Investigation of the Impact of Concrete Surface Treatment Methods on the Interfacial Bond Strength

Paper in proceeding, 2023

Fibre-reinforced shotcrete (FRS) tunnel linings in hard rock are structurally intricate due to the complexity of rock mechanics and the interaction between shotcrete and rock. As the consequences of tunnel failure can be severe, maintaining the structural safety is vital in an operating tunnel. However, to satisfy safety demands, design- and maintenance measures can be conservative, as empirical methods and worst-case scenarios are commonly used in design and repair to account for the complexity of the system. A novel method for verification of tunnel linings, combining experiments, state-of-the-art distributed optical fibre sensing systems embedded in the shotcrete, advanced finite element modelling, and machine learning algorithms is in development. For the experiments, the characterisation of bond strength between rock and FRS are crucial to simulate trustworthy data. In this paper, a literature review and experiments are presented aiming to characterise a high and a low, non-zero, interfacial bond strength between two layers of concrete. The properties of the substrate surface in terms of roughness, microcracks, cleanliness and free water are investigated before casting. The experiments study four different surface treatments, including jackhammering, grinding, grinding and adding a layer of sand to the surface, and grinding and adding free water to the surface. The literature review showed hydrodemolition as a surface treatment method to have a consistently high bond strength, while the experiments showed a lower bond strength for ground surfaces than for jackhammered specimens. The inclusion of sand did not significantly decrease the bond strength, while the addition of water slightly increased the strength.