Plastic shrinkage cracking in cementitious repairs and overlays

A study was undertaken to investigate the influence of mixture proportions on plastic shrinkage cracking in cementitious repairs and overlays. The following variables were studied: water-cement ratio (w/c), sand-cement ratio (s/c) aggregate-cement ratio (a/c), fly-ash content, and the presence of a shrinkage reducing admixture (SRA). A recently developed bonded overlay method was used. In this method, the cementitious material to be investigated is cast on a substrate base with protuberances and the entire assembly is subjected to drying in an environmental chamber. With a high degree of restraint, cracking develops in the overlay over time which is then characterized with the help of magnification device and image analysis software. Results indicate that a lower water-cement ratio significantly reduces crack widths and crack areas and also reduces the ‘time to first crack’. The exact influence of s/c on cracking could not be established. The study further demonstrated that increasing the a/c provided internal restraint and significantly reduced early-age cracking. The addition of Class C fly-ash increased the amount of cracking but also increased the ‘time to first crack.’ This was however true only to a certain threshold value of fly-ash content beyond which fly-ash can be effective in reducing early age shrinkage cracking. Finally, the addition of a shrinkage reducing admixture was effective in reducing cracking but the cracks occurred sooner.