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The early-age strength development of concrete containing slag cement has been investigated to give guidance for its use in fast-track construction. Measurements of temperature rise under adiabatic conditions have shown that high levels of slag cement-for example, 70% of the total binder-are required to obtain a significant reduction in the peak temperature rise. Despite these temperature rises being lower than those for portland cement mixtures, however, the early-age strength under adiabatic conditions of slag cement concrete can be as high as 250% of the strength of companion cubes cured at 20 °C (68 °F). The maturity and, hence, strength development were calculated from the adiabatic temperature histories based on several maturity functions available in the literature. The predicted strength development with age was compared with the experimental results. Maturity junctions that take into account the lower ultimate strengths obtained at elevated curing temperatures were found to be better at predicting the strength development.

This paper highlights the beneficial effects of temperature on the strength development of slag concrete. The in situ temperature rise occurring inside thick structural elements has been shown to produce a significant improvement in early-age strength development compared to standard curing. The slag concrete in this study had 1-day strength 1.75 times higher than standard cured cube strength in temperature matched curing tests. For summer curing conditions, in the high strength concretes studied in this paper, the magnitude for this effect was sufficient for 35% slag concrete to have early-age strengths similar to those of portland cement concrete. In winter ambient conditions, the strength development of both concretes is slowed down and the effect is greater for slag concrete that for portland cement concrete.

A full-scale, seven-story, reinforced concrete building frame was constructed in-place at the Building Research Establishment's Cardington Laboratory, which encompassed a range of different concrete mixtures and advanced construction techniques. This provided an opportunity to assess in-place nondestructive test methods, namely the pullout test, and more specifically the Danish version, which has been known as the Lok test, on a systematic basis during the construction of the building. It was used in conjunction with both standard and temperature-matched cube specimens to assess its practicality and accuracy under site conditions. Strength correlations were determined using linear and power function regression analysis. Strength predictions from these were found to be in very good agreement with the compressive strengths of temperature-matched cube specimens. When a general correlation is used, however, estimates for compressive strength are likely to have 95% confidence limits of around ′20% of the mean value of four results.