Physical evidence of swelling as the cause of anomalous capillary water uptake by cementitious materials

Water uptake by capillarity is widely used for characterising the durability properties of building materials. However, cementitious materials are generally reported with an anomalous behaviour in relation to other construction materials: during capillary water uptake they show a non-linear evolution with the square root of time. Several hypotheses have been proposed to explain this anomaly, among which the most sound seems to be the hygroscopic nature of cementitious materials, since the swelling of the calcium silicate hydrates when in contact with water could limit the water flow. Using strain gauges attached to mortar and concrete samples, we monitored their deformations during capillary water uptake. This paper presents those results and their connection with water ingress. Experimental data registered during water uptake allow validating the swelling hypothesis. The idea of cementitious materials as rigid materials during capillary water uptake seems incomplete for a comprehensive description of the transport process.