Recycled concrete at early age

Abstract : This chapter presents the methodology and results of a comprehensive experimental study about early age behavior of concrete with recycled concrete aggregates (RCA). This study includes the determination of RCA total absorption and absorption rate, the behavior of fresh concrete under standard and severe drying conditions, and the assessment of properties of hardening concrete. The RCA used in this study (provided by RECYBÉTON Project) showed high absorption rate. They reached 90% of the nominal absorption within the fjrst 2 h, and their absorption was even faster in fresh cement paste. The monitoring of RCA water saturation in cement paste also showed a release of absorbed water when exposed to severe drying. When shifting from standard to severe drying, the increase of evaporation rate did not result in increased shrinkage to mass-loss ratios. However higher shrinkage magnitudes were reached because plastic shrinkage development was accelerated in severe drying conditions thus it could develop before setting. The air entry pressure concept was used to analyze the cracking sensitivity of fresh concrete. It was assessed on various concrete mixtures with and without RCA. A strong correlation was found between air entry pressure and negative capillary pressure at cracking time. The Free water to Binder (W f /B) ratio was found to influence directly the air entry pressure. The (W f /B) ratio accounts for the effects of initial water saturation of RCA. The evolution of plastic shrinkage, Young’s modulus, and tensile strength were continuously monitored from fresh state to 24 h. The initial water saturation of RCA had limited influence on these properties. Restrained shrinkage tests would be necessary to assess the influence of RCA proportion on cracking sensitivity and estimate relaxation.