Mixture proportioning for internal curing

The concept of internal concrete curing is steadily progressing from the laboratory to field practice. In terminology currently being considered by ACI Committee 308, Curing Concrete, “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water.” The additional internal water is typically supplied by using relatively small amounts of saturated, lightweight, fine aggregates (LWA) or superabsorbent polymer (SAP) particles in the concrete. Benefits of internal curing include increased hydration and strength development, reduced autogenous shrinkage and cracking, reduced permeability, and increased durability. The impact of internal curing begins immediately with the initial hydration of the cement, with benefits that are observed at ages as early as two days. Internal curing is especially beneficial in low watercement ratio (w/c) concretes because of the chemical shrinkage that accompanies portland cement hydration and the low permeability of the calcium-silicate hydrates. Because the water that is chemically bound and adsorbed by the cement hydration products has a specific volume less than that of bulk water, a hydrating cement paste will imbibe water (about 0.07 g water/g cement) from an available source. While in higher w/c concretes, this water can be, and often is, supplied by external (surface) curing, in low w/c concretes, the permeability of the concrete quickly becomes too low to allow the effective transfer of water from the external surface to the concrete interior. This is one justification for internal curing. Additional water that can be distributed somewhat uniformly throughout the concrete will be more readily able to reach unhydrated cement.