Quantitative microstructural characterisation of concrete cured under realistic temperature conditions

The curing temperature is known to influence the strength and durability of concrete. In general, curing at elevated temperature results in high early strength gain, but impairs the long-term strength and transport properties. Previous investigations have not provided a complete understanding of these effects; in particular, no quantitative relation between microstructural and macroscopic changes has been developed. This work has adopted a combination of macro and micro characterisation to further a comprehensive and quantitative understanding of the property development under different curing conditions, through approaches which include both qualitative and quantitative microstructural analysis with SEM, EDS, XRD, NMR and TG. A systematic study on several mix designs and curing regimes has been carried out, to relate microstructural properties such as degree of hydration, C-S-H composition, C-S-H relative density and capillary porosity to compressive strength and water sorptivity. The results show that the Ca/(Si+Al) ratio of C-S-H is constant with temperature, and the increase in relative C-S-H brightness is thus attributed to the higher C-S-H density and lower chemically bound water at higher temperatures. The early hydration of concretes is accelerated by high temperatures, but the ultimate value is not limited, as opposed to widely reported lower values in the literature. This discrepancy is explained by the fact that previous workers have assumed that the bound water is constant for different curing temperatures whereas this is not the case. Linear relationships were established between the degree of hydration and the compressive strength of concretes, which depend on curing temperatures and mix design. Through their influence on the density and distribution of hydration products, elevated temperatures increase the ultimate capillary porosity of concretes. The relationship between capillary porosity and compressive strength seems independent of temperature to a first approximation, but no unique relationship is shown between sorptivity and compressive strength. Materials which were exposed to higher temperatures for only short periods of time, such as might be experienced in prefabrication or in the curing of large masses, showed some recovery in both microstructure and mechanical properties from the detrimental effects of high temperature.