###### Abstract  
Leakage in below-grade concrete structures are repaired using various types of grout-injection materials, but the selection of optimal material types with a consideration of the environmental degradation factors are not conducted. Different degradation factors can act on the waterproofing membranes or grout-injection materials simultaneously. Especially in the early stages of installation, the injected grout materials in the cracks for leakage repair or for reforming damaged waterproofing layers are subject to complex forms of degradation factors. In such cases, physical property changes to the materials can reduce the waterproofing performance of the grout-injection materials. In this study, a technical evaluation regime is proposed for selecting the optimal repair material to be used in underground concrete structure leakage cracks. In this study, six environmental degradation factors (thermal stress, chemical corrosion, erosion due to ground water flow, hydrostatic pressure, substrate movement, and humidity on concrete surface) are identified. Corresponding evaluation methods based on the ISO TS 16774 test method series were used for each factor to assess the performance evaluation of four different types of grout-injection materials (acrylic resin, epoxy resin, polyurethane foam, and synthetic polymerized rubber gel). Based on the test results, a new comprehensive evaluation regime is presented that allows a quantitative performance comparison between each type of grout-injection material.  
membrane layer is nonexposed. In such case, when the waterproofing membrane is damaged and allows the ingress of water into the interior of the concrete structures, the waterproofing layer is not directly accessible for repairs [1]. Therefore, grout injection is used to ensure the leakage crack is sealed or to fully reform the waterproofing layer in the positive side of the wall. However, numerous cases show that when the grout-injection materials are applied in below-grade concrete structures, a reopening of the leakage and defects can occur [2].  
Some of the main causes are a continuous exposure to humidity, water flow, the hydrostatic pressure from underground water, the thermal stress, chemical corrosion, and the substrate movement (six types of degradation) from the surrounding environment. While cases are rare where all of these factors are simultaneously present with enough degree to affect the structure durability, these environmental degradation factors affect the structural integrity of the concrete structure simultaneously and their individual effects are difficult to predict and assess [3]. To compensate for this predicament, the materials selected for repair should be evaluated to ensure that they possess the individual resistance or response property to each of the possible degradation factors.  
In underground spaces such as basements, tunnels, and below-grade structures where the existing waterproofing system or the defect source causing leakage is not exposed and accessible for facilitated repair, this is particularly problematic. If suboptimal repair materials that are easily susceptible to physical properties change are used, the waterproofing performance can be affected during the installation and early stages of their lifetime [4]. Refer to Figure 1 below for an illustrated concept of the six types of degradations in below-grade structures.