Evaluation of Strand Transfer and Development Lengths in Pretensioned Girders with Semi-Lightweight Concrete

摘要

Indiana has been using lightweight aggregate consisting mostly of expanded shale in the production of prestressed concrete bridge girders for several projects. The lightweight aggregate has been used as partial replacement for regular gravel or crushed limestone coarse aggregate. This semi-lightweight concrete weighs around 2080 kg/m .In a study sponsored by the FHWA, the current AASHTO equations for the calculation of transfer and development lengths of prestressing strand were found to be unconservative in the case of lightweight concrete members with unit weight less than 1 920 kg/m3 . This finding raises a question regarding the applicability of the same equations to semi-lightweight concrete members.The objective of this study co-sponsored by the Indiana Department of Transportation and the Federal Highway Administration is to determine if the current AASHTO Specifications are applicable to semi-lightweight pretensioned concrete bridge members. The study addresses the transfer and development lengths of 13.3 mm and 15.2 mm pretensioned strand in semi-lightweight concrete girders with concrete compressive strength of 48 MPa for the 1 3.3 mm strand and 69 MPa for the 1 5.2 mm strand. Two strand types were considered in this study.The results of the laboratory evaluation suggest that the current transfer length of 50 strand diameters is adequate for semilightweight concrete if longitudinal cracks are not present along the transfer length. In the presence of longitudinal splitting cracks, the measured transfer length increased to approximately 70 strand diameters. It is recommended that the current requirements for strand development length be enforced at a distance \u22dp \u22 from the point of maximum moment towards the free end of the strand. The quantity \u22dp\u22 is the distance from the extreme compression fiber to the centroid of the prestressing steel but not less than 80% of the overall member height, hi the specimens tested, web-shear cracks, intercepting the transfer length near the member ends, were not present.