Time-zero and deformational characteristics of high performance concrete with and without superabsorbent polymers at early ages

Internal curing with superabsorbent polymers (SAP) can help lower early-age cracking risk of high performance concrete, by reducing both autogenous shrinkage and thermal strain. In this paper, early-age properties of a concrete mixture (with water-to-binder ratio of 0.25) with and without SAP were examined. First, a practical and convenient method for determining time-zero of concrete with and without SAP based on the rate of temperature change induced by cement hydration was proposed. The validity of the proposed method was demonstrated through the good agreement between its determined time-zero values and the corresponding time (i) of divergence between chemical and autogenous shrinkage as well as (ii) when stress due to restrained deformation started to develop. Second, autogenous shrinkage of concrete with different SAP amounts was studied, together with the temperature dependency of autogenous and self-desiccation shrinkage of concrete containing 0.33% SAP. The addition of 0.33% SAP was found to have completely mitigated autogenous shrinkage of early-age concrete. Third, the coefficient of thermal expansion (CTE) of concrete with and without SAP was reliably measured. It is shown that both internal curing (using SAP) and external curing can effectively reduce the CTE, thereby reducing the associated thermal stresses and the risk of thermal cracking. In addition, the inclusion of SAP was found to compromise mechanical properties of concrete during the first 7 days, but such adverse effects became rather small at later ages.