Abstract:
With the increased use of high performance concrete a lower water-to-binder ratio is employed to improve the properties of concrete and mineral and chemical admixtures are introduced. With a low water-to-binder ratio, considerable early-age volume changes can occur and compromises the durability of the concrete. Early age shrinkage is often not tested in practice due to the difficulty in measuring and the lack of standardisation of measuring techniques.
The tensile capacity of concrete is lowest at an early age and therefore the most susceptible to cracking and material faults when stresses are induced by, for instance, drying shrinkage. Drying shrinkage can be prevented by proper placing and curing techniques but even then, cracking has been observed soon after casting. This occurrence can be attributed to factors such as high thermal gradients and autogenous shrinkage. Autogenous shrinkage is defined as a change in volume while no moisture is allowed to leave the material system and is present at low water-to-binder ratios. The influence of supplementary cementitious materials (SCMs) on autogenous shrinkage is not well understood but is well documented. SCMs are used to improve the properties of concrete but significantly affect the exothermic hydration process of cement. The strength and heat of hydration is altered, and this is dependent on the water-to-binder ratio but also on the type of SCM used and the amount present. Autogenous shrinkage is caused by hydration reactions and the heat of hydration is linked to strength development. There may thus exist a relationship between autogenous shrinkage, the thermal processes and strength development.
The primary objective of this study was to investigate the relationship between autogenous shrinkage, the thermal processes and the strength development of concrete. The influence of binder composition and content on the early age behaviour of concrete was studied. The results indicated a correlation between autogenous shrinkage and cumulative heat released where the autogenous shrinkage observed increased with cumulative heat released. A weak correlation was found between compressive strength and autogenous shrinkage. Generally, the shrinkage increased with concrete strength and the early age strength attributed more to autogenous shrinkage than later age strength. A correlation was observed between the compressive strength and cumulative heat released. SCM replacement led to a lower cumulative heat but SCM addition was detrimental in that more heat was released for an equivalent strength than for SCM replacement samples.
Notably, GGBS replacement led to more shrinkage with a lower cumulative heat released and a lower strength as well. This was mitigated by additional CSF replacement which was also not expected as CSF is known to exacerbate autogenous shrinkage. It was believed that the CSF used in this study conglomerated to behave as a material with a larger particle size.
