Performance Recovery of Fire-damaged Concrete by Impregnation of NaOH-added Lithium Silicate

When concrete is subjected to the elevated temperatures of a fire, cracking occurs and the mechanical performance and durability deteriorate. Current crack repair methods using epoxy or polymer cement cannot repair microscopic damage and cannot restore the performances of whole concrete. There is no effective method to promote the recovery of concrete properties after fire. In this study, the authors proposed to use a highly permeable and alkaline silicate surface impregnation solution (NW-LS) to repair fire-damaged concrete, which is composed of sodium hydroxide (NaOH), lithium silicate (LS) and water and can penetrate in the inside of concrete. The concretes used in the experiments had the compressive strengths of 24.4 to 60.0 MPa before heating, and were heated at different temperatures (300, 500, 650°C) and then cooled in water or air. The changes in the performances, chemical compositions, and internal structure of the concretes after repairing were investigated in detail. The experimental results indicate that the repair densified the internal structure of heated concretes by the reaction between the silicate of the solution and the Ca(OH)2 of the heated concretes, and greatly increased the compressive strength, carbonation resistance, and freezing-thawing resistance of the heated concretes. In addition, the alkalinity of heated concrete was restored by the NW-LS solution.