Tunnel thermoplastic membranes are widely used in road tunnels to divert water leakage from the rock. The Swedish Transport Administration specifies several functional requirements for these membranes, including fire safety. Higher fire performance often compromises other properties such as strength and durability, which is why membranes with lower fire classification, E, are commonly used. An E- class membrane is tested according to EN ISO 11925-2, involving exposure to a small flame for 15 seconds, with flame spread limited to 150 mm within 20 seconds.

During construction, membranes are typically installed in large sections before shotcrete application, leaving them temporarily unprotected. This raises concerns about fire risks, as construction vehicles can produce heat release rates of several megawatts. In installation spaces, membranes may remain exposed in the finished tunnel, but the potential fire load is smaller (e.g., an electrical cabinet of about 200 kW).

This project evaluated the fire behavior of E-classified membranes through tunnel fire tests. Six thermoplastic (PVC, polyethylene, and polyolefin) membranes, 1.1–2.1 mm thick and surface density 1.1–2.7 kg/m², were tested in a square tunnel with side 2 m and length 60 m under four fire scenarios. All membranes met class E requirements for surface and edge exposure with a small flame. No ignition occurred in the radiation scenario (200 kW at 0.8 m distance). For direct exposure of 200 kW (equivalent to 3 MW full scale), membrane contribution was marginal. At 750 kW (equivalent to ~13 MW full scale), contribution was brief before melting created a natural fire break. No continuous fire propagation along the membrane section was observed.