Accelerated microwave curing of concrete: A design and performance-related experiments

Microwave (MW)-accelerated curing has emerged as an innovative and popular curing method for concrete materials. This paper reports the results of a study to model the horn antenna used for the MW irradiation of a workpiece with a mobile MW-accelerated concrete curing unit, based on a coupled thermal and electromagnetic analysis. The mathematical models were useful for evaluating the heat generation within a horn antenna and as a basis for constructing a mobile MW-accelerated curing unit with an operating frequency of 2.45 GHz and a MW power level of 800 W. Further, the early-age compressive strength development and volume stability of MW-cured concrete were investigated in terms of its shrinkage and compared to the properties of autoclave-cured concrete. The design results showed that under the concept of the allowable maximum temperature for the concrete workpiece, which was controlled to less than 80 °C, a horn antenna that was 216.70 mm wide, 333.68 mm long, and 273.0 mm high produced a uniform thermal distribution in a concrete workpiece. Moreover, experimental investigations showed that the application period for curing using a mobile MW-curing unit was considerably shorter than that in autoclave curing methods. The appropriate delay time (time after concrete mixing) was 30 min, and MW irradiation for 45 min could improve the maximum 8-h early-age compressive strength of MW-cured concrete, whereas an application time of 15 min produced the 28-day compressive strength. When a concrete workpiece was cured at high temperature using MW energy for more than 15 min at a temperature greater than 80 °C, the effect was a continuous increase in the early-age compressive strength, which was greater than that achieved by autoclave curing. In terms of volumetric stability, MW-curing for 30 and 45 min increased the ultimate shrinkage to a greater extent than that by autoclave curing and vice versa in the case of a MW application time of 15 min.