Understanding the Effects of Maturity on Strength Development and Prediction in Self-Compacting Concrete Incorporating Supplementary Cementitious Materials

Abstract

Self-compacting concrete (SCC) is becoming a popular alternative to traditional vibrated concrete because of its excellent flowability and reduced energy requirements during construction. The higher paste content of SCC results in a microstructure that differs from that of conventional concrete, leading to distinct mechanical properties when exposed to varying curing temperatures. However, there is a scarcity of studies on the influence of curing temperature on the performance of SCC containing supplementary cementitious materials (SCMs) as a partial substitute for cement. This PhD work is structured into three main parts. The first part of this thesis focuses on the necessary treatments applied to the supplied rice husk ash (RHA) to make it suitable for use in SCC. The RHA, which was obtained from uncontrolled burning, showed poor pozzolanic activity because of high levels of unburned carbon and a porous structure. To enhance its effectiveness and make it suitable for use with cement binder in concrete applications, further treatments such as reburning and grinding were carried out. Experimental work was performed to investigate the effects of prioritising reburning versus grinding treatments on the properties of resulting RHA. Managing the treatment procedure was found to be important for producing effective material. It was demonstrated that reburning, followed by grinding, transformed the RHA into a more effective SCM. The second part of the thesis examines the effects of curing temperature on the hydration reaction and important hardened concrete properties, including the strength and porosity of high-strength SCC containing treated RHA, silica fume (SF), fly ash (FA), or ground granulated blast-furnace slag (GGBS) as partial replacement of cement. The results demonstrate that concrete gains strength quickly at high temperatures and initially develops a denser, less porous microstructure; however, this leads to lower long-term strength and slower refinement of pores. Conversely, low curing temperatures produce concrete with slower initial strength gain and a more porous microstructure but result in greater long-term pore refinement and enhanced strength development. The strength development of specimens cured at different temperatures in the first part of the study was predicted using a maturityiv function, which revealed that both the presence of SCMs and the curing temperature influenced the predictions. The third part of the thesis focuses on improving the fib Model Code’s maturity function to be more comprehensive and robust. Novel approaches are proposed to determine the apparent activation energy based on the fib Model Code, which is essential for calculating the equivalent age in the maturity function. This calculation is crucial for predicting the strength development of high-strength SCC containing different SCM as partial cement replacements, as validated by the results reported in the second part of this thesis. Two new methods are described for calculating the apparent activation energy using the fib Model Code’s maturity function based on compressive strength development of the samples, respectively. One method uses different constant activation energy values for each material, irrespective of temperature and time of the concrete, while the other employs variable values that depend on both time and the temperature. The prediction of compressive strength using the fib Model Code’s maturity function is also detailed.