close

Вход

Забыли?

вход по аккаунту

?

mcmbaaa.28227.0047

код для вставкиСкачать
THEME FIVE:
DURABILITY
ENHANCING
ADMIXTURES
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Keynote
Paper
DURABILITY
HOW FAR CAN
IT BE EXTENDED WITH ADMIXTURES
M Corradi
R Khurana
MAC S.p.A. Modern Advanced Concrete
Italy
A B S T R A C T . The durability of concrete structures was considered to be an intrinsic property
of the material. It is now evident that it is very much dependent on the environmental
conditions in which the structure is situated.
Some basic requirements, such as good quality concrete, which is caracterized by a low water
cement ratio and hence a low permeability, adequate cover to reinforcement, proper
compaction and curing of the concrete, are of fundamental importance for its durability.
Besides the design and construction practice, material properties and exposure conditions are
of great importance.
The causes of concrete deterioration are manifold and may be broadly classified as of
physical and chemical nature. Admixtures added to the concrete mix during mixing will help
to mitigate the effects of the degradation agents. Several families of admixtures are now
available to enhance the durability of concrete. In this paper these admixtures presented in
relation to their effectiveness against these deteriorations.
Keywords: Admixtures, Air entraining, Alkali aggregate reaction, Biological attack,
Chloride diffusion, Corrosion inhibitors, Cracking, Durability, Exposure conditions, Freeze
thaw resistance, Porosity, Reinforcement corrosion, Shrinkage, Superplasticizers.
D r M a r i o C o r r a d i is the Managing Director and Director of Research & Development of
MAC S.p.A. (Modern Advanced Concrete), Treviso/Italy which is a part of the MBT-SKW
Group. He is actively involved since 1970 in the development and use of admixtures to
improve the quality of concrete structures and has authored several papers on this subject.
Ing R a b i n d e r K h u r a n a is the Technology Director at MAC S.p.A. (Modern Advanced
Concrete), Treviso/Italy. He has been involved for more than 25 years for enhancing the role
of admixtures in concrete construction. He has an active member of CEN TCI04 SC3
"Admixtures for concrete" and CEN TCI04 SC8 "Products and systems for the protection of
concrete structures".
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
476 Corradi, Khurana
INTRODUCTION
Concrete is surely the most common and widely used man made material. Estimates indicate
that every year one cubic meter of concrete is made for every two human beings. Its wide
distribution is due to its low cost and the availability of the basic raw materials. Good quality
concrete is normally considered to have durability as an intrinsic property. But the enormous
amount of money being spent all over the world to repair concrete structures clearly indicates
that much has to be still done to improve the durability and maintain the serviceability of
concrete structures.
Factors influencing durability
The factors which determine the durability of concrete may be classified under four
categories; for example design, construction practice, material properties and exposure
conditions. Errors in design or carelessness in detailing may lead to stress concentrations and
cracking. These should be of concern to the design engineer. The environmental conditions to
which the concrete structure is exposed will determine the eventual cause of its deterioration.
These exposure conditions may vary over a wide range which include sheltered, indoor
concrete, cyclic freeze thaw conditions, hot and dry desert ambient. Contact with, or the
presence of certain chemicals in concrete such as sulphates, chlorides, sulphides, acids,
carbon dioxide etc., will also lead to deterioration. Higher ambient temperatures will
accelerate the reaction rate. Micro-organisms may also attack the concrete. Mechanical
abrasion or erosion by water or wind will also effect the service life of a structure. These
factors may act singly or in combination and the degradation mechanism may be either a
physical effect such as shrinkage, creep, expansion, erosion, etc. or a chemical reaction like
sulphate attack, carbonation, reinforcement corrosion, alkali silica reaction, etc. Examples of
erosion by cavitation and severe cracking caused by delayed ettringite formation are shown
in Figure 1 and Figure 2 .
Scale 1:100
Figure 1 Erosion of concrete in a stilling basin of a dam caused by cavitation
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Extended Durability with Admixtures
477
The constituent materials of concrete determine its ability to resist certain aggressions. Photo
no. 3 shows concrete deterioration caused by thaumasite. Rock excavated from a highway
tunnel was crushed to produce aggregates which were used to produce concrete for the tunnel
lining. During some phase of the tunnel construction sulphate bearing rocks were encountered
and used as aggregates.Therefore, the choice of sound and good quality materials is of prime
importance. Another basic requirement for a high quality and durable concrete is its low
permeability. The permeability depends upon the quality of cement or binder and aggregates,
water binder ratio, degree of hydration, compaction and curing and the absence or presence of
cracks. The right choice of cement type and aggregates may exclude certain kinds of
aggressions such as sulphate attack, alkali silica reaction, etc. The reduction of the water
cement ratio will reduce the quantity of pores and their size in the concrete. Effective
compaction (related to the workability of the concrete mix) and efficient curing will
determine the degree of hydration, which influences the dimensions of the pore structure.
Therefore, by reducing the amount of pores and their dimensions it is possible to reduce the
permeability and hence the rate of ingress of moisture borne aggressive agents.
Scale 1:5
Figure 2 A prestressed concrete railway sleeper cracked by delayed ettringite formation
Admixtures and durability
European Standard pr EN 934 part 2 defines admixtures as "Material added during the mixing
process of concrete in a quantity not more than 5% by mass of the cement content, to modify
the properties of the mix in the fresh and/or hardened state". Amounts exceeding 5% are
considered to be "additions" according to prEN 206.
As mention earlier, concrete is subjected to numerous types of aggression. Different types of
admixtures are now available to control and block their deleterious effects and therefore,
enhance the durability of the concrete. Since the permeability of the cementitious matrix
plays a fundamental role in the deterioration process, the water cement ratio is the crucial
parameter to be controlled. To reduce the water cement ratio, either the cement content has to
be increased or the water content has to be decreased. An increase in the cement content leads
to an increase of materials cost, heat of hydration and shrinkage . Also, reducing the water
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
478 C o r r a d i , K h u r a n a
Scale 1:50
Figure 3 This concrete column just a few months after its construction is rendered unfit due
to the formation of thaumasite. Sulphate rocks were inadvertently used for making
concrete. The structure had to be demolished and remade
content to decrease the water cement ratio will cause a reduction of the workability of the
concrete mix. The stiffer is the concrete mix, more the compactive effort it will require to
achieve full compaction. This will require powerful compaction equipment and longer
placing times. Therefore, the first category of admixtures that will be considered is the water
reducing type.
Water reducing admixtures
These admixtures may be classified as water reducing/plasticizing admixtures or as high
range water reducing/superplasticizing admixtures (water reduction more than 12%) which
may have other functions such as set retardation, set acceleration, hardening acceleration and
air entrainment. These multifunctional admixtures effect several properties of the fresh
concrete at the same time. Modification of the setting properties of the concrete help avoid
construction problems such as cold joints, thus improving the durability. Commercially
available water reducing admixtures may be classified in polymers of five generic families:
i)
Modified ligninsulfonate (MLS)
ii) Sulfonated melamine formaldehyde condensate (SMF)
iii) Sulfonated naphthalene formaldehyde condensate (SNF)
iv) Carboxylic ethers (CE)
v) Others
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Extended Durability with Admixtures
479
Typical performances of these admixtures are reported in Table 1.
As mentioned earlier, the workability of the concrete at the time of its placement is also of
fundamental importance. A loss in workability of the concrete mix during the time interval
from mixing to placing means that the concrete has to be placed and compacted with greater
care and effort. If this is not done properly, the hardened concrete will result to be more
porous and hence, less durable. If water is added to the concrete mix to restore its workability
(retempering), this will lead to an increase in the water cement ratio and consequently, a
higher porosity and permeability of the hardened concrete. Innovative types of
superplasticizers, based on polymers of polycarboxylic ethers (CE) are now available which
allow the concrete mix to retain its workability for 90 minutes and even more, if required.
Table 1 Performances of various water reducing admixtures
FAMILY
Modified ligninsulfonate
Sulfonated melamine
formaldehyde
Sulfonated napthalene
formaldehyde
Polycarboxlic ethers
TYPICAL DOSAGE, %
WATER REDUCTION, %
0.2 - 0.6
up to 10
1-3
up to 25
0.8-1.5
up to 25
0.5-1.5
up to 40
Therefore, the role of water reducing admixtures having secondary functions, such as long
slump retention, are of extreme importance for reducing the water cement ratio and hence as a
consequence, the porosity and permeability of the hardened concrete. All durability factors
relating to the porosity and permeability such as chloride diffusion, sulphate ingress, rate of
carbonation, etc. are, therefore, improved.
Air e n t r a i n i n g a d m i x t u r e s
These are admixtures that allow a controlled quantity of small, uniformly distributed air
bubbles to be incorporated in the concrete mix during the mixing and which remain in it after
hardening (prEN 934 part 2). Air entraining admixtures are surfactants whose molecules are
adsorbed at the air-water and solids-water interface. They may be of anionic or cationic
nature. The commercially available air entraining admixtures are usually salts of wood resins
(neutralized vinsol resin or synthetic detergents (alkyl aryl sulfonates). Other types of air
entraining agents include salts of petroleum acids, proteinaceous materials, fatty and resinous
acids and sulfonated hydrocarbons. Air entrainment reduces strengths of the hardened
concrete. However, this reduction can be offseted by the use of water reducing admixture in
the same mix.
The amount of air entrained, containing bubbles of the right size and spacing, will control the
damage to concrete cause by frost and de-icing salts (see figure 4 and 5).
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
480
Corradi, Khurana
Scale 1:100
Figure 4 A concrete retaining wall heavily damaged by frost action
In a properly air entrained concrete, the air content should be 4-8% (depending on the
maximum size of the aggregate), bubble frequency of 300 to 600 per meter, specific surface
of 24-48 mm" and a spacing factor of 100 to 200 microns.
1
Scale 1:50
Figure 5 Deterioration of concrete guard rails caused by de-icing salts
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
E x t e n d e d D u r a b i l i t y w i t h A d m i x t u r e s 481
C o r r o s i o n inhibitors
Reinforcement corrosion is one of the major cause of deterioration of concrete structures. A
wide spread increase in the use of chlorides (sodium and calcium) for the removal of snow
and ice from roads, pavements and bridge decks in cold climates has aggravated the situation
(see figure 6).
Scale 1:50
Figure 6 Reinforcement corrosion and heavy spalling in a beam of
a bridge deck caused by chloride ingress
Methods for preventing or controlling reinforcement corrosion in new or existing structures
include the use of epoxy coated steel bars, waterproofing membranes to prevent chloride ion
ingress, impregnation of concrete with polymers, removal of chloride ions and cathodic
protection. For new structures, corrosion inhibiting admixtures may be added to the concrete
mix to combat reinforcement corrosion. These admixtures are either inorganic salts or organic
materials.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
482
Corradi, Khurana
The inorganic type are grouped in three categories, namely anodic, cathodic or mixed types,
depending upon their interaction to the corrosion process. They include calcium and sodium
nitrite, sodium benzoate, sodium chromate, zinc oxide and disodium phosphate. Organic
materials showing corrosion inhibiting action include hydrazine hydrate, chelating
compounds, carboxylic acids, analine and petrolactam compounds. Other parameters, such as
permeability, cracks, resistance to chloride ions, etc. also influence the corrosion of steel. Use
of superplasticizers and supplementary cementitious materials such as fly ash, slag and silica
fume are also beneficial.
Alkali a g g r e g a t e reaction inhibiting a d m i x t u r e s
Alkali aggregate attack is an example of damage caused by chemical reactions occuring
within the concrete. The expansion is caused by reaction between the alkali hydroxides,
usually derived from cement, and reactive silicious or carbonate aggregates. This reaction
involves a formation of a gel of variable composition. This gel absorbs water and swells. The
stresses caused by this swelling may cause cracking and or localized "pop outs"(see figure 7).
Scale 1:200
Figure 7 Alkali silica reaction has caused intensive cracking in a concrete dam.
Attempts to repair it by injecting with epoxy resins was made
The major cause of the alkali aggregate reaction is the alkali content in the concrete. Major
source of alkalis is the cement. Recently, due to the pressure from the environmental lobby,
all the effluents from the cement production are recycled back to the production process. This
has led to an increase of the alkali content (sodium equivalent) of the cement and in some
cases it is more than 1%. As a consequence, even in countries where alkali silica reaction was
unknown, it is now quite frequent. Most international specifications limit the sodium
equivalent content in cement: the ASTM C150 limit is 0.6%. Other standard specifications
for example BS, EN. etc. limit the total alkali in concrete, derived from all sources, to 3
kg/m . Besides controlling the nature of the aggregates and the alkali content of the cement,
3
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
E x t e n d e d D u r a b i l i t y w i t h A d m i x t u r e s 483
fly ash, blast furnace slag, silica fume, rice husk ash and recently, finely ground glass have
also been used as additions to prevent alkali silica reaction. Chemical admixtures that reduce
or eliminate the expansion caused by the alkali silica reaction include lithium compounds
(hydroxide, carbonate, nitrate and floride) sodium silica floride and alkyl alkoxy silane.
Biological a t t a c k resistance a d m i x t u r e s
Concrete structures made with durable concrete can suffer severe damage due to the action of
bacteria, fungi and insects. The most common manifestation is in the sewage structures (see
figure 8).
Scale 1:20
Figure 8 Acid corrosion caused by bacteria action in a concrete sewer
The metabolic activity of certain bacteria and fungi releases corrosive chemicals which
promote the deterioration of concrete. In some cases, protection of the concrete surface may
be required for resistance to acid produced by these organisms. Admixtures to be added to
the concrete mix to combat the action of fungi, bacteria and termites are also available. They
do not effect the properties of fresh and hardened concrete. Bactericide admixtures are
polyhalogenated phenols, sodium benzoate, benzaalkonium chloride and copper compounds.
Emulsified dieldrin chemical added to the concrete mix has a potent toxic effect on termites.
Shrinkage reducing admixtures
Concrete, right from its placement and finishing is subjected to volume changes such as
settlement, plastic, thermal and drying shrinkage. All these shrinkages may lead to some form
of cracking in the concrete structure. Cracks are preferable paths for ingress of aggressive
agents into the concrete. Settlement shrinkage can best be taken care of by an appropriate mix
design. Plastic shrinkage is combatted with appropriate curing and use of synthetic fibers.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
484
Corradi, Khurana
Thermal shrinkage is related to construction techniques and is normally minimized by
controlling the thermal gradient along the cross section of the concrete pour. Drying
shrinkage is caused by the loss of moisture from the cementitious paste and takes place over
the life span of the structure. To reduce the drying shrinkage and thus reducing the possibility
of crack formation, shrinkage reducing admixtures are now available. These admixtures
include oligoalkylen glycols, polyalchols and derivatives. Their mechanism is based on the
reduction of the surface tension of the pore water and therefore lowering the stresses caused
by drying which could lead to cracks.
CONCLUSIONS
The durability of a structure depends upon the quality of the concrete and the exposure
condition to which it is subjected. The right choice of the constituent materials of concrete
make it less vulnerable to certain aggressions. Correct construction techniques and adequate
specifications also enhance the durability. The role of water reducing admixtures and
superplasticizers is of fundamental importance as they reduce the porosity of the concrete,
thereby making it less penetrable to aggressive agents which may undermine its durability.
Air entraining admixtures, incorporating the right quality and quantity of air voids in the
hardened concrete, renders the structure resistant to freeze thaw cycles and deicing salts.
Corrosion inhibitors provide extra protection against reinforcement corrosion. Specific
admixtures, along with additions like fly ash, slag and silica fume ensure protection against
alkali aggregate reaction. The tendency of concrete to crack due to drying shrinkage is
reduced if shrinkage reducing admixtures are incorporated into the concrete mix. Even attack
by microorganisms and insects can be counteracted by appropriate admixtures. For certain
aggressions such as abrasion, acid attack and specific chemicals, concrete must be protected
with coatings or linings. Admixtures that help improve construction practices such as
underwater concreting, construction in cold climates, pumping aids, shotcreting, improving
bond, damp proofing and water resisting, etc. are also available but are not directly related to
the durability of the concrete structure. Durability is enhanced if the existing knowledge is
applied and ensured by adequate quality control and quality assurance systems. Admixtures
for known aggressions are readily available and new developments are always happening.
Admixtures play a fundamental role for extending the durability of concrete structures so that
they may arrive intact into the fourth millennium.
REFERENCES
1. RAMACHANDRAN, V.S., Concrete Admixtures Handbook, Noyes Pubblications,
Second Edition, 1995.
2. MAILVAGANAM, N.P., Repair and Protection of Concrete Structures, CRC Press, 1991.
3. MEHTA, P.K., Concrete Structure, Properties and Materials, Prentice Hall, 1986.
4. NEVILLE, A.M., Properties of Concrete, Pitman Publishing, Fourth Edition, 1995.
5. MALHOTRA, V.M., Superplasticizers: A Global Review with Emphasis on Durability
and Innovative Concretes, American Concrete Institute SP-119, 1989.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Документ
Категория
Без категории
Просмотров
2
Размер файла
980 Кб
Теги
mcmbaaa, 28227, 0047
1/--страниц
Пожаловаться на содержимое документа