A sprayed waterproofing membrane whose base material is a polymer has a higher initial strength and faster construction time than conventional sheet-type waterproofing materials. Because of its high adhesion and tensile strength, the shotcrete or concrete reinforced with waterproofing membrane shows characteristics like composite materials. In this study, to consider an application to the conventional design method, the numerical method was used. In the numerical analysis, material and contact properties were adopt from previous laboratory studies. Because the contact properties between membrane and other supporting materials at the interface is important parameter to understand the supporting mechanism of the sprayed waterproofing membrane, contact properties were mainly considered at the interface. In addition, the conventional design method, the allowable stress design for the tunnel lining, was reviewed with numerical results.
Waterproofing is one of the main components to be checked to maintain structural safety and serviceability. Groundwater is an important consideration in engineering projects because it can weaken and degrade a structure causing subsidence near an excavated section (Nakashima et al., 2015). EFNARC (2008) and ITAtech (2013) proposed the use of sprayed waterproof membranes and thin spray-on liners (TSLs) for tunnels and underground structures. The sprayed waterproof membranes and TSLs can prevent water penetrates into lining structure. Polymer-type membranes are quick to deploy and have better construction time than conventional sheets. TSLs have a similar composition to waterproof membranes, but TSLs were mainly used as a supporting material along with a shotcrete primary lining (Holter, 2015).
The most obvious way to assess the performance of a sprayed waterproof membrane is to perform field and laboratory-scale tests (Tannant, 2001). However, a few test results on the interface properties of sprayed waterproof membranes have been reported (Verani and Aldrian, 2010; Holter, 2015; Johnson et al., 2016; Su and Bloodworth, 2016, 2018).
In this study, the membrane properties and interface properties were obtained from experiments and numerical method instead of the commonly used "composite beam" approach and numerically simulated both pre- and post-interface debonding stage. Then the structural effects of a sprayed waterproof membrane on concrete lining were analyzed. The membrane's reinforcing effects of the concrete lining were evaluated based on the results of numerical analyses simulating three-point bending tests varying conditions (material properties, maximum stress of damage initiation, cohesive stiffness, fracturing energy, softening effects) and their results are discussed.
To evaluate the material properties of the sprayed waterproof membrane, the tensile test was conducted according to ASTM-D638 standards (ASTM, 2010), as shown in Fig. 1(a). The dimensions of the Type-4 membrane specimen from the ASTM standards (thickness 3 mm) are shown in Fig. 1(b). The specimen was molded in a mold corresponding to Type-4 according to ASTM-D638, and the tensile test was carried out after 28 days of curing age.