CONFINED SLAB SPALLING TEST (CSST): A SCREENING TOOL TO ASSIST CONCRETE MIX DESIGN IN TUNNEL PROJECTS

Explosive spalling is one of the most challenging aspects in the design of segmental tunnel linings, due to the high concrete quality entailed by precast production, the severe design fire curves imposed by the design codes and the sustained compressive load borne by the tunnel vault. Since no established predictive models are yet available to account for the many variables governing the incidence of this phenomenon, experimental fire tests are generally regarded as the reference tool for optimizing the mix design and implementing the required mitigation measures (e.g. polypropylene fibre addition). A viable solution consists in testing square flat slabs under a biaxial confinement load. This symmetric configuration is more severe than uniaxial confinement and better replicates the isotropic effect of restrained thermal dilation in the central region of a lining segment. In the proposed Confined Slab Spalling Test setup (CSST), a 0.20 to 0.30 m thick slab is installed in the load frame and exposed to the prescribed fire curve on a 1.0 m side square area. The loading system comprises 8 hydraulic jacks restrained by a compact octagonal tense frame. The exerted load can range from 0 to 3 MN per axis, thus allowing to replicate the actual service conditions of most installations. Different sensors and monitoring systems can support a thorough analysis of the test results. In the paper, the testing setup is described together with the results of the validation tests performed.