Toughening of cement and other brittle solids with fibres

Abstract
In common with other brittle solids, cements are toughened much more by the incorporation of fibres than by inclusions of other geometries. The largest energies required to break a specimen are found when multiple fracture of the specimen occurs before final failure. Theoretical models of a crack moving normal to a set of parallel fibres will be considered, to show that the crack spacing and first cracking strain should depend on the area of fibre-matrix interface per unit volume of composite. The first cracking strain is shown to increase for all fibre volumes provided that the fibre spacing is less than the critical flaw size according to the Griffith’s equation. The theoretical models are compared with experiment and the practical difficulties of defining first cracking strain and interfacial area mentioned. The best practical means of assessing the resistance to failure of the composite is the work done per unit volume of the specimen in separating it into two distinct pieces. The maximum values of toughness attainable - some 10 6 J m -3 - can decrease with time under external weathering, owing to continuing hydration of the matrix and consequent increase in the critical volume fraction of fibres.

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