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

2.4.2 Studies of End-Region Transverse-Reinforcement StrainsOf the two important questions: “What starts a crack?” and “What stops acrack?”, the first one cannot be answered with sufficient ease and accuracy. Thenew approach… [is] more concerned with the second question. It [attempts] tofind the effects and the propagation of the anchorage zone cracks… [leading] tothe examination of the role of the transverse reinforcement in confining the crack(Gergely & Sozen, 1967, p. 48).The “new approach” taken by experimental researchers in the 1960s was tomeasure strains of transverse reinforcement in the end region. While bursting andspalling effects often cause pretensioned concrete beam end regions to crack, they rarelycause the transverse bars there placed to yield. Hence, while the concrete in the endregions cannot be said to respond elastically, the steel placed there can, and conversion oftransverse tensile strains to stresses proceeds without errors due to inappropriatelyassumed elastic behavior.Since the early 1960s, five significant studies have examined end-region behaviorof pretensioned concrete beams at transfer of prestress by means of measuring transversebarstrains. (Another study, examining post-tensioned concrete beams, is also reviewed inthis section.) As with studies of surface strains, these studies have focused on I-beams(80% of the specimens reporting transverse stress variation, i.e. 37 of 45 beam endregions). The experimental methods and findings of these studies are detailed in thissection. Data from these studies have been compiled along with that from the presentstudy into a database. Findings based on analysis of this database will be described inSection 4.3; figures showing the results of the individual studies are provided inAppendix A.The key for interpretation of these studies’ findings lies in the placement of thestrain gages. Strain gages placed far from crack locations are likely to return low strainsas the adjacent uncracked concrete will aid in resisting the transverse tensile stresses.42