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

4.2.3.3 Beam 2: Transverse-Bar Bursting & Spalling StressesBursting and spalling stresses in transverse reinforcement and crack patterns fromtransfer for Beam 2 are shown in Figure 4.9.Measured stresses were somewhat higher than in Beam 1, but still low comparedto the AASHTO limit (20 ksi) in general. Spalling-zone stresses decreased with distancefrom the beam end as observed in Beam 1. However, in three bursting zones of Beam 2(Locations 2A, 2D and 2E in Figure 4.9), measured stresses peak at the third bar from thebeam end. This behavior (stresses that reach their maximum some distance into the beam)is characteristic of bursting.As for Beam 1, reasonable correlation exists between visible cracking andmoderate to high stresses in reinforcement. In some locations (e.g. Location 2D), cracksformed between lines of gages; there stresses in the transverse reinforcement closer to thecracks (than the instrumented locations) are expected to be higher than those measured.The AASHTO stress limit was surpassed for multiple bars in both the spalling andbursting zones. Near mid-depth at the square end of the beam (Location 2F inFigure 4.9), spalling stresses in the first two bars averaged 22 ksi (10% beyond theAASHTO limit). A crack 15 in. in length, but less than 0.005 in. in width, formed alongthe line of these strain gages.More interesting is the location in which bursting stresses exceeded the AASHTOlimit (Location 2A). Here stresses peaked at 30 ksi, 50% past the code limit. Stressesdissipated to nominal levels by one transfer length (30 in.) into the beam. The adjacentcracks formed diagonally from the corner of the beam, in the same direction as shearcracks. It should be noted that these high bursting stresses were measured within the largetriangular end block. Whatever beneficial effect that the large end block may have had in163