Early age properties and thermal cracking of concrete structure

Abstract

Despite the concrete has been widely used because of its favourable structural properties, durability and competitive cost, the concrete volume instability due to temperature variation and shrinkage effect, especially during early age, is recognised as a vulnerable disadvantage among many other construction materials. Following that, early-age cracking of concrete structures due to restrained conditions in practice has been a critical issue that may seriously compromise the structural integrity, durability and aesthetics, thereby causing significant economic impact.To date, lot of research about early age concrete properties and thermal cracking phenomenon have been carried out, yet there is a knowledge gap remains unsolved. A reliable prediction model for each shrinkage component is essential, however, due to limited data available for tensile properties and lack of study about many other influencing factors, the existing models are not sufficiently reliable. In fact, the creep and autogenous shrinkage determination described in Australian Standard, AS3600, is based on compressive data at age generally older than 14 days (Khan, 2017). More importantly, no literature provided remarkable discussion and application about reference temperature known as zero-stress temperature, thereby estimation of absolute thermal effect and early age thermal cracking is desired. In this research, a new concept of zero-thermal stress temperature (Tz-thermal) based on pure thermal stress and a better understanding of early-age concrete properties under different temperature are considered as paramount to bridge the knowledge gap. To achieve this aim, a Temperature-Stress Test Machine, an advanced early-age concrete test machine developed at the University of Queensland, has been utilized to obtain reliable data and then a step-by-step numerical method is used to analyse early-age behaviour of concrete. As a consequence, a significant dependency of Tz-thermal on the concrete temperature is found. This indicates the possibility of early age thermal crack control by controlling the concrete temperature, thereby Tz-thermal and thermal stress. The initiation time of thermal loading significantly influenced the magnitude of the Tz-thermal. Moreover, Tz-thermal at heating phase is greater than that at cooling phase. This is thought to be due to different effects of early age concrete properties (such as coefficient of thermal deformation and Young’s modulus) at different phases.