Development of end-region cracks in pretensioned concrete I-girders employing 0.7-in. diameter strands

Although 0.5- and 0.6-in. diameter strands are commonly used in the prestressing industry, there is a growing interest in the implementation of 0.7-in. diameter strands. However, the greater prestressing force induced poses several potential implications, particularly when the strands are placed on a 2- by 2-in. grid. One such issue is end-region cracking, an occurrence that is common in pretensioned girders, regardless of strand size. These cracks tend to grow in width, length, and number over time due to time-dependent effects such as shrinkage or creep. Additionally, the cracks tend to close under an applied load when placed in a service-state condition. End-region crack widths are often used to evaluate the condition of pretensioned girders, so a thorough understanding of the development of these cracks is essential to applying crack width criteria appropriately. A multifaceted experimental program was conducted at the Ferguson Structural Engineering Laboratory at the University of Texas at Austin. A series of seven Texas bulb-tee girders employing 0.7-in. diameter strands was fabricated, monitored, and load tested under shear-critical conditions. The end-region cracks of three specimens were measured immediately after prestress transfer and monitored for at least 28 days, showing that the crack widths grew significantly over time. This growth corresponded closely with the shrinkage strain measured at midspan of each girder, indicating that shrinkage is the primary cause of end-region crack growth. A significant amount of transverse reinforcement is placed in end-regions to restrict cracks immediately after prestress transfer, but this same reinforcement also provides a large amount of restraint against concrete shrinkage, exacerbating crack growth. End-region cracks were also measured during the shear-critical load test for two specimens. Although they closed in a linear manner, they were not completely closed at an expected service load. At ultimate load, the cracks never closed entirely, as the imperfect concrete surfaces bore against each other shortly after initial diagonal shear cracking. Based on both of these findings, future end-region crack widths can be more accurately predicted from any point in the lifespan of a pretensioned girder, allowing for more appropriate applications of permissible crack width limits.