Thermal History

A key concern for concrete in the field is thermal cracking at early ages [47,48]. Because the cement hydration reactions are exothermic, large temperature gradients (on the order of 50 ^oC) may be generated within a concrete structure. The microstructure model may be used to investigate the propensity for this phenomena by predicting the adiabatic heat signature curve for a specific concrete mix proportion [37]. Using a measured activation energy (typically on the order of 40 kJ/mol [7]), the Arrhenius function, and the maturity method, the temperature rise with time for a specimen hydrated under totally adiabatic conditions may be predicted [37]. Figures 6 to 8 provide a comparison of model and measured results for an OPC concrete, and the same system with silica fume [37] and fly ash [49]. For all three concretes, the model predictions are seen to lie within a few degrees Celsius of the experimental results. In both cases, the addition of pozzolanic materials is seen to significantly increase the temperature rise, causing the long term temperature of the concrete to exceed 70 ^oC so that secondary ettringite formation may become a concern [3]. By coupling this model with a finite element model for a specific concrete structure, the propensity for thermal cracking under various curing regimes could be predicted.

  

Figure 6: Comparison of experimental (data points) and simulated (solid line) adiabatic heat signature curves for a w/c=0.65 OPC concrete.

  

Figure 7: Comparison of experimental (data points) and simulated (solid line) adiabatic heat signature curves for a w/c=0.65 OPC concrete with 10 % by mass silica fume.

  

Figure 8: Comparison of experimental (data points) and simulated (solid line) adiabatic heat signature curves for a w/c=0.65 OPC concrete with 50 % by mass fly ash.