Impact of Larger Diameter Strands on AASHTO/PCI Bulb-Tees

This thesis consists of the analytical study and the experimental investigation of larger diameter strands in AASHTO Type I girders. The main purpose of this study was to verify that the 2 inch minimum spacing recommended by ACI 318-08 and AASHTO (2008) can be used for 0.7 inch diameter strands by comparing various effects in girders using 0.7 and 0.6 inch diameter strands. Based on the parametric analysis it was concluded that by using 0.7 inch strands there was a considerable saving in the material. For example, an AASHTO BT-72 with 0.6 inch strand could be replaced with AASHTO BT-54 with 0.7 inch strand for the same span capacity. In order to fully realize the benefits and to verify the adequacy of 2 inch spacing, a three dimensional finite element analysis was performed with two full-scale AASHTO Type I girders with 0.6 inch and 0.7 inch diameter strands. Only the effects due to the prestressing force at transfer were studied in the two models. The maximum principal stress and the axial stress in the concrete along the direction of the strands were determined. Based on the analytical results from the FE model it was found that the girder with the 0.7 inch diameter strand was more vulnerable to cracking at the transition zone between the bottom flange and the web. This defect could be overcome by placing the required amount of confinement reinforcement at the end zone of the girder. Based on the analytical study, two I-girder specimens, one with larger 0.7 in. strand and other with high strength 0.62 in. strand were cast. The transfer lengths of both the girders were measured and compared with the current AASHTO 2008 and ACI 318-08 equations. It was found that both strands exhibited a shorter transfer length than obtained in the equations. Based on these experimental results further studies are to be carried out for the implementation of these highly efficient strands.