Controlling capillary pressure in concrete to prevent plastic shrinkage cracking

Abstract

Plastic shrinkage cracking occurs in concrete members with large exposed surface areas, such as concrete pavements, bridge decks and floor slabs. Current research has shown that a notable increase in plastic shrinkage was observed in the engineering practice, leading to considerably more cracking problems than in the past. Plastic shrinkage cracking occurs when the plastic shrinkage of fresh concrete is restrained, leading to tensile stresses, which result in cracks when the tensile strength is exceeded. Plastic shrinkage is caused by negative capillary pressure build-up in fresh concrete. Controlling the negative capillary pressure build-up makes it possible to reduce the risk of plastic shrinkage cracking. The aim of this study was to develop a model that uses live in-situ capillary pressure measurements in fresh concrete to control the capillary pressure build-up to prevent plastic shrinkage cracking at any evaporation rate. A model was developed that calculates a critical pressure limit for when an action is needed to prevent plastic shrinkage cracking. The model uses the negative capillary pressure build-up area between two crucial time points in plastic shrinkage to determine this limit. The proposed model was tested and verified in two phases. The testing and verification of the model were conducted on a low bleed concrete having a water/cement ratio of 0.5 and a self-compacting concrete with a water/cement ratio of 0.4. Tensiometers were used to measure the capillary pressure build-up in concrete. The first phase consisted of determining the parameters required for the model. The second phase used the determined parameters to test the model with the two concrete mixtures at various evaporation rates. The results showed that the model could determine a critical pressure limit relevant to the concrete and evaporation rate. The proposed model proved to be a valuable tool in controlling the capillary pressure and preventing plastic shrinkage cracking in low bleed and self-compacting concrete.