Crack Control On Continuous Concrete Flexural Beams Post-tensioned By Unbonded Tendons

The unbonded post-tension(UPT) method has been often applied to continuous flexural members(e.g., slabs and beams), where tendons are typically placed continuously through the supports. Little research has been performed on continuous UPT members. Determining normal ultimate load-bearing capacity is important in design of concrete continuous beams prestressed with unbonded tendons. Tests on continuous unbounded prestressed concrete beams show that bending moment redistribution can occur. As the few existing studies on continuous beams mainly aimed at strength prediction, the results of these studies are not applicable to examine the service load behavior of UPT members. They also did not reflect the moment redistribution phenomenon, secondary moment and shear behavior in continuous beams, which is quite important to accurately predict the flexural strength and crack propagation behavior of continuous UPT beams.In this thesis, investigated structures for crack diagnosis were two-span beams post-tensioned by unbonded tendons. In order to investigate the cracking behaviour of concrete beams, 16 large size beams have been tested to failure in which measurements the crack development, crack spacings and crack widths, stress change of unbonded tendons at different load levels. The two-span beams had varying span lengths, reinforcement ratios and degrees of prestressing.According to results of the test, stress increase of unbounded tendons while nonprestressed longitudinal tension reinforcement yielding increases linearly with the increase of bending moment at service state when the concrete cracks to the nonprestressed longitudinal tension reinforcement yields. It is found that in case when prestressing reinforcement index is constant, if global reinforcement index increase then prestressing stress also increases. In case when prestressing over global reinforcement indexes ratio is constant, if global reinforcement index increase then prestressing stress has to decrease. In case when global reinforcement index is constant, if prestressing reinforcement index increase then prestressing stress has to decrease. It is also found that stress increment of unbounded tendon is affected by the span-to-depth ratio and the ratio of the global reinforcement index at two critical sections of inner support and the mid-span.Beam test data analysis shows the cracking pattern and deformation capacity at service load state of flexural regions in prestressed members. Given the analysis of crack behavior over critical regions and value of test beam failure loads which were indicated by crushing of the concrete on the compression zone.Based on the parameters of the global reinforcement index and prestressed reinforcement index, the calculation method of stress increase of unbonded tendons while nonprestressed longitudinal tension reinforcement yielding is presented.Meanwhile, the calculation method also contain the span-to-depth ratio and the ratio of the global reinforcement index at two critical sections of inner support and the mid-span. Thus, the stress increment of unbonded tendons under service load can be expressed by yielding moment, cracking moment, stress increment of unbonded tendons at the moment when nonprestressed longitudinal tension reinforcement yielding.Through selecting the average of redistribution ratio during the period when the concrete cracks to the nonprestressed tension longitudinal reinforcement yields, the calculation formula of redistribution ratio is presented based on the global reinforcement index. Further the yielding moment can be calculated according to the elastic bending moment and the redistribution ratio. By considering and containing the shear force effect on the cracks, the formula for calculating the crack spacing is established. According to the measured results of average cracks spacing and cracks width, the calculation method of crack width in unbounded prestressed concrete beams is presented which can reasonably consider the effect of prestress as external load.