Bond and Material Properties of Grade 270 and Grade 300 Prestressing Strands

Abstract

The first objective of this thesis was to determine the material properties of grade 270 and grade 300 prestressing strand of various sizes. Tension tests were performed on each type of strand. The data from these tests was used to determine modulus of elasticity, yield stress, ultimate stress, and ultimate elongation for each strand. The yield stresses and ultimate stresses for many of the strands did not meet the requirements found in ASTM A416. The ultimate elongation results far exceeded the requirements and the measured elastic moduli were near the modulus recommended by AASHTO LRFD. A secondary objective from the tension tests was to evaluate a gripping method which used aluminum tubing to cushion the strands against notching. The grips performed very well. Most of the strand breaks did not occur in the grips and when a strand did break in the grips, the failure occurred after significant post-yield elongation. The second objective was to evaluate the bond properties of grade 270 and grade 300 prestressing strands. The North American Strand Producers (NASP) Bond Test and Large Block Pullout Test (LBPT) were performed on six different strand grade and strand size combinations. Both of the tests are simple pullout tests on untensioned strand. The results for each strand type were compared to one another as well as to measured transfer and development lengths from beams using the strand from the same reel. All of the strands showed sufficient bond in the beams, but one strand type did fail both the NASP Test and the LBPT. Both pullout tests were acceptable methods to evaluate strand surface condition and the benchmarks set for 0.5 in. diameter regular strand were conservative for the strands used in this thesis. Little difference was evident in the bond performance of grade 270 and grade 300 prestressing strand.