Director of engineering research, National Sand and Gravel Association, National Ready Mixed Concrete Association, Silver Spring, MD
Proposed changes to the initial curing of standard strength cylinders under ASTM Making and Curing Concrete Test Speciments in the Field (C 31) were studied. Cylinders were cured for periods of one or two days under four simulated field conditions: (1) 16°C (60°F) water, (2) 16°C (60°F) air, (3) 27°C (80°F) water, and (4) 27°C (80°F) air. After this initial curring period, all cylinders were given standard curing until 28-days age. Strength results were significantly affected by initial curing environment. Initial water curing at 16°C (60°F) resulted in 28-day strengths on the order of 5 to 7 MPa (700 to 1000 psi) more than concrete from the same batch cured initially in air at 27°C (80°F). It was also found that cement source has an influence on the strength difference because of initial curing environment and, also, on the temperature rise of the concrete cylinders during the first 6 h after molding. Increasing the initial curing period from one to two days only reduced compressive strength by about 1%.