Restrained shrinkage behaviour of rapid hardening fibre reinforced concrete repairs

The functionality and durability of concrete overlays is compromised by delamination and large cracks that result from excessive shear and tensile stresses due to restrained shrinkage. Expansive cements could mitigate shrinkage problems, but as they are usually brittle, they still develop cracks under mechanical loads. Manufactured Steel fibres (MSF) can be used to control crack widths of repairs. However, to promote the sustainability of repairs, recycled fibres extracted from un-vulcanised rubber belt off-cuts can be used. They are also more cost effective than MSF. Currently, there is no accepted design approach to limit crack widths or to accurately quantify the effect of fibres on crack widths and crack spacings of overlays.
The aim of this study is to contribute to the understanding of flexural performance and restrained shrinkage and subsequent deterioration of plain and recycled fibre reinforced rapid hardening overlays, especially the fibre effect on crack widths of overlays, and to promote more sustainable, yet efficient solutions. A combination of experimental, analytical and numerical investigation is employed to study: a) the effect of recycled clean steel fibres (RCSF) on the compressive and flexural behaviour of rapid hardening mixes, b) the effect of RCSF on the crack development of overlays and shear stresses at the interface and c) the effect of non-uniform shrinkage distribution across the depth of overlays on the tensile stress development, and therefore, on the risk of cracking in overlays.
It was found that the RCSF are efficient in bridging cracks, resulting in flexural hardening properties. The RCSF reduce crack widths in overlays by about 60%. The available methods for predicting crack widths are found to be inaccurate. Therefore, a modified crack width equation is proposed and validated, and a new equation for estimating crack spacing is derived. The fibres are also found to positively contribute in reducing the risk of delamination. They are shown to enhance the shear strength and proven to reduce the shear stress development after crack development and reduce the level of deterioration of shear interface by controlling crack widths. The assumption of uniform shrinkage distribution in overlays underestimates the extent of hygral tensile stresses. An empirical equation to consider this effect is proposed.
This work is expected to enable better and more sustainable designs for overlay repairs and strengthening.