Evaluation of spalling of concrete pieces from tunnel lining employing joint shear model

This paper was conducted to quantify the spalling of tunnel lining concrete. This paper firstly modeled the spalling of tunnel lining concrete and examined ways to quantitatively evaluate the safety of the tunnel lining concrete against spalling by comparing the shear stress acting on the joint surfaces of lining concrete cracks with the shear capacity of the joint surfaces. Secondly, based on double shear tests simulating the spalling phenomenon, it was found that wider crack widths as well as greater tapered angles of the joint surfaces resulted in lower shear capacities. Thirdly, a method for simulating the shearing process of joint surfaces was proposed. The method was shown to be able to roughly estimate the relationship between the joint aperture and the shear capacity. Using the improved method, the shear capacity was considered using various widths and tapered angles of the cracks. Furthermore, it was found that the JRC value of the joint surface samples collected from actual tunnels was between the JRC value of the mortar beam joint surfaces that were made in the shear test and that of the joint surfaces of the concrete beam, made in the shear tests with the maximum aggregate size measuring 40 mm. A parameter study was also conducted under hypothetical conditions for the relationship between the width of the cracks and the safety of the tunnel lining against spalling.