close

Вход

Забыли?

вход по аккаунту

?

mcmbaaa.28227.0005

код для вставкиСкачать
FACTORS INFLUENCING THE RATIO OF MORTAR TO
CONCRETE STRENGTHS
G K Moir
Blue Circle Industries PLC
United Kingdom
ABSTRACT. The factors which influence the ratio of EN 196-1 mortar to BS 4550 concrete
strengths have been examined using data derived from the testing of random audit samples
taken as part of the BSI Kitemark Scheme. Selected cements with a range of chemical and
physical characteristics were also tested in EN 196-1 mortar, ASTM C 109 mortar and a
range of concretes prepared using BS 4550 aggregates. It is concluded that no single test,
whether in mortar or concrete, can be expected to reliably predict concrete strength over a
range of cement contents. Nevertheless for a given cement source a significant change in
mortar strength is likely to be reflected in a significant change in concrete strength.
Keywords: EN 196-1 mortar, ASTM C 109 mortar, BS 4550 concrete, Alkalis, Cement
content, Portland cement.
Dr G K Moir is Chief Chemist at the Blue Circle Industries PLC Technical Centre at
Greenhithe in Kent. His main responsibilities and interests concern cement production
quality control and the factors which influence both the short and long term performance of
cements in use. He is currently chairman of CEN TC51/WG10 (masonry cement) and of
CEN TC51/WG12/TG1 (sulfate resistance test method).
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
50 Moir
INTRODUCTION
All national cement standards have minimum strength requirements which are determined
utilising a standardised test procedure. Cement type may also be classified according to
strength and the current British cement standards, in common with the proposed European
Standard for Common Cement (prEN 197-1) has several strength classes with well defined
upper, as well as lower strength limits.
In the world today most national standards utilise one of 4 test procedures whose main
characteristics are summarised below:
Table 1 Standardised strength tests for cement used internationally
DESIGNATION
MIX PROPORTIONS
CA:FA:C:W*
SPECIMEN
DIMENSIONS mm
CURING TEMP
°C
BS 4550 concrete*
EN 196-1/ISO 679 mortar
ASTM C109 mortar
BS 4550 mortar
3.5 : 2.5 : 1 : 0.60
0 : 3 : 1 : 0.50
0 : 2.75 : 1 : 0.485
0 : 3 : 1 : 0.40
100x 100x 100
4 0 x 4 0 x 160
5 0 x 5 0 x 50
70.7 x 70.7 x 70.7
20
20
23
20
* CA = Coarse aggregate, FA = Fine Aggregate, C = Cement, W = Water
J Now withdrawn but maintained as BS 1881 : Part 131 and as the UK Cement Industry Test
Procedure.
In the UK the vibrated mortar cube test was introduced in 1940 but was never popular with
either cement users or manufacturers. The cement industry developed its own 4 inch cube
(~ 100 mm) concrete method and this was standardised in 1978 with the publication of
BS 4550 : Part 3.4. This method formed the basis of quality control testing within the
industry until the introduction of BS 12 : 1991 which required strength testing using the EN
196-1 mortar prism method. The adoption of the new testing procedure meant that
customers were supplied with test certificate data based on mortar rather than concrete, and
an ongoing debate has continued regarding the significance of these results to concrete
production quality control data.
Schramli [1] has investigated the relationship between standard mortar test results and
standard concrete results with the results of ready-mixed concrete strength testing. It was
found that the standardised concrete test was no better than the standard mortar test for
predicting the strength of production concrete. This was attributed to the relatively low
variability of the cement used in the investigation, and the poor reproducibility of the
concrete test.
Gaynor [2] has analysed ASTM C 109 mortar and field concrete data and concluded that
provided reliance is not placed on the results of single tests, mortar results do give a
meaningful guide to concrete strengths. The relationships improve markedly if cement
variability is increased, and/or repeat testing and average results are used.
De Hayes [3] investigated the relationship between ASTM C 109 mortar strengths and
concrete cylinder strengths. Although a good correlation was found between 1 day mortar
and concrete strengths, correlations at later ages were poor.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Mortar and Concrete Strengths
51
Bloomer [4] has investigated the factors which affect the ratio of EN 196-1 mortar to
concrete strengths. It was concluded that cement properties other than strength have little
effect on this ratio.
This paper compares strength data obtained when cements from different sources were tested
in mortar (ASTM C 109 and EN 196-1) and concrete. Factors responsible for differences in
the ratio of mortar to concrete strength are examined.
SOURCES OF DATA
In the preparation of this paper, data have mainly been drawn from 3 sources:
•
•
•
Cement plant routine quality control data
Audit sample testing at Blue Circle Industries PLC central laboratory
An experimental programme utilising 17 cements from different plants in the UK and
overseas.
REPRODUCIBILITY OF TEST METHOD
If a test method is to yield meaningful results, it is essential that the inherent variability of the
method is relatively low in relation to cement variability. The reproducibility of the
EN 196-1 and ASTM C 109 methods can be assessed from the annual round-robin testing
exercises organised respectively by the French Cement industry association (ATILH) and the
US Cement and Concrete Research Laboratory (CCRL).Typical mean values in recent years
for all participating laboratories are given in Table 2
Table 2 Reproducibility of EN 196-1 and ASTM C 109
METHOD
EN 196-1
ASTM C 109
COEFFICIENT OF REPRODUCIBILITY
3%
7%
Within the cement industry itself, a higher level of reproducibility and repeatability is
normally achieved, and within laboratory coefficients of variation of 2% and 4% should be
achievable for the EN and ASTM methods respectively.
Thus, the European method has a level of inherent variability, which is approximately half
that of the ASTM method. This is borne out by quality control data for US plants, where
despite similar levels of control of cement chemistry, fineness and mineralogy, the
coefficients of variation of the mortar test results are approximately twice those of UK plants
which employ the EN 196-1 mortar test procedure. These differences in apparent strength
variability are illustrated in Figure 1.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
52 Moir
UK
UK
UK
UK
UK
UK
H UK
£ UK
<S UK
J UK
EN 196-1 MORTAR
NA
NA
NA
NA
NA
NA
NA
ASTM C 109 MORTAR
10
cv%
Figure 1 Coefficient of variation of 28 day strength results for 1997
Apr-94
Dec-94
May-95
Nov-95
May-96
Nov-96
May-97
Nov-97
May-98
DATE
Figure 2 Works A audit samples tested at Central Laboratory
70
OH
PC 42.5
•
SR42.5
60
s
PC 52.5
50 40
-
<
^
E a c h p o i n t r e p r e s e n t s the
m e a n o f 2 3 tests on c e m e n t s
O
sampled over a 4 year period
~7 30
ON
^
10
2010
,
20
I
30
•
- J
40
1
50
1
60
BS 4550 CONCRETE MPa
Figure 3 Relationship between EN 196-1 mortar and BS 4550 concrete strengths
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Mortar and Concrete Strengths
53
The BS 4550 concrete testing procedure has been withdrawn, but continues within the UK
cement industry as the Cement Industry Test Procedure and in other laboratories as BS 1881 :
Part 131. The method uses 10 mm crushed granite aggregate from a single source and
selected quartz sands. In a competent laboratory the coefficient of variation of 28 day
strength determined using this method is as good as that for the EN 196-1 mortar method,
that is - 2 % .
C O R R E L A T I O N B E T W E E N E N 196-1 M O R T A R S T R E N G T H S
A N D B S 4550 C O N C R E T E S T R E N G T H S
Parallel testing of cement samples in mortar and concrete has demonstrated that a significant
change in mortar strengths will always be reflected in concrete strengths, and vice versa.
This relationship is illustrated in Figure 2 where data from the testing of audit samples from a
cement source which shows higher than normal variability are plotted.
Since March 1994 the 6 random despatch audit samples taken each year for 16 cements,
(13 PC 42.5, 2 sulfate resisting PC 42.5 and 1 PC 52.5), as part of the requirements of the BSI
Kitemark Scheme have been tested in concrete, as well as mortar, at the Blue Circle UKAS
accredited central laboratory. The sand used during the period March 1994 - December 1997
was Normensand from Germany and the method of compaction used was vibration. Figure 3
illustrates the relationship between mortar and concrete 2 day and 28 day strengths. Each
point represents the average result obtained for 23 samples from each cement source/type.
The relationship appears satisfactory, but when the data are examined, it can be seen that
there are systematic differences in mortar:concrete (m/c) ratio for different products. At 28
days the m/c ratios range from 1.19 (PC 52.5) to 1.35 (relatively coarse PC 42.5). Figure 4
illustrates that there is clearly a relationship between m/c ratio and cement surface area. The
surface area of PC 42.5 cement reflects the fineness of grinding required to achieve a mean
28 day EN 196-1 mortar strength of -59 Mpa. The fineness of grinding required is
influenced by
•
•
•
the cement milling equipment (eg. open or closed circuit)
the clinker chemistry (silicates content, alkali level, etc.).
clinker mineralogy
and tends to be characteristic of a given plant.
The reduction in m/c ratio with increasing fineness is related to the influence of cement
fineness on the degree of compaction achieved during specimen preparation. Prism densities
were determined by weighing the mortar specimens in air and water immediately following
demoulding at an age of 24 hours.
In Figure 5 it can be seen that prism density decreases with increasing cement surface area.
This suggests that mortar compaction and the elimination of entrapped air is inhibited with
finer cements, at least with the vibration method of compaction.
Figure 6 confirms that m/c ratio increases with increasing mortar density. It can be deduced
that mortar compaction is influenced by cement fineness to a greater extent than is the case
with concrete compacted using the BS 4550 hand tamping
procedure.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
5 4 Moir
2,310
1.4
PC 42.5 SRC 42.5 PC 52.5
PC 42.5 SRC 52.5 PC 52.5
2,300
1.35
B 2,290
P
1-3
% 2,280
in
1.25
Q
2,270
1.2
2,260
—I i I i I i I 1 1 1 1 1 L_
2,250
320 340 360 380 400 420 440 460 480
_l
i I i L_
1.15
320 340 360 380 400 420 440 460 480
2
2
S S A m /kg
S S A m /kg
Figure 5 Relationship between prism
density and cement SSA
Figure 4 Relationship between m/c
strength ratio and cement SSA
1.4
m / c ratio = 1.73 - 0 . 0 0 1 S S A
- 0.008C3A
•
1.35
•
•
1.3 • •
U
< 1.25
-
.
•
•
1.2
•
PC 42.5 SRC 42.5 PC 52.5
•
%
•
2,250 2,260 2,270 2,280 2,290 2,300 2,310
DENSITY kg/m
PC 42.5 SRC 42.5 PC 52.5
•
@
•
1.15
1.15
i
1.2
I
1.25
.
1.3
I
,
1.35
,
!
1.4
3
Figure 6 Relationship between m/c strength
density and mortar density
PREDICTED
Figure 7 Actual v predicted m/c
Strength ratio
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Mortar and Concrete Strengths
55
A secondary influence identified using multiple linear regression analysis is cement C3A
content. Figure 7 illustrates that m/c ratio can be satisfactorily predicted using the expression
m/c = 1.73 -0.001 SSA -0.008 C A.
3
Thus the 28 day m/c ratio is not the same for all cements, but unless significant changes are
made to cement fineness or clinker chemistry the ratio remains a characteristic of that
particular cement.
INFLUENCE O F M O R T A R TEST P R O C E D U R E AND
CONCRETE CEMENT CONTENT
Whilst it has been shown that m/c ratio is not the same for all cements, the implications for
the prediction of concrete strength need to be assessed taking into consideration the other
important influences on concrete strength, such as:
•
•
•
Cement content
w/c ratio
Aggregate type
In order to investigate the influence of mortar test method and concrete cement content, 19
different cements were obtained from the UK, US, Australia and South Africa. This selection
enabled a much wider range of cement characteristics to be investigated, than would have
been the case had only UK class 42.5 cements been included in the programme.
All of the cements were tested in concretes prepared from BS 4550 aggregates, with nominal
cement contents of 200, 260, 310, 360, 430 and 550 kg/m . The concretes were gauged to a
constant slump of 60 mm at w/c ratios ranging from 1.10 to 0.36. In addition, 17 of the
cements were tested in BS 4550 concrete (w/c 0.60) and using the EN 196-1 and ASTM C
109 mortar methods.
3
Figures 8 and 9 respectively illustrate the relationship between EN 196-1 and ASTM C 109
mortar strengths, and those given by the BS 4550 procedure. Note that these are the results
of single pairs of tests and thus the scatter is considerably greater than in Figure 3.
Figure 10 illustrates that if the cements are ranked according to BS 4550 28 day strength the
overall agreement with both ASTM and EN 196-1 strengths is relatively poor. However, it
can also be seen that the agreement of BS 4550 concrete strengths with those of concretes
having different cement contents is also poor.
Several factors affect the response of different cements to changes in cement content and thus
w/c ratio. One of the most important is cement alkali content. Figure 11 illustrates that in the
rich 550 kg/m mix the cement alkalis have the expected negative influence on 28 day
strength. However, in the lean 200 kg/m mix, where the w/c was —1.1, the cement alkalis
had a positive influence on 28 day strength. Thus, no single test method, whether based on
mortar or concrete, can be expected to predict performance in a range of production
concretes. It is likely that the closer the w/c of the concrete to that used in the standardised
test, the better will be the relationship.
3
3
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
56 Moir
70
E a c h result is a s i n g l e test
60
OH
S
50
2
§
4 0
30
28
2 8 ddays
a v s 7 days 2 days
20
10 10
20
30
50
40
10
20
BS 4550
<
cu
EN 196-1
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
H
A
B
C
D
E
F
G
H
ASTM C 1 0 9
I
sex
z
z
36 38 40 42 44 46 48
MPa
50
200 kg/m3 (60 mm slump)
40
Figure 9 Relationship between
ASTM C 109 mortar
and BS 4550 concrete
Figure 8 Relationship between EN 196-1
mortar and BS 4550 concrete
z
30
BS 4550 MPa
BS 4550 MPa
<
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
46 48 50 52 54 56 58 60 62
MPa
36
310 kg/m3 (60mm slump)
38 40
MPa
42
44
550 kg/m3 (60mm slump)
|
A
B
C
D
E
F E
G
1
= n —
1
i
J
K
Z
<
K H
L H
M H
N ET
O EZ
p inn
Q D
M
N
O
PE
Q
10
12
MPa
14
34
^
,
1
1
^
, • ,.. i
'
=
^
,
i
-
;
,
i
1
\
i
i
1 :
36
38
46
40
MPa
75
MPa
80
Figure 10 Influence of test procedure and cement content on 28 day strength
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
M o r t a r and Concrete Strengths
8
0
0.2
0.4
0.6
0.8
1
0
1.2
EQ Na 0
2
0.2
0.4
0.6 0.8
EQ Na 0
1
2
Figure 11 Influence of cement alkali content on concrete 28 day strength
80
BS 4550
AGGREGATES
EN 196-1
MORTAR
RESULTS ARE M E A N S OF TESTS USING 17
60
C E M E N T S FROM DIFFERENT S O U R C E S
ASTM C109
MORTAR
40
Cu
20
0.2
0.6
BS 4550 aggregates
Concretes gauged to constant slump of 60mm.
0.8
1.2
W/C RATIO
Figure 12 Relationship between cement content and 28 day strength
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
57
1.2
58 Moir
Apart from its lower inherent variability a further advantage of the EN 196-1 procedure,
compared to the ASTM C 109 procedure, is that the strengths obtained are similar to those
given by a BS 4550 type concrete mix, proportioned to give a w/c of 0.50. The ASTM test
gives much lower strengths, despite the lower w/c of 0.485. This difference is illustrated in
Figure 12 using average data for the 17 cements. The main reason for the much lower
strengths given by the ASTM method is the much higher level of air in the mortar. The
ASTM C 109 mortars have typical air contents of 10%, whilst in EN 196 mortar the level
should be less than 4%, and in BS 4550 concretes less than 2%.
CONCLUSIONS
1. The ratio of EN 196-1 mortar to BS 4550 concrete strength is mainly influenced by
cement surface area. A secondary influence is cement C3A content.
2. The ratio of EN 196-1 mortar to BS 4550 concrete at 28 days strengths can be estimated
using the expression
m/c - 1.73 -0.001 SSA-0.008 C A
3
3. The decrease in m/c ratio with increasing cement surface area is associated with reduced
mortar prism density.
4. For any given cement source a significant change in EN 196 mortar strength is likely to
be reflected in a significant change in concrete strength.
5. It is not possible to predict concrete strengths over a range of cement contents from any
single strength test either based on standard mortar or concrete.
6. The EN 196 mortar strength is more satisfactory for quality control purposes than the
ASTM C 109 test procedure, on account of its lower inherent variability, and the much
better agreement between strengths obtained and those expected for a concrete of similar
w/c ratio.
REFERENCES
1. SCHRAMLI, W, and WOLTER, H. Determining the quality of cement by means of
standard mortar or standard concrete testing - an investigation. Zement Kalk Gips, Vol
41, No 10, 1988, pp 403-408.
2. GAYNOR, R D. Cement strength and concrete strength - an apparition or a dichotomy.
Cement Concrete and Aggregates, Vol 15, No 2, 1993, pp 135-144.
3. De HAYES, S M. C 109 vs concrete strengths: a preliminary investigation. Proceedings
of the 12 International conference on cement microscopy, Vancouver, 1990, pp 246-263.
th
4. BLOOMER, S J. Effect of cement properties on the strength performance of various
cements in commercial concrete. Advanced Concrete Technology Project 95/4. Institute
of Concrete Technology, 1995.
Downloaded by [ Griffith University] on [25/10/17]. Copyright © ICE Publishing, all rights reserved.
Документ
Категория
Без категории
Просмотров
0
Размер файла
412 Кб
Теги
mcmbaaa, 28227, 0005
1/--страниц
Пожаловаться на содержимое документа