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

2.4 EXPERIMENTAL STUDIES OF BURSTING & SPALLING EFFECTSOver the past 50 years, researchers have used two distinct experimentalapproaches for the study of bursting and spalling. In one of these, strains on the surface ofa concrete specimen have been measured through use of photoelastic materials ormechanical gages (e.g. Demec or Whittemore). Such experiments have proved useful inchecking analytical solutions based on the theory of elasticity or examining theprobability of cracking in members for which transverse reinforcement is rarely provided(e.g. hollow-core slabs). However, attempts to extend these tests’ results to designrecommendations been largely unsuccessful.The other approach involves the measurement of strains in provided transversereinforcement. The experimental database of studies of this sort is highly skewed towardI-beams, though some rectangular and inverted T-beams are included. (No U-beams, norbeams of any cross section with an end block, have been tested.) Such an approach is notuseful for checking theoretical solutions for end-region stresses because in order for thereinforcement to be engaged, cracks (visible at the surface or not) must form, taking thespecimen into an inelastic region of response. That said, studies taking this approach(measuring reinforcement strains) have been able to develop empirical equations for thedesign of transverse reinforcement adequate for control of end-region cracks toreasonable widths and lengths. The equation developed by Marshall and Mattock (1962)forms the basis of the U.S. code provisions relevant to bursting and spalling effects inpretensioned concrete members.2.4.1 Studies of End-Region Surface StrainsIn the 1940s and earlier, limited photoelastic studies were carried out by a numberof researchers as experimental justification of their two-dimensional theoretical solutionsfor end-region stresses. The results of many of these studies are reported by Guyon24