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

Most beams observed to crack did so near the section centroid (in this case, middepth),at which height maximum spalling strains were measured at the beam end.Interestingly, a number of beams cracked at top flange/web junction (40% of the beamswith observed cracking, 7 of 18), perhaps due to bursting effects from the prestressingforce applied to the top flange.The maximum spalling strain in each end region was reported by the researchers.It can be observed that the mean of the maximum spalling strains in “uncracked” beams(370 microstrain) was strikingly similar to that for the “cracked” beams (380microstrain). The spalling strains for beams without observed cracking ranged from 170to 900 microstrain; all were above the nominal 150-microstrain cracking strain. AsArthur and Ganguli stated, “Some of these strains are so high that it is suspected thatcracking had in fact occurred, although there was no evidence of it” (p. 95).Unlike other studies, Arthur and Ganguli also reported time-dependent behaviorof their measured strains. From measurements made immediately after prestress transfer,24 hours after transfer and 48 hours after, the researchers reported that strains had atendency to decrease (100 microstrain over 48 hours) except at locations of web cracking.Where gages spanned cracks, strains increased with time.2.4.1.6 Krishnamurthy, 1970Krishnamurthy performed a similar study to that Arthur and Ganguli, also atGlasgow. Krishnamurthy’s study investigated the effect of detensioning method (i.e. timespent detensioning) on transverse tensile strains and transfer length in 12-in. I-beams.Strains were measured on the concrete surface with a small mechanical gage, whosetarget points covered the spalling zone: the middle portion of beam, within h/2 of thebeam end (Figure 2.25).38