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This study investigates the efficacy of post-fire curing using carbonation with a 20% carbon dioxide concentration and a relative humidity cycle set between 40% and 90% for restoring the mechanical properties of thermally damaged high-performance concrete (HPC) specimens containing 0%–40% silica fume. The HPC specimens were exposed to temperatures of 600, 800, and 1000 °C for 1 h, and the compressive strength recovery was measured. The microstructure, porosity, pore size distribution, and chemical composition of the HPC specimens were analyzed to explore the strength recovery mechanism. After exposure to elevated temperatures, the average compressive strength of samples without silica fume decreased by 49.2 MPa. Subsequent carbonation recuring resulted in a significant recovery of 73.9 MPa in the average compressive strength. This recovery surpassed the original strength for the samples heated to 600 and 800 °C, attributable to the filling and coalescing effects of calcium carbonate polymorphs formed through the carbonation of residual cement particles and β-C 2 S. The samples containing 20% silica fume exhibited the second highest average strength recovery of 34.1 MPa. However, the strength recovery for the samples with 40% silica fume exposed to 800 and 1000 °C was negligible, as the microcracks exceeding 1 μm in width had barely been restored by the carbonation of the low-calcium calcium silicates with low reactivity. Overall, this study presents an exciting future prospect for the labor and cost-effective restoration of thermally damaged concrete structures through the use of carbonation curing. [ABSTRACT FROM AUTHOR]

Explosive spalling consists in the sudden separation of concrete layers or pieces from the surface exposed to fire, due to (a) stress caused by thermal gradients and external loads and (b) pore pressure rise induced by water vaporization. As regards the material properties, the phenomenon is affected by some meso- and micro-structural aspects mainly concerning the interaction during heating among cement paste, aggregate and fibre, thus influencing porosity and permeability variation at high temperature. Focusing on the effect of pore pressure in fracture behaviour of Normal-Strength and High-Performance Concretes, an experimental campaign has been planned, aimed at investigating the role played by the mix design. Nine concrete mixes have been studied considering three grades (f c ≥ 40, 60 and 90 MPa), three aggregate types (silico-calcareous, basalt and calcareous aggregates) and different fibre types and contents (monofilament or fibrillated polypropylene fibres). The experimental study, based on indirect tension tests performed under different levels of sustained pore pressure, clearly shows that the influence of pressure on concrete fracture response during heating strongly depends on mix design. [ABSTRACT FROM AUTHOR]