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

Though Marshall and Mattock placed some strain gages at various depths(Figure 2.20), such gages were applied only to the end face of the member. As burstingstrains do not reach their maximum value until some distance into the beam, it can besurmised that only spalling strain variation was measured.2.4.1.4 Gergely, Sozen & Siess, 1963Gergely, Sozen and Siess, also noted for their contributions to the understandingof end-region behavior using other methods, studied surface strain distribution and crackpropagation in 10 eccentrically post-tensioned beams. The beams were of bothrectangular and I-section, were small-scale (6 in. maximum breadth, 12 in. deep), andwere unreinforced in the transverse direction. Transverse tensile strains were measuredusing foil strain gages applied to the concrete surface at 0.5 in. spacing. Instrumentationwas placed along the line of prestress, at the section centroid and in between; bothbursting and spalling effects were measured.Typically, as prestress was applied, beams would crack in both their bursting andspalling zones. Bursting cracks would form 1 to 2 in. from the beam end face. Spallingcracks would propagate from the end face just below the centroid. Beams alsoexperienced flexural cracking along their top edge, but such cracks occurred far enoughfrom the ends not to affect the transverse strain readings.Transverse strains were observed to vary nonlinearly with applied prestressingforce at very low levels of prestress (25% of the prestressing force at observed cracking).The behavior of the spalling strains for lines along the section centroid was interesting.These strains would typically increase with load, but if a crack should form at anotherlocation (e.g. in the bursting zone, or just below the gage line), the strains woulddecrease, sometimes becoming compressive. This behavior is shown in Figure 2.22.34