Some state highway agencies have implemented high-performance concrete (HPC) for bridge decks for durability rather than strength. While the low permeability of HPC is used to protect reinforcing steel and prevent corrosion, any concrete cracks can diminish the intended protection. One unintended consequence of HPC can be early-age cracking. This paper explores two ways to prevent bridge deck cracking - internal curing, and paste reduction by using an aggregate blend with a larger maximum size of aggregate. Results also apply to other classes of concrete used in transportation and other infrastructure. Mixtures were tested with large and small coarse aggregate. Fine lightweight aggregate (LWA) was added to some of the mixtures to reduce cracking tendency. The ring test, modifi ed from ASTM C1581, was used to determine the time of cracking of a concrete specimen due to drying and autogenous shrinkage against the restraint of the steel ring. Tests were carried out until cracking or for a maximum of 90 days. The strongest effect on cracking was due to the replacement of a small maximum size coarse aggregate (No. 8) with an aggregate blend of No. 8 and No. 57. Increasing the coarse aggregate absorption level from low to medium had a less dramatic effect, as did the introduction of LWA for internal curing to the low absorption coarse aggregate.

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.

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 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.