Title: Behavior of High-Strength High-Performance Concrete Bridge Girders
Author(s): J. F. Stanton, P. Barr, and M. O. Eberhard
Publication: Symposium Paper
Appears on pages(s): 71-92
Keywords: bridge girders; camber; high-performance concrete; high-strength concrete; instrumentation; prestress; strains; thermal effects
This paper describes the measured behavior of a bridge made with precast, prestressed, high-performance concrete (HPC) girders. The concrete was considered high-performance, because it was specified to have a compressive strength of 51 MPa (7400 psi) at release and 69 MPa (10,000 psi) at 56 days. By using HPC instead of normal-strength concrete, the bridge designer was able to reduce the number of girder lines from seven to five. These girders were the first to be constructed in Washington State using HPC. To monitor the girders, vibrating-wire strain gages with thermistors were installed in five girders, and camber was monitored by various means, including a stretched-wire system that could be monitored by the data-aquisition system. Temperatures measured during fabrication indicate the presence of a large and unexpected temperature gradient over the height of the girder. As a result the concrete strength at release may have been lower at the bottom of the girder than at the top. The long-span girders were stressed to an initial bottom stress of approximately 28 MPa (4000 psi). The strength of the concrete was higher than usual and permitted the high initial stress. However elastic modulus rises only with the square root of strength, so elastic shortening strains, and creep strains that are assumed to depend on them, are higher for high-strength concrete. In these girders, elastic shortening and creep dominated the loses. The measured losses were compared with the predicted losses form two standard methods, but neither method was able to provide a universally superior match. In general, the measured losses exceeded the calculated losses initially, but with time, the discrepancies decreased.