Fire safety is one of the major concerns with regard to repair work in the Mont Blanc Tunnel. The spalling of concrete is a phenomenon which can affect special concrete or concrete that has been heated quickly and/or to a high temperature. The regulations applicable to tunnels require the performance of the concrete to be tested, using the HCM curve, in relation to this phenomenon.
Slab of the Mont Blanc tunnel
The spalling phenomenon depends on several factors, such as the water content of the concrete, the compactness of the concrete, the type of aggregate used, the heating speed, the stress state of the element, and the geometry of the element.
In 2017 Efectis France carried out a characterization test in the laboratory at Les Avenières of the concrete spalling of the slabs used in the Mont Blanc tunnel.
There are many constraints on the type of test, such as:
- Reproduction of an element representative of the real element (similar stress state)
- Fire with high intensity (1300°C)
- Risk of breakage of the element during the test
- Loading conditions
- General dimensions of the furnace
- Implementation of many thermocouples in the concrete
Set-up of the test
Several methods exist to determine the spalling depth:
- Reading the thermocouple temperatures in order to assess the matter loss of the exposed face
- Measuring the spalling after cooling of the tested element in order to confirm the thermocouple measurements
- Finally, making a 3D scan of the tested elements in order to get a precise cartography of the spalling
Example of the spalling of a concrete slab
The test confirmed the spalling depth used by the designer for the structural fire design.
Contact: Pierre Lonchampt – [firstname.lastname@example.org]