Transfer Length and Prestress Losses of a Prestressed Concrete Box Girder with 18 mm Straight Strands

Despite the potential advantages of 18 mm strands, the limited research on the behavior of girders with larger-diameter strands hinders the application in bridges. Transfer length and prestress losses are two important indicators. In this research, a 32.6 m long prestressed concrete box girder with 18 mm straight strands and 15 mm harped strands was produced, and the transfer length and the prestress losses were studied. The transfer length was calculated based on the existing equations in codes and previous research. Three beam specimens were fabricated, and strain gauges were pasted on the concrete surface to measure the transfer length of 18 mm strands. It indicated that the average measured transfer length was 700 mm. This value was smaller than the transfer lengths predicted by AASHTO LRFD 2017 and ACI 318-19, while Mitchell’s equation offered the closest prediction to the average measured transfer length. Additionally, the prestress losses at different stages were evaluated. A one-end stressing test was conducted to analyze the effect of strand harping on the loss of tensile force. In comparison with the actual measured loss based on the concrete strain and the longitudinal shortening, the instantaneous prestress loss calculated using the AASTHO LRFD 2017 alternative equation was appropriate. The time-dependent prestress losses due to shrinkage, creep, and relaxation were predicted using two different methods addressed in AASHTO LRFD 2017. The time-dependent predicted losses of 69.2 MPa at 28 d using the refined method were 37% higher than the measured losses 47.4 MPa at 28 d, indicating an overestimation of AASHTO LRFD 2017. The accumulation of the total losses over time revealed that the prestress losses developed in the first two months occupied the majority of the total losses in the long term. The research may provide guidelines for the design of a pretensioned concrete box girder with 18 mm strands.

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