Flexural Performance of Reinforced Concrete Beams Subjected to Mild-to-Severe Premature Loading

Abstract

This research investigates the flexural behavior of reinforced concrete (RC) beams subjected to loading prior to attaining the design 28th days strength. Five groups of RC beams have been cast, loaded at age of 7 days to different levels and then tested until failure at 28 days. The loading at 7 days and testing until failure at 28 days were performed by a fourth-point bending testing. Three beam samples of the first group (the control group) have been tested until failure at age of 7 days to determine the ultimate strength of beams at this age. The other three samples of the control group were tested at age of 28 days to determine the strength of beams at this age and whether the beams had any strength gain between the ages of 7 and 28 days. The second to fifth groups have been loaded at age of 7 days up to 25, 50, 75 and 100% of their nominal flexural capacity at that age, respectively. All of the loading levels exceed the experimental cracking loading of the beams. These loading levels also correspond to tension steel stresses of 30, 61, 100 and 116% of the yield strength of steel bars used. The beam samples of the second to fifth groups were tested until failure again at age of 28 days to determine whether premature loading had negatively affected the flexural behavior of the beams. Flexural performance indices such as ductility, crack stiffness and toughness have also been determined and compared against the control beams. Test results indicated that premature stressing of RC beams at 7 days and up to 75% of the nominal capacity has not affected the flexural capacity of beams. At 100% premature loading, the nominal capacity has decreased sharply. The obtained test results support the assumptions of ignoring the contribution of the cracked concrete underneath the neutral axis of the section, and considering the yielding stress of steel in most of the international design procedures such as ACI-318. However, mid-span deflection of the loaded beams was relatively larger than the unloaded beams. As the premature loading level increased, the corresponding deflection at 28 days is increased as well.

Keywords- Premature loading, early loading, flexural strength, RC beams.