Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

Figure 2.28 I-beams studied by Marshall & MattockPrestress was transferred by flame cutting. Immediately after transfer, 18 of the 25beams were determined to crack by visual observation or measurement of reinforcementstrain. The cracking typically occurred near the section centroid at the end face of thebeams and extended a 2 to 4 in. into the beam. Because the cracks were so fine (0.004 in.wide or less), cracks length were reported as the distance at which the measured strainsdecreased to an assumed cracking strain (125 microstrain). Up to the end of the crack,spalling strains decreased approximately linearly with distance from beam end(Figure 2.29). The extent of cracking observed in all beams was judged acceptable.Separating the prestressing force into two relatively equal strand groups (in thetop and bottom flanges) was found to contribute to observed cracking. Beams in thisstudy had 10 to 50% of the total strands in the top flange. Every beam with more than25% of the strands in the top flange cracked; half of those (7 of 14) with fewer strands atthe top did not crack.44