Passive fire protection of concrete structures

Passive fire protection of concrete structures is chiefly aimed at combating explosive thermal spalling, which relies on two underlying mechanisms: (a) the build-up of pore pressures and (b) the development of thermal stresses. The former could be alleviated, or eliminated, by the use of polypropylene (PP) fibres in the mix. Thermal stresses could be reduced by the use of thermally stable aggregates of low thermal expansion—not always practical when the choice of aggregates is limited at the location in question. The second, more effective, but much costlier solution is the use of thermal barriers that reduce the heat flow into the substrate concrete and hence reduce concrete temperature (and the resulting pore pressures and thermal stresses). Thermal barriers, therefore, address both underlying mechanisms of explosive thermal spalling. Criteria for determining the thickness of thermal barriers are currently based on specified maximum temperatures at the barrier/concrete interface. This is fundamentally flawed. The current author proposes that authorities should consider instead other criteria, which would include the heating rate at the interface (because explosive spalling is more a function of heating rate than maximum temperature) as well as permeability (strength), silica fume content, initial moisture content and, if possible, aggregate type. This approach is scientifically more correct and provides a more conservative (i.e. safer) result. In new designs, a judicial cost-effective use of both thermal barriers and PP fibres is recommended.