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Curing requirements of silica fume and fly ash mortars
CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 1480-1490, 1993. Printed in the USA. 0008-8846/93. $6.00+00. Copyright © 1993 Pergamon Press Ltd.
CURING
REQUIREMENTS FLY ASH
OF SILICA MORTARS
FUME
AND
M o h a m m a d Shamim Khan and Michael E. Ayers School of Civil Engineering Oklahoma State U n i v e r s i t y Stillwater, OK 74078-0327 (Communicated by C.D. Pomeroy) (Received March 17, 1993)
ABSTRACT The curing requirements of silica fume and fly ash mortars were investigated in this study. Silica fume and fly ash mortar specimens were moist cured for periods of 0, 3, 7, 14 and 28 days. After each of the five periods, the moist curing was interrupted by o v e n - d r y i n g the specimens at a temperature of ll0°C for 3 days. The specimens were later tested for compressive strength and absorptivity. In this study, it was also d e t e r m i n e d whether the losses in strength and i m p e r m e a b i l i t y of silica fume and fly ash mortars due to an i n t e r r u p t i o n in curing could be regained by recuring. The test results clearly indicate that the curing r e q u i r e m e n t of silica fume m o r t a r is less than that of plain cement mortar, while in the case of fly ash m o r t a r it is higher than that of plain cement mortar. Introduction Extensive investigations have been carried out on the use of fly ash and silica fume in concrete during the past two decades and have c o n s e q u e n t l y lead to their w i d e s p r e a d application in the c o n s t r u c t i o n industry (1-6). The curing r e q u i r e m e n t of these concretes is an issue which has not received s u f f i c i e n t attention of the researchers. Some recent studies (7-9) have a d d r e s s e d the c u r i n g sensitivity of fly ash concretes, and indicated that fly ash concretes are more susceptible to poor curing conditions than plain cement concretes. There is a lack of such type of data in the case of silica fume concrete.
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In the U n i t e d States, the field application of silica fume concrete started in 1983, m o s t l y in bridge decks and p a r k i n g structures. A r e v i e w of silica fume concrete use in bridge decks was p r e s e n t e d by Luther (i0), and in parking structures by Weil (ii). From these publications, a number of similarities can be noted among d i f f e r e n t silica fume concrete projects in terms of mixing, p l a c i n g and finishing. However, most of the projects differ on the point of c u r i n g of silica fume concrete. For example, Bunke (12) on the basis of his experience with silica fume concrete projects at the Ohio D e p a r t m e n t of Transportation recommends a continuous w a t e r curing of 3 days for silica fume concrete. On the other hand, Holland (13) recommends a wet curing of 7 days as an absolute minimum. The v a r i a t i o n s in the curing practice of silica fume concrete may be largely a t t r i b u t e d to the lack of standard specifications, which are tied to insufficient b a c k g r o u n d data on silica fume concrete, in general, and its curing in particular. The ACI C o m m i t t e e 308 report (14) provides guidelines for the curing of normal p o r t l a n d cement concrete, but does not include concretes made with p o z z o l a n i c materials such as fly ash and silica fume. The main objective of the present study is to investigate the c u r i n g r e q u i r e m e n t of silica fume concrete. A fly ash mix is included in the study for comparison. Also i n v e s t i g a t e d is w h e t h e r the losses in strength and impermeability of silica fume and fly ash concretes due to an interruption in moist curing could be regained by recuring. The effects of interrupted curing and r e c u r i n g in the case of plain cement concrete are d o c u m e n t e d by the authors elsewhere (15). The available literature lacks data on the effects of recuring in either plain cement concrete or concretes c o n t a i n i n g silica fume and fly ash.
Experimental Details
M a t e r i a l s and Mix Design The m a t e r i a l s used in this study were ASTM Type I p o r t l a n d cement, silica fume, class F fly ash, natural sand, potable w a t e r and a high range w a t e r reducing admixture (HRWA). The sand had a water absorption of 0.28% and its fineness modulus was 2.74. The g r a d a t i o n curve of the sand was almost in the middle of A S T M C-33 g r a d a t i o n limits for fine aggregates. Two silica fume m o r t a r mixes with w a t e r - t o - c e m e n t i t i o u s material (W/C) ratios of 0.44 and 0.49, and one fly ash m o r t a r mix with w a t e r - t o - c e m e n t i t i o u s material ratio of 0.44 were used (Table I) in this study. The cementitious material content was the c o m b i n e d w e i g h t of cement and pozzolanic material (silica fume or fly ash). In all the silica fume and fly ash m o r t a r mixes, the silica fume or fly ash content was 15% of the total cementitious material content. C a s t i n g and C u r i n g The c a s t i n g of the m o r t a r specimens was done in small batches w h i c h
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TABLE I Mix Proportions for Six Mortar Specimens (50-mm or 2-inch Cubes) Constituents Mix W/C* designation ratio Water (gm)
Cement Silica (gm) fume
(gm)
Fly ash
Sand (gm)
HRWA** (ml)
(gm)
SF-0.44
0.44
220.8
425
75
1504.2
SF-0.49
0.49
245.8
425
75
1504.2
FY-0.44
O. 44
220.8
425
75
1504.2
* W/C ratio means w a t e r - t o - c e m e n t i t i o u s material ratio ** HRWA means high range water reducing admixture Note: 1 Ib = 453.592 gm; 1 fl oz = 29.57353 ml yielded 6 specimens (50-mm or 2-inch cubes). After casting, the molds c o n t a i n i n g the specimens were covered w i t h a plastic sheet and stored in normal laboratory conditions. The specimens were demolded after 24±1 hours. After demolding, the specimens were subjected to curing under conditions of complete immersion in water for five different periods of time; 0, 3, 7, 14 and 28 days. After each m o i s t curing period, the specimens were removed from water and o v e n - d r i e d for 3 days at a temperature of ll0°C (230°F). This was done to, practically, stop the hydration of the specimens after each curing period, because the specimens were not tested immediately after their removal from water. Note that the specimens c o r r e s p o n d i n g to 0 day of moist curing were subjected to oven-drying immediately after demolding. There were six specimens for each curing p e r i o d from each mix. After the oven-drying, three specimens were stored in the normal dry conditions of the laboratory and were later used to assess the strength and permeability c h a r a c t e r i s t i c s of silica fume and fly ash mortars for different periods of moist curing. The curing of these specimens has been referred to as d i s c o n t i n u o u s curing in this paper. In order to determine the effect of recuring after an interruption in moist curing, the other three specimens of each mix and each curing p e r i o d were reimmersed in water for a p e r i o d of time, w h i c h if added to the m o i s t curing period before interruption was equal to 28 days. This means that all the recured specimens had a total moist curing p e r i o d of 28 days, but had an interruption in their moist curing (oven-drying at ll0°C for 3 days). It is to be noted that after oven-drying, the specimens were allowed to cool for 24 hours before immersing them in water for recuring. Testinq At the end of the recuring period, the recured specimens along with
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the c o r r e s p o n d i n g specimens subjected to d i s c o n t i n u o u s c u r i n g were tested for a b s o r p t i v i t y and compressive strength. The a b s o r p t i v i t y was d e t e r m i n e d by using the following relationship, o r i g i n a l l y suggested by Powers and Brownyard (16) and later u s e d by others
(17): (q/A) =(Kat)~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (i) Where q/A is the amount of w a t e r absorbed per unit area of e x p o s e d surface in time t. Ka, expressed in cm2/sec, is the c o e f f i c i e n t of absorptivity. The r e l a t i o n s h i p between the c o e f f i c i e n t of absorptivity, p e r m e a b i l i t y is as follows (16):
Ka, and
k a = [(k2) c o Sc]/~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) Where (K2)c is the coefficient of p e r m e a b i l i t y through the c a p i l l a r y s y s t e m of the cement paste, ~ is the surface tension of water, ~ is the v i s c o s i t y of water, and S c is the surface area of gel. S c is a d i f f i c u l t p a r a m e t e r to measure, and it is s u g g e s t e d by Powers and B r o w n y a r d (16) that K a can be used as a relative m e a s u r e of permeability. In Eq. (i), the amount of water absorbed (q) was d e t e r m i n e d by o v e n - d r y i n g the specimens at a temperature of ll0°C (230°F) for 24 hours, c o o l i n g them in air tight containers for another 24 hours, and then soaking in water for 60 minutes. Equation (i) is v a l i d for an a b s o r p t i o n time of 30 to 60 minutes (16). A f t e r the a b s o r p t i v i t y test, the same specimens were used for c o m p r e s s i v e strength measurement. But before c o m p r e s s i v e s t r e n g t h measurement, the specimens were oven-dried at ll0°C (230°F) for 24 hours and then c o o l e d in air tight containers for 24 hours. This was done in an attempt to keep the c a p i l l a r y m o i s t u r e of all the specimens at the same level before testing. It has been d e m o n s t r a t e d by Popovics (18) that small differences in the moisture content of concrete specimens cause s i g n i f i c a n t d i f f e r e n c e in their compressive strength. The more usual p r a c t i c e of t e s t i n g the specimens in moist condition was not followed in this study, because that m i g h t have changed the hydration status of the specimens s u b j e c t e d to discontinuous curing. Also, this was the reason for the selection of a p e r m e a b i l i t y test involving a short exposure time (60 minutes) of the specimens to water.
Test Results and Discussion The c o m p r e s s i v e strength and absorptivity data for the silica fume and fly ash m o r t a r specimens cured for different periods of time and under two d i f f e r e n t curing conditions, d i s c o n t i n u o u s c u r i n g and recuring, are shown in Table II. The averages, standard deviations, and coefficients of v a r i a t i o n shown in Table II are for 3 specimens. The individual compressive strength and a b s o r p t i v i t y values were c h e c k e d for outliers. In identifying the o u t l y i n g measurements, both the highest and lowest m e a s u r e m e n t s were t e s t e d according to the T-statistic procedure, d e s c r i b e d in A S T M E-178,
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TABLE
Vol. 23, No. 6
II
Compressive Strength and Absorptivity Data for Silica Fume and Fly Ash Mortar Specimens Compressive strength Specimen designation
Average! Standard C.O.V. deviation (%) (MPa) (MPa)
Coefficient of absorptivity Average x i0 "° (cmZ/sec)
C.O.V. Standard (%) deviation x I0 6 (cm2/sec)
SF-0.44-0-D SF-0.44-0-R SF-0.44-3-D SF-0.44-3-R SF-O. 44-7-D SF-0.44-7-R SF-0.44-14-D SF-0.44-14-R SF-0.44-28
58.5 71.0 67.6 74.0 75.3 78.5 74.3 74.3 74.1
0.39 3.13 2.13 5.21 1.75 3.92 2.72 3.52 2.93
0.66 4.41 3.16 7.04 2.32 4.99 3.66 4.73 3.96
3.73 2.17 1.81 1.40 1.74 1.62 1.79 1.63 2.11
0.291 O.O55 0.077 0.091 0.114 0.016 0.083 0.058 0.167
7.80 2.54 4.25 6.51 6.58 0.98 4.61 3.52 7.91
SF-0.49-0-D SF-O. 49-0-R SF-0.49-3-D SF-0.49-3-R SF-0.49-7-D SF-0.49-7-R SF-0.49-14-D SF-0.49-14-R SF-0.49-28
55.3 64.7 58.9 62.1 59.8 62.9 66.9 64.7 65.0
0.81 0.39 0.39 3.25 3.30 1.25 3.13 3.08 2.07
1.46 0.60 0.66 5.23 5.51 1.98 4.68 4.75 3.18
3.59 2.14 2.62 2.05 2.60 2.22 2.58 2.04 3.05
0.163 0.095 0.i00 0.031 0.053 0.067 0.020 0.064 0.117
4.54 4.41 3.82 1.50 2.04 3.00 0.77 3.15 3.84
FY-0.44-0-D FY-0.44-0-R FY-0.44-3-D FY-0.44-3-R FY-0.44-7-D FY-0.44-7-R FY-0.44-14-D FY-0.44-14-R FY-0.44-28
36.0 60.3 53.7 68.4 58.5 70.4 66.0 72.0 70.1
1.03 1.61 0.81 1.21 1.03 1.61 2.13 1.62 3.61
2.84 2.67 1.50 1.77 1.75 2.29 3.23 2.26 5.15
4.59 2.70 3.87 2.54 3.84 2.56 3.47 2.58 3.23
0.138 0.108 0.135 0.151 0.222 0.140 0.069 0.122 0.086
3.01 4.01 3.50 5.93 5.77 5.45 2.00 4.71 2.65
Note: C.O.V. means coefficient of variation; 1 c m 2 / s e c = 0 . 1 5 5 in2/sec
1 MPa
=
145
psi;
for an u p p e r 5% s i g n i f i c a n c e level. The m e a s u r e m e n t s t h a t did not m e e t the u p p e r 5% s i g n i f i c a n c e level c r i t e r i a w e r e d i s c a r d e d . In addition, the i n d i v i d u a l c o m p r e s s i v e s t r e n g t h m e a s u r e m e n t s w e r e also c h e c k e d for t h e i r 10% v a r i a b i l i t y f r o m the average. The i n d i v i d u a l c o m p r e s s i v e s t r e n g t h m e a s u r e m e n t s m o r e t h a n 10% h i g h e r or lower t h a n the a v e r a g e w e r e d i s c a r d e d . This is one of the r e l i a b i l i t y c r i t e r i a for the s t r e n g t h r e s u l t s of 5 0 - m m (2-inch) cubes, as specified by ASTM C-I09. There were very few m e a s u r e m e n t s t h a t did not m e e t the u p p e r 5% s i g n i f i c a n c e level or the 10% v a r i a b i l i t y criteria. The v a r i a t i o n of c o m p r e s s i v e s t r e n g t h of the s i l i c a fume and fly ash m o r t a r s p e c i m e n s w h e r e m o i s t c u r i n g was d i s c o n t i n u e d a f t e r 0, 3, 7, 14 and 28 days is i l l u s t r a t e d in Fig. 1 (W/C = 0.44) and Fig. 2 (W/C = 0.49). In these figures, the d a t a s h o w n for p l a i n c e m e n t m o r t a r s p e c i m e n s h a v e b e e n t a k e n f r o m the s t u d y by the a u t h o r s (15), c i t e d earlier, for c o m p a r i s o n . The d e s i g n s of the
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CURING, SILICA FUME, FLY ASH, MORTARS
Compressive Strength, MPa
1485
_ Compressive Strength. MPa
80
60
40
20
0 0 Day
m
3 Days 7 Days 14 Days P e r i o d ol Moist C u r i n g Aller D e r n o l d l n ~
sttica Fume Mortar
~
28 Days
Fly A|b Mortar
0 Day
m
3 Days ? Days 14 Days Period o! Moist Curing Alter Dernoldlng Silica Fume Mortar
~
28 Days
Plain Cement Mortar
Plain Cement Mortar
FIG. 1 Compressive strength of silica fume, fly ash and plain cement m o r t a r specimens with W/C = 0.44
FIG. 2 Compressive strength of silica fume and plain cement mortar specimens with W/C = 0.49
plain cement mortar mixes with W/C ratios of 0.44 and 0.49 were similar to those of the c o r r e s p o n d i n g silica fume m o r t a r mixes except that the plain cement m o r t a r mixes c o n t a i n e d a cement content e q u i v a l e n t to the combined weight of cement and silica fume in the silica fume m o r t a r mixes. Note that the silica fume m o r t a r mixes u t i l i z e d 425 gm (0.94 ib) of cement and 75 gm (0.17 ib) of silica fume for 6 specimens (Table I). Similar number of plain cement m o r t a r specimens utilized only 500 gm (i.i0 ib) of cement. The d i f f e r e n c e in the curing sensitivity of silica fume, fly ash and plain cement mortars can be clearly observed in these figures. The c o m p r e s s i v e strength of the silica fume m o r t a r specimens ovendried immediately after demolding (no moist curing) was found to be 79-85% ( W / C = 0.44, 0.49) of that of specimens c o n t i n u o u s l y moist cured for 28 days. The c o r r e s p o n d i n g strength in the case of fly ash m o r t a r specimens (W/C = 0.44) is 51% and in case of plain cement m o r t a r specimens ( W / C = 0.44, 0.49) 62-65%. In silica fume mortar specimens, the m a x i m u m compressive strength corresponds to moist curing periods of 7 to 14 days, while in case of fly ash and plain cement mortar specimens, a continuous gain in s t r e n g t h is o b s e r v e d up to moist curing period of 28 days. Care should be exercised in comparing the compressive s t r e n g t h data p r e s e n t e d in Table II with other data in the literature. The compressive strength values in Table II are for dry m o r t a r specimens. In most instances, the compressive strength values are r e p o r t e d for wet specimens. In a limited scale experiment, the authors found the compressive strength of dry concrete cubes (50-mm or 2-inch) 13-19% higher than that of wet concrete cubes. The d e v e l o p m e n t of r e l a t i v e l y high confining stresses near the specimen ends due to increased friction between specimen and steel platens is one of the reasons for the higher strength of dry concrete or m o r t a r specimens compared to wet specimens (19). A c c o r d i n g to Roy (20), the hydration rate of cement c o n t a i n i n g silica fume is greater than that of ordinary p o r t l a n d cement, while that of cement c o n t a i n i n g fly ash is slower than that of o r d i n a r y
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Vol. 23, No. 6
Compressive Strength, MPa
Coellicient of Absorptivity (IE-06)
I00[ 80~ 60 40 20
0 Day
l
3 Days 7 Days 14 Days Period o! Moist Curing Alter Derno|ding
Silica Fume Mortar
J
Fly ASh Mortar
28 Days
E ~ Plain Cement Mortar
=
0 Day
3 Days 7 Days 14 Days Period of Moist C u r i n g A/ler D e m o l d t o g m
D i s c o n t i n u o u s Curing
m
28 Days
Recuring
FIG. 4 Compressive strength of silica fume mortar specimens with W/C = 0.44, subjected to discontinuous curing and recuring
FIG. 3 Coefficient of absorptivity of silica fume, fly ash and plain cement mortar specimens with w/c
0
0.44
portland cement. The same order of hydration rate, as interpreted from the strength development data, has been observed in this study. A high hydration rate of cement containing silica fume is attributed to the fact that silica fume particles accelerate the hydration of C3S (Ca3SiO5, alite), the cement compound primarily responsible for strength development at early ages (20). However, the strength development in silica fume mortar specimens which did not have any moist curing is not accompanied by a proportional reduction in permeability (Fig. 1 and Fig. 3). The coefficient of absorptivity of these silica fume mortar specimens is not significantly different than that of plain cement and fly ash mortar specimens. But once the silica fume mortar specimens receive a moist curing of 3 days, there is a significant reduction in their absorptivity compared to plain cement and fly ash mortar specimens. This implies that a harsh curing environment at early stages would not affect the strength of silica fume concrete significantly, but it would have a significant effect on its permeability. Although, careful visual examination did not reveal any macro surface cracks in either silica fume mortar specimens or fly ash and plain cement mortar specimens, there is a p o s s i b i l i t ~ o f microcracking of the specimens subjected to oven-drying (ll0VC or 230°F) immediately after demolding. One of the factors that might have caused the micro-cracking of the specimens, particularly in the surface region, is the loss of some chemically bound water along with capillary water. Lankard et al. (21) measured a loss of chemically bound water on the order of 10% in 50-mm (2-inch) cubes of neat cement and mortar after heating the specimens at a temperature of 121°C (250°F) for 58 days. The shrinkage associated with the loss of chemically bound water is believed to disrupt the bonding configuration between the solid hydrated cement phases, and thus causes the micro-cracking of the cement paste (21). The
incompatibility
in
the
strength
and
permeability
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characteristics of silica fume mortar specimens subjected to a harsh curing environment at an early stage is in agreement with the recent findings of Bentur and Jaegermann (22). Their results indicate that the adverse effects of inadequate water curing are more evident on concrete permeability rather than on strength. This is attributed to the fact that permeability is largely a function of the quality of the surface region of the concrete specimen, while strength is a function of the quality of the concrete specimen as a whole. The surface region of the concrete specimen is the one which is affected most by an adverse curing environment. Here it is to be noted that even from the standpoint of permeability, the curing requirement of silica fume mortar specimens is lower than that of plain cement and fly ash mortar specimens. Figures 4-6 illustrate the effect of recuring on the compressive strength of silica fume and fly ash mortar specimens. The beneficial effects of recuring, as reported by Khan and Ayers (15) for plain cement mortar specimens, are clearly noticed in silica fume and fly ash mortar specimens in this study. Relatively small strength gain in silica fume mortar specimens after recuring, further emphasizes that silica fume mortar specimens gain strength quite early and their curing requirement is low. The compressive strengths of the recured silica fume mortar specimens after curing interruptions at 0, 3 and 7 days are 17-21%, 6-10% and 4-5%, respectively, higher than those of the corresponding specimens whose curing was discontinued after interruption. In the case of silica fume mortar specimens where curing was interrupted after 14 days, the strengths of the specimens subjected to recuring and discontinuous curing are similar. The strength of the recured fly ash mortar specimens is significantly higher than that of the specimens subjected to discontinuous curing, particularly at early periods of curing interruption. The strengths of the recured fly ash mortar specimens at curing interruptions of 0, 3, 7 and 14 days are 67%, Compressive Strength. MPu 70
80
Compressive Strenglh, MPa
60 60
50 40
40
30 20
20
l0 0 0 Day
3 Days 7 Days 14 Days Period oi Moist Curing Alter Dernolcllng
Disconlinuous Curing
~
28 Days
Recurlng
FIG. 5 Compressive strength of silica fume mortar specimens with W/C = 0.49, subjected to discontinuous curing and recuring
0
0 Day
3 Days 7 Days 14 Days P e r i o d of Moist Curing Alter Demoiding i
Discontinuous Curing
m
26 Dug
Recuring
FIG. 6 Compressive strength of fly ash mortar specimens with W/C = 0.44, subjected to discontinuous curing and recuring
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27%, 20% and 9%, respectively, higher than those of the specimens subjected to discontinuous curing. The corresponding percentages in the case of plain cement mortar specimens have been found to be 41%, 18%, 9% and 2% (15). A harsh environment for curing interruption (oven-drying at ll0°C or 230°F for 3 days) was selected in order to obtain highly visible effects of interrupted curing and recuring. Drying of concrete to such an extent is unlikely in the field, because even in hot regions o f ) t h e world the ambient temperature rarely goes beyond 50~C (122°F . Thus in practice the percent of concrete strength that can be regained by recuring should be at least equal to what was achieved in the present study and the previous study (15), if not more. The beneficial effects of recuring on the permeability of both silica fume and fly ash mortar specimens can be noted in Table II. The coefficient of absorptivity values of the recured silica fume and fly ash mortar specimens whose curing was interrupted after 0 day of moist curing are 70% lower than those of the companion specimens subjected to discontinuous curing. There were doubts that the volumetric changes resulting from the hydration of the unhydrated cement particles during the recuring period may significantly stress the hardened mortar matrix, cause its cracking, and thus increase the permeability. The data obtained in this study rule out this possibility. The possible mechanism through which the hydration of the remaining unhydrated cement particles took place during the recuring period was that the hydration products occupied the capillary pores left at the time of curing interruption, and thus did not stress the matrix to an extent that could cause its cracking. Increased hydration resulted in strength gain, and filling of the capillary pores resulted in the reduction in absorptivity or permeability. The hydration products during recuring might even have healed the micro-cracks developed during the curing interruption (oven-drying) of the specimens. Conclusions The results obtained in this study suggest that from the viewpoint of compressive strength and permeability, the minimum length of moist curing required for silica fume concrete is less than that of plain cement concrete. This is in contrast with the curing requirement of fly ash concrete. The number of concrete properties investigated in this study (compressive strength, absorptivity) are not enough to recommend that shorter length of moist curing would be sufficient for silica fume concrete. However, there are indications that the current industry practice of over-curing the silica fume concrete is unnecessary. Silica fume concretes moist cured according to the curing requirements of portland cements with which they are blended (for example, 7 days with Type I cement, 3 days with Type III cement, and 14 days with Type V cement, according to ACI 308) are expected to yield desirable results. Another
important
finding
of this
study
is that
if due
to
some
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1489
reason the moist curing of a concrete, including silica fume, fly ash and plain cement concretes, is interrupted and the concrete dries out, the resulting loss in strength and impermeability could be regained significantly by recuring the concrete. It is important to mention here is that the above statements are based on the assumption that the silica fume concrete is fully protected from any undesirable exposure during the first 24 hours. In this study, all the mortar specimens including silica fume mortar specimens were placed in the normal laboratory condition and covered with a plastic sheet during the first 24 hours after casting. Acknowledgments The authors greatly acknowledge the support of the School of Civil Engineering of Oklahoma State University, Stillwater, Oklahoma, where this research was conducted.
References i.
ACI Committee 226, Use of Fly Ash in Concrete, Concrete Institute, Detroit, Michigan (1987).
2.
ACI Committee 226, ACI Materials Journal,
3.
V.M. Malhotra (ed.), Fly Ash, Silica Fume, Slag & Other Mineral By-Products in Concrete, ACI SP-79, American Concrete Institute, Detroit, Michigan (1983).
4.
V.M. Malhotra (ed.), Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-91, American Concrete Institute, Detroit, Michigan (1986).
5.
V.M. Malhotra (ed.), Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-II4, American Concrete Institute, Detroit, Michigan (1989).
6.
V.M. Malhotra (ed.), Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-132, American Concrete Institute, Detroit, Michigan (1992).
7.
M.D.Ao Thomas, J.D. Mathews and C.A. Haynes, The effect of curing on the strength and permeability of PFA concrete, In: Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-I14, American Concrete Institute, Detroit, Michigan, 191 (1989).
8.
M.K. Gopalan and M.N. (1987).
9.
M.N. Haque, R.L. Day and B.W. Langan, ACI Materials Journal, 85, 241 (1988).
i0.
M.D. Luther, Transportation Research Record, 1204, ii (1988).
Haque,
American
84, 158 (1987).
ACI Materials
Journal,
84,
14
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Vol. 23, No. 6
Ii.
T.G. Weil, Transportation Research Record,
12.
D. Bunke, Transportation Research Record,
13.
T.C. Holland, Transportation Research Record, 1204, 1 (1988).
14.
ACI Committee 308, Standard Practice for Curing Concrete, American Concrete Institute, Detroit, Michigan (1981).
15.
M.S. Khan and M.E. Ayers, (1992).
16.
T.C. Powers and T.L. Brownyard,
17.
E. Senbetta and C.F. Scholer, ACI Journal,
18.
S. Popovics,
19.
S. Mindess and J.F. Young, Concrete, Prentice-Hall, Englewood Cliffs, New Jersey, 410 (1981).
20.
D.M. Roy, Fly ash and silica fume chemistry and hydration, In: Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-II4, American Concrete Institute, Detroit, Michigan, 117 (1989).
21.
D.R. Lankard, D.L. Birkimer, F.F. Fondriest and M.J. Synder, Effects of moisture content on the structural properties of portland cement concrete exposed to temperatures up to 500 F, In: Temperature and Concrete, ACI SP-25, American Concrete Institute, Detroit, Michigan, 59 (1971).
22.
A. Bentur and C. Jaegermann, ASCE Civil Engineering, ~, 252 (1991).
ACI Journal,
1204, 8 (1988). 1204,
27 (1988).
Concrete International, ACI Journal,
14(12),
25
4__33, 865 (1947).
8]_, 82 (1984).
8/3, 650 (1986).
Journal
Inc.,
of Materials
in
CURING
REQUIREMENTS FLY ASH
OF SILICA MORTARS
FUME
AND
M o h a m m a d Shamim Khan and Michael E. Ayers School of Civil Engineering Oklahoma State U n i v e r s i t y Stillwater, OK 74078-0327 (Communicated by C.D. Pomeroy) (Received March 17, 1993)
ABSTRACT The curing requirements of silica fume and fly ash mortars were investigated in this study. Silica fume and fly ash mortar specimens were moist cured for periods of 0, 3, 7, 14 and 28 days. After each of the five periods, the moist curing was interrupted by o v e n - d r y i n g the specimens at a temperature of ll0°C for 3 days. The specimens were later tested for compressive strength and absorptivity. In this study, it was also d e t e r m i n e d whether the losses in strength and i m p e r m e a b i l i t y of silica fume and fly ash mortars due to an i n t e r r u p t i o n in curing could be regained by recuring. The test results clearly indicate that the curing r e q u i r e m e n t of silica fume m o r t a r is less than that of plain cement mortar, while in the case of fly ash m o r t a r it is higher than that of plain cement mortar. Introduction Extensive investigations have been carried out on the use of fly ash and silica fume in concrete during the past two decades and have c o n s e q u e n t l y lead to their w i d e s p r e a d application in the c o n s t r u c t i o n industry (1-6). The curing r e q u i r e m e n t of these concretes is an issue which has not received s u f f i c i e n t attention of the researchers. Some recent studies (7-9) have a d d r e s s e d the c u r i n g sensitivity of fly ash concretes, and indicated that fly ash concretes are more susceptible to poor curing conditions than plain cement concretes. There is a lack of such type of data in the case of silica fume concrete.
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CURING, SILICA FUME, FLY ASH, MORTARS
1481
In the U n i t e d States, the field application of silica fume concrete started in 1983, m o s t l y in bridge decks and p a r k i n g structures. A r e v i e w of silica fume concrete use in bridge decks was p r e s e n t e d by Luther (i0), and in parking structures by Weil (ii). From these publications, a number of similarities can be noted among d i f f e r e n t silica fume concrete projects in terms of mixing, p l a c i n g and finishing. However, most of the projects differ on the point of c u r i n g of silica fume concrete. For example, Bunke (12) on the basis of his experience with silica fume concrete projects at the Ohio D e p a r t m e n t of Transportation recommends a continuous w a t e r curing of 3 days for silica fume concrete. On the other hand, Holland (13) recommends a wet curing of 7 days as an absolute minimum. The v a r i a t i o n s in the curing practice of silica fume concrete may be largely a t t r i b u t e d to the lack of standard specifications, which are tied to insufficient b a c k g r o u n d data on silica fume concrete, in general, and its curing in particular. The ACI C o m m i t t e e 308 report (14) provides guidelines for the curing of normal p o r t l a n d cement concrete, but does not include concretes made with p o z z o l a n i c materials such as fly ash and silica fume. The main objective of the present study is to investigate the c u r i n g r e q u i r e m e n t of silica fume concrete. A fly ash mix is included in the study for comparison. Also i n v e s t i g a t e d is w h e t h e r the losses in strength and impermeability of silica fume and fly ash concretes due to an interruption in moist curing could be regained by recuring. The effects of interrupted curing and r e c u r i n g in the case of plain cement concrete are d o c u m e n t e d by the authors elsewhere (15). The available literature lacks data on the effects of recuring in either plain cement concrete or concretes c o n t a i n i n g silica fume and fly ash.
Experimental Details
M a t e r i a l s and Mix Design The m a t e r i a l s used in this study were ASTM Type I p o r t l a n d cement, silica fume, class F fly ash, natural sand, potable w a t e r and a high range w a t e r reducing admixture (HRWA). The sand had a water absorption of 0.28% and its fineness modulus was 2.74. The g r a d a t i o n curve of the sand was almost in the middle of A S T M C-33 g r a d a t i o n limits for fine aggregates. Two silica fume m o r t a r mixes with w a t e r - t o - c e m e n t i t i o u s material (W/C) ratios of 0.44 and 0.49, and one fly ash m o r t a r mix with w a t e r - t o - c e m e n t i t i o u s material ratio of 0.44 were used (Table I) in this study. The cementitious material content was the c o m b i n e d w e i g h t of cement and pozzolanic material (silica fume or fly ash). In all the silica fume and fly ash m o r t a r mixes, the silica fume or fly ash content was 15% of the total cementitious material content. C a s t i n g and C u r i n g The c a s t i n g of the m o r t a r specimens was done in small batches w h i c h
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Vol. 23, No. 6
TABLE I Mix Proportions for Six Mortar Specimens (50-mm or 2-inch Cubes) Constituents Mix W/C* designation ratio Water (gm)
Cement Silica (gm) fume
(gm)
Fly ash
Sand (gm)
HRWA** (ml)
(gm)
SF-0.44
0.44
220.8
425
75
1504.2
SF-0.49
0.49
245.8
425
75
1504.2
FY-0.44
O. 44
220.8
425
75
1504.2
* W/C ratio means w a t e r - t o - c e m e n t i t i o u s material ratio ** HRWA means high range water reducing admixture Note: 1 Ib = 453.592 gm; 1 fl oz = 29.57353 ml yielded 6 specimens (50-mm or 2-inch cubes). After casting, the molds c o n t a i n i n g the specimens were covered w i t h a plastic sheet and stored in normal laboratory conditions. The specimens were demolded after 24±1 hours. After demolding, the specimens were subjected to curing under conditions of complete immersion in water for five different periods of time; 0, 3, 7, 14 and 28 days. After each m o i s t curing period, the specimens were removed from water and o v e n - d r i e d for 3 days at a temperature of ll0°C (230°F). This was done to, practically, stop the hydration of the specimens after each curing period, because the specimens were not tested immediately after their removal from water. Note that the specimens c o r r e s p o n d i n g to 0 day of moist curing were subjected to oven-drying immediately after demolding. There were six specimens for each curing p e r i o d from each mix. After the oven-drying, three specimens were stored in the normal dry conditions of the laboratory and were later used to assess the strength and permeability c h a r a c t e r i s t i c s of silica fume and fly ash mortars for different periods of moist curing. The curing of these specimens has been referred to as d i s c o n t i n u o u s curing in this paper. In order to determine the effect of recuring after an interruption in moist curing, the other three specimens of each mix and each curing p e r i o d were reimmersed in water for a p e r i o d of time, w h i c h if added to the m o i s t curing period before interruption was equal to 28 days. This means that all the recured specimens had a total moist curing p e r i o d of 28 days, but had an interruption in their moist curing (oven-drying at ll0°C for 3 days). It is to be noted that after oven-drying, the specimens were allowed to cool for 24 hours before immersing them in water for recuring. Testinq At the end of the recuring period, the recured specimens along with
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CURING, SILICA FUME, FLY ASH, MORARS
1483
the c o r r e s p o n d i n g specimens subjected to d i s c o n t i n u o u s c u r i n g were tested for a b s o r p t i v i t y and compressive strength. The a b s o r p t i v i t y was d e t e r m i n e d by using the following relationship, o r i g i n a l l y suggested by Powers and Brownyard (16) and later u s e d by others
(17): (q/A) =(Kat)~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (i) Where q/A is the amount of w a t e r absorbed per unit area of e x p o s e d surface in time t. Ka, expressed in cm2/sec, is the c o e f f i c i e n t of absorptivity. The r e l a t i o n s h i p between the c o e f f i c i e n t of absorptivity, p e r m e a b i l i t y is as follows (16):
Ka, and
k a = [(k2) c o Sc]/~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) Where (K2)c is the coefficient of p e r m e a b i l i t y through the c a p i l l a r y s y s t e m of the cement paste, ~ is the surface tension of water, ~ is the v i s c o s i t y of water, and S c is the surface area of gel. S c is a d i f f i c u l t p a r a m e t e r to measure, and it is s u g g e s t e d by Powers and B r o w n y a r d (16) that K a can be used as a relative m e a s u r e of permeability. In Eq. (i), the amount of water absorbed (q) was d e t e r m i n e d by o v e n - d r y i n g the specimens at a temperature of ll0°C (230°F) for 24 hours, c o o l i n g them in air tight containers for another 24 hours, and then soaking in water for 60 minutes. Equation (i) is v a l i d for an a b s o r p t i o n time of 30 to 60 minutes (16). A f t e r the a b s o r p t i v i t y test, the same specimens were used for c o m p r e s s i v e strength measurement. But before c o m p r e s s i v e s t r e n g t h measurement, the specimens were oven-dried at ll0°C (230°F) for 24 hours and then c o o l e d in air tight containers for 24 hours. This was done in an attempt to keep the c a p i l l a r y m o i s t u r e of all the specimens at the same level before testing. It has been d e m o n s t r a t e d by Popovics (18) that small differences in the moisture content of concrete specimens cause s i g n i f i c a n t d i f f e r e n c e in their compressive strength. The more usual p r a c t i c e of t e s t i n g the specimens in moist condition was not followed in this study, because that m i g h t have changed the hydration status of the specimens s u b j e c t e d to discontinuous curing. Also, this was the reason for the selection of a p e r m e a b i l i t y test involving a short exposure time (60 minutes) of the specimens to water.
Test Results and Discussion The c o m p r e s s i v e strength and absorptivity data for the silica fume and fly ash m o r t a r specimens cured for different periods of time and under two d i f f e r e n t curing conditions, d i s c o n t i n u o u s c u r i n g and recuring, are shown in Table II. The averages, standard deviations, and coefficients of v a r i a t i o n shown in Table II are for 3 specimens. The individual compressive strength and a b s o r p t i v i t y values were c h e c k e d for outliers. In identifying the o u t l y i n g measurements, both the highest and lowest m e a s u r e m e n t s were t e s t e d according to the T-statistic procedure, d e s c r i b e d in A S T M E-178,
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TABLE
Vol. 23, No. 6
II
Compressive Strength and Absorptivity Data for Silica Fume and Fly Ash Mortar Specimens Compressive strength Specimen designation
Average! Standard C.O.V. deviation (%) (MPa) (MPa)
Coefficient of absorptivity Average x i0 "° (cmZ/sec)
C.O.V. Standard (%) deviation x I0 6 (cm2/sec)
SF-0.44-0-D SF-0.44-0-R SF-0.44-3-D SF-0.44-3-R SF-O. 44-7-D SF-0.44-7-R SF-0.44-14-D SF-0.44-14-R SF-0.44-28
58.5 71.0 67.6 74.0 75.3 78.5 74.3 74.3 74.1
0.39 3.13 2.13 5.21 1.75 3.92 2.72 3.52 2.93
0.66 4.41 3.16 7.04 2.32 4.99 3.66 4.73 3.96
3.73 2.17 1.81 1.40 1.74 1.62 1.79 1.63 2.11
0.291 O.O55 0.077 0.091 0.114 0.016 0.083 0.058 0.167
7.80 2.54 4.25 6.51 6.58 0.98 4.61 3.52 7.91
SF-0.49-0-D SF-O. 49-0-R SF-0.49-3-D SF-0.49-3-R SF-0.49-7-D SF-0.49-7-R SF-0.49-14-D SF-0.49-14-R SF-0.49-28
55.3 64.7 58.9 62.1 59.8 62.9 66.9 64.7 65.0
0.81 0.39 0.39 3.25 3.30 1.25 3.13 3.08 2.07
1.46 0.60 0.66 5.23 5.51 1.98 4.68 4.75 3.18
3.59 2.14 2.62 2.05 2.60 2.22 2.58 2.04 3.05
0.163 0.095 0.i00 0.031 0.053 0.067 0.020 0.064 0.117
4.54 4.41 3.82 1.50 2.04 3.00 0.77 3.15 3.84
FY-0.44-0-D FY-0.44-0-R FY-0.44-3-D FY-0.44-3-R FY-0.44-7-D FY-0.44-7-R FY-0.44-14-D FY-0.44-14-R FY-0.44-28
36.0 60.3 53.7 68.4 58.5 70.4 66.0 72.0 70.1
1.03 1.61 0.81 1.21 1.03 1.61 2.13 1.62 3.61
2.84 2.67 1.50 1.77 1.75 2.29 3.23 2.26 5.15
4.59 2.70 3.87 2.54 3.84 2.56 3.47 2.58 3.23
0.138 0.108 0.135 0.151 0.222 0.140 0.069 0.122 0.086
3.01 4.01 3.50 5.93 5.77 5.45 2.00 4.71 2.65
Note: C.O.V. means coefficient of variation; 1 c m 2 / s e c = 0 . 1 5 5 in2/sec
1 MPa
=
145
psi;
for an u p p e r 5% s i g n i f i c a n c e level. The m e a s u r e m e n t s t h a t did not m e e t the u p p e r 5% s i g n i f i c a n c e level c r i t e r i a w e r e d i s c a r d e d . In addition, the i n d i v i d u a l c o m p r e s s i v e s t r e n g t h m e a s u r e m e n t s w e r e also c h e c k e d for t h e i r 10% v a r i a b i l i t y f r o m the average. The i n d i v i d u a l c o m p r e s s i v e s t r e n g t h m e a s u r e m e n t s m o r e t h a n 10% h i g h e r or lower t h a n the a v e r a g e w e r e d i s c a r d e d . This is one of the r e l i a b i l i t y c r i t e r i a for the s t r e n g t h r e s u l t s of 5 0 - m m (2-inch) cubes, as specified by ASTM C-I09. There were very few m e a s u r e m e n t s t h a t did not m e e t the u p p e r 5% s i g n i f i c a n c e level or the 10% v a r i a b i l i t y criteria. The v a r i a t i o n of c o m p r e s s i v e s t r e n g t h of the s i l i c a fume and fly ash m o r t a r s p e c i m e n s w h e r e m o i s t c u r i n g was d i s c o n t i n u e d a f t e r 0, 3, 7, 14 and 28 days is i l l u s t r a t e d in Fig. 1 (W/C = 0.44) and Fig. 2 (W/C = 0.49). In these figures, the d a t a s h o w n for p l a i n c e m e n t m o r t a r s p e c i m e n s h a v e b e e n t a k e n f r o m the s t u d y by the a u t h o r s (15), c i t e d earlier, for c o m p a r i s o n . The d e s i g n s of the
Vol. 23, No. 6
CURING, SILICA FUME, FLY ASH, MORTARS
Compressive Strength, MPa
1485
_ Compressive Strength. MPa
80
60
40
20
0 0 Day
m
3 Days 7 Days 14 Days P e r i o d ol Moist C u r i n g Aller D e r n o l d l n ~
sttica Fume Mortar
~
28 Days
Fly A|b Mortar
0 Day
m
3 Days ? Days 14 Days Period o! Moist Curing Alter Dernoldlng Silica Fume Mortar
~
28 Days
Plain Cement Mortar
Plain Cement Mortar
FIG. 1 Compressive strength of silica fume, fly ash and plain cement m o r t a r specimens with W/C = 0.44
FIG. 2 Compressive strength of silica fume and plain cement mortar specimens with W/C = 0.49
plain cement mortar mixes with W/C ratios of 0.44 and 0.49 were similar to those of the c o r r e s p o n d i n g silica fume m o r t a r mixes except that the plain cement m o r t a r mixes c o n t a i n e d a cement content e q u i v a l e n t to the combined weight of cement and silica fume in the silica fume m o r t a r mixes. Note that the silica fume m o r t a r mixes u t i l i z e d 425 gm (0.94 ib) of cement and 75 gm (0.17 ib) of silica fume for 6 specimens (Table I). Similar number of plain cement m o r t a r specimens utilized only 500 gm (i.i0 ib) of cement. The d i f f e r e n c e in the curing sensitivity of silica fume, fly ash and plain cement mortars can be clearly observed in these figures. The c o m p r e s s i v e strength of the silica fume m o r t a r specimens ovendried immediately after demolding (no moist curing) was found to be 79-85% ( W / C = 0.44, 0.49) of that of specimens c o n t i n u o u s l y moist cured for 28 days. The c o r r e s p o n d i n g strength in the case of fly ash m o r t a r specimens (W/C = 0.44) is 51% and in case of plain cement m o r t a r specimens ( W / C = 0.44, 0.49) 62-65%. In silica fume mortar specimens, the m a x i m u m compressive strength corresponds to moist curing periods of 7 to 14 days, while in case of fly ash and plain cement mortar specimens, a continuous gain in s t r e n g t h is o b s e r v e d up to moist curing period of 28 days. Care should be exercised in comparing the compressive s t r e n g t h data p r e s e n t e d in Table II with other data in the literature. The compressive strength values in Table II are for dry m o r t a r specimens. In most instances, the compressive strength values are r e p o r t e d for wet specimens. In a limited scale experiment, the authors found the compressive strength of dry concrete cubes (50-mm or 2-inch) 13-19% higher than that of wet concrete cubes. The d e v e l o p m e n t of r e l a t i v e l y high confining stresses near the specimen ends due to increased friction between specimen and steel platens is one of the reasons for the higher strength of dry concrete or m o r t a r specimens compared to wet specimens (19). A c c o r d i n g to Roy (20), the hydration rate of cement c o n t a i n i n g silica fume is greater than that of ordinary p o r t l a n d cement, while that of cement c o n t a i n i n g fly ash is slower than that of o r d i n a r y
1486
M.S. Khan and M.E. Ayers
Vol. 23, No. 6
Compressive Strength, MPa
Coellicient of Absorptivity (IE-06)
I00[ 80~ 60 40 20
0 Day
l
3 Days 7 Days 14 Days Period o! Moist Curing Alter Derno|ding
Silica Fume Mortar
J
Fly ASh Mortar
28 Days
E ~ Plain Cement Mortar
=
0 Day
3 Days 7 Days 14 Days Period of Moist C u r i n g A/ler D e m o l d t o g m
D i s c o n t i n u o u s Curing
m
28 Days
Recuring
FIG. 4 Compressive strength of silica fume mortar specimens with W/C = 0.44, subjected to discontinuous curing and recuring
FIG. 3 Coefficient of absorptivity of silica fume, fly ash and plain cement mortar specimens with w/c
0
0.44
portland cement. The same order of hydration rate, as interpreted from the strength development data, has been observed in this study. A high hydration rate of cement containing silica fume is attributed to the fact that silica fume particles accelerate the hydration of C3S (Ca3SiO5, alite), the cement compound primarily responsible for strength development at early ages (20). However, the strength development in silica fume mortar specimens which did not have any moist curing is not accompanied by a proportional reduction in permeability (Fig. 1 and Fig. 3). The coefficient of absorptivity of these silica fume mortar specimens is not significantly different than that of plain cement and fly ash mortar specimens. But once the silica fume mortar specimens receive a moist curing of 3 days, there is a significant reduction in their absorptivity compared to plain cement and fly ash mortar specimens. This implies that a harsh curing environment at early stages would not affect the strength of silica fume concrete significantly, but it would have a significant effect on its permeability. Although, careful visual examination did not reveal any macro surface cracks in either silica fume mortar specimens or fly ash and plain cement mortar specimens, there is a p o s s i b i l i t ~ o f microcracking of the specimens subjected to oven-drying (ll0VC or 230°F) immediately after demolding. One of the factors that might have caused the micro-cracking of the specimens, particularly in the surface region, is the loss of some chemically bound water along with capillary water. Lankard et al. (21) measured a loss of chemically bound water on the order of 10% in 50-mm (2-inch) cubes of neat cement and mortar after heating the specimens at a temperature of 121°C (250°F) for 58 days. The shrinkage associated with the loss of chemically bound water is believed to disrupt the bonding configuration between the solid hydrated cement phases, and thus causes the micro-cracking of the cement paste (21). The
incompatibility
in
the
strength
and
permeability
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CURING, SILICA FUME, FLY ASH, MORTARS
1487
characteristics of silica fume mortar specimens subjected to a harsh curing environment at an early stage is in agreement with the recent findings of Bentur and Jaegermann (22). Their results indicate that the adverse effects of inadequate water curing are more evident on concrete permeability rather than on strength. This is attributed to the fact that permeability is largely a function of the quality of the surface region of the concrete specimen, while strength is a function of the quality of the concrete specimen as a whole. The surface region of the concrete specimen is the one which is affected most by an adverse curing environment. Here it is to be noted that even from the standpoint of permeability, the curing requirement of silica fume mortar specimens is lower than that of plain cement and fly ash mortar specimens. Figures 4-6 illustrate the effect of recuring on the compressive strength of silica fume and fly ash mortar specimens. The beneficial effects of recuring, as reported by Khan and Ayers (15) for plain cement mortar specimens, are clearly noticed in silica fume and fly ash mortar specimens in this study. Relatively small strength gain in silica fume mortar specimens after recuring, further emphasizes that silica fume mortar specimens gain strength quite early and their curing requirement is low. The compressive strengths of the recured silica fume mortar specimens after curing interruptions at 0, 3 and 7 days are 17-21%, 6-10% and 4-5%, respectively, higher than those of the corresponding specimens whose curing was discontinued after interruption. In the case of silica fume mortar specimens where curing was interrupted after 14 days, the strengths of the specimens subjected to recuring and discontinuous curing are similar. The strength of the recured fly ash mortar specimens is significantly higher than that of the specimens subjected to discontinuous curing, particularly at early periods of curing interruption. The strengths of the recured fly ash mortar specimens at curing interruptions of 0, 3, 7 and 14 days are 67%, Compressive Strength. MPu 70
80
Compressive Strenglh, MPa
60 60
50 40
40
30 20
20
l0 0 0 Day
3 Days 7 Days 14 Days Period oi Moist Curing Alter Dernolcllng
Disconlinuous Curing
~
28 Days
Recurlng
FIG. 5 Compressive strength of silica fume mortar specimens with W/C = 0.49, subjected to discontinuous curing and recuring
0
0 Day
3 Days 7 Days 14 Days P e r i o d of Moist Curing Alter Demoiding i
Discontinuous Curing
m
26 Dug
Recuring
FIG. 6 Compressive strength of fly ash mortar specimens with W/C = 0.44, subjected to discontinuous curing and recuring
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Vol. 23, No. 6
27%, 20% and 9%, respectively, higher than those of the specimens subjected to discontinuous curing. The corresponding percentages in the case of plain cement mortar specimens have been found to be 41%, 18%, 9% and 2% (15). A harsh environment for curing interruption (oven-drying at ll0°C or 230°F for 3 days) was selected in order to obtain highly visible effects of interrupted curing and recuring. Drying of concrete to such an extent is unlikely in the field, because even in hot regions o f ) t h e world the ambient temperature rarely goes beyond 50~C (122°F . Thus in practice the percent of concrete strength that can be regained by recuring should be at least equal to what was achieved in the present study and the previous study (15), if not more. The beneficial effects of recuring on the permeability of both silica fume and fly ash mortar specimens can be noted in Table II. The coefficient of absorptivity values of the recured silica fume and fly ash mortar specimens whose curing was interrupted after 0 day of moist curing are 70% lower than those of the companion specimens subjected to discontinuous curing. There were doubts that the volumetric changes resulting from the hydration of the unhydrated cement particles during the recuring period may significantly stress the hardened mortar matrix, cause its cracking, and thus increase the permeability. The data obtained in this study rule out this possibility. The possible mechanism through which the hydration of the remaining unhydrated cement particles took place during the recuring period was that the hydration products occupied the capillary pores left at the time of curing interruption, and thus did not stress the matrix to an extent that could cause its cracking. Increased hydration resulted in strength gain, and filling of the capillary pores resulted in the reduction in absorptivity or permeability. The hydration products during recuring might even have healed the micro-cracks developed during the curing interruption (oven-drying) of the specimens. Conclusions The results obtained in this study suggest that from the viewpoint of compressive strength and permeability, the minimum length of moist curing required for silica fume concrete is less than that of plain cement concrete. This is in contrast with the curing requirement of fly ash concrete. The number of concrete properties investigated in this study (compressive strength, absorptivity) are not enough to recommend that shorter length of moist curing would be sufficient for silica fume concrete. However, there are indications that the current industry practice of over-curing the silica fume concrete is unnecessary. Silica fume concretes moist cured according to the curing requirements of portland cements with which they are blended (for example, 7 days with Type I cement, 3 days with Type III cement, and 14 days with Type V cement, according to ACI 308) are expected to yield desirable results. Another
important
finding
of this
study
is that
if due
to
some
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reason the moist curing of a concrete, including silica fume, fly ash and plain cement concretes, is interrupted and the concrete dries out, the resulting loss in strength and impermeability could be regained significantly by recuring the concrete. It is important to mention here is that the above statements are based on the assumption that the silica fume concrete is fully protected from any undesirable exposure during the first 24 hours. In this study, all the mortar specimens including silica fume mortar specimens were placed in the normal laboratory condition and covered with a plastic sheet during the first 24 hours after casting. Acknowledgments The authors greatly acknowledge the support of the School of Civil Engineering of Oklahoma State University, Stillwater, Oklahoma, where this research was conducted.
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