ABSTRACT

Abstract High levels of autogenous shrinkage have important implications for both cast in situ and precast concrete construction. Some of the practical issues that arise from autogenous shrinkage include dimensional incompatibility between concrete elements and cracking that is partially or wholly induced by autogenous movements. In all real structural elements, even of moderate thicknesses, the early age behaviour is characterised by a combination of moisture-and temperature-related curing effects. In this paper, results obtained from unrestrained autogenous shrinkage measurements on a range of high strength concretes with and without silica fume will be described. Measurements were made under standard conditions and also under conditions of moderate temperature development to simulate concrete elements of medium thickness. Significant unrestrained autogenous shrinkage strains in excess of 600 jjs were measured within 1 day of casting. The use of 10% silica fume in a Portland cement binder is shown to significantly increase the autogenous shrinkage when compared to a plain Portland cement concrete. However, the thermal expansion that accompanies even a moderate net temperature rise of 15°C is shown to substantially reduce the net autogenous shrinkage in some concretes. A discussion on current methods of evaluating shrinkage in high performance concretes is provided in the context of the results obtained. Keywords: Autogenous shrinkage, low water/binder, silica fume, temperature, expansion.

1 Introduction