Experimental and analytical investigation of end zone cracking in BT-78 girders

In the past decade, with a growing interest in long span bridge girders, designers are using an increasing number of prestressing strands. This scaling up has led to cracking issues in the end zone of narrow stemmed bulb-tees and I-girders, impacting the service life and possibly design capacities in case of excessive cracking. In a project funded by the Alabama department of transportation (DOT), a 180 ft long girder design was developed by modifying a standard Bulb-Tee girder and using a 10 ksi self-consolidating concrete mix. The new girder is 78 in. deep and has 66-0.6 in. strands. The impact of the draping angle and debonding on the end zone cracking was first evaluated using a 3 D finite element model (FEM) developed in ABAQUS. Subsequently, the critical stresses were monitored during the detensioning process of four 54 ft long full-scale girders with different end zone details. It was found that the combination of limiting the draping angle and debonding the strands resulted in minimizing the end zone cracking. The detailed FEA model which was developed is also verified using experimental field data. The results of this experimental and analytical investigation will be presented in this paper.