Total and external chemical shrinkage of low w/c ratio cement pastes

Abstract

Practical use of high performance concrete (w/c ≤ 0·40) has shown that it is sensitive to cracking at early ages. Based on a belief that early volume contraction due to chemical shrinkage is a driving force for cracking, experiments to determine total chemical shrinkage and external chemical shrinkage have been carried out on cement pastes in the w/c range 0·30-0·50 over a 48 h period, using two types of cements. Measurements of true external chemical shrinkage are very difficult to carry out because of the disturbance of bleeding. To avoid this, the samples were placed in very thin walled, elastic rubber containers and rotated continuously under water. The results show that the total chemical shrinkage was largely independent of w/c for the first 48 h, but occurred with a faster rate at lower w/c during the first 12 h. The external chemical shrinkage was equal to the total chemical shrinkage until it ‘flattened out’, demonstrating that a semi-rigid skeleton was established which led to empty contracting pores during further hydration (i.e. shrinkage). The ‘flattening out level’ (vol %) increased with decreasing w/c. Furthermore, the transition from fluid paste to semi-rigid skeleton was more gradual at lower water-to-cement ratios. It is proposed that both these effects of external chemical shrinkage indicate that high performance concrete is more sensitive to cracking at early ages under conditions of some degree of external restraint.