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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.

Ultrasonic wave reflection (UWR) has been used to monitor hydration and strength development of concrete. UWR measures the changes in reflected ultrasonic waves at the interface between a buffer material and hydrating cement paste. To monitor the subtle changes during early hydration it is necessary to use a buffer with low acoustic impedance, close to that of cement paste. In this research, UWR measurements on hydrating Type I portland cement are performed using a high impact polystyrene (HIPS) buffer. Both S-waves and P-waves are analyzed simultaneously to develop and extend the use of UWR to monitor early stiffening of cement paste. The penetration resistance test (ASTM C 403) and temperature rise of cement paste are used to correlate stiffening characteristics. The UWR responses show good correlation with results from temperature rise and penetration resistance. The onset of stiffening is the same for penetration resistance and both P- and S-wave UWR, and nearly the same for temperature rise. It is found that the HIPS buffer can provide sensitive measurement on the early age stiffening of cement paste.

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.

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.