2001

Holt, Erika

Volume change of concrete resulting from structural and environmental factors is an acceptable phenomenon. In the majority of cases this volume change, or shrinkage, is assumed to begin at the time of loading or drying. In reality, a volume change commences immediately after the cement and water come in contact during concrete mixing. These early age volume changes are typically ignored in design of concrete structures since their magnitude can be much less than shrinkage resulting from drying. But even when the concrete curing conditions are ideal, the first day shrinkage can significantly contribute to the ultimate shrinkage and thus the cracking risk. The goal of this work was to establish a clearer understanding of the mechanisms causing autogenous shrinkage of concrete during the early ages. Autogenous shrinkage is a volume change resulting when there is no moisture transfer to the surrounding environment. It is most prominent in high strength, or high performance concrete where the water-to-cement ratio is under approximately 0.42. Autogenous shrinkage at later ages has been well documented and explained by self-desiccation behavior but the presence of autogenous shrinkage during the first day of concrete hardening has not been theoretically explained. This work aimed at developing a test method to assess the shrinkage occurring immediately after mixing the concrete and continuing for the first 24 hours. Once a test method was established it was possible to investigate the chemical and physical phenomena causing the autogenous shrinkage in the first hours for neat paste, mortar and concrete. The shrinkage was well correlated to the cements' chemistry and the development of internal capillary pressure within the cement paste. Material parameters influencing the magnitude of the early age autogenous shrinkage were studied, such as the use of superplasticizer and different cement types. The three factors most significantly contributing to the autogenous shrinkage were identified as the concrete's chemical shrinkage, amount of bleeding, and time of hardening. Finally, a generalized model was proposed for reducing early age autogenous shrinkage in future practice.