Investigation on air-voids structure and compressive strength of concrete at low atmospheric pressure

This study has investigated the characteristics of air-voids structure and compressive strength of air-entrained concrete at normal atmospheric pressure (P = 1 atm) area and low atmospheric pressure (0.7 P = 0.7 atm) area. The hardened air-voids structure analyzer and the scanning electron microscope (SEM) were employed to evaluate the structure and the micromorphology of air-voids. Results showed that when specimens were formed and tested at 0.7 P, the fresh and hardened air content were separately 13.8%-41.3% and 20%-53% lower; the spacing factor and the average air-voids diameter were 11.6%-37.4% and 14.6%-30.3% larger respectively; the proportion of air-voids within 400-2500 mu m was 7%-21% higher; the air-voids shell was much thinner; and the compressive strength of concrete at 3 days, 7 days and 28 days under 0.7 P were respectively 19%-21%, 5%-24% and 14%-19% lower than that at P. It confirmed that when at 0.7 P, the nucleation and the development of air bubbles were harder, the air-voids structure was abnormal, thus the compressive strength of concrete was deteriorated.