Size and Sustained Load Effects in Concrete Columns

Columns were tested with sizes ranging from 3-3/4-in. to 7-1/2-in. square. Slenderness ratio and percentage reinforcement were kept constant for all columns. Three levels of load were applied representing, respectively 55%, 65%, and 75% of short-term capacity. The age of concrete at initial application of sustained load was 14 days, 60 days, and 120 days. The sustained load test was conducted in an environmentally controlled space. The average duration of sustained loading was approximately 200 days. The volume/surface ratio effect is shown by column midheight moment amplification defined as the ratio of midheight moment under sustained to the same moment at initial loading. This effect varies with loading level and concrete age. The comparison with the ACI Code indicates that the Code approach is conservative for the columns tested.