The applicability of ultrasonic pulse velocity(UPV) method to in-situ monitor setting and hardening process of foamed concrete(FC) was systematically investigated. The UPVs of various FC pastes were automatically and continuously measured by a specially designed ultrasonic monitoring apparatus(UMA). Ultrasonic tests were performed on FC mixtures with different density(300, 500, 800 and 1 000 kg/m3), and different fly ash contents(0%, 20%, 40% and 60%). The influence of curing temperatures(20, 40, 60 and 80°C) was also studied. The experimental results show that three characteristic stages can be clearly identified during the setting process of an arbitrary FC paste: dormant stage, acceleration stage, and deceleration stage. Wet density, fly ash content, and curing temperature have great impact on setting behavior. A stepwise increase of the wet density results in shorter dormant stage and larger final UPV. Hydration reaction rate is obviously promoted with an increase in curing temperature. However, the addition fly ash retards the microstructure formation. To aid in comparing with the ultrasonic results, the consistence spread test and Vicat needle test(VNT) were also conducted. A correlation between ultrasonic and VNT results was also established to evaluate the initial and final setting time of the FC mixtures. Finally, certain ranges of UPV with reasonable widths were suggested for the initial and final setting time, respectively.
