Field and laboratory evaluation of precast concrete bridges
Terry J. Wipf
The precast concrete deck bridge (PCDB) is a short span bridge that was commonly used on Iowa's secondary roads approximately forty years ago. Each PCDB span consists of eight to ten simply supported precast panels ranging in length from 19 ft to 36 ft. The panels resemble a steel channel in cross-section; the web is orientated horizontally and forms the roadway deck and the legs act as shallow beams. Bundled reinforcing bars in each leg act as the primary flexural reinforcement. Many of the approximately 600 PCDBs in Iowa show signs of significant deterioration. Typical deterioration consists of spalled concrete cover and corrosion of the bundled primary reinforcement. The objective of this research was to access the structural sufficiency of the deteriorated PCDBs through field and laboratory testing. Four deteriorated PCDBs were instrumented with strain gages to measure strains in both the concrete and reinforcing steel and transducers to measure vertical deflections. Response from loaded trucks was recorded and analyzed. Test results revealed that all measured strains and corresponding stresses were well within acceptable limits. Likewise, measured deflections were much less than the recommended AASHTO value. Load fractions, a measure of live load distribution, were calculated from the deflection data and compared for the four PCDBS. The two PCDBs tested without shear connectors were found to have load fractions considerably higher than those with shear connectors. Laboratory testing consisted of loading twelve deteriorated panels to failure in a four point bending arrangement. Although all panels exhibited significant deflection prior to failure, the experimental capacity of eleven panels exceeded their theoretical capacity. The experimental capacity of the twelfth panel, an extremely distressed panel, was only slightly below its theoretical capacity. Additional laboratory testing consisted of service tests performed on a four panel laboratory PCDB to determine the effect of various shear connection configurations on transverse load distribution. The laboratory PCDB was also utilized in an investigation into the behavior of an overloaded PCDB. Finally, a strengthening retrofit was applied to several panels and tested for its effectiveness.