Study on the Mechanical Properties of Retrofitted Concrete Damaged under Fire and Rapid Cooling

The present study concentrated on the mechanical performance and microstructure characterization of fire-affected concrete cylinders. The enhancement of properties of fire-damaged cylinders through the retrofitting technique was also addressed. A total of 39 concrete cylinders (100-mm diameter×200-mm high) were cast and cured. Among the 39 samples, three are used as the control (CC). The remaining 36 samples were grouped into two, one group without adopting retrofitting and the other by adopting the retrofitting technique. The ISO 834 fire curve was followed to attain the three different temperatures: low—300°C (300_GC_NW; 300_GC_W), moderate—600°C (600_GC_NW; 600_GC_W), and high—900°C (900_GC_NW; 900_GC_W). The samples were maintained at the corresponding temperature for 30, 20, and 15 min respectively. Three from each temperature regime were subjected to sudden cooling by dipping them in water and the remaining three were allowed to cool gradually. The retrofitting of fire-damaged cylinders was carried out using basalt sheets and basalt fiber-based engineered geopolymer composites (BFEGCs). An axial compression test on all samples was performed, followed by a parametric analysis. The confinement offered by the wrapping in specimens 300_GC_W, 300_RC_W, 600_GC_W, and 600_RC_W restored the characteristic compressive strength of 20 MPa. The ultimate load, elastic modulus, and toughness modulus of rapidly cooled and retrofitted specimens 300_RC_W and 600_RC_W showed promising values. The microstructural analysis of the rapidly cooled samples showed the bridging of gaps due to additional calcium silicate hydrate gel formation. The cohesive core made the confinement effective, thus enhancing the properties of fire-damaged concrete.