Design of polymeric capsules for autonomous healing of cracks in cementitious materials

Abstract

Now, most of the capsules used to contain polymeric healing agents in self-healing concrete, are made of glass. However, glass capsules cannot be mixed in concrete and are therefore placed manually into the moulds during concrete casting in laboratory tests. This represents a major drawback for an eventual industrialisation. In this study, polymeric capsules were designed to meet three requirements: breakage upon crack appearance, compatibility with the polymeric healing agent and survival during concrete mixing. Three different polymers with a low glass transition temperature (Tg) were selected (PLA – PS – P(MMA-n-BMA)). These polymers are brittle at 20°C, and consequently have the possibility to break upon crack appearance, but are rubbery above their glass transition temperature and, consequently, can survive mixing upon heating. Differential Scanning Calorimetry and Dynamic Mechanical Analysis were performed to define the glass transition temperature of the selected polymers and to quantify the evolution of their mechanical properties with increasing temperature. Concrete mixing tests were performed both at 20°C and at a temperature above the Tg of the capsules. Mixing at increased temperature was done by previously heating the capsules and the concrete components. The survival rates increased drastically when the capsules and the concrete components were heated. Even capsules with a thin wall (thickness 0.4 mm) resisted a 2 minute concrete mixing process, whereas none of them survived at 20°C. In addition, the compatibility of the capsules with a two-component polyurethane healing agent was studied. The pre-polymer hardened after some days. This research revealed that suitable design of polymeric capsules can help to meet the requirements for self-healing concrete even though further research is needed before a possible use in industry.