Effect of axial preloads on confined concrete

Specimens measuring 54 mm x 110 mm were used to conduct 110 triaxial tests. Experimental variables included unconfined concrete strength, lateral confining pressure, level of axial preload and load path. The concretes used had unconfined strengths of 36, 84 and 120 MPa. Confinements ranged between 0.05f'c and 0.4f' c and the axial preloads were as high as 0.8f' c. The effects of axial preloads on concrete prior to applying confinement are evaluated to determine whether columns subjected to significant axial loads should be retrofitted. Results of the testing program are used to analyze the expansion ratio (lateral strain to axial strain) under various conditions and to develop an analytical model. Results indicated that axial preloads up to 0.8f' c had no significant effect on performance at peak and only a minor effect on ductility. The expansion ratio had two unique behaviours and was modelled based on the results obtained.