Assessment of Post Fire Structural Strengths of Normal Strength Concrete Subjected to Cyclic Thermal Loadings

Abstract

Concrete is a composite construction material consisting basically of a binder, aggregates, water and with or without admixture to modify either or both its physical and chemical properties. The rate at which concrete structures especially buildings are gutted by fire hazards is on the increase and this has adverse effect on the strength, hence, the need to  assess the post fire structural strengths of Normal Strength Concrete(NSC). NSC of grade 50 was produced and cast into specimens of sizes 150mm cubes and 100mm x 100mm x 500mm reinforced concrete beams. The concrete cubes and beams were subjected to elevated cyclic thermal loadings after 7, 14 and 28 days of curing, while the rate of heating was maintained at 1^oC/min until the target temperature of (100, 130, 160, 200 and 250)^o C where attained and this was maintained for one hour and then allowed to cool at 1^oC/min to room temperature of 32^oC.         Unstressed Residual Uniaxial Compressive Test (URUCT and Flexural Strength Test (FST) were conducted on the cubes and reinforced concrete beams, respectively. Direct Tensile Strength Test (DTST) was performed on the high yield (460N/mm²) steel reinforcement. The result showed that the residual compressive strengths of concrete cubes decreases with increase thermal loadings. The expected strengths of 0.69 and 0.97 of the 28 days strength for 7 and 14 days respectively were met. The flexural strengths decrease with increase in thermal loadings. The flexural strength of the concrete beams at 28 days of 15.25N/mm² at thermal loading of 32^oC was reduced to 8.16/mm² at thermal loading of 160^oC; while the strength at 14 days of

12.19 N/mm² at thermal loading of 32^oC was reduced to 7.62N/mm² at thermal loading of 160^oC and finally, the strength at 7 days of 10.13N/mm² at thermal loading of 32^oC was reduced to 5.82N/mm² at thermal loading of 160^oC. All the results met the specification for high yield steel of 12% elongation at fracture. It was concluded that within the thermal loading range adopted, the tensile strength of the reinforcement was not impaired beyond the limit specified by the code.

Keyword: Normal Strength Concrete, Thermal loadings, Unstressed Residual  Strength Test, Fire hazard.