Quantifying Volume Change, Stress Development, and Cracking Due to Self-Desiccation

This paper describes recent experimental efforts to quantify autogenous volume change, stress development, and cracking in low water-to-cement ratio (w/c) mixtures. A non-contact laser was used to measure length changes from the time that the mortar was placed in the forms. An analytical solution was used to illustrate how the restrained ring test could be used to quantify the level of residual stress that develops from the time of casting. Piezoelectric sensors were used to measure the acoustic activity that develops in the mortar. It is hypothesized that this acoustic activity is generated by microcracking at the aggregate interface caused by differential shrinkage. Length change measurements show an initial period of shrinkage before setting, a slight expansion during setting, and continued shrinkage after final set. The residual stresses were observed to develop several hours after the time of set, suggesting that some movement may be permitted while the structure of the paste is developing. Acoustic activity was observed to occur a few hours after residual stresses developed, suggesting that some critical strain level is needed for microcracking to occur.