Enhancing the mechanical behaviour of cement based materials

A mechanism for the toughening of ceramic materials has been successfully developed in the refractories industry using residual stresses at the microstructural scale. This paper addresses the application of this approach to cement-based systems. In the cement case, the growth of expansive phases has been controlled, along with the set characteristics of the paste, to establish internal stresses which, while significant, have not been sufficient to cause expansive failure. Macro-crack propagation, initiated by external loading, has been suppressed either by the release of residual stresses which act to close the crack or by grain bridging. Because such stresses are dispersed throughout the matrix, this toughening mechanism would add to the value of steel reinforcement in the strengthening of concrete structures. This paper describes some early work concerning the factors which control internal stresses in cement microstructures and the experimentally measured stress–strain and fracture behaviour of pastes and mortars which confirm the dramatic effect of microstructural engineering.