A novel approach for modifying air-voids in concrete by short-term low-air pressure intervention

If it is desired to experimentally verify the regularity in the effects of air-voids on the properties of concrete, methods of altering voids must be relied on. Consequently, this research explored a method that reduced the curing air pressure after mixing concrete so as to modify air-void characteristics. In addition, in order to avoid introducing variables besides air-voids, a limit was placed on the duration of low-pressure maintenance, which was restoring the air pressure to atmospheric pressure after the formation of the concrete's air pressure transient resistant strength. The results showed that if the reduction in air pressure was insufficient, the differential pressure between the inside and outside of the original bubbles was not enough to cause them to break through energy barriers resulting in expansion. Only bubble regeneration due to the decrease in gas solubility was observed. When the differential pressure exceeded the energy barriers, bubble expansion could be achieved. In addition, it was possible to achieve the air pressure transient resistant strength in short time, which was achieved in only 7 h for the concrete used in this paper. Water loss calculation, X-ray diffraction, thermogravimetry-derivate thermogravimetry, scanning electron microscope, and mercury intrusion porosimetry were used to evaluate the hydration of the concrete confirming that the low-air pressure curing of 7 h had almost no effect on hydration. The method proposed can provide an effective means of specimen preparation for scientific research involving the effects of air-voids on the properties of concrete.