Effects of Increasing the Allowable Compressive Stress at Prestress Transfer

The maximum allowable compressive stress limit at prestress transfer was first introduced by the American Association of State Highway and Transportation Officials (AASHTO) in the 1961 Bridge Design Specifications. The limits prescribed by AASHTO in 1961 have not changed since their inception. Over the last decade, a substantial amount of research and discussion has been published related to increasing the allowable release stress in compression. In 2007, Dolan and Krohn conducted a survey of Prestressed Concrete Institute (PCI) professional and producer members [2007]. Thirty of forty-four respondents reported regularly using compressive stresses at release in excess of 0.60f'_ci. The majority of respondents reported no difficulties with higher release stresses. However, some respondents associated the following problems with higher release stresses: excessive camber, concrete splitting around the strand end, excessive strand slip, minor cracks at the ends, short horizontal cracks at the top of the web. Also, the results of the survey indicated that greater compression transfer stresses were typically used in building products than in bridge products. The authors recommended increasing the allowable stress in ACI-318 to 0.70f'_ci. Also, they stated that refined camber and prestress loss calculation methods may be needed as a result.