A simple but challenging experiment was carried out to measure concrete temperature, air content, unit weight, slump, setting (penetration resistance), heat release, maturity, and compression strength over a 28-day period beginning with discharge from the chute of a concrete truck. It was thus demonstrated that concrete’s transition from liquid to solid is represented continuously by maturity and by heat release, but it is more commonly recorded in terms of three phases in concrete development: slump loss, setting, and strength gain. The paper describes how these phases overlap each other and are related to concrete temperature, heat release, and maturity.

The propensity for early-age shrinkage cracking in low w/c concretes has spawned the development of new technologies that can reduce the risk of cracking. One such technology is internal curing. Internal curing uses saturated lightweight aggregate to supply ‘curing water’ to low w/c paste as it hydrates. Significant research has been performed to determine the effects of internal curing on shrinkage and stress development in sealed samples. However, relatively little detailed information exist about how water is released from the lightweight aggregate to the surrounding cement paste. This study examines the timing of moisture release from saturated lightweight aggregate (LWA). Specifically this paper focuses on fluid transport around the time of set. X-ray absorption is used to trace the time at which water moves from the lightweight aggregate to the paste. X-ray observations are compared with results from the Vicat needle, autogenous shrinkage, and acoustic emission tests. These results are contextualized in terms of structure formation and vapor space cavitation in the cement paste.

SP-259CD The American Concrete Institute (ACI) Committee 231, Properties of Concrete at Early Ages, has sponsored a full-day technical session on the Transition from Fluid to Solid: Re-examining the Behavior of Concrete at Early Ages at the ACI Spring Convention, San Antonio, Texas, March 15-19, 2009. This special publication contains the twelve papers presented at this session. The subject matter of these papers includes: (1) the development of concrete properties and microstructure at early ages, (2) test methods for assessing early-age volume change and cracking potential, (3) construction operations timing, (4) computer simulations of early-age behavior, and (5) mechanisms that end the concrete dormant period.

Drying shrinkage cracking can occur in concrete due to volumetric changes caused by temperature and moisture gradients. The purpose of this study is to acquire fundamental data on the relationship between internal relative humidity and drying shrinkage cracking. The detection of drying shrinkage cracking was performed using the Acoustic Emission method. It was found that the AE measurement technique was successful at detecting drying shrinkage cracking.

Slag cement concrete is characterized by many advantages, which leads to its intensive use in the construction industry in Belgium. However, it may exhibit a high sensitivity to cracking at early age in case of restrained shrinkage. The understanding of this behavior involves an in-depth analysis of the early age deformations. Firstly, an experimental investigation of the mechanical properties (compression strength, elastic modulus) and the microstructure evolution (hydration kinetic and hydrates development) was performed on three concretes containing different slag proportions (0%, 42% and 71% of the mass of binder), but with identical total binder content, in order to understand the effect of slag on these parameters. Secondly, the autogenous deformations were measured from casting time on concrete cylinders under isothermal conditions. The apparent activation energy and the time of initial set were also evaluated in order to analyse these deformations. The apparent activation energy is used to convert the actual age into equivalent age to express the concrete properties independently of the temperature variations. The time of initial set from which the strains are expressed is determined by ultrasonic detection and by the Kelly-Bryant method.