Introduction

Internal curing provides a modern twist on good curing practice by providing water to the cementitious matrix after setting. Internal curing improves the performance of concrete by increasing the reaction of the cementitious materials. However, unlike conventional curing that supplies water from the surface of concrete, internal curing provides curing water from the aggregates within the concrete (Figure 1). This is very beneficial since the depth that external water can penetrate is limited for any concrete, while internal curing water is dispersed throughout the depth of the concrete. In North America, this water-filled inclusion is typically an expanded lightweight aggregate, although superabsorbent polymers, cellulose fibers, or recycled concrete have been used.

The water that is absorbed in the lightweight aggregate does not contribute to the classic definition of the water-to-cement ratio. The water-to-cement ratio is a descriptor of structure of the matrix and pores that develop in the fluid concrete system. Once the concrete sets, the structure and pore network have been established, and water can only aid in hydration. The water in the lightweight aggregate will desorb (leave) the pores of the lightweight aggregate as the negative pressure in the pore fluid develops with setting and increases thereafter.

Internal curing also can reduce autogenous shrinkage, since water from the lightweight aggregate will work to fill pores that otherwise can lead to autogenous shrinkage. The water from the pores may eventually be lost to the environment; however, there are benefits in reducing the rate of shrinkage. Internal curing also can reduce transport properties (permeability, diffusion, and sorption) through increased hydration of the interfacial transition zone around the lightweight aggregate. Internal curing is especially beneficial for mixtures containing high volumes of supplementary cementitious materials that may require longer times to hydrate. Internal curing can also make concrete less susceptible to thermal cracking, as the “built-in” stress caused by autogenous shrinkage is substantially reduced. Many additional benefits are being investigated, such as benefits in terms of reduced curling.