Internal Curing of Concrete Quiz

Hint

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.

Hint

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.

Hint

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.

Hint

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.

Hint

Internally cured concrete has mostly been used in bridge deck applications to date; however, researchers have investigated extending the use of internal curing to pavements. The following sections describe recent activity in continuously reinforced concrete pavements, jointed plain concrete pavements, concrete patches, and concrete overlays.

Hint

Internal curing in jointed plain concrete pavements appears to have positive effects on issues known to influence structural longevity and durability. Small reductions in unit weight, elastic modulus, and coefficient of expansion together with a small increase in strength promote improved structural integrity. The combined effect of these small improvements results in a significant positive impact on slab fatigue damage and associated slab cracking in jointed concrete pavements. The extended moisture supply provided by internal curing improves concrete pavement durability by reducing net moisture loss and improving hydration.

Hint

While the vast majority of field trials have focused on the use of internal curing for bridge decks, field trials performed in Indiana in 2014 used internal curing with expanded slag aggregate in high early-strength, full-depth concrete pavement patches (Figure 3). The application of internal curing in the high early-strength patches provided a concrete with two distinct benefits when compared with conventional concrete: 1) reduced built-in stress caused by the restraint of shrinkage, which resulted in reduced cracking, and 2) increased water curing (from inside the concrete) after the patches are covered with curing compound and opened to traffic.

Hint

Except for the lightweight aggregate, internally cured concrete mixture design generally is identical to that of conventional concrete with similar air content, water content, and coarse aggregate content. Currently, internal curing in North America is typically achieved by replacing a portion of the conventional fine aggregate (i.e., sand) with a pre-wetted lightweight fine aggregate. This practice occurs mainly due to lightweight aggregate availability, economics, and the fact that the fine lightweight aggregates already appear on the approved materials lists of many state highway agencies. Fine lightweight aggregate is generally preferable to coarse lightweight aggregate because:

Hint

• The fine lightweight aggregate can provide sufficient distribution of the curing water across the cross section to enable the internal curing water to reach the matrix.
• The use of conventional coarse aggregate provides conventional stiffness and fracture properties for the concrete.
• The fine lightweight aggregate is generally less expensive than the coarse lightweight aggregate.
• The fine lightweight aggregate can be evaluated using ASTM C1761, although expanded clay, shale, slate, and slag have all been used effectively for internal curing.

Hint

The two key benefits determined for internally cured concrete in concrete pavement are structural longevity and durability. Structural longevity is improved with internally cured concrete due to its small reduction in unit weight, elastic modulus, and coefficient of expansion, and a small increase in strength. These small effects, when combined, amount to a significant positive impact on slab fatigue damage and associated slab cracking in jointed concrete pavements. Likewise, internally cured concrete leads to tighter crack openings and reduced punchout failures for CRCP. Several case studies were analyzed using the Pavement ME Design procedure, and the results indicated improved performance and longer lives for internally cured concrete sections. Life-cycle cost analyses for these projects showed generally lower costs for internally cured concrete as compared to conventional concrete.