Concrete to the Test of Fire in Tunnels Bored by Tunnel Boring Machines

Concrete might appear like a solid, impervious material. But when it is exposed to extreme heat, it can react in unexpected ways: explode.

The phenomenon is called explosive spalling: the breaking away of pieces from the surface of a concrete structure. The sound of it resembles the popping of expanding cork kernels when popcorn is made.

The explosion is caused by water inside the concrete turning into vapour and expanding to create enough pressure to make its surface go POP!

This is a risk widely known in the industry, especially when it comes to precast concrete segments, which line the walls of tunnels after they are excavated by tunnel-boring machines (TBMs).

The heat resistance of these segments are always put to the test. But it is usually done after the concrete mixture has been chosen and the machines to produce the segments have been already calibrated.

But Webuild engineers have decided to change things around: testing the heat resistance before the final mixture of concrete has been chosen and the machines prepared for their production. They figured it would be more effective and cost less.

Major fires like the one in the Channel Tunnel between England and France in 1996 have served as grim reminders of concrete’s brittleness – and the dangers that it poses. When explosive spalling occurs, it can compromise the structure of a tunnel, as well as put at risk people escaping the blaze or fire fighters trying to extinguish it.

The lessons from these fires were quickly learned by the tunnelling industry.

With safety being paramount, it looked at alternative ways of making precast concrete segments. And it found it in steel and polypropylene fibres. By mixing them with concrete, they eliminated the need to insert a steel rebar cage into the segment to maintain its shape and strength. This method came to be known as Steel Fibre Reinforced Concrete.

Of course, the extent to which there is exploding spalling depends on a number of factors. But the polypropylene fibres definitely mitigate it,

Although their use has become the standard in the industry, they have yet to be considered for major projects in Italy. This is something else that Webuild’s engineers are determined to change.

The usual way of doing a precast concrete segment for a tunnel wall is to insert a steel rebar cage into a mould and pour the prepared concrete mixture over it. The mould is then slid into a furnace to cook the concrete. The resulting segment is then left out to cool and dry.

When the concrete mixture contains the fibres, however, a cage is no longer needed.

The steel fibres give the segment its strength, or load bearing capacity. They reduce the risk of cracking. They also do not corrode like steel rebar. The polypropolene fibres, meanwhile, make the segment more fire resistant, minimising the risk of explosive spalling.

Using these fibers in the concrete is cheaper and greener because it eliminates the need to make steel rebar cages. It also makes it easier and safer to make segments because factory workers do not need to install by hand each cage into the mould.

An excavated tunnel will nevertheless have both kinds of precast concrete segments. Although there will be fewer of those with steel rebar cages, they are essential wherever geological pressure points along a tunnel wall are particularly intense.

Precast concrete segments are usually tested for their fire resistance after the fact: when the mixture has been chosen, the machines at the factory calibrated, and production started.

But this limits the possibility of making improvements to the concrete mixture should it be required after testing. So Webuild’s engineers went to work with the Politecnico di Milano to find a way to do the testing before all of that.

After about three years of research, they came up with the Confined Slab Spalling Test, or CSST.

Since it allows Webuild to perform the test at a facility in Italy rather than ship a precast concrete segment to one located abroad, the CSST is a faster, easier and cheaper alternative. It also provides more comprehensive results.

CSST entails putting a square slab of sample concrete into a steel frame, fastening hydraulic jacks on all four sides, applying 450-600 bars of pressure, affixing sensors, placing the slab in front of a furnace, exposing it to temperatures of up to 1,350 degrees Celsius – and seeing what happens.

Putting pressure on all sides– known as a biaxial confining load – is what makes the test resemble the conditions to which a concrete segment is exposed when it is part of a wall. The usual way to do it – uniaxial confining load – applies pressure only on two sides of an actual precast concrete segment, making the test results less accurate.

The slab measures 1.3 metres by 1.3 metres, with a thickness between 0.2 and 0.3 of a metre, making it slightly thinner than a normal segment.

If the damage caused to the segment’s surface by exploding spalling is limited to a depth of 5 centimetres, it is deemed adequately fire resistant. This is also the case for current projects.

By performing the tests in Italy, Webuild can reduce the cost to less than a third of the usual amount.

But it does not stop there.

The other objective set by Webuild’s engineers was to develop a concrete mixture that contained fewer polypropylene fibres than normal.

The industry standard for this mixture is 40 kilograms of steel fibres per cubic metre, and 2.0 kilograms per cubic metre of polypropylene. But the engineers worked to reduce the amount of polypropylene to 1.5 kilograms or less without weakening the concrete’s fire resistance.

The result is a mix that is easier to work with. It also leads to a cheaper mix to make concrete segments.

With client approval, they have been testing a variety of concrete mixtures for the production of precast segments for the walls of tunnels being excavated at major projects across Italy. They include the railway that will link Messina, Catania and Palermo in Sicily; the high-speed/high-capacity line between Naples and Bari; Circonvallazione di Trento and the one between Fortezza and Ponte Gardena, an extension of the Brenner Base Tunnel that will be the longest railway tunnel in the world under the Alps.

Currently, this method is beingused, and in the coming months this type of fibre-reinforced concrete will be deployed for the first time in Italy for segments designed to line tunnel walls.