphalt cement film in some
cases. Second, aggregate is the part of a mixture that generates heat when the mixture is
exposed to microwaves. The final reason is the ease of producing several levels of water
content and the ease of accurately determining the actual water content
Two gradations of aggregate that represent coarse aggregate and sand were used as
shown in Table 5.2. The difference in gradation was selected to provide a wide range of
water content. Sand usually holds more moisture than coarse aggregate, due to small
123
Table 5.2 Gradation of Coarse aggregate and Sand.
Sieve Size
3/8 "
1/4"
no. 4
no. 8
no. 10
no. 40
no. 80
no. 200
Coarse Agg.
% passing
100
56.5
24.0
0.5
Sand
% passing
100
100
97
89
87
40.5
4.0
0.5
124
voids between particles that hinder water drainage and to its greater surface area and
associated surface tension.
B. Testing
In order to achieve the objective of this part of the work, a simple testing
procedure was followed. The test involved heating equally-weighted aggregate samples
with varying moisture content in the microwave oven for prescribed periods, then
measuring the core temperature with a thermocouple. To monitor how the change of
moisture content caused by drying during heating affects microwave heating efficiency,
individual samples of sand of the same moisture content were heated for periods of time
ranging from 3 minutes to IS minutes depending on the moisture condition. In the case of
coarse aggregate, heating time ranged from 1.5 minutes to 7 minutes.
C. W ater Content Range
In order to cover a wide range of water content that may include cases that were
described by others, samples with three distinct water contents were prepared as follows:
Dry Materials: Sand and aggregate were dried in a forced draft oven at 250° F for
two days. Dry hot materials were kept in air tight containers until they cooled to room
temperature for testing.
Wet Materials: Aggregate and sand were immersed in water in buckets for three
days and kept underwater until testing time. Materials were scooped out from buckets
immediately into heating container after allowing the free water to flow away.
Damp Materials: Amounts of wet sand and aggregate were spread in the
laboratory and left to surface dry at room temperature overnight.
125
D. Testing procedures
1- From the prepared large batch of water-material, an amount of approximately
1000 gm. was taken and mixed for uniformity.
2- In a Teflon heating mold, 700 gm of material was weighed loose, then
heated in the microwave oven for the prescribed time.
3- The maximum temperature of the heated sample at the core was measured by
inserting the probe from the mid-point side hole of the mold to the point of
highest temperature.
4- Weight of sample and mold before and after microwave heating were also
recorded. Vapor condensed on the inside wall of the mold nullified results.
5- Samples for the calculation of the actual moisture content were made from
the rest of the 1000 gm. batch. Each sample was heated to a constant weight
at 280 F in order to determine its moisture content..
III. Test Results and Evaluation
Results of tests on sand and coarse aggregate are listed in Tables 5.3 and 5.4. The
effect of moisture content on the rise in temperature of samples heated in the microwave
oven for 3, 5 and 7 minutes are shown in Fig. 5.1 for sand and in Fig. 5.2 for coarse
aggregate. In order to monitor the change in the effect of moisture due to drying ,
temperature versus heating time for the three mixture cases was plotted for sand and
coarse aggregate in Fig. 5.3 and Fig. 5.4 respectively.
The results show that the temperature of heated specimens increased as the water
content increased, reaching the peak at about 1 percent moisture content and then
dropping when the water content increased excessively, resulting in a wet material as
shown in Fig. 5.1 and 5.2.
A very small amount of moisture, lower than 1 percent, was sufficient to cause a
significant improvement on the rate of microwave heating. If it is assumed that the
126
Table 5.3
Results of microwave heating of sand at different moisture contents.
Wet Sand
Moist Sand
Dry Sand
Heating
time (min.)
w/c
(%)
Temp,
(°F)
w/c
(%)
Temp.
(°F)
w/c
(%)
Temp.
(°F)
3
3
3
13.46
210
0.76
0.69
0.46
326
333
346
0 .0 0
0 .0 0
162
166
5
5
5
5
14.80
14.80
210
211
0.62
0.53
0.42
0.42
402
399
405
. 414
0 .0 0
0 .0 0
222
222
7
7
11.70
210
0.69
0.43
488
492
0.11
288
10
10
13.00
12.60
335
320
0.11
361
12
12.40
405
15
13.80
485
127
Table 5.4
Results of microwave heating of coarse aggregate at different moisture
contents.
Wet Agg.
Heating
time (min.)
w/c
(%)
Temp.
TO
Moist Agg.
w/c
(%)
Temp.
CD
Dry Agg.
w/c
(%)
Temp.
CD
1.5
1.5
4.07
5.68
201
0 .6
207
0.67
179
174
0 .0 0
0 .0 0
114
128
3
3
3
6.35
4.67
4.75
210
210
210
0.75
0.95
0.95
303
283
287
0 .0 0
185
163
186
5
5
6.35
6.35
270
286
0.92
0.75
418
420
0 .0 0
0 .0 0
222
7
7
6.35
6.35
399
421
0.67
0.74
514
525
0 .0 0
0 .0 0
315
332
0.13
0 .1 0
244
128
H eating T im e fm in.t
Temperature (F)
500
400
300
200
100
0
2
4
10
12
8
Initial Water Content (%)
6
14
16
Figure 5.1 Effect of moisture content on microwave heating of sand.
129
H eating T im e fm in.t
7
O
5
♦
3
■
600
Temperature (F)
500
400
300
200
100
2
4
6
8
10
12
Initial Water Content (%)
14
16
Figure 5.2 Effect of moisture content on microwave heating of
coarse aggregate.
130
500
Wet
400 ■Temperature (F)
D am p
300 • -
200
■-
100
0
2
4
10
6
8
Heating Time (min.)
12
14
16
Figure 5.3 Effect of moisture content and exposure time on microwave
heating of sand.
131
600 r -
Temperature (F)
500
400 ■Damp
Wet
300 -200 '
■-
100
- -
0
1
2
3
4
Heating Time (min.)
5
6
7
Figure 5.4 Effect of moisture content and exposure time on microwave heating
of coarse aggregate.
132
desired temperature was 300 0 F, then heating sand with less than 1 percent moisture
would take about 2.7 minutes, compared to 7.5 minutes for diy sand ( a 64% saving) and
would require 3 minutes instead of 6.5 minutes for coarse aggregate ( a 54% saving).
The response and behavior of the different material-water mixtures used here
were found to be as follows:
Damp Materials: Both sand and coarse aggregate exhibited superior heating rates
compared to dry materials. The combination of modified higher dielectric properties and
low specific heat had resulted in a rapid microwave heating rate. The difference in
temperatures of dry and damp materials seems to increase with time. This might have
been caused by the increase in the dissipation factor of water, which increases with
temperature [32]. In the case of coarse aggregate and as shown in Fig. 5.4, the slope of
the damp materials' curve was less than that of wet aggregate's curve between 0 and 2
minutes. This suggests that for temperatures below 212 F, damp materials are not as
good as water. In the case of sand, Fig 5.3 does not show the same pattern. It is possible
that the wet sand heated faster than damp sand between 0 and 3 minutes. A minimum
amount of surface water probably would be beneficial in initially boosting temperature to
212 F.
Wet materials: When surface water was present, the temperature of both sand and
coarse aggregate did not exceed 212° F. This confirms that excess surface water was
absorbing all the energy and thus heating only the water. After all the surface moisture
was evaporated, sand and coarse aggregate became similar to damp materials and a
sudden increase in the heating rate occurred . Curves of temperature rise vs. time of
heating for wet materials became almost parallel to the damp material' curves for both
sand and coarse aggregate after excess water was evaporated, as shown in Fig. 5.3 and
5.4. The time (energy) required for this behavior to occur depends on the amount of
surface water present. If the amounts of surface water in those materials had been greater
133
than what they actually were, segment AB of the wet material curves would be expected
to shift to the right. However, when surface water is zero, AB will coincide on damp
material's curves
Dry Materials: Dry sand and coarse aggregate had low dielectric properties that did
not enable them to heat faster than damp materials. However, when compared to wet
sand, dry sand initially reached a higher temperature then wet sand. Dry sand was utilizing
all the energy it absorbed to raise its temperature while energy that was absorbed by
wet sand was wasted in evaporating the surface water, as shown in Fig. 5.2. Eventually,
and when all surface water was evaporated, wet sand became damp sand and employed
the energy more efficiently. The heating of wet aggregate is similar to wet sand but it must
be noted that water in wet aggregate was in the range of 4.07 to 6.35 percent while in the
wet sand it ranged from 11.7 to 14.8 percent. The amount of water in wet coarse
aggregate was not sufficient to allow intersection between wet and dry aggregate curves in
Fig. 5.4, as was the case with the sand.
IV. W ater-M aterial Com binations and Limits
Based on the above discussion, kinds of plain aggregate ready to be heated with
microwave energy might be classified according to amount and form of moisture present
as 1) dry materials, 2) materials with absorbed moisture and, 3) wet materials. These
materials are defined with their limits below:
1) Dry Materials: These materials would have zero percent water content (w/c
0%). Thus, microwave energy will be utilized totally to heat the materials. These
materials would have lower dielectric properties and hence, their microwave heating
efficiency would also be low.
2) Materials with Absorbed Moisture: These materials contain some moisture in
pores. The highest limit of absorbed water is the saturated surface dry condition and the
134
lowest is when material is completely dry . Dielectric properties of the material are
modified by the presence of water, and the resultant improved properties are responsible
for improved microwave heating efficiency of these materials. During heating, after all
moisture in the materials is removed, they will behave as if dry but with high initial
temperature. Fig. 5.5 shows the general relationship of temperature and heating time for
case 1 and case 2 .
3)
thin
Wet Materials: The lowest limit of the water content in this case would be a
film of water on the surface. The highest limit would be when the material is
completely immersed in water. In this type of material, initial energy input will be
utilized in evaporating water and the temperature will not exceed 212 F. As the water
evaporates, the material is shifting closer to being saturated surface dry. However, this
change depends primarily on the amount of surface water. If it is assumed that the material
is non-porous, then only surface water will be present, and as water evaporates, the
material approaches dry material with an initial temperature of 212 F. The general
relationship of temperature to heating time for case 3 and expected changes is illustrated in
Fig 5.6.
135
wc1
Case 2: Damp
Temperature
wc2
wc1 > wc2 > wc3
wc3
Case 1: Dry wc « 0%
Microwave Heating Time
Figure 5.5 Time-temperature relationship for heating wet aggregate:
Cases 1 and 2.
136
T=
Temperature
Absorbed Water
(permeable voids)
Wet
Aggregate
Dry Aggregate
(no voids)
Water
212 F
Microwave Heating Time
Figure 5 .6 Time-temperature relationship for heating wet aggregate:
Case 3.
CHAPTER SIX
MICROWAVE HEATING ENHANCERS FOR PAVEMENT MATERIALS
I. Introduction
A number of chemicals additives have been used to modify asphalt cement or
added to asphalt mixtures to cut costs and to improve properties of mixtures and
performance of roads. Asphalt cement has been, for example, extended with sulfur [51]
and lignin [52] ,and has been modified with polymers [53], recycling agents [5] and
manganese [55]. Depending on their dielectric properties and contents in the mixture, they
may affect the microwave heating efficiency of asphalt pavement mixtures. The presence
o f such materials may cause an unexpected increase in microwave heating efficiency,
which may result in the overheating of the mix. Table 6.1 lists the heating characteristics
of chemical compound samples ranging in weight between 10 gm. for dark compounds to
2 00 gm. for light-colored compounds when heated in a microwave oven with two 800
watt 2450 MHz magnetrons [56]. These high temperatures are signals of possible damage
to asphalt mixtures that could occur when such chemicals are present with microwave
heating. Once the positive role of the additive is known, then microwave application time
can be reduced and thus energy and construction time savings realized. In Chapter Two,
it was shown that some types of aggregate do not heat very well when treated with
microwaves. In areas that lack high loss aggregate (aggregate with high dielectric
properties), the addition of chemical modifiers that would boost the dielectric properties of
the mix and thus augment microwave heating efficiency might be necessary for
economical application of microwave energy in heating asphalt pavement materials. The
addition of water to plain aggregate could be considered as a microwave heating enhancer
as described in Chapter Five. However, the use of water in constructed pavement is not
guaranteed, since the absorption of water by asphalt-coated material is very low. A
microwave heating modifier needs to be part of the mix when it is initially laid in order to
facilitate the economical future heating of pavement as in recycling. In this chapter the
13?
Table 6.1 Microwaves heating times of various chemical compounds [55]
Compound
Color
Heating Time (min.)
Max. Temp. (° C)
AI2O3
White
24.0
1900
C (Charcoal)
Black
0 0 .2
1000
CaO
White
40.0
200
C02 O3
Black
3 .0*
900
CuO
Black
4.0
800
CuS
Dark blue
5.0
600
Fe2 0 3
Red
6 .0
1000
Fe3 0 4
Black
0.5
500
FeS
Black
6 .0
800
MgO
White
40.0
1300
M np 2
Black
*
M 0 O3
Pale green
M 0 S2
46.0
750
Black
0.1
900
Ni20 3
Black
3.0*
PbO
Yellow
U 02
ZnO
1300
13.0
900
Dark green
0.1
1100
White
4.0
1100
* Bumped out of the container at maximum power input.
139
concept of microwave heating enhancement through the addition of a selected chemical
will be demonstrated by identifying, then testing, one chemical additive that is expected to
hasten microwave heating of asphalt pavements and mixtures.
H. Identifying Microwave Heating Improvers (Additives)
Any chemical, to be used for the purpose of improving microwave heating
efficiency of asphalt pavement materials, should satisfy the following criteria:
1.
Added material should improve the heating of asphalt mixtures with
microwaves compared to similar mixtures without it
2.
Microwave heating enhancement characteristics of the added material should
be permanent
3.
Addition of modifier should not cause any undesirable effect on the quality of
the mixture and hence shall not affect performance or life of the pavement
4.
The use of the modifier must be economically justifiable.
5.
The additive must be commercially available.
6.
The additive must not endanger human health or environment
7.
The use of such a chemical should not create complex and difficult
construction procedures.
An ideal material for this purpose would be one that has been previously added to
asphalt pavement mixtures for other reasons and has very high dielectric properties, i.e.,
high e' and tan 8 . The addition of this material should raise the overall dielectric
properties of the mix.
In addition to the chemicals listed in Table 6.1, materials that possess high
dielectric properties were sought. From Von Hippel's Dielectric Materials and
Applications. Table 6.2 was generated. It shows that as the amount of carbon in
140
Table 6.2 Examples of materials of high dielectric properties [32].
Material
T(°C)
Frequency (Hz) e'/e"
tanS xlO-4
Water
25
3xl0 8
3xl0»
77. 5
76.7
Ethylene Glycol
25
3x10*
3xl0 9
39
1600
12
10,000
3xl0 8
3xl0 9
24
16
2800
2200
3xl0 8
3xl0 9
17
14.8
3000
3600
Thiokol
type FA
23
type ST
25
160
1570
Polystyrene plus fibers
Polystyrene 91%
Carbon
9%
25
3xl0 8
3xl0 9
Polystyrene 70%
Carbon
30%
lxlO 9
3xl0 9
11
25
Polystyrene 50%
Carbon
50%
25
(molded at 1000 psi)
lxlO 9
3xl0 9
25.9
20.8
7300
5600
Polystyrene 50%
Carbon
50%
25
(molded at 10,000 psi)
lxlO 9
3xl0 9
36
25
4600
6200
Tam Ticon BS
Barium
3.81
3.6
9.1
310
386
2300
2500
25
79%
3xl0 8
7000
3xl0 9
2000
900
Strontium Titanate 21%
5000
Tam Ticon B
Barium Titanate
25
3xl0 8
3xl0 9
1100
600
500
300
141
polystyrene increased, the dielectric properties of that material increases. A number of
carbon rich products such as tire rubber and carbon black have been used as asphalt
mixture additives [57,58,59,60]. The response of different types of rubber to microwave
heating is shown in Fig. 6.1. However both the dielectric properties and microwave
heating efficiency are a function of the carbon black content [61], as illustrated in Fig. 6.2.
Furthermore, carbon black is commercially available with great uniformity (97% elemental
carbon). The addition of carbon black to asphalt mixtures does not require any special
arrangement It is usually mixed with hot aggregate before the addition of hot asphalt in
the pugmill. These factors have made carbon black a prime candidate for this study.
There are two possible mechanisms by which carbon black is efficient in
increasing the dielectric properties of the host material. The first is associated with the
surface chemistry of the carbon black particles and the second is related to its dielectric
conductivity. Several dipolar groups were found to be present on the surface of the
carbon black aggregates as a result of oxidation during during its manufacture [62,63].
There are five major functional groups : carboxylic, quinonic, lactonic, phenolic and
carbon-bond hydrogen groups. Also, carbon black was found to adsorb moisture from
the air and that its moisture content was a function of the surface area and the type of
carbon black [64]. The presence of the dipolar groups and the water, which is also
dipolar, are believed to be responsible for the dipolar polarization component of the
dielectric constant of the carbon black. The high conductivity of the carbon black adds a
further component to the dielectric constant [35], resulting in higher dielectric properties
overall, and thus more efficient microwave heating.
III. Use of Carbon Black in Asphalt Mixtures
Carbon black is a class of carbon produced by partial combustion of residual tars
from petroleum refining or petrochemical processes. The fine particle size of carbon black
and thus its large surface area result from its formation as a smoke in flames at high
142
Neoprene
120
Isoprene
Pre-heating with
1000 watts at 2450 MHz
Temperature Increase
100
S.B.R.
80
60
Natural
Rubber
40
20
0
0
1
2
3
4
5
Heating Time (min.)
Figure 6.1 Temperature of microwave heated rubbers [61].
143
36%
32%
120
100
Temperature Increase
11 %
80
60
40
Pre-heating with
500 watts at 2450 MHz
20
0
0
1
2
3
Heating Time (min.)
Figure 6.2 Effect of carbon black on microwave heating of rubber [61].
144
temperature. Carbon black particles are collected from the smoke in bag filters, densified,
and packaged. Carbon black is used in manufacturing tires, black inks, paints, paper, and
plastics [65].
Because of its fine particle size, carbon black has been used as an asphalt cement
reinforcing agent. The mean particle diameter of carbon black ranges from 19 to 500
nanometers (nm), while asphalt cement in compacted pavements has an average film
thickness of about 5000 nm. Thus, the carbon black can be dispersed and completely
imbedded in the asphalt film. The addition of carbon black in the amount of 10 to 15 % of
the asphalt cement weight has been shown to improve the durability and wear resistance of
pavement, and the temperature-viscosity susceptibility characteristics of the asphalt
[59,66]. It was also reported that the addition of carbon black to asphalt cement reduced
rutting at high temperatures, and improved creep characteristics at high temperatures and
long loading times [67]. The same study showed that fatigue resistance and fracture
strength of tested mixtures were not adversely affected by the addition of carbon black.
Carbon black is added to asphalt mixtures in a pelletized form containing fluxing
oil to facilitate rapid dispersion and compensate for the stiffening effect resulting from its
addition. Carbon black pellets containing 8 and 25 percent flux oil are commercially
available. Carbon black with 8 percent fluxing oil from Cabot Inc. costs about 310 per
pound [68 ].
IV.
Experiment
The experiment consisted of heating fabricated specimens in the microwave oven
for a fixed time then measuring their temperatures. Specimens made from mixtures with
different amounts of carbon black were compared to specimens made from mixtures
without carbon black. The following paragraphs outline the experiment
145
A. Materials
Aggregate and asphalt cement used in this portion of the research were similar to
those used in the tests of Chapter Three. Asphalt cement content was 4.5 percent of the
total weight of asphalt and aggregate. Added carbon black was in pellet form, containing
8 percent fluxing oil (Microfil 8 from Cabot Coip). Microfil 8 was added to hot dry
aggregate in the amount of 2 ,5 ,1 0 , and 15 percent of the asphalt cement weight. Those
amounts comprised 1.84 , 4.6, 9.2, and 13.8 percent of carbon black out of the asphalt
weight respectively.
B.
Equipment
In addition to the equipment used in Chapter Four an infra-red temperature
measuring gun (Omega Scope - Series 2000) was used. The device was capable of
measuring temperatures in the range -25 to 2500 °F. The emissivity of the surface whose
temperature is being read by the gun must be known and entered into the gun before
testing. Darker surfaces should have a high emissivity that approaches 1.0. All asphalt
mixture specimens in this portion of the work were assumed to have emissivities of 1.0 .
Surface temperatures could be measured within 5 seconds from aiming with the gun,
which reduced the effect of heat loss to the air and heat transfer within specimens.
C.
P rep aratio n of specimens
Specimens were prepared according to Marshall procedures (ASTM D-1559).
Carbon black was added to dry hot aggregate and mixed before the addition of asphalt
cement
146
D. Test procedures
Test procedures were similar to those used in Chapter Four, except that surface
and bottom temperatures were measured with the infra-red gun, while core temperatures
were measured with the probe. Another deviation from procedures in Chapter Four was
in specimen's arrangement in the microwave oven. The amount of absorbant aggregate
below the bottom of the mold of heated specimen was reduced to allow for more air
separating the layer between the bottom of the specimen and the absorbant aggregate.
V. Tests Results and Evaluation
Results are summarized in Table 6.3 and Fig. 6.3, Fig. 6.4, Fig. 6.5 and Fig. 6 .6 .
A significant increase in the temperature of mixtures containing carbon black compared to
mixtures without carbon black was obtained. The rise in temperature is proportional to the
percentage of carbon black. However, the 9.2% and 13.8% levels essentially had the
same effect The wide scatter of points at these two levels of carbon black concentration,
as shown in Fig. 6.3, hints at a lack of uniform dispersal of carbon black in the mix. If a
hot spot existed in a sample due to a carbon black lump, it would have been detected only
if it happened to be at the predetermined temperature measuring point. The highest
temperature could represent a hot spot or well-dispersed carbon black. The lowest
temperature, on the other hand, could be a cold spot or, again, the case of well-dispersed
carbon black. The addition of carbon black to asphalt mixtures improved their absorbancy
of microwaves. However, this treatment reduced microwave penetration into the
specimen. Penetration of microwaves decreased in direct proportion to amount of carbon
black. Fig. 6.6 shows that the difference between temperatures of the core and the bottom
of specimen increased as the content of carbon black increased. This suggests that lower
and lower levels of energy were allowed to reach the bottom of the specimen as the carbon
black content increased. However, the presence of increasing amounts of carbon black
147
Table 6.3
CB
(%)*
0 .0
1.48
4.6
9.2
13.8
Effect of carbon black content on microwave heating and penetration in
asphalt mixtures after 10 minutes of heating.
Spec.
no.
Wt.
(gm)
Ht.
(in)
IniL Temp.
(F)
1021
Final Temperature (F) at
Surface Core** Bottom
1
2
1018
2.28
2.26
72
72
292
286
362
345
305
316
Avg.
1020
2.27
72
289
353
310
1
2
1029
1037
2.34
2.33
75
76
327
317
387
381
313
345
Avg.
1033
2.33
76
322
384
328
1
2
1031
1041
2.33
2.34
76
76
351
358
403
412
337
327
Avg.
1036
2.34
76
355
408
332
1
2
3
1035
1027
1035
2.26
2.28
2.26
74
74
74
366
377
418
411
429
484
342
362
368
Avg.
1032
2.27
74
387
441
357
1
2
3
1039
1041
1036
2.34
2.31
2.32
72
72
72
422
351
367
483
385
438
364
340
366.
Avg.
1039
2.32
72
380
435
357
i .
* Carbon black as % of asphalt cement weight.
** The core is at the center of the 4" dimeter specimen, and 1.5 " from the bottom.
Temperature at the core of specimen (F)
148
500
480
460
440
420
400
380
360
340
320
300
0
2
4
6
8
10
CB content (% of asphalt)
12
Figure 6.3 Effect of adding carbon black to asphalt mixtures on
microwave heating.
14
Change in temperature (F)
149
400 ----- ----------------------------------------------------------- ——
367
363
332
— —
p i *
308
"
lllflf
28 ft3 0 0 ----- 280------------—I— :
11111 —
§ !
%*•
200
- V
1 0 0 - -
0
'
:
*
K -
I"-
i
0
'
-
i
— \
'•
4
'
—
" 1
i
i fo'"1'1
'
—
i
—
.
-
'"""''••I ) H !¥ f :?t i
1.84
4.6
9.2
CB contenet {% of asphalt)
13.8
Figure 6.4 Effect of carbon black content on the change of the temperature
at the core of microwave heated specimens (averages).
150
CB Content
500
450
9.2%
i3.8%
4.6%
tr
400
350
784%
0%
300
250-1----surface
core
Points of temperature measuring
Figure 6.5 Effect of carbon black addition to asphalt mixtures on
microwave penetration in compacted specimens.
bottom
151
t
79
Change in Temperature (F)
100
1.84
4.6
9.2
13.8
CB content (% of asphalt)
Fig. 6 .6 Effect of carbon black content on difference between the higher
temperature of the core and that of the bottom of microwave
heated specimen (distance between core and bottom =1.5 in.).
152
had compensated for the decreasing levels of penetrating waves as shown in Fig. 6.5.
Thus the relation between temperature and carbon black concentration that was found
earlier is still valid. If such additives can be incorporated in asphalt cement, microwave
heating of it will be possible. Matching the heating rate of the binder to the aggregate
should eliminate microwave-preferential heating; thus, simultaneous uniform and fast
heating of both asphalt and aggregate may occur. The maximum temperature recorded,
which was 440 F, is a warning of possible overheating of the binder when such a
chemical is present in the mixture. This problem can be avoided if the time (energy) of
heating is reduced. Cutting microwaves application time and associated costs are the
t
goals of using such enhancers. However, cost-benefit analysis is needed to determine
whether the use of such an additive is feasible.
CHAPTER SEVEN
SUMMARY, CONCLUSIONS AND RECOM M ENDATIONS
I. S u m m ary
This research has focused on the material aspects of microwave heating of asphalt
pavement mixtures. A review of the literature provided some basic information about the
quality of pavement materials as far as microwave heating is concerned. Some general
conclusions have been drawn, based on this review. However, that review has revealed
the shortage of information that describes the relationship between microwave heating and
asphalt pavement mixtures, loose and compacted, in specific terms. This research.has
addressed the interaction between the heated material and microwave energy absorption
from two complementing sides.
First, the effect of microwave heating on mixture quality was evaluated by the
diametral resilient modulus, the water stripping resistance and the split tensile strength.
Three different groups of laboratory-prepared mixtures were subjected to testing. Those
mixtures were: 1) virgin mixtures, 2 ) virgin mixtures with a polar anti-stripping agent and
3) artificially aged materials. Present and anticipated heating applications of microwaves in
treating asphalt materials were represented in the laboratory. Full microwave heating,
supplementing conventional heating with microwaves, and later and brief treatment
(zapping) of conventionally heated mixtures with microwaves were the three basic heating
procedures used in this study.
Second, the effect of several mixture variables on microwave heating efficiency of
pavements was also investigated. The variables that were part of this study are the asphalt
content, the aggregate gradation, the compaction effort, the initial temperature of the
material, and finally the moisture content of aggregate.
154
Furthermore, enhancement of microwave heating efficiency was demonstrated to
, be possible through the addition of selected chemicals.
During the course of the study, a home microwave oven was the source of
microwave energy. Heating molds had to be fabricated from Teflon to facilitate heating in
the microwave oven. In an effort to simulate vertical application of microwaves inside the
microwave oven, Teflon molds were wrapped with an aluminum foil around their outer
circumference to prevent microwave penetration from the sides.
II. C o n c lu sio n s
The following are several conclusions that appear to be warranted:
1.
The microwave heating efficiency of aggregate depends on its chemical
composition. High metallic content (high specific gravity) and low
quartz content are characteristics of good aggregate for microwave
heating.
2.
The microwave heating efficiency of aggregate is also a function of
moisture content. The efficiency increases as the amount of absorbed
water increases. About 1 percent of absorbed water could reduce heating
time by 50 percent of that required for dry materials.
3.
Distinction must be made between absorbed water and surface water
when describing moisture content of materials to be heated with
microwave energy.
4.
Excess surface water wastes energy in evaporation, and the more the
surface water, the more the energy wasted.
Completely dry aggregate and sand should be avoided in microwave
heating or otherwise modified with water.
The efficiency of heating asphalt cement and mixture by microwaves can
be significantly improved by the addition of selected chemicals.
Carbon black is a very effective microwave heating enhancer for asphalt
mixtures.
The enhancement is in direct proportion to carbon black content for
amounts below 10 % of asphalt weight
Use of highly absorbant additives may decrease penetration of
microwaves. Depending on the type of the additive, its absorbancy may
compensate for the low level of energy allowed to penetrate.
Heating of plain aggregate in microwave oven alone does not produce a
uniform temperature. Consequently adhesion of asphalt to aggregate will
not be uniform and severe stripping or high loss in strength may occur
even with the use of polar anti-stripping agents.
Zapping of hot asphalt mixtures, virgin or recycled, in the microwave
oven could result in a higher MR and St. Increase in these values is
proportional to zapping time.
The microwave supplemental heating produced recycled mixtures with
strength properties (Mr and St) in between those of conventionallyheated and microwave-heated recycled mixture.
156
13.
Increase in the density, whether achieved by using dense-graded
aggregate mix or higher compaction effort, increases heating efficiency
of these materials.
14.
Increase in the asphalt content appears to reduce heating efficiency of
mixtures slightly.
15.
Initial temperature of dry compacted mixtures within the range tested do
not affect microwave heating efficiency measured by the difference
between initial and final temperatures.
16.
The use of a kitchen-type microwave oven as a source of microwaves
facilitated quantitative comparison among laboratory materials.
17.
Only a qualitative evaluation of Held materials can be obtained by the
microwave oven at this time. While close resemblance to microwave
heating ducts might be provided by the microwave oven, the vertical
application is not possible.
m . R eco m m en d atio n s
The use of the microwave oven in laboratory to heat and test asphalt mixtures is
very attractive due to fast heating. However, this research has revealed some drawbacks,
most important of which is the lack of a uniform temperature in the heated substance,
which the user should be aware of. The following are some recommendations that
should reduce these shortcomings:'
1. Microwave heated material should be stored at the desired temperature in a
convection oven for a short period of time to promote temperature equalization.
The storage time needs to be determined.
157
2. When microwave heating is used in sequence with conventional heating,
microwaves should be applied first. This arrangement would allow the
microwaves to benefit from any moisture that might be present in the material
for fast heating. Also, this sequence would insure temperature equality of
heated material before discharge.
3. When heating asphalt mixtures in a microwave oven, it is recommended to
cover the heated mixture to reduce exposure to any circulating air in the oven
that might age the asphalt. Covering should not to be very tight in order to
allow any evaporating water to escape.
IV. Needed R esearch
This study, being the first of its kind in the microwave heating of asphalt mixtures,
cannot emphasize enough the need for additional research. Although this study was
intended to be more specific, it has resulted in describing general trends and relations that
need a second effort to be statistically sound. However, testing research findings in the
field would be the best approach.
Again, it is recommended that the results of this research be field tested with actual
pavement microwave heaters.
It is also recommended that a mix design procedure and design criteria appropriate
for microwave-heated mixtures be developed. A method of rapid heating by microwaves
which does not age asphalt cement as much as the lengthy conventional methods make this
step of high priority, especially for recycled mixtures. The potential benefit of early
strength gain for treated mixtures demands work in this area too. The effect of microwave
heating on fatigue is also a good topic for further research.
158
Correlating field and laboratory data should lead to developing standard
procedures of using microwave ovens in the laboratoiy for mix design and quality control
purposes, especially since many laboratories use them for rapid heating without taking
into account the difference between microwave and conventional heating. This in turn
mandates more studies that correlate microwave oven heating to the effect of convection
heating on asphalt mixtures. This step would permit the use of existing data bases, and
would allow exchange and interpretation of data between laboratories that use different
heating systems.
The cause and the extent of the loss of strength of microwave heated materials due
to water action need further investigation. Since most of that loss has been attributed to
the lack of temperature uniformity in the heated aggregate, it would be of interest to see if
the vertical application of microwaves in the field would reduce that problem. The early
strength benefits need further investigation and comparison with available techniques,
such as the modification of asphalt through the addition of chemicals.
Fundamental studies on the effect of microwaves on polarization of asphalt cement
and polar additives need to be pursued. Testing procedures that examine the compatibility
of asphalt cement and rejuvenator or polar additive in the presence of microwaves are
required in order to avoid creating double layers which weaken adhesion. The possible
adhesion and coating improvement mechanisms described in Chapter Three need to be
isolated and studied in more detail. The extent of the effectiveness of each one in relation
to materials used needs to be researched. Viscosity and polarity of asphalt cement in
relation to these mechanisms need further study too. These studies should facilitate
selection and design of mixtures with polar additives and in utilizing and optimizing the
other benefits of microwave energy.
159
Though it is believed that findings on the role of water in microwave heating of
plain aggregate in Chapter Five could be extended to asphalt mixtures, work on reclaimed
pavement material would be of a great value. The effect of moisture on microwave
heating of compacted materials containing moisture also needs to be studied. The
presence of water as ice also needs investigation since responses of water and ice to
microwaves are different. Water responds much better than ice. A highly absorbant
aggregate is recommended to be used in future testing. Optimum water content for
microwave heating for the different mixtures and aggregate types needs to be determined.
Identifying more microwave heating efficiency improvers should continue. The
optimum amount for these additives in the mix also needs determination. The economic
impact of the use of these materials needs to be illustrated in conjunction with commercial
microwave heating systems.
The proposed microwave heating equipment for paving are of a high power levels
that could be 125 times the power of a home oven. Therefore, the safety of operators and
workers should be a major concern. In addition to what has been done there are several
steps through which a safer use of microwaves might be achieved. These steps may
include: 1) the education and the training of operators and workers, 2 ) the development of
microwave emission standards suitable for such highly powered equipment, or the
revision of existing standards that have been used for home ovens, 3) communicating
safety concerns to the equipment manufacturers, and the health experts on the effect of
microwaves on human body, at this stage of adapting microwaves to pavem ent
applications, and 4) development of sensitive emission detecting devices.
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VITA
Abdulaziz Al-Ohaly was bora in Onaiza, Saudi Arabia in 1954. He received
his high school Diploma in 1973 from Al-Assema Institute in Riyadh, the capital of
Saudi Arabia. In 1978 he graduated from King Saud University (formerly
University of Riyadh) with a B.S. in civil engineering. He worked as a teaching
assistant at the College of Architecture and Planning of King Faisal University in
Dammam, Saudi Arabia for one year. In December 1979, King Faisal University
granted him a scholarship for graduate studies in the United States. In 1982 he
received an M.S. in civil engineering from Wayne State University in Detroit,
Michigan. He was awarded the Ph.D. degree in civil engineering by the University
of Washington in Seattle, Washington in 1987.
His permanent address is:
College of Architecture and Planning
P.O.Box 2397
Dammam, Saudi Arabia.
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