Investigating early age thermal cracking of concrete

Abstract

This thesis presents the results of a comprehensive study conducted to develop a practical tool to assist concrete specialists in i) evaluating the risk of early age thermal cracking of concrete for a particular concrete element, ii) identifying the optimal concrete mix design to reduce the risk of early age thermal cracking, iii) evaluating the effectiveness of construction strategies, including sequential placement and internal cooling using embedded pipes, in reducing the risk of early age thermal cracking. This thesis addresses the gaps in the available literature regarding early age thermal cracking including evaluating the accuracy of the available hydration models for Australian concrete, a lack of existence of a comprehensive numerical simulation for evaluating early age thermal cracking and mix design optimisation with the aim of minimizing the risk of early age thermal cracking. A comprehensive experimental study was conducted to address the first gap by evaluating the precision of existing hydration heat models through extensive calorimetry tests. To address the second gap, an advanced three-dimensional numerical simulation model was developed and verified by real-world site measurements using COMSOL Multiphysics. This model allows direct use of calorimetry data as well as existing hydration models as the heat source and is capable of modelling the effect of reinforcement, thermal and mechanical boundary conditions, etc. This model is also modified to numerically analyze common construction methodologies for reducing early age thermal cracking such as sequential concrete pouring and using of embedded cooling pipes in concrete. While the proposed numerical simulation model makes available a means of evaluating the effect of different mixes on early age thermal cracking, try and error to identify an optimal mix design which minimizes the risk of early age thermal cracking is highly time-consuming. To address this issue, a genetic algorithm multi-objective mix design optimisation method was developed to mathematically identify the optimal mix design that minimizes the risk of early age thermal cracking in a particular element, while considering the practical constraints. The proposed optimisation algorithm was developed in MATLAB and was designed to include an embedded finite difference model to allow its use as a stand-alone tool.