Compressive strength of concrete after early loading

In modern construction projects it is necessary to work with concrete at a time before its strength can be fully developed. Applying large construction loads to a structure can lead to a reduction in strength if the serviceability limit has been exceeded. However, the effects of smaller compressive loading on concrete cubes, below the serviceability limit state, have been found to have a positive impact on the 28 day ultimate strength. The results from this study indicate the 28 day strength of wet cured concrete cubes increased on average by 6% when specimens were loaded up to 90% of their ultimate strength at 1, 3 or 7 days after casting. Concrete specimens under the same conditions loaded past the point of maximum stress at an early age displayed a reduction in strength from 5% or greater, depending upon the extent of the loading. This phenomenon of increased strength after loading and subsequent curing has been reported in the literature for many years, but the use of modern compressive test apparatus has enabled the present authors to show that the final strength has a high level of correlation with the displacement during the initial early loading. The experiments carried out in the present study were to simulate high construction loading at early ages to better understand the effects of early loading and the changes in ultimate strength at later age.