Tunnel fires have attracted increasing attention in recent years, after a series of disastrous fires in road and rail tunnels. Some of the most catastrophic examples are the Mont Blanc tunnel fire in 1999 [39 fatalities - inoperable for 3 years], the Tauern bridge in the same year [12 fatalities - three-month closure] and the more recent one in China [31 fatalities, mostly children]. A key element in the spreading of a fire and its effect on the stability of an underground space is the design and performance of the concrete lining. Specifically, under fire, concrete linings spall (i.e. surface abrasions—explosive behaviour), resulting in the collapse of the tunnel structure, which causes a significant scourge in the economy, society, and the environment. Apart from the strength of the concrete (compressive and tensile), spalling is affected by the properties of the concrete such as permeability, age, mix homogeneity, size of aggregates, moisture content, pore pressure, pre-existing cracks, type of reinforcement (if any), size and shape of the exposed specimen.
In summary, the design of tunnel concrete linings is based on thermal calculations, which often ignore spalling. Based on such calculations new types of more durable, strong and hence denser concrete have been introduced on the market recently that are much more probable to spall due to their lower permeability. Reinforcing the concrete specifically with polypropylene fibres results in them melting during tunnel fires (fibres melt at approximately 160oC, before the concrete starts to spall) creating voids, through which water vapour can dissipate eliminating the expansion of pressure in the concrete and thus, spalling. However, the addition of fibres tends to decrease the workability and the environmental impact of the concrete and potentially its strength and durability too. The need to reduce CO2 emissions, or use sustainable materials to increase the life cycle of urban construction has led to the development of alternative types of concrete with properties such as self-compaction, self-healing, low-density, etc. Replacing limestone aggregates with lightweight ones, such as pumice, and therefore, introducing larger voids into the mortar mass, might be the solution to reducing spalling without using polycarbonate fibres, as well as making the concrete greener. The higher porosity values and subsequent lower density offer several advantages, including reduced construction costs, a high strength-to-weight ratio, a low coefficient of thermal conductivity and improved durability, yet cracking at early ages poses a significant concern for its structural use.
To enhance our understanding of the mechanisms involved in cracking and spalling it is necessary to employ full-field imaging techniques to study the size, distribution and orientation of the aggregates and pores within the concrete matrix and relate them to its thermo-mechanical response. This has not been done before and will contribute to the improvement of the design and performance of concrete linings. Specifically, in FiRe2C we have employed a holistic approach between state-of-the-art numerical (Discrete Element Method) and full-field experimental methods (X-ray computed tomography [XCT], utilising quantitative 3D experimental measurements at different length-scales to validate accurately the material response of the simulated material. Our main objectives were:
1) Manufacture a new type of high-strength, lightweight, fire-resistant concrete to minimise the need for additional costly fire protective measures (e.g. fireproof boards) and reduce the time and costs required for the restoration of a tunnel after a fire has occurred. The response of this material both under uniaxial compression and exposure to fire was compared against the response of normal-weight concrete and polycarbonate-fibre reinforced concrete.
2) Study experimentally the effect of size, distribution and orientation of the aggregates, pores and fibres on the strength and fire-resistance of concrete linings, by employing full-field imaging techniques (XCT) pre- and post-fire.
3) Create a novel DEM model to predict the mechanical response of concrete exposed to fire, utilising quantitative 3D experimental measurements to validate accurately the material response
