Microstructure of Steam Cured Concretes Deteriorated by Alkali-Silica Reaction
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Mtermials Model concretes were made with five different cements (table 1). Their behavior regarding the alkali-aggregate reaction was tested after thermal treatment and compared to those of standard samples kept at 20TC before the AAR tests. The mix design of the concretes is in table 2:
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Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
--C3A Na20
Table 1 : Chemical composition of cements D B C 8.2 10.6 8.1
A 8.1 0.860
0.369
0.369
0.810
0.830
E 8.2
Table 2 :Mix design of concretes Cement Water Coarse aggregates Sand
: : :
Retarder Air entraining agent
: :
446 179.4 1028 706
kg/m3 kg/m3 kg/m3 kg/m3
928 ml/m3 77 ml/m3
Water/Cement ratio : 0.4
Thermal cycles of the steam cured concretes Two different thermal cycles of the steam curing were applied to concrete prisms of 7 x 7 x 28 cm. The maximun temperature was 60 0 C for the first cycle and 80'C for the second one (fig 1) 80 70 60 50 40 Temperature 30 0 C If%
cycle H cycle I
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Figure 1 : Thermal cycles used
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Alkali - Aggregate Reaction The alkali aggregate reactivity was evaluated with a special equipment standardized in France; it is a modified version of the Canadian A23-2-14A accelerated test. This equipment maintains the relative humidity at 100% and the temperature at 38°C with a double regulation system [8]. Linear expansion Linear expansions have been measured up to 270 days (table 3).The results show that all the cements behaved in the same way. However, the expansions of cements A, B and D exceed the limit of 0.4%.
r
Table 3 : Linear expansion (%), 2 0 days ,
first cycle second cycle standard
A
B
C
D
E
0.65% 0.78 % 0.48 %
0.95% 0.55 % 0.41%
0.74% 0.28 % 0.27 %
0.75% 0.74 % 0.41%
0.85% 0.36 % 0.22 %
NUMERICAL STUDIES In order to simulate the effect of the thermal treatment on concrete ties we have used two models developed at our laboratory. The first evaluates the predamaging zones induced by the thermomechanical effect due to the cement hydration. This model is built in the framework of thermodynamics of irreversible processes [9]. The second one evaluates the expansion of concrete bars affected by alkali silica reaction. In this model, the temperature effect on kinetics of reaction is represented by Arrhenius function. This model is built using a probabilistic approach [10] Thermomechanical computations During the hydration of the cement paste, some phenomena occur (heat generation and volume changes). They can result in the cracking of a concrete structure. From the mechanical point of view, one one hand, the Poisson's ratio, which is 0.5 during the dormant period, decreases during the setting and hardening periods. On the other hand, the Young's modulus increases. Simultaneously, strains appear due to self-dessiccation, thermal gradient, environmental effects (humidity, temperature) and concrete composition. Using the thermodynamics of irreversible processes we have developed a global model capable of describing this main phenomena. The starting point is the free energy potential which
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