Experimental and Theoretical Studies on Behavior of Unbonded PC Beams at the Ultimate State

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The present study consists of two test series. The purpose of series A tests is to examine the effect of basic parameters such as amount of prestressed steel, amount of nonprestressed bonded steel, compressive strength of concrete and length of shear span on the fundamental flexural properties of unbonded beams without bond of prestressing steel in comparison with those of bonded beams. In the series B tests, on the other hand, the ratio of lateral confinement steel and initial tensioning stress of prestressing steel were chosen as the main test variables and then the effect of lateral confinement on the ultimate flexural strength, maximum attainable stress in prestressing steel and ductility of unbonded and bonded beams were studied.The results of series A tests showed that the stress increment of unbonded steel due to applied load up to the maximum flexural strength is significantly affected by all the test variables and that the measured stress increment agreed well with the calcula … More ted one when the strain compatibility factor was assumed to be 0.2-0.3, depending upon the shear span length. The provision of the minimum amount of nonprestressed steel for the unbonded beams, as recommended by the ACI Code, was confirmed to be very effective for improving the flexural properties at the ultimate state as well as the distribution of cracks. Use of high- strength concrete could increase well both of stress increment of unbonded steel and flexural ductility. And then, it was found from series B tests that the maximum flexural strength as well as the maximum attainable steel stress was significantly increased due to lateral confinement in the unbonded beam compared to bonded beam and that such tendency became remarkable with decreasing initial tensioning stress of steel. In the unbonded beams, the provision of lateral confinement with 1% by vol. could not well increase the stress increment of prestressing steel and the beam strength at the ultimate state but significantly improve the beam ductility. The behavior of unbonded beam including the falling branch region after the maximum strength could be estimated well using the strain compatibility factor as mentioned above. Less