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

Horizontal cracks in both the bursting and spalling zone wrapped around the beamends, and seemed to engage the end-block transverse reinforcement, especially at squareend (Location 1B). Flexural cracks from the top edge of the beam began 3 ft. from thebeam end (outside the beam end region). These cracks were 0.025 in. in width or less,with most measuring 0.010 in.4.2.2.2 Beam 1: Strand StressesAt transfer, gages on prestressing strands recorded decreases in strain, fromwhich, through multiplication by an experimentally determined modulus (determinedthrough testing reported in Section 3.4.2), the prestress loss at instrumented sections canbe inferred. Prestress losses are represented in Figure 4.4 by circle area.Losses, subtracted from the initial prestress, give effective prestress. Within thetransfer length, the effective prestress increases with distance into the beam. As theeffective prestress stabilizes 30 in. into the beam (Figure 4.5), the experimentallydetermined transfer length matches the code value (30 in. for both the ACI 318 andAASHTO LRFD equations, as discussed in Section 3.2.1).The prestress loss measured one transfer length into the beam can be taken as theprestress loss from beam elastic shortening. The average values at the stressing and fixedends were similar (22 and 23 ksi, Figure 4.4) and comparable with the code-predictedvalue (25 ksi, reported in Appendix C). Past studies of prestress losses in Texas U-beams(Gross & Burns, 2000; Tadros et al., 2003) have similarly shown the elastic-shorteningloss provisions to be accurate.153