Effect of Detensioning on Rupture of Stainless Steel Strands in Pretensioned Concrete Members

Precast prestressed concrete members are widely used in civil engineering structures around the world with growing demand in the last few years due to the rapid urbanization and industrialization driving an increase in infrastructure projects. Precast pre-tensioned concrete members are especially beneficial for accelerated bridge construction (ABC) methods because of the reduction of cost, less traffic interference, and faster project development that are enabled by precasting the elements off-site. New technologies and emerging materials (e.g. high strength stainless steel strands) have recently been introduced to the construction industry as an alternative to corrosion resistance prestressing. However, the detensioning procedures during the load transfer in the precasting yard have not yet been studied for the implementation of such materials. In the casting beds, once the strands have been tensioned to the desired prestressing force and the concrete has been placed and achieved the necessary compressive strength, then the detensioning of strands takes place. This process transfers the prestressing force from the bulkhead of the precasting bed to the concrete member. This detensioning procedure can be accomplished by saw cutting the strands with a hydraulic pump, or by means of a flame torch. If the strands are abruptly cut using the saw cut method, the sudden release of pretensioning force may cause damage at the end of the member, slippage, and an increase in strand development length. Therefore, it is generally recommended to gradually detension the strands using a torch while following a symmetrical pattern to guarantee that the force is being transferred to the member with no damage. For efficiency reasons and to improve the economy of the process, multiple concrete members are usually cast in series using a long casting bed, in which all strands are placed along the length of the bed and tensioned. Once the concrete has reached the required strength for transfer (in some cases 24h after casting), the strands are cut in between and at the ends of the members. This transfers the load to each concrete member.For large members with multiple layers of prestressing strands, the strands sometimes break between the end of the concrete member and the bulkhead during the detensioning process. The strand rupture occurs due to the accumulation of elastic shortening of the concrete member as each strand transfers its force into the concrete. In consequence, the successively increased shortening of the concrete member amplifies the tensile strain in the remaining uncut strands. For concrete elements in harsh environments, high-strength stainless steel (HSSS) prestressing strands are being implemented with the goal of producing longer lasting structures. However, HSSS strands have some disadvantages compared to the generally used carbon steel strands. HSSS strands elongate more compared to regular carbon steel strands and have a much lower ultimate breaking strain. The risk of yielding leading to breakage during the detensioning process needs to be studied, as precasters are unfamiliar with this new material. The reduction in ultimate yielding strain significantly affects the pretensioning properties of the stainless steel strands, and it may increase the risk of rupture arising from the elastic shortening of the concrete member. This study aims to evaluate the yielding of the remaining uncut strands during the detensioning process based on the properties of both carbon steel strands and high strength stainless steel strands in large precast prestressed concrete members. Characteristics such as the number of strands, free length between the member and bulkhead, detensioning procedures, and the concrete member span length are considered the driving factors for the potential yielding and failure of strands during the load transfer process due to the additional tensile strain in the uncut strands caused by the shortening of the concrete member. A parametric study was created to extensively analyze the elastic shortening of the concrete using different beam sizes and number of strand. Finally, the strains in the uncut strands were compared to the yielding strain of HSSS strands to determine how many strands could be cut before the uncut strands yield. The yielding strain was chosen as the limiting strain, because thereafter strains are in the nonlinear range and cause permanent deformation, and because relatively little strain capacity remains after yielding until breakage occurs -- compared to carbon steel strands. Ultimately, this study provides conclusions and recommendations to improve the practices during the detensioning process to help ensure a smooth implementation of high-strength stainless steel strands in the precast prestressed concrete industry.