A stereological approach to roughness of fracture surfaces and tortuosity of transport paths in concrete

Relatively weak interfaces between aggregate grains and the cementitious matrix initiate the damage evolution process leading to fracture. Coalescence between nearby interface cracks is promoted by the small nearest neighbour distances in a dense random packing of the aggregate. The fracture surface is therefore modelled as a dividing plane from which particles protrude. Assuming spherical aggregate, roughness is obtained as the global geometrical-statistical expression for the increase in fracture surface area due to a multitude of dome-like caps of various sizes. Transport phenomena in concrete are equally influenced by the aggregate, because traversing molecules or ions have to go around the dense grains. This route is additionally promoted by the relatively high porosity in the ITZ. Premature 'interface' cracking further reinforces this phenomenon. The planar and linear concepts of tortuosity in the transport path are analogous to those of roughness. Hence, the same estimates as developed for roughness will hold for tortuosity. (C) 2000 Elsevier Science Ltd. All rights reserved.