Self-repairing performance in cementitious composites

Self healing cementitious composites can be regarded as a broad category of smart construction materials to which strong and highly qualified research efforts are currently being devoted worldwide, with the aim of providing a sound scientific background to their consistent, and – designwise – “consciously safe”, use in the engineering practice.

Though it is recognized that concrete and cement based materials do inherently possess an autogeneous capacity of self healing minor cracks and damages, mainly if they occur in their early ages, effects of self healing on the engineering properties are from the quantitative point of view, highly scattered and hence cannot be easily quantified to be relied upon, e.g., in a design perspective. Tailored additions aimed at making the aforementioned capacity less scattered and hence more reliable have been developped and widely employed, among which the so-called crystalline admixtures,play a prominent role. Crystalline admixtures consist of proprietary active chemicals provided in a carrier of cement and sand, which, because of their hydrophilic nature, react with water and cement particles in the concrete to form calcium silicate hydrates, increasing the density of the CSH phase, and/or pore-blocking precipitates in the existing microcracks and capillaries. The mechanism is analogous to the formation of CSH and the resulting crystalline deposits become integrally bound with the hydrated cement paste, thus contributing to a significantly increased resistance to water penetration under pressure. It can be furthermore argued that, as hairline cracks form over the life of concrete, crystalline admixtures continue to activate in the presence of moisture and seal additional gaps, even if cracks may still develop that exceed the self sealing capacity of the concrete.

Natural fibres, a waste product of food and agriculture industry to which a great potential of use as dispersed reinforcement in cementitious composites has been recognized, can also act as promoters and facilitators of self healing phenomena has been recently hypothesized and confirmed by preliminary investigations. As a matter of fact, thanks to their porous microstructure, natural fibres are able to create a porous network through which the moisture, as absorbed by the same fibres, can be fdistributed throughout the cementitious matrix and activate the delayed hydration reactions which, together with carbonation ones, can be responsible of the autogeneous healing of cracks and damages.

In this presentation the results will be summarized of the investigation currently carried out at Politecnico di Milano (Italy) also in cooperation with Indian Institute of Technology Madras, Chennai (India), Universitat Politecnica de Valencia (Spain) , Universidade Federal do Rio de Janeiro (Brasil) and Universidad de Buenos Aires (Argentina).

Normal Strength plain Concrete (NSC), conventional Fibre Reinforced Concrete (FRC), High Performance Fibre Reinforced Cementitious Composite (HPFRCC), with steel and natural fibres, have been considered and the self healing capacity has been investigated with reference to durability (permeability) and mechanical properties (strength, stiffness and, in case, deformation capacity), taking advantage of testing methdologies tailored to this purpose.

Suitable “healing indices” have been also defined and calculated from the experimental results to quantify, e.g. in a durability based design framework, the effect of self healing on the recovery of the aforementioned properties of the different tested materials, also as a function of the preexisting damage, exposure conditions and durations etc.