Porosity generation arising from steel fibre in concrete

The present study concerns a measurement of porosity at the steel fibre interface in mortar. Mortar cast with a centrally located steel fibre (Ø0.39 × 15.0 mm) was segmented for microscopic observation at a backscattered electron (BSE) image. In the BSE image, the interfacial zone was defined as long as 270 μm from the surface of steel fibre, where the porosity was measured by counting pixels after binarising the image for porosity. As a result, it was found that the pores generated in the vicinity of the steel fibre ranged from 5.40% to 11.45%, equivalent to 0.51-1.08% of air void in a bulk concrete. The high porosity at the interface may impose a reduction of concrete strength (i.e. up to a debondment with cement paste) and a potential risk of further supply of aggressive ions through the pores, leading to a lower durability against chemically severe environments.