While typically used to reduce early-age autogenous shrinkage and cracking, internal curing will also strongly influence the microstructure that is produced in cement-based materials. In this paper, the microstructure of a set of three different blended cement high-performance mortars produced with and without internal curing will be compared. For these mortars with a watercementitious material ratio of 0.3 by mass, internal curing has been provided by the addition of pre-wetted lightweight fine aggregates. Their microstructures have been examined after 120 days of sealed curing using scanning electron microscopy of polished surfaces in the back-scattered electron imaging mode. Clear distinctions between the microstructures produced with and without internal curing are noted, including differences in the unreacted cementitious content, the porosity, and the microstructure of the interfacial transition zones between sand grains (normal and lightweight) and the hydrated cement paste. These microstructural observations will be related to previously measured performance attributes such as autogenous deformation and compressive strength development.

This research explores the influence of internal curing on time dependent strains under no water exchange with the environment, i.e., basic creep and autogenous shrinkage. The behavior of high-performance concrete mixtures (HPC) containing pre-wetted lightweight aggregate (HPLC-1) for internal curing was compared to companion mixtures containing air-dried lightweight aggregate (HPLC-2). It was found that internally stored water reduced basic creep in direct relation to the amount of water held in the aggregate. Reductions in basic creep, up to 49%, were found between dry and prewetted lightweight aggregate mixtures. Further, internally stored water reduced autogenous shrinkage to the extent that actual expansion occurred proportional to the amount of internal water.

The effects of internal curing, type of blended cement and coarse aggregate size on earlyage expansion, autogenous shrinkage, and strength of high-performance concrete were investigated. To do so, 12 high-performance concrete mixtures were developed and tested under sealed and room temperature conditions. The results were statistically analyzed using the paired comparison design method. It was shown that internal curing of HPC with presaturated porous lightweight aggregate allowed signifi cant autogenous expansion and resulted in considerable reduction in net autogenous shrinkage. The type of cement used in concrete, which was either ordinary portland cement, silica fume blended cement, or slag/silica fume blended cement, had a strong effect on early-age expansion, autogenous shrinkage, and the effectiveness of internal curing. For instance, the concrete specimens made with silica fume blended cement, which yielded the largest autogenous shrinkage strains under sealed conditions, obtained the best reductions in autogenous shrinkage when tested under an internal curing condition.

The partial substitution of natural sand by lightweight sand has been used to reduce autogenous shrinkage in concretes with a low water/binder ratio, but when this substitution is combined with quasi-adiabatic curing conditions during the first 24 hours, it has been found that autogenous shrinkage can be mitigated and controlled. During an experiment done at Sherbrooke University on large concrete blocks measuring 0.6 × 0.6 × 0.6 m (2 × 2 × 2 ft) where 28% by volume of the natural sand in the concrete was replaced by the same volume of saturated lightweight sand, with absorption of about 20%, it was found that autogenous shrinkage was mitigated within the concrete blocks. Moreover, it has been found that the compressive strength and the elastic modulus of the substituted concrete were not affected by this substitution. For the first time in large concrete specimens, it can be reported that autogenous shrinkage can be mitigated and controlled without the help of any chemical product added to the concrete to induce an initial expansion to neutralize autogenous shrinkage. It seems that quasi-adiabatic conditions favor the development of large crystals that result in swelling of the apparent volume of the concrete block, and that the temperature increase also contributes to reduce chemical shrinkage. This could explain why Le Chatelier found more than 100 years ago that when a paste was cured under water, after a certain time, it swells enough to break the vase in which it had been placed.

The construction of a five-mile section of State Highway (SH) 121 north of Dallas is currently underway by the Texas Department of Transportation (TxDOT) and involves the conversion of a nonfreeway into a freeway facility. This technical paper presents the field experience for mixing, placing, finishing, and testing of slipform mainline concrete paving, utilizing rotary kiln expanded lightweight aggregate as both an intermediate gradation and as a reservoir to provide water to enhance the cement hydration of a typical paving mix. There is abundant laboratory research on internal curing of concrete including but not limited to numerous studies conducted at the National Institute of Standards and Technology.1 Construction of SH 121 represents the next logical step, following research, taking the laboratory to the field in the form of this major highway project. TxDOT anticipates higher strengths, leading to reduced paste content, reduced drying shrinkage cracking, and possibly less susceptibility to freeze thaw damage.