Characterisation of fibre distribution uniformity in steel fibre-reinforced concrete under microwave-induced heating during casting

Traditional active microwave thermography (AMT) tests for hardened steel fibre-reinforced concrete (SFRC). The surface temperature of SFRC derived from AMT cannot fully characterise the distribution of steel fibres in the SFRC. This study proposes a microwave-induced heating method combined with temperature sensors (MI-TS) to estimate the fibre dispersion of an SFRC mixture during casting. The principles and testing process are discussed, and a numerical model is developed to evaluate the temperature distribution of the SFRC mixture following microwave exposure. Through MI-TS and numerical models, the temperature in the SFRC mixture with uniform fibre distribution, fibre clustering and fibre sinking is investigated, and the results are compared with those obtained by AMT and destruction technology to evaluate the feasibility of the proposed method. The results show that in the same specimen, the temperature difference between areas with similar fibre content falls in a very narrow range (e.g., 3.5-4.5 degrees C for a 100 x 100 x 100 mm specimen exposed to 600 W of microwave irradiation for 180 s). When the fibre content in a given area is more than 0.5 vol%, a large temperature difference forms between this and other areas. When the equivalent fibre content of the specimen is ignored, the temperature difference between the two areas is directly proportional to the difference in fibre content. Therefore, the area that shows differences in fibre clustering and fibre content can be easily determined found based on the temperature difference derived from MI-TS.