Effect of sand grain size and fibre size on macro–micro interfacial bond behaviour of steel fibres and UHPC mortars

In this paper, the macro–micro interfacial bond behaviour of steel fibres embedded in ultra-high-performance concrete (UHPC) matrix was investigated using a single-fibre pull-out test with the parameters of fibre size, maximum sand grain size and fibre type. Test results showed that the maximum sand grain size (1·18 and 2·36 mm) had a slight effect on the pull-out responses while increasing the maximum sand grain size tended to change the microscopic interfacial failure mode according to observations made using scanning electron microscopy (SEM). Increasing the fibre size led to an increase in the maximum pull-out load, average bond strength and pull-out work but a decrease in the equivalent bond strength. The percentage of end hook contribution to the total pull-out energy ranged from 54·0 to 60·3%, indicating a significant mechanical anchorage contribution. A new evaluation index on the fibre–matrix interfacial bond behaviour was firstly proposed using the concept of the contribution of unit volume from the perspective of structural demands. It was found that, according to the proposed index, hooked-end fibres with a smaller diameter would be a better choice for structural applications.