SP-256CD In the absence of adequate curing, early-age self-desiccation and consequent autogenous shrinkage may be problematic, particularly in concretes with a low water-to-cementitious material ratio. In 2003, a Federal Highway Administration survey regarding the most common distresses in high-performance concrete estimated that up to 60% of bridge decks have experienced early-age cracking, most likely due to autogenous shrinkage. Internal curing has been proposed as a potential technique to mitigate autogenous shrinkage and earlyage cracking. Internal curing is accomplished by the incorporation of water-absorptive materials in low permeability (that is, high performance) concretes, where external curing may not be sufficient to maintain saturation of the concrete member. Within the past decade, internal curing techniques have begun to move from laboratory research to field applications, with tremendous success. The papers contained in this publication were presented at the Fall 2007 American Concrete Institute Convention in Fajardo, Puerto Rico. The two half-day technical sessions brought together engineers and material scientists from around the world to discuss laboratory research, case studies, and practical applications related to internal curing of high-performance concretes. This publication, co-sponsored by ACI Committees 236, Material Science of Concrete, and 231, Properties of Concrete at Early Ages, offers a unique state-of-the-art perspective regarding this evolving topic.

The purpose of this study was to quantify the degree to which moisture-bearing lightweight aggregate can contribute to cement hydration in low, intermediate and high water/cement ratio "normal" density concrete. This was accomplished using chemical testing methods (x-ray fluorescence and thermal gravimetric analysis) supplemented by physical strength testing. To study the influence of lightweight aggregate as a function of paste density water/cement ratio), three cement contents were chosen to represent low, intermediate, and high water/cement ratio concrete. The three water/cement ratios examined were 0.37, 0.47, and 0.57. Each series consisted of a control mixture containing no lightweight aggregate, and a test mixture containing 6 ft3 of presoaked 3/8 in. to No. 8 intermediate lightweight aggregate used in substitution of a portion of the coarse and fine aggregate. The results of this study provide quantitative validation to the theory that the addition of an effectively preconditioned lightweight aggregate will provide moisture for cement hydration in the concrete. The significance of the findings present an improvement of the performance characteristics of concrete by providing additional internal moisture minimizing the effect of self-desiccation by maintaining a fairly high degree of saturation during the critical strength gain period.

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.

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 benefits of using lightweight aggregate (LWA) to replace a portion of the normalweight aggregates in concrete mixtures have been investigated by many researchers. The main purpose of this substitution has been to provide a source of moisture for internal curing that will promote more complete hydration of the cementitious materials. The adequate initial moisture conditioning of the LWA is the most crucial step in the ready mixed concrete production cycle. Once the LWA has been satisfactorily saturated, the potential for field problems is insignificant. Any shortcuts in this fundamental procedure can result in the failure of the concept and reluctance on the part of the concrete contractor to adapt this technology. The problems can range from yield issues to slump loss, segregation, finishability, and pumpability. The slow release of moisture from the lightweight aggregate to the concrete matrix has resulted in the mitigation or elimination of plastic and drying shrinkage cracking, as well as limiting the effects of self-desiccation. Enhanced workability and better consolidation due to an improved total grading provided by the use of an intermediate aggregate is also evident; contractors have reported that it reduces the total placing time.