Rapid Microwave Curing of Precast Concrete Slab Elements

The speed of curing is often a critical issue in the manufacture of precast concrete elements. For some products, the curing cycle consumes up to 70% of the total production cycle. To improve the speed of production, heat curing is often used to accelerate the hardening of precast concrete. Conventional heating techniques rely on thermal conduction. Microwave energy offers potential to increase the rate of bulk heating in precast concrete through its relatively deeper penetration, which allows quicker through-depth heating and maturing. Research on microwave curing of concrete has been ad hoc in the past and a wide range of issues remain unresolved. These encompass materials-microwave interactions, process design and control, hardware and logistics as well as the impact of microwave curing on concrete properties. In this paper, progress on research on microwave curing is described with reference to work carried out at CSIRO. In particular, results from pilot-scale heating of slab-type elements are discussed in relation to heating characteristics, process control, set acceleration, strength development and process efficiency. The authors' results show that for the same bulk heating rates, microwave heating produces significantly lower temperature gradients when compared to steam heating. Using rapid curing cycles of less than six hours, compressive strengths in excess of 25 MPa can be achieved in high quality precast concrete. Doubling the bulk heating rate using microwaves does not result in any deterioration in near-surface quality as was the case with conventional steam heating.