Characterisation of differential shrinkage of bonded mortar overlays subjected to drying

Abstract

This work presents an experimental technique developed to assess sensitivity of cracking due to the property gradients in the layers of cement-based materials used as a bonded overlay. The non-uniform moisture distribution in materials causes differential length changes as a result of the combination of shrinkage, creep and strain relaxation effects. In this study, several tests are used for the purpose of predicting the differential behaviour (hydric, hydration, porosity and strain gradients) of material subjected to drying. Two formulations are investigated. Hydric and hydration gradients are measured by using the “slice test” and strain gradients are measured at various positions in the material layer using embedded strain gauges. The material is subjected to drying along a preferential direction into a controlled atmosphere (relative humidity = 65%; temperature = 20°C). Also, in order to study the influence of the overlay–substrate interface, specimens are cast directly on to either a non-adhesive substrate to simulate length changes for the free shrinkage case or an adhesive substrate to simulate length changes for the restrained shrinkage case. For both, the behaviour of the material differs significantly according to depth. So, the strain gradients and consequently the related gradients of mechanical properties are closely linked to hydric heterogeneities generated by non-uniform drying in the layer of material.