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.

An ultrasonic method for continuously assessing changes in the shear modulus of hydrating cementitious materials after casting, through setting and early strength gain, is presented. In the test method, reflected shear waves from the interface between the material of the form and the cementitious material are monitored. The test procedure for obtaining the ultrasonic test data and the inversion subroutines for assessing the shear modulus of the cementitious material at different stages of hydration are described. Results from a test program showing the response of ultrasonic signals of 1 MHz frequency reflected from the interface between the form and mortar are presented. A form made of polymethyl-methacrylate (PMMA), was used in the study. The observed experimental trends are explained considering reflection at the interface between two visco-elastic materials. It is shown that shear modulus can be determined immediately after casting and the increase in shear modulus can be sensitively monitored through setting and early strength gain. The shear modulus assessed at 1 MHz exhibits a five orders of magnitude increase in the first 24 hours after casting. The rate of increase in the shear modulus is the most rapid before initial set. The rate of modulus decreases steadily through final set and early strength gain. It is shown that there is a complete phase reversal in the reflected waves with time. The reversal corresponds in time with the final setting time determined using ASTM C 403 and it occurs when the shear modulus of the mortar is almost equal to the shear modulus of PMMA.

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.

Assessing the fl uid-to-solid transition in cementitious systems at early-ages is crucial for scheduling construction operations, for determining when laboratory testing can begin, and for assessing when computer simulations of restrained stress development should be initiated. This transition has been traditionally assessed using mechanical penetration techniques (e.g., Vicattest), which, though easy to perform, do not directly relate to the evolution of fundamental material properties or the microstructure. This paper assesses the fl uid-to-solid transition of a cementitious material at early ages using measures that relate to the formation of a solid-skeleton in the material. The increase in the ultrasonic wave velocity is correlated to the percolation of a solid structure that occurs during the fl uid-to-solid transition. Results of computer modeling (using CEMHYD3D) indicate that solidifi cation as determined from the percolation of the solids is similar to experimental observations (Vicat test). It is noted that the rate of change in the pulse velocity is not a rigorousmethod for assessment of the time of solidifi cation, especially in systems containing air. Rather, an increase in the pulse velocity beyond a threshold value appears to be a more appropriate method to assess structure formation. Further, the isothermal calorimetry (heat release) response is observed to not correspond to a fundamental aspect related to solid percolation or structure formation in the material.