Investigation of the cracking problem of short type IV girders
Tuchscherer, Robin Garrett
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A research project was conducted at the Phil M. Ferguson Structural Laboratory at the University of Texas at Austin investigating the allowable tensile stress limits of prestressed concrete. The project was funded by TxDOT after field observations were made of flexural cracking in the end regions of AASHTO Type IV beams at the time of prestress transfer. Girders that exhibited cracking were relatively short in length (20 to 60-ft) with highly eccentric strand configurations resulting in tensile stresses in the range of 6 f 'c to 7.5 c f ' .A thorough review was conducted of the documentation and research relating the tensile and compressive strengths of concrete to one another; followed by the material testing of Type III concrete mixes at a very early-age (less than 24 hours). Seven full-scale AASHTO Type IV beam specimens were fabricated and tested at the Ferguson Laboratory at the University of Texas at Austin. Strains were measured in the end regions of each beam; resulting in 14 separate tests. The purpose of measuring the strains was to validate the assumptions of the mechanics of prestress transfer. Beams with an extreme fiber tensile stress greater than 4.5 c f ' exhibited cracking at the time of release. In addition to the full-scale beam tests, an extensive amount of material data was collected through testing and literature review. Properties measured included the compressive strength, split cylinder tensile strength, flexural tensile strength (modulus of rupture), and the modulus of elasticity. Over 60 cylinders were collected on separate occasions from four precast prestressed beam manufacturers (over 240 total cylinders) and tested between 6 to 24-hours after batching. Also, in combination with the fabrication of each Type IV beam, anywhere from 25 to 72 cylinders were prepared and tested. Cylinders were electronically match-cured, cured next to a curing beam, and cured at the ambient laboratory temperature. Split cylinder and modulus of rupture tests did not accurately represent the tensile strength of concrete in a Type IV beam specimen. Specimens fabricated as part of this study cracked with an applied stress less than half of the strength determined per material tests. Based on the full-scale beam and material tests conducted in this study, it was concluded that limiting the extreme fiber tensile to 4 f 'c will prevent cracking at release.