Abstract
Explosive spalling of concrete exposed to fire consists in the violentexpulsion of shards from the hot surface due to the interaction between cracking andpore pressure build-up. Fire spalling relevantly increases the overall thermaldamage of a structure exposed to fire, thus leading to much higher costs in therepair intervention, and in some cases it can even jeopardize the structuralstability due to loss of reinforcement protection and reduction of the bearingcross-sections. High-performance concrete is particularly sensitive to spallingphenomenon due to inherent material features, such as the unstable fracturebehaviour and the low permeability (favouring high values of pore pressure). In thiscontext, an experimental campaign has been carried out on high-performance concrete(fc ≈ 60 MPa withsilico-calcareous aggregate), without or with one of three different fibre types(steel fibre, monofilament or fibrillated polypropylene fibres). Tests wereperformed by means of a special test setup developed at Politecnico di Milano, basedon slabs (800 × 800 × 100 mm) subjected to Standard Fire at the bottom and tobiaxial compressive loading in the mid-plane, while monitoring pore pressure,temperature and deflection. Explosive spalling was observed in both plain concreteslabs and in one of the two slabs with steel fibre, this casting some doubts on theuse of steel fibre alone against spalling. No detachment was observed whenpolypropylene fibre was added to the mix.
Highlights
Structural behaviour can be strongly affected by fire due to the decay of material properties at high temperature and the introduction of indirect actions induced by restrained thermal dilation [1]
Fire duration was set in the range 100–120 min in absence of spalling, otherwise heating was stopped when the slab was no more able to sustain the external biaxial compression (34–35 min)
The concretes at issue have been extensively studied in a previous experimental campaign focused on the compressive behaviour of concrete cylinders at high temperature and in residual conditions, proving that the decay of elastic modulus as a function of temperature is just slightly influenced by the addition of polypropylene fibre [35, 36]
Summary
Structural behaviour can be strongly affected by fire due to the decay of material properties at high temperature and the introduction of indirect actions induced by restrained thermal dilation [1]. Even though numerical models have been developed addressing fire spalling progression in concrete, the reliable prediction of its evolution is significantly affected by several uncertainties such as the mutual interaction between pore pressure and concrete fracture behaviour (as reported in [3, 4]), and porosity and permeability variation with temperature. Further details on the main features of the test setup are reported in Lo Monte and Felicetti [14]
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