Microwave heating in concrete analysis

Abstract

AbstractMicrowave heating has been shown to be a valuable method for weakening concrete prior to the separation of aggregate in the analysis of hardened concrete. Quantitative determinations were made of the strength reductions of concrete samples after microwave radiation using a 5 kW laboratory microwave generator and a 1.5 kW (later 2.5 kW) microwave oven. Aggregate separation was much easier after heating for 5 and 20 to 30 min respectively. The modulus of elasticity correlated best with subjective assessments of the ease of aggregate removal. Thirty minutes irradiation in the microwave oven reduced the modulus of elasticity to one fifth of its original value, compressive strength was reduced to 80% of the unheated value and the tensile strength to 55 to 65%. Tests with ‘miniature’ concrete showed that the reduction in impact strength was similar to the compressive strength. Conducting aggregate does not preclude the use of microwave heating although large metallic inclusions (reinforcement) should be avoided. Irradiated concretes containing an appreciable proportion of clay may yield increased amounts of soluble silica during subsequent chemical analysis. Comparison with conventional heating shows that microwave heating can produce the same strength reductions in about half the time, but the cohesiveness of the material is affected more markedly by microwave irradiation. Appendices give notes on the use of microwave heating in the analysis of concrete and some observations on the behaviour of different types of concrete heated during routine investigations.