MICROSTRUCTURE OF CONCRETE AND ITS INFLUENCE ON THE MECHANICAL PROPERTIES (INTERFACE)

This report presents the results of experimental and theoretical studies on the microstructure of concrete and its influence on the mechanical properties.;The hydration of the cement paste is studied using frozen hydrated scanning electron microscopy. This technique allows the use of bulk specimens at very early hydration; results for cement pastes hydrated for 1-hour are presented.;The mechanical properties of the transition zone were analyzed by microhardness measurements of the interface and by splitting tension tests. Using a probabilistic treatment an analysis of the aggregate-cement paste bond strength was carried out.;A generalized continuum for concrete which includes the porosity of the matrix and the composite nature of concrete is developed. General constitutive postulates are discussed, and the line theory is analyzed as a particular case.;The microstructure of the transition zone between cement paste and aggregate was studied in detail. Over 250 specimens were used to investigate the characteristics of the transition zone as influenced by aggregate type, cement type, amount of pozzolan (silica fume), and age of hydration. The main findings are as follows: (a) the mechanism of cement hydration is a through solution process; (b) reduction in the calcium hydroxide at the interface increased the strength of concrete. This was confirmed from studies on a Type K cement and an alite cement. In the former case the hydration process prevented the formation of a continuous film of calcium hydroxide at the interface whereas with the latter case a reaction between calcite and calcium hydroxide formed a basic calcium carbonate hydrate.;The microstructure of the steel-cement paste transition zone as affected by age and level of silica fume additions was investigated. Additions of silica fume caused a remarkable densification of the transition zone. Pull-out tests using 16 concrete mix designs and two types of reinforcing steel bars were performed to study the influence of the existing microstructure on the mechanical properties. For the same compressive strength, concrete with silica fume had a higher bond strength, which shows that the densification of the transition zone has a major influence on the mechanical properties.