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Effects of coarse aggregate on relative permeability of concrete
Effects of coarse aggregate on relative permeability of concrete F. D. Lydon* and D. K. Broadley! School of Engineering, UWCC, PO Box 925, Newport Road, Cardiff CF2 1 YF, UK Received 18 February 1994; revised and accepted 15 April 1994 Concretes made with three types of coarse aggregates, of nominal porosities of 3, 34 and 69% and volume fractions of 35 and 50%, with free water/cement ratios of 0.37 and 0.54, were subject to three curing regimes until 14 days old. After drying at 50 'C, relative permeability was measured and, again, after further drying at 105 'C. The previous interpretation of permeability parameters was confirmed as was the important role of the moisture content of concrete. Concrete made with the most porous aggregate suffered the least apparent damage after oven drying. Air drying may be best for conditioning but allowance for concrete moisture content may be deemed necessary. Keywords: coarse aggregate; permeability; concrete
Various aspects of mass transfer through concrete have been studied over several years, the method of test depending on the mechanism of interest'. Such tests are usually done in the context of durability, generally being concerned with the permeation of aggressive or potentially damaging agents into the concrete. Crucial to the permeation process is the availability of accessible pore space, which, in structural concrete, is normally in the paste or mortar; thus low water/cement ratio and water curing are taken to indicate reduced accessibility and low permeability". However, the permeability of paste in concrete is likely to be increased owing to the presence of mismatched aggregate particles distributed through it. (Mismatching arises because E, #- E p , where Ea , Ep are the moduli of aggregate and paste respectively.) Microcracking- and porous aggregate/paste transition zones" will usually provide a significantly easier passage for the permeating fluid. Additionally, the aggregate inclusions themselves may be quite permeable' and, at least potentially, may be sources of even easier passage. Thus the 'quality' of the paste, its curing history and the type and amount of coarse aggregate can be expected to influence the permeability. A limited test series was designed with these parameters in mind.
Scope of testing programme Three aggregate types, of nominally 10 mm single size and known to have different porosities and accessible pore structures", were used. Some properties are given in Table 1. Two free water/cement ratios, 0.54 and 0.37, were used *Correspondence to Mr F. D. Lydon 'Former MSc student
Table 1 Coarse aggregates used Aggregate
30 min absorption (% volume)
Relative density (oven dry)
l.l 14.6 10.3
2.74
3
1.51
34 69
A B C
Porosity(%)
0.70
'Water accessible
Table 2 Concrete mixes used Aggregate A B C
Mix proportions' by mass V,b=0.50 V.=0.35
1:2:4.38:0.54 1:2:2.65:0.54 1:2:1.29:0.54
1:2:2.09:0.37 1:2:1.26:0.37 1:2:0.60:0.37
'Cement: sand: coarse aggregate: water, by mass bV. is the volume fraction of coarse aggregate (cement contents were about 310 and 455 kg m- 3 respectively)
to vary paste quality but free water content was constant at 170 kg m - 3. The sand/cement ratio was also constant at 2:1 by mass. Mix proportions are given in Table 2. Three curing regimes were used; from each mix the test specimens, 100 mm cubes, were demoulded the day after making and pairs were stored in the laboratory and allowed to become air dry, or (2) water cured at 20°C for 1 day before being allowed to become air dry in the laboratory, or (3) sealed in two layers of plastic film, to prevent evaporation, for 1 week and then unsealed to become air dry in the laboratory. (I)
0950-0618/94/03/0185-05
© 1994 Butterworth-Heinemann Ltd
Construction and Building Materials 1994 Volume 8 Number 3
185
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
10
Oven dried at 50°C for 1 week
8 ~
I..
Nitrogen gas bottle
III
.a ...., 6
Aggregate
0..
~
I..
Two-pen
y/t plotter
Figure 1
---
:J
II
III III
Q)
I.. Q..
Aggregate 2
Aggregate A
Relative permeability test set-up. (Equipment is duplicated)
0
When the cubes were 14 days old they were still, slowly, losing weight and had high moisture contents, indicating that permeability would have been very low"; to reduce drying time they were then oven dried at 50 "C for I week, tested and then dried at 105 OC for at least a week and retested.
Testing technique The procedure was a modified version of Martin's" technique, described elsewhere", and was essentially as in Figure J. Gas was allowed to escape from the reservoir, charged initially to 10 bar, to the drilled cube in the pressure cell; as permeation occurred through the concrete to atmosphere the plotter recorded the pressure decrease with time. Nitrogen gas was used as a convenient, safe and unreactive fluid. The test does not measure intrinsic permeability but rather relative permeability, i.e. it allows comparisons between concrete on some arbitrary (but sensible) basis. Given the form of pressure/ time decay suggested", P= 10'-mt, at least three bases can usually be used: (I) (2) (3)
H
A = P dz where A is the area under the plot of P, the gas pressure, versus I, the time, m = (I - 10gP)/I, where m is the slope of the plot of log P versus t, and 11/ 2, the half-life, i.e. the time for P to decay from 10 to 5 bar pressure.
These are interrelated and of the form predicted although there are some departures from the prediction".
Results Typical plots of Pit are shown in Figure 2. Figure 3 shows plots of (tl/z, m) and (A, m) compared with those expected. Values of {1/2 are shown in Figure 4 for each concrete and curing history while Figure 5 gives values of {1/2 after 50 OC oven drying plotted against those after 105 186
0
500
1000 Time (min) Figure 2
Typical plots of gas pressure P versus time
I
DC. Unsurprisingly, similar behaviour is illustrated by the other parameters, A and m.
Discussion of results The drying of concrete changes, irreversibly, its internal structurex'? because of changes in hydrates and because of cracking due to the presence of aggregate inclusions I I. Slow drying rates will allow the readjustment of the structure, probably with less damage, but takes longer, allows hydration to proceed further (depending on internal relative humidity) and results in higher moisture content at equilibrium, all of which affect permeability. Oven drying obviously increases the drying rate but accelerates structural changes and causes more damage and, especially at lower temperatures, can result in increased hydration's; equilibrium moisture content is also changed. The choice of drying regime is thus rather arbitrary although there may be sound practical reasons for choosing a particular one. Here it was decided to dry, initially, by air drying in the laboratory, followed by drying at 50°C and 105 DC. (See the first section.) Further drying at 105°C confirmed the equilibrium values reached after the first drying period. For Va= 0.50 mixes with aggregates A and B, where Va is the coarse aggregate volume fraction, it was noticed that, after 50°C drying, cubes initially air dried from demoulding contained several tiny cracks which, of course, prevented meaningful measurements of 'permeability'. Cracking was worse after 105°C drying. This behaviour was unexpected and was probably due to the particular combination of mix proportions and curing history. With a high volume fraction of coarse aggregate and especially with Ea/Em» I (where Em is the mortar modulus) as with aggregate A, there is naturally an increased risk of cracking on drying. But aggregate B
Construction and Building Materials 1994 Volume 8 Number 3
Effects of coarse aggregate on concrete permeability: F D. Lydon and D. K. Broadley Area, Half-life
A
t 1/ 2 2) (mm (min) lOll 10 3
A=
t 1/ 2 =
0.3
m x 10-
;--
5
8.71 (m
x 10- 5 ) ° . 97
(from Ref. 7)
(from Ref. 7)
Figure 3 Correlation between half-life t ' 12 (min), area A (mm-) and m= logP(t(IO-S): +, mixes with aggregate A (V, aggregate B (V, = 0.50 and 0.35); x, mixes with aggregate C (V, = 0.50); 0, mixes with aggregate C (V, = 0.35)
surprisingly exhibited somewhat similar behaviour even though E; < Em 13. This aspect is being studied further. After drying at 50°C different moisture contents were retained in the different concretes (which had not reached equilibrium). Table 3 gives ranges of values, calculated from the 105 °C data, for each cube. Because of differences in the densities of the concretes moisture contents should be expressed on a volume basis (i.e. volume of moisture per unit volume of concrete) which allows a strictly comparable basis between them's. Since it was previously found? that moisture content (although conveniently expressed on a mass basis for normal weight concretes) was linearly related to the logarithm of A, m and 11/2> Figure 6 shows plots of moisture content (per cent of volume) versus log 11/ 2 for the three concretes here. Although the data are very limited, three distinct trends are evident, one for each type of coarse aggregate. For example, with 11/ 2 = 100 min, concretes with aggregates A, Band C have moisture contents of about 4.5, 6.4 and 7.3%. The distribution of moisture between coarse aggregate and mortar is unknown but, making some simple assumptions, it is possible to have an estimate (even if rather crude) of some values. Let it be assumed that mortar has the same porosity and pore size distribution in the three concretes and is of volume fraction 0.65; the coarse aggregate volume fraction is 0.35. If aggregate A retains, say, 0.5% moisture content by volume, then with the above concrete moisture contents, Table 4 shows that aggregates Band C will retain about 5.9 and 8.5% moisture by volume respectively. But Table 1 shows that they absorbed 14.6 and 10.3% initially, and therefore either
=
0.50 and 0.35); . , mixes with
the mortar with C retained more moisture than that with B, or the water retentivity of C was greater than that of B. Although there are different pore structures between B and C6 they are hardly relevant to the short-term values discussed here; hence, contrary to the initial assumption, it may well be that the structures of the mortar are different. Greater water retentivity could arise from the less damaging effects of oven drying with the more porous aggregate, which also has the lowest Ea. Taking a moisture content of concrete of 4.5% by volume, the 11/ 2 values are 100 and 23 min, respectively, for concretes with A and that with B. This seems to indicate that drying at 50°C removes enough moisture to allow increased access to the nitrogen gas, because of more cracking in the mortar and/or the greater porosity of B. Likewise, at a moisture content of, say, 7%, 11/ 2 is about 150 min for concrete with Band 80 min for concrete with C, again pointing to greater permeation through C once sufficient moisture is removed from the mortar. Despite the improvement of the aggregate/paste bond with porous particles such as Band C, and possibly the improved hydration due to absorbed water, together with less cracking, internally, due to the mismatch of E, and Em, it is apparent that drying at 50°C - even for a fairly short time - allows easier passage of the gas in concretes containing such particles. Either the process 'exposes' the aggregates to access or increased permeability arises in the mortar. Drying at 105°C reduced differences between the concretes and curing regimes, although the sealed and watercured cubes with Va = 0.50 for aggregate C were the least permeable. Further drying at 105°C resulted in a con-
Construction and Building Materials 1994 Volume 8 Number 3
187
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
b
+ x Aggregate
100 (a) (b)
(c)
(a)
(b)
ABC
ABC
ABC
C
(c)
50 300
0 ABC
400
ABC
ABC
a A, B, C: coarse aggregates
c
'E
Aggregate B
300
o
°o
(Table 7)
It'l
~
-N
c
...... ~
200
OJ
'>' L.
AT;
"0
2On-
x
L.
...,
J..
Ql
4rtl
rtl
:r:
100
(a) (b)
(c)
(a)
(b)
c
(c)
'E
-N
ABC V a
= 0.50
IT
ABC
ABC
ABC
V a
I
100
A
+
I!
B
1/
ABC
•
Y
= 0.35
C
+
x
Figure 4 Values of t'!l (min) for each concrete and curing condition after (a) 50 T and (b) 105 °C drying. Curing regimes: (a), air dried after demoulding; (b), water cured for I day then air dried; (c), sealed for I week then air dried
=0.35 =0.50 =0.35 =0.50 V = 0.35 a V =0.50 a
V a \1 V a 0 V a V a 6.
100
siderable increase of permeability (discounting the cracked cubes referred to previously) without loss of water, implying more internal cracking.
t 1 / 2 (min) after drying at 105°C Figure 5 Values of
till
(min) after 50 °C drying versus those after 105
T drying
Conclusions Results obtained after drying at 50°C and at 105 °C confirmed correlations, obtained previously with normal weight concretes, between A, the area under the pressure decay/time curve, m, the slope of the log pressure/time plot, and t 1/2, the half-life. 2 Although very limited results were available, they confirmed the form of relationship previously found between moisture content (here expressed as per cent by volume, because of large differences in densities between the three types of concrete tested) and the logarithm of the relative permeability parameters. A different linear equation was found for concretes made with each type of aggregate. 3 Drying at 50°C for 1 week, after 2 weeks of previous conditioning since demoulding, indicated that the concrete containing 50% by volume of the most porous aggregate, with a free water/cement ratio of 0.54 and sealed-eured for 1 week, was the least permeable of all specimens. The next least permeable was the concrete, also sealed-eured for I week, with 188
Table 3
Moisture content of cubes after 50 °C drying
Aggregate type
Concrete moisture content range" (% mass)
(% volume)
A
3.~.7
B
3.7-8.5 6.0-8.6
C
1.4-2.0 2.1-4.5 3.8-5.7
"Determined from oven drying at 105 °C
the aggregate of intermediate porosity, with 35% by volume, and a free water/cement ratio of 0.37. 4 The three types of concrete had different moisture contents after drying at 50°C which affected the permeability values. At comparable moisture contents the least permeable was that made with the least porous aggregate and the most permeable was that made with the most porous aggregate. It is suggested that partial drying removes enough water to allow access to permeable aggregates for gas flow. 5 In general, it seemed that concrete made with the
Construction and Building Materials 1994 Volume 8 Number 3
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
Qi-
+ V = 0.35 Aggregate C x Va = 0.50
9
x
a
E :J
o
> '+-
o
7
dP ~
+-'
C
B
Ql +-'
C
o
U
5
~'teA
Ql l..
:J
+-'
VI
o
~
3
'0
20
50
20
'00 Log
t,
o Va =0.35 • V = 0 .50 a
=0.35 V = 0.50 a
6. V
\l
amount in equilibrium with the exposure conditions can increase significantly the apparent permeability. To allow comparison between materials it is necessary to ensure that their moisture content is taken into account. It is recommended that whenever testing is done the moisture contents of test specimens are measured and reported with the test results.
References
a
50
200
2 3
/2
Figure 6 Effect of test specimen moisture content (per cent by volume) on l vn values
4 5
most porous aggregate suffered the lowest increase in perme ability after 105 °C dr ying compared with 50 DC. It may be preferable to condition concretes at some acceptable relative humidity which does not cause 'artificial' damage , even if this results in higher moisture contents.
Recommendation
6 7 8 9
to
Because of the important and serious effects of conditioning on the permeability of concrete it is strongly recommended that whatever method is used should, if possible , cause the minimum damage to the internal structure of the material. For concretes exposed to particular environmental conditions it may be best to test them under closely similar conditions in order to assess their likely performance. This may be especially important for concretes made with very porous aggregates, where removal of aggregate pore water below the normal
II 12 13 14
Concrete Society Working Party . Permeability testing 01 site concrete - a review 01 methods and experience, Concrete Society, Lond on. 1988 Parrott , L. J. Effect of changes in UK cements upon strength and recommended cur ing times. Concrete 1985, 19, No.9, 22 Shah , S. P. and Slate, F. O. Internal microcracking, mort ar-aggregate bond and the stress-strain curve of concrete. Proc. Int. Con! on The Structure 01 Concrete. London, September 1965 (ed. A. E. Brooks and K . Newman), Cement and Concrete Association , London, 1968,p. 82 Goldman, A. and Bentur , A. Bond effects in high-strength silicafume concretes. ACI Mater. J. 1989,86 ,440 Zhang, M. H. and Gjerv , O. E. Char acteristic s of lightweight aggregates for high strength concrete . ACI Mater. J. 1991 ,88,150 Lydon, F. D. and Al-Mahfoudh, H. H. Water absorpti on by aggregates. Constr . Build. Mater . 1989,3,2 Lydon , F. D. The relative permeability of concrete using nitrogen gas. Constr. Build. Mater. 1993,7, 213 Mart in, G. R. A method for determin ing the relative permeability of concrete using gas. Mag. Concr. Res. 1986,38,90 Powers, T. C. Mechanisms of shrinkage and reversible creep of hardened cement paste. Proc. Int. Conf. on The Stru cture 01 Concrete. London, Sept ember 1965 (ed. A. E. Brooks and K. Newman), Cement and Concrete Association, London, 1968, p. 319 Ishai, O. The time-dependent deform ati onal behaviour of cement paste, mortar and concrete. Proc. lilt. Conf. on The S tructure 01 Concrete, London, September 1965 (ed. A. E. Brooks and K . Newman), Cement and Concrete Association, Lond on, 1968, p. 345 Hsu, T. T. C. Mat hematical analysis of shrinkage stresses in a model of hardened concrete. J . ACI Proc, 1963,60,371 Byfors, J. Plain concrete at early ages. Res. Rep. £03, Swedish Cement and Conc rete Research Institute, Stockholm, 1980 Lydon, F. D. and Balendran, R. V. Some observations on elastic properties of plain concrete. Cern. Concr. Res. 1986, 16, 314 The Institution of Structural Engineers/The Concret e Society. Guide to the Stru ctural Use 01 Lightweight Aggregate Concrete, Institution of Stru ctur al Engineers, London, 1987
Table 4 Estimate of moisture co ntents of coarse aggregates Band C, in concrete dried at 50 ' C, if A retains 0.5% by volume. (Values are per cubic metre of concrete ) Coarse aggregate
Volume of coarse aggregate (I)
Moisture in coa rse aggregate
Moisture in concrete
Moisture in mort ar
(I)
(I )
(I )
Moisture in aggregate (% volume)
350 350 350
1.75' 20.75' 29.75
45 64 73
43.25' 43.25 43.25
0.5 5.9" 8.5
A B C ' 0.5% x 350 I ' (45- 1.75) I '(64-4 3.25) I "(20.75/350) 100%
Construction and BUilding Materials 1994 Volume 8 Number 3
189
Various aspects of mass transfer through concrete have been studied over several years, the method of test depending on the mechanism of interest'. Such tests are usually done in the context of durability, generally being concerned with the permeation of aggressive or potentially damaging agents into the concrete. Crucial to the permeation process is the availability of accessible pore space, which, in structural concrete, is normally in the paste or mortar; thus low water/cement ratio and water curing are taken to indicate reduced accessibility and low permeability". However, the permeability of paste in concrete is likely to be increased owing to the presence of mismatched aggregate particles distributed through it. (Mismatching arises because E, #- E p , where Ea , Ep are the moduli of aggregate and paste respectively.) Microcracking- and porous aggregate/paste transition zones" will usually provide a significantly easier passage for the permeating fluid. Additionally, the aggregate inclusions themselves may be quite permeable' and, at least potentially, may be sources of even easier passage. Thus the 'quality' of the paste, its curing history and the type and amount of coarse aggregate can be expected to influence the permeability. A limited test series was designed with these parameters in mind.
Scope of testing programme Three aggregate types, of nominally 10 mm single size and known to have different porosities and accessible pore structures", were used. Some properties are given in Table 1. Two free water/cement ratios, 0.54 and 0.37, were used *Correspondence to Mr F. D. Lydon 'Former MSc student
Table 1 Coarse aggregates used Aggregate
30 min absorption (% volume)
Relative density (oven dry)
l.l 14.6 10.3
2.74
3
1.51
34 69
A B C
Porosity(%)
0.70
'Water accessible
Table 2 Concrete mixes used Aggregate A B C
Mix proportions' by mass V,b=0.50 V.=0.35
1:2:4.38:0.54 1:2:2.65:0.54 1:2:1.29:0.54
1:2:2.09:0.37 1:2:1.26:0.37 1:2:0.60:0.37
'Cement: sand: coarse aggregate: water, by mass bV. is the volume fraction of coarse aggregate (cement contents were about 310 and 455 kg m- 3 respectively)
to vary paste quality but free water content was constant at 170 kg m - 3. The sand/cement ratio was also constant at 2:1 by mass. Mix proportions are given in Table 2. Three curing regimes were used; from each mix the test specimens, 100 mm cubes, were demoulded the day after making and pairs were stored in the laboratory and allowed to become air dry, or (2) water cured at 20°C for 1 day before being allowed to become air dry in the laboratory, or (3) sealed in two layers of plastic film, to prevent evaporation, for 1 week and then unsealed to become air dry in the laboratory. (I)
0950-0618/94/03/0185-05
© 1994 Butterworth-Heinemann Ltd
Construction and Building Materials 1994 Volume 8 Number 3
185
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
10
Oven dried at 50°C for 1 week
8 ~
I..
Nitrogen gas bottle
III
.a ...., 6
Aggregate
0..
~
I..
Two-pen
y/t plotter
Figure 1
---
:J
II
III III
Q)
I.. Q..
Aggregate 2
Aggregate A
Relative permeability test set-up. (Equipment is duplicated)
0
When the cubes were 14 days old they were still, slowly, losing weight and had high moisture contents, indicating that permeability would have been very low"; to reduce drying time they were then oven dried at 50 "C for I week, tested and then dried at 105 OC for at least a week and retested.
Testing technique The procedure was a modified version of Martin's" technique, described elsewhere", and was essentially as in Figure J. Gas was allowed to escape from the reservoir, charged initially to 10 bar, to the drilled cube in the pressure cell; as permeation occurred through the concrete to atmosphere the plotter recorded the pressure decrease with time. Nitrogen gas was used as a convenient, safe and unreactive fluid. The test does not measure intrinsic permeability but rather relative permeability, i.e. it allows comparisons between concrete on some arbitrary (but sensible) basis. Given the form of pressure/ time decay suggested", P= 10'-mt, at least three bases can usually be used: (I) (2) (3)
H
A = P dz where A is the area under the plot of P, the gas pressure, versus I, the time, m = (I - 10gP)/I, where m is the slope of the plot of log P versus t, and 11/ 2, the half-life, i.e. the time for P to decay from 10 to 5 bar pressure.
These are interrelated and of the form predicted although there are some departures from the prediction".
Results Typical plots of Pit are shown in Figure 2. Figure 3 shows plots of (tl/z, m) and (A, m) compared with those expected. Values of {1/2 are shown in Figure 4 for each concrete and curing history while Figure 5 gives values of {1/2 after 50 OC oven drying plotted against those after 105 186
0
500
1000 Time (min) Figure 2
Typical plots of gas pressure P versus time
I
DC. Unsurprisingly, similar behaviour is illustrated by the other parameters, A and m.
Discussion of results The drying of concrete changes, irreversibly, its internal structurex'? because of changes in hydrates and because of cracking due to the presence of aggregate inclusions I I. Slow drying rates will allow the readjustment of the structure, probably with less damage, but takes longer, allows hydration to proceed further (depending on internal relative humidity) and results in higher moisture content at equilibrium, all of which affect permeability. Oven drying obviously increases the drying rate but accelerates structural changes and causes more damage and, especially at lower temperatures, can result in increased hydration's; equilibrium moisture content is also changed. The choice of drying regime is thus rather arbitrary although there may be sound practical reasons for choosing a particular one. Here it was decided to dry, initially, by air drying in the laboratory, followed by drying at 50°C and 105 DC. (See the first section.) Further drying at 105°C confirmed the equilibrium values reached after the first drying period. For Va= 0.50 mixes with aggregates A and B, where Va is the coarse aggregate volume fraction, it was noticed that, after 50°C drying, cubes initially air dried from demoulding contained several tiny cracks which, of course, prevented meaningful measurements of 'permeability'. Cracking was worse after 105°C drying. This behaviour was unexpected and was probably due to the particular combination of mix proportions and curing history. With a high volume fraction of coarse aggregate and especially with Ea/Em» I (where Em is the mortar modulus) as with aggregate A, there is naturally an increased risk of cracking on drying. But aggregate B
Construction and Building Materials 1994 Volume 8 Number 3
Effects of coarse aggregate on concrete permeability: F D. Lydon and D. K. Broadley Area, Half-life
A
t 1/ 2 2) (mm (min) lOll 10 3
A=
t 1/ 2 =
0.3
m x 10-
;--
5
8.71 (m
x 10- 5 ) ° . 97
(from Ref. 7)
(from Ref. 7)
Figure 3 Correlation between half-life t ' 12 (min), area A (mm-) and m= logP(t(IO-S): +, mixes with aggregate A (V, aggregate B (V, = 0.50 and 0.35); x, mixes with aggregate C (V, = 0.50); 0, mixes with aggregate C (V, = 0.35)
surprisingly exhibited somewhat similar behaviour even though E; < Em 13. This aspect is being studied further. After drying at 50°C different moisture contents were retained in the different concretes (which had not reached equilibrium). Table 3 gives ranges of values, calculated from the 105 °C data, for each cube. Because of differences in the densities of the concretes moisture contents should be expressed on a volume basis (i.e. volume of moisture per unit volume of concrete) which allows a strictly comparable basis between them's. Since it was previously found? that moisture content (although conveniently expressed on a mass basis for normal weight concretes) was linearly related to the logarithm of A, m and 11/2> Figure 6 shows plots of moisture content (per cent of volume) versus log 11/ 2 for the three concretes here. Although the data are very limited, three distinct trends are evident, one for each type of coarse aggregate. For example, with 11/ 2 = 100 min, concretes with aggregates A, Band C have moisture contents of about 4.5, 6.4 and 7.3%. The distribution of moisture between coarse aggregate and mortar is unknown but, making some simple assumptions, it is possible to have an estimate (even if rather crude) of some values. Let it be assumed that mortar has the same porosity and pore size distribution in the three concretes and is of volume fraction 0.65; the coarse aggregate volume fraction is 0.35. If aggregate A retains, say, 0.5% moisture content by volume, then with the above concrete moisture contents, Table 4 shows that aggregates Band C will retain about 5.9 and 8.5% moisture by volume respectively. But Table 1 shows that they absorbed 14.6 and 10.3% initially, and therefore either
=
0.50 and 0.35); . , mixes with
the mortar with C retained more moisture than that with B, or the water retentivity of C was greater than that of B. Although there are different pore structures between B and C6 they are hardly relevant to the short-term values discussed here; hence, contrary to the initial assumption, it may well be that the structures of the mortar are different. Greater water retentivity could arise from the less damaging effects of oven drying with the more porous aggregate, which also has the lowest Ea. Taking a moisture content of concrete of 4.5% by volume, the 11/ 2 values are 100 and 23 min, respectively, for concretes with A and that with B. This seems to indicate that drying at 50°C removes enough moisture to allow increased access to the nitrogen gas, because of more cracking in the mortar and/or the greater porosity of B. Likewise, at a moisture content of, say, 7%, 11/ 2 is about 150 min for concrete with Band 80 min for concrete with C, again pointing to greater permeation through C once sufficient moisture is removed from the mortar. Despite the improvement of the aggregate/paste bond with porous particles such as Band C, and possibly the improved hydration due to absorbed water, together with less cracking, internally, due to the mismatch of E, and Em, it is apparent that drying at 50°C - even for a fairly short time - allows easier passage of the gas in concretes containing such particles. Either the process 'exposes' the aggregates to access or increased permeability arises in the mortar. Drying at 105°C reduced differences between the concretes and curing regimes, although the sealed and watercured cubes with Va = 0.50 for aggregate C were the least permeable. Further drying at 105°C resulted in a con-
Construction and Building Materials 1994 Volume 8 Number 3
187
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
b
+ x Aggregate
100 (a) (b)
(c)
(a)
(b)
ABC
ABC
ABC
C
(c)
50 300
0 ABC
400
ABC
ABC
a A, B, C: coarse aggregates
c
'E
Aggregate B
300
o
°o
(Table 7)
It'l
~
-N
c
...... ~
200
OJ
'>' L.
AT;
"0
2On-
x
L.
...,
J..
Ql
4rtl
rtl
:r:
100
(a) (b)
(c)
(a)
(b)
c
(c)
'E
-N
ABC V a
= 0.50
IT
ABC
ABC
ABC
V a
I
100
A
+
I!
B
1/
ABC
•
Y
= 0.35
C
+
x
Figure 4 Values of t'!l (min) for each concrete and curing condition after (a) 50 T and (b) 105 °C drying. Curing regimes: (a), air dried after demoulding; (b), water cured for I day then air dried; (c), sealed for I week then air dried
=0.35 =0.50 =0.35 =0.50 V = 0.35 a V =0.50 a
V a \1 V a 0 V a V a 6.
100
siderable increase of permeability (discounting the cracked cubes referred to previously) without loss of water, implying more internal cracking.
t 1 / 2 (min) after drying at 105°C Figure 5 Values of
till
(min) after 50 °C drying versus those after 105
T drying
Conclusions Results obtained after drying at 50°C and at 105 °C confirmed correlations, obtained previously with normal weight concretes, between A, the area under the pressure decay/time curve, m, the slope of the log pressure/time plot, and t 1/2, the half-life. 2 Although very limited results were available, they confirmed the form of relationship previously found between moisture content (here expressed as per cent by volume, because of large differences in densities between the three types of concrete tested) and the logarithm of the relative permeability parameters. A different linear equation was found for concretes made with each type of aggregate. 3 Drying at 50°C for 1 week, after 2 weeks of previous conditioning since demoulding, indicated that the concrete containing 50% by volume of the most porous aggregate, with a free water/cement ratio of 0.54 and sealed-eured for 1 week, was the least permeable of all specimens. The next least permeable was the concrete, also sealed-eured for I week, with 188
Table 3
Moisture content of cubes after 50 °C drying
Aggregate type
Concrete moisture content range" (% mass)
(% volume)
A
3.~.7
B
3.7-8.5 6.0-8.6
C
1.4-2.0 2.1-4.5 3.8-5.7
"Determined from oven drying at 105 °C
the aggregate of intermediate porosity, with 35% by volume, and a free water/cement ratio of 0.37. 4 The three types of concrete had different moisture contents after drying at 50°C which affected the permeability values. At comparable moisture contents the least permeable was that made with the least porous aggregate and the most permeable was that made with the most porous aggregate. It is suggested that partial drying removes enough water to allow access to permeable aggregates for gas flow. 5 In general, it seemed that concrete made with the
Construction and Building Materials 1994 Volume 8 Number 3
Effects of coarse aggregate on concrete permeability: F. D. Lydon and D. K. Broadley
Qi-
+ V = 0.35 Aggregate C x Va = 0.50
9
x
a
E :J
o
> '+-
o
7
dP ~
+-'
C
B
Ql +-'
C
o
U
5
~'teA
Ql l..
:J
+-'
VI
o
~
3
'0
20
50
20
'00 Log
t,
o Va =0.35 • V = 0 .50 a
=0.35 V = 0.50 a
6. V
\l
amount in equilibrium with the exposure conditions can increase significantly the apparent permeability. To allow comparison between materials it is necessary to ensure that their moisture content is taken into account. It is recommended that whenever testing is done the moisture contents of test specimens are measured and reported with the test results.
References
a
50
200
2 3
/2
Figure 6 Effect of test specimen moisture content (per cent by volume) on l vn values
4 5
most porous aggregate suffered the lowest increase in perme ability after 105 °C dr ying compared with 50 DC. It may be preferable to condition concretes at some acceptable relative humidity which does not cause 'artificial' damage , even if this results in higher moisture contents.
Recommendation
6 7 8 9
to
Because of the important and serious effects of conditioning on the permeability of concrete it is strongly recommended that whatever method is used should, if possible , cause the minimum damage to the internal structure of the material. For concretes exposed to particular environmental conditions it may be best to test them under closely similar conditions in order to assess their likely performance. This may be especially important for concretes made with very porous aggregates, where removal of aggregate pore water below the normal
II 12 13 14
Concrete Society Working Party . Permeability testing 01 site concrete - a review 01 methods and experience, Concrete Society, Lond on. 1988 Parrott , L. J. Effect of changes in UK cements upon strength and recommended cur ing times. Concrete 1985, 19, No.9, 22 Shah , S. P. and Slate, F. O. Internal microcracking, mort ar-aggregate bond and the stress-strain curve of concrete. Proc. Int. Con! on The Structure 01 Concrete. London, September 1965 (ed. A. E. Brooks and K . Newman), Cement and Concrete Association , London, 1968,p. 82 Goldman, A. and Bentur , A. Bond effects in high-strength silicafume concretes. ACI Mater. J. 1989,86 ,440 Zhang, M. H. and Gjerv , O. E. Char acteristic s of lightweight aggregates for high strength concrete . ACI Mater. J. 1991 ,88,150 Lydon, F. D. and Al-Mahfoudh, H. H. Water absorpti on by aggregates. Constr . Build. Mater . 1989,3,2 Lydon , F. D. The relative permeability of concrete using nitrogen gas. Constr. Build. Mater. 1993,7, 213 Mart in, G. R. A method for determin ing the relative permeability of concrete using gas. Mag. Concr. Res. 1986,38,90 Powers, T. C. Mechanisms of shrinkage and reversible creep of hardened cement paste. Proc. Int. Conf. on The Stru cture 01 Concrete. London, Sept ember 1965 (ed. A. E. Brooks and K. Newman), Cement and Concrete Association, London, 1968, p. 319 Ishai, O. The time-dependent deform ati onal behaviour of cement paste, mortar and concrete. Proc. lilt. Conf. on The S tructure 01 Concrete, London, September 1965 (ed. A. E. Brooks and K . Newman), Cement and Concrete Association, Lond on, 1968, p. 345 Hsu, T. T. C. Mat hematical analysis of shrinkage stresses in a model of hardened concrete. J . ACI Proc, 1963,60,371 Byfors, J. Plain concrete at early ages. Res. Rep. £03, Swedish Cement and Conc rete Research Institute, Stockholm, 1980 Lydon, F. D. and Balendran, R. V. Some observations on elastic properties of plain concrete. Cern. Concr. Res. 1986, 16, 314 The Institution of Structural Engineers/The Concret e Society. Guide to the Stru ctural Use 01 Lightweight Aggregate Concrete, Institution of Stru ctur al Engineers, London, 1987
Table 4 Estimate of moisture co ntents of coarse aggregates Band C, in concrete dried at 50 ' C, if A retains 0.5% by volume. (Values are per cubic metre of concrete ) Coarse aggregate
Volume of coarse aggregate (I)
Moisture in coa rse aggregate
Moisture in concrete
Moisture in mort ar
(I)
(I )
(I )
Moisture in aggregate (% volume)
350 350 350
1.75' 20.75' 29.75
45 64 73
43.25' 43.25 43.25
0.5 5.9" 8.5
A B C ' 0.5% x 350 I ' (45- 1.75) I '(64-4 3.25) I "(20.75/350) 100%
Construction and BUilding Materials 1994 Volume 8 Number 3
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