Structural behavior of un-bonded post-tensioned two span T-beams using high strength concrete with different tendon profiles

The research planned on testing a series of two span un-bonded post-tensioned beams using high performance concrete represents the supplementation of a previous investigative effort dealing with similar types of concrete members (Etien Frett and Michael Ladouceur, 2004) at the CEE Structures Lab of CSUF. In addition to the complicated behavior of continuous beam, the behavior of un-bonded post-tensioned beams become unpredictable as the member reaches failure, because the unbounded post-tensioning does not reach its specified ultimate strength. Though the ACI Code specified approximate allowable ultimate stresses of post-tensioned strands, it is not yet clearly explained how much stress at the unbounded post-tensioning can be obtained before the beam collapse. Most of the stress equations developed, are based on single span beam tests. Another item that complicates the calculation of ultimate strength of a beam is the existence of secondary moment. A more in-depth effort is required to predict the behavior of the beams while also examining the effects of secondary moment in all the T-bears. In addition this research will study the change in the post-tensioning force in the cable during loading, as well as investigating the effect of cable eccentricity at the internal support. Furthermore, an attempt will be made to examine the changes on the reactions at internal and external supports due to the difference in cable profile. Study of crack propagation, change of deflection and camber, and final ultimate strength will be made in this experimental study.;For this purpose, three un-bonded post-tensioned two span T-beams having different cable profiles were constructed. T-Beam 1 (TB 1) designed with double harp tendon with no eccentricity at the internal support. TB2 and TB3 had same cable profile but with eccentricities at the internal support. High performance concrete has been selected for the construction of all three beams to investigate its possible contribution to the overall behavior of the members. An improvement has been observed in the overall behavior of the tested beams compare to Etien Frett and Michael Ladouceur (2004) beams. In addition, the results indicated the secondary moment and a conclusion on not including the secondary moment in the ultimate moment calculation has been suggested.