Risk Mitigation

Mobile furnace for fire tests on tunnels

The CSTB’s mobile furnace (2.5 t) makes it possible to control the rise in temperature of a tunnel wall up to 1300°C to determine its fire rating.

The CSTB has a mobile furnace, unique in France, capable of performing in situ fire tests on tunnel walls. The City of Paris requested testing of two tunnels of the Boulevard Périphérique beltway. Objective: document their fire ratings, and optimize fire protection costs for the structures.

As part of its maintenance works to ensure the safety of civil engineering structures, the City of Paris is renovating tunnels to bring them into compliance with regulations. After the tragedy of the Mont Blanc Tunnel fire in 1999, standards were revised and now all tunnels longer than 300 meters must comply. The City of Paris is examining the six tunnels of its beltway, known as Le Périph, and it called on the CSTB to conduct tests on two of them.

Wall heated to 1300°C

In 2016, the City of Paris asked the CSTB to determine the fire rating of a vertical wall of the Courcelles Tunnel (580 m). The mobile furnace has unique dimensions. The test is applied to an area of 1.25 x 1.25 m and lasts two hours. A burner with 1.5 MW of thermal power generates a temperature of 1300°C in steady state within a box attached to the tunnel wall. At this temperature, concrete can spall, leaving exposed reinforcement, which no longer provides enough mechanical strength to ensure structural stability.

For the Courcelles Tunnel, spalling was limited to a depth of 70 mm. Based on its calculations, the CSTB declared the fire rating of the concrete compliant. Some tunnel walls will therefore not require additional protection.


The City of Paris is continuing its tunnel checks using the CSTB’s fire test engineering and mobile furnace. In February 2017, it requested another test, this time for the Lac Supérieur Tunnel, near Porte de Passy. Analysis is underway, and the results will be delivered in April.


Bernard Verbeke, assistant head of the Seine and Civil Engineering Division and manager of the Tunnels Subdivision, City of Paris

« We contacted the CSTB because it is the only institution with a mobile furnace that can perform on-site fire tests on large works that cannot be moved. For us, the purpose of the test in the Courcelles Tunnel was to document its fire rating with the French National Commission on Evaluating Road Construction Safety (CNESOR). The City of Paris has about 16 tunnels that are over 300 meters long. Having reliable test results before launching work is essential so that we can make considerable savings when carrying out fire protection operations. »