Shotcrete walls play a crucial role in modern infrastructure, serving as vital structural and protective elements. Widely used in transit systems, shotcrete walls provide support and durability in underground tunnels, retaining walls, and various transportation facilities. While shotcrete construction practice have improved over the years, there are many instances where the shotcrete is not executed properly, leading to damage in the waterproofing membrane, presence of voids, and vertical joints between the shotcrete layers.  Without proper evaluation and maintenance, these issues can compromise the safety, durability, and functionality of these structures.

The following case study outlines the application of advanced non-destructive evaluation (NDE) methods to assess the quality and integrity of a shotcrete wall in a transit infrastructure project. The assessment focused on identifying potential defects within the shotcrete wall. By leveraging state-of-the-art NDE technologies such as Impact-Echo and Ultrasonic Pulse Echo, the project aimed to ensure the long-term reliability of the structure.

Objective

The assessment aimed to determine the overall quality of the shotcrete wall by identifying significant anomalies (defects or flaws) that could compromise its structural integrity, mapping surface and subsurface cracks and voids to evaluate their impact on performance, and validating findings through physical sampling of core extractions to ensure the accuracy of non-destructive evaluation methods. A grid-based testing approach was also employed, enabling systematic and precise data collection across the area for detailed mapping of the wall’s condition.

Methodology

To achieve the objectives of the study, a combination of advanced non-destructive evaluation techniques and intrusive methods were employed. These methods were chosen for their ability to provide detailed insights into the condition of the wall without causing damage to the structure.

1. Impact-Echo Testing of Shotcrete Wall

The Impact-Echo (IE) test is a non-destructive method used to assess the internal condition of concrete structures by analyzing stress waves. In this project, a mechanical impact was applied to the surface, generating waves that reflected at interfaces such as voids, cracks, or boundaries. These echoes were recorded by a transducer, which converted the displacement into electrical signals for analysis. Learn more on Impact-Echo

2. Ultrasonic Pulse Echo Tomography of Shotcrete Wall

Ultrasonic Pulse Echo (UPE) tomography was the primary technique used for this evaluation. It is a well-established NDE method that analyzes the internal condition of concrete and shotcrete structures. The process involves transmitting ultrasonic waves into the material and analyzing their reflections to detect irregularities, voids, cracks, or changes in density. Learn more on UPE

The reflections occur when the waves encounter interfaces with different acoustic properties, such as cracks, voids, rebar, or the far side of the wall. This technique is particularly effective in locating defects, determining wall thickness, and assessing the structural integrity of concrete and shotcrete elements. In this project, UPE tomography was used to identify anomalies across the test area and create a 3D model of the subsurface conditions.

Non-destructive Evaluation of Shotcrete Walls

3. Ground Penetrating Radar (GPR)

Ground Penetrating Radar was employed during the project to provide critical information about the location of steel reinforcement and other potential subsurface objects. By identifying the positioning of rebar, the GPR scans enhanced the interpretation of UPE results, particularly for near-surface reflections.

Shotcrete Wall with GPR Overlay_Resize

4. Validation Through Core Sampling

As part of the evaluation, core samples were extracted from selected locations identified during the UPE analysis. These samples provided direct, physical evidence of the wall’s condition, allowing for the validation of the non-destructive testing results. The core samples were carefully examined for voids, cracks, material density, and other indicators of structural health.

Key Findings

The evaluation revealed important insights into the condition of the shotcrete wall. These findings highlight both the strengths and areas of concern within the structure.

1. Quality and Integrity of Shotcrete Walls

The assessment indicated that the majority of the wall was in good condition. The shotcrete material exhibited consistent density and uniformity, with no significant structural defects observed in most of the evaluated area. This suggests that the overall construction quality of the wall was satisfactory.

Localized surface anomalies were detected within the shotcrete, indicating small voids or irregularities in the material. These defects were limited in size and distribution, accounting for a small percentage of the wall’s volume.

2. Validation

The findings from the core samples aligned closely with the results of the UPE analysis. Core samples extracted from sound areas of the wall confirmed the absence of significant defects, while cores from regions with detected anomalies exhibited corresponding voids, air pockets, or minor irregularities. This validation reinforced the accuracy and reliability of the non-destructive evaluation methods used in the project.

By combining innovative NDE techniques with physical validation, the project provided a comprehensive understanding of the wall’s condition. These insights not only address immediate concerns but also support long-term planning to ensure the structure’s durability and reliability for years to come.

 

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