Autogenous Deformation and Internal Curing of Concrete

Abstract

High-performance concrete (HPC) is generally characterized by a low water/binder ratio and by silica-fume addition, which guarantee a low porosity and a discontinuous capillary pore structure of the cement paste. Modern concretes possess some highly advantageous properties compared to traditional concrete, such as good workability in the fresh state, high strength, and low permeability. However, they have also shown to be more sensitive to early-age cracking than traditional concrete. Early-age cracking mainly occurs due to the fact that the deformations of the concrete member are restrained by adjoining structures. In addition, internal microcracking may occur, due to restraint offered to the shrinking paste by the non-shrinking aggregates. A main source of early-age deformations in HPC is autogenous deformation. Autogenous deformation is the self-created deformation of a cement paste, mortar or concrete during hardening. In traditional concretes it is negligible compared to drying shrinkage. However, the low water/binder ratio and the addition of silica fume in HPC cause a significant drop of the internal relative humidity (RH) in the cement paste during sealed hydration and the occurrence of autogenous shrinkage. Despite the growing interest in autogenous shrinkage, no consensus has been reached in the scientific community about its mechanisms neither about measuring methods. Moreover, different strategies aimed at limiting the autogenous shrinkage are debated at the moment. In this thesis, autogenous deformation of cement pastes, Normal Weight Concrete (NWC), and Lightweight Aggregate Concrete (LWAC) were measured. Both Portland and Blast Furnace Slag (BFS) cement were studied. A model for calculating self-desiccation shrinkage of cement paste was proposed and validated with experiments. Shrinkage of NWC was derived with a composite model and early-age expansion of LWAC, a puzzling phenomenon up to now, was explained. Finally, transport of water from saturated lightweight aggregates (LWA) to hardening cement paste was measured with x-ray absorption.