Setting Bar- Bending Requirements for High-Strength Steel Bars in Cold Temperature
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Demand for high-strength reinforcing bars has recently increased in the U.S. construction industry. Due this demand, bars with yield strengths up to 120 ksi are being developed and produced in the United States. High strength bars, however, are expected to require larger bend diameters than grade 60 bars having a specified yield strength of 60 ksi, to achieve similar bend performance. As high strength bars will be used in colder climates, there is also concern that their bends may be more brittle at low temperatures. To examine the relationship between bend diameter and temperature on bend performance under loading, a program consisting of bend/re-bend (or re-straightening) tests was conducted on grade 60, 80 and 100 bars bent both at cold (-23°C) and room temperature (+20°C). Tests were conducted at two different temperatures +20°C and as close to -40°C as practical. Tests results demonstrated a significant difference in bend performance due to bar grade and bend diameter, such that a larger bend diameter is required for higher grade bars to achieve the same performance during re-bending. The temperature at which bars were bent did not affect the behavior of the bends when tested at a relatively warm ambient temperature. Cold temperature tests were not successful in this study as bars always fractured at the grips of the testing machine, thereby invalidating results. Future testing should consider alternative gripping mechanisms for cold temperature bend tests.