Statistical evaluation of transfer and development length of low-relaxation prestressing strands in standard I-shaped pretensioned concrete beams

The Federal Highway Administration (FHWA) placed a moratorium on the use of 0.6-inch diameter prestressing strand for pretensioned concrete beams in October of 1988. This moratorium was imposed because of some apparent unconservatism in the code requirements addressing transfer and development lengths of seven-wire prestressing strand.

The research conducted at Texas Tech University (TTU) and the results reported herein are an integral part of a larger, joint research project conducted with The University of Texas at Austin (UT) designed to provide additional test data for consideration toward Iifl;ing the FHWA moratorium. This joint study involved 36 fullscale AASHTO Type I (Texas Type A) I-beams. Six of these beams were tested at TTU. Concrete strengths in three ranges were investigated in the joint TTU / UT project. The three concrete strength ranges were 5,000-7,000 psi, 9,500-11,500 psi, and 13,000-15,000 psi. The strand surface conditions of the 0.6-inch diameter strand were mill bright or msty. For each combination of concrete strength and strand surface condition, beams were tested for three other variations in the strand; all strand fully bonded, 50% of the strand debonded, and 75% of the strand debonded.

TTU portion of the study covered transfer and development length tests of 0.6-inch prestressing strand with rusty surface condition in fully bonded, 50% debonded and 75%) debonded pretensioned concrete beams with low strength concrete 5,000-7,000 psi. Results from this study were used to evaluate the current ACI, AASHTO-Standard and AASHTO-LRFD requirements for the transfer and development lengths, as well as the transfer and development equations proposed by C. Dale Buckner and Susan N. Lane. Research results showed that ACI and AASHTO-Standard code equations for transfer length are slightly unconservative. AIso, it showed that Buckner and AASHTO-LRFD equations are less conservative compared to Lane equation for both transfer and development length.

Due to these observations, it was determined that a new transfer and development length equation needed to be developed. The new transfer and development length equation should provide conservative results for all concrete strength and prestressing strand diameter of 0.5 and 0.6-inch but yet not be overiy conservative for high-strength concrete. The new equations for the transfer and development lengths were formulated based on the resuhs obtained from this study and the results from other research studies to make sure that the equations would be representative of large number of data.