Search

Early-age thermal cracking and delayed ettringite formation (DEF) are short-term and long-term major issues to the durability of mass concrete and require a high level of consideration in the development of a suitable concrete mix design in placement and curing strategies, particularly in mixes of high cement content. Where DEF risk proves to be significant, contractors are required to adopt risk mitigation strategies including lowering concrete's temperature during placement by adding cool water or ice, for example, to the mix, embedding cooling pipes in the cast element and/or sequential placement of the concrete. These strategies can be implemented at different levels, as needed, to minimise the risk of cracking and DEF. Determining the most suitable and cost-efficient risk mitigation strategy, however, together with its implementation, is a complex task that requires consideration of each complex strategy under project-specific conditions. While a significant research effort has been invested to develop an improved understanding of the effectiveness of different early age thermal cracking and DEF risk mitigation strategies, the lack of reliable models capable of simulating and comparing the effectiveness of all scenarios remains elusive and considerably limits the ability of the practitioners in identifying the optimal approach. This paper presents a Multiphysics model developed to predict the effect of early-age thermal cracking and DEF risk mitigation scenarios. The model is validated using four real-life project Case Studies and is demonstrated to provide reliable results. In addition, the paper studies the comparative effectiveness of key risk mitigation strategies and the important parameters influencing their effectiveness.

Early-age thermal cracking and delayed ettringite formation (DEF) are major durability risks associated with mass concrete casting with high cement content. Among various strategies to mitigate the risk of early-age thermal cracking and DEF, concrete mix optimisation is favoured by the industry due to its minimal required operational adjustments and insignificant implementation cost. The existing concrete mix design optimization methods are, however, incapable of accounting for the risk of early-age thermal cracking and DEF; and have been designed solely to meet the mechanical performance targets. Mitigating the risk of DEF could be challenging mainly because limiting the temperature rise in concrete requires moderating the hydration heat/rate to minimize the risk of early-age thermal cracking and DEF which could contradict the traditional objectives associated with the concrete's mechanical performance. Further, early-age thermal cracking can be influenced by project-specific factors including the dimensions of the concrete element, ambient conditions, and external restraints which do not come into the picture in a conventional mix design approach. These issues add to the complexity of the mix design problem. This paper presents a mix design approach based on integrated numerical simulation of heat transfer in concrete and genetic algorithm optimisation that minimises the risk of early-age cracking within the limits specified for the mechanical performance of concrete. The results of the optimization framework applied to three case studies show a significant decrease in peak temperature, as well as the ratio of induced tensile stresses to developed tensile strength for optimized concrete mixes.