Internal Curing with Crushed Returned Concrete Aggregates

High performance concrete (HPC) requires a low water-to-cementitious materials mass ratio (w/cm), often with the inclusion of supplemental cementitious materials such as silica fume in the mixture, thus necessitating the use of a superplasticizer. Because of the low w/cm and rapid reaction at early ages, proper curing from the earliest time possible is very essential in HPC. Internal curing has been developed and demonstrated to substantially reduce autogenous shrinkage and minimize early-age cracking of HPC. In 1991, Philleo suggested this new concept of “water-entrained” concrete with the addition of saturated lightweight fine aggregates (LWAS) as a remedy to provide an internal source of water to offset the chemical shrinkage that occurs during hydration of the paste. This article explores the use of less expensive crushed returned concrete aggregate (CCA) as an internal curing material. In this investigation, CCAs in the 1000 psi (6.9 MPa), 3000 psi (20.7 MPa) and 5000 psi (34.5 MPa) strength range were prepared for evaluation as internal curing agents. The best performing CCA was then selected and further tested in combination with LWAS. Test results showed a significant reduction in net autogenous shrinkage (79 % and 70 % reduction of the control at the ages of 1 d and 56 d, respectively) found with the CCA/LWAS blended mixture, while the compressive strength reduction was negligible (6 % and 0 % reduction of the control at the ages of 28 d and 56 d, respectively).