Validation of Strengths and Limitations of Seismic Methods in Quality Management and Assessment of Shotcrete Liners

Abstract

Shotcrete liners are being utilized more frequently in transportation infrastructure as retaining structures and tunnel covers. Despite their economic advantages to concrete, the initial quality of construction and long-term durability of shotcrete liners are of concern among highway agencies. The quantitative construction quality management is needed to ensure adequacy and uniformity of strength and thickness. Also, periodic inspection of shotcrete liners is necessary to assess possible material deterioration and to detect the onset of defects. The impact echo and ultrasonic surface wave methods are two nondestructive sonic methods that have been proven effective for those purposes in concrete structures. Unlike concrete slabs, shotcrete slabs exhibit spatial heterogeneity. In addition to heterogeneity, surface roughness of shotcrete adds some complications into data interpretation in terms of thickness measurement. The practical effectiveness of applying these methods to shotcrete liners using a commercially-available nondestructive testing device was evaluated in three actual tunnels. This paper focuses on validating the effectiveness of these methods through a well-controlled testing on a number of shotcrete specimens constructed with and without known defects. Results from this study demonstrate that the stiffness of shotcrete layer and the presence of delamination and voids within the shotcrete liners can be detected well by using these seismic methods in combination.