Full-Depth Precast Concrete Bridge Deck System: Phase II

The Minnesota Department of Transportation (MnDOT) developed a design for a precast composite slab span system (PCSSS) to be used in accelerated bridge construction. The system consists of shallow inverted-tee precast beams placed between supports with cast-in-place (CIP) concrete placed on top, forming a composite slab span system. Suitable for spans between 20 and 60 ft., the MnDOT PCSSS is useful for replacing a large number of aging conventional slab-span bridges throughout the United States highway system. Originally developed in 2005, the PCSSS had three distinct design generations in the 12 bridges that were constructed by MnDOT between 2005 and 2011. The objective of this investigation was to evaluate the field performance of a sample of the existing bridges through detailed crack mapping and core analysis and through continued monitoring of data obtained from one of the original PCSSS bridges (Bridge No. 13004) instrumented during construction in 2005. A parametric design study was also conducted to investigate the effects of continuity design on the economy of the PCSSS.;Overall, the field inspections indicated that the changes made between each design generation improved the performance of the PCSSS. Bridge No. 13004 in Center City, MN from the first design generation showed many short, longitudinal cracks on the deck surface with very little transverse and map cracking. The longitudinal cracks were located primarily over the precast beam web, corresponding to what appeared to be insufficient consolidation of the CIP concrete around the stirrups projecting vertically from the section to facilitate composite action, which had little clearance above the precast webs. In the second generation, more clearance was provided under the stirrups projecting from the surface. Bridge Nos. 33005 and 33008 near Mora, MN from the second generation did not show the short cracks over the webs from the first generation, but more transverse cracks and longer longitudinal cracks were observed.;For the third design generation, the thickness of the precast beam flanges was decreased and the trough reinforcement spacing (consisting of trough hooks projecting horizontally from the beams across the joint, as well as a drop-in cage) was decreased from a maximum 10 in. center-to-center to 6 in. center-to-center to better control reflective cracking. The decreased spacing was accomplished by staggering the trough hooks from adjacent precast beams. Bridge Nos. 49007 and 49036 near Little Falls, MN from the third generation did not exhibit longitudinal cracking over the precast beam joints, indicating that the design changes may have had a positive impact, though not conclusively. The most significant issue observed with the third generation was shrinkage cracking, indicated by longitudinal cracks located over the precast beam webs and more extensive transverse and map cracking. Generally, bridges with a larger length to width aspect ratio (i.e., L/W) had more transverse cracking, which could be related to more longitudinal shrinkage restraint.;In addition to the field inspections, strain data from the instrumentation of Bridge No. 13004 was analyzed to evaluate performance. The bridge was instrumented in 2005 to monitor reflective cracking and continuous system behavior. Six years of strain and temperature data showed a progression of reflective cracking in several locations and significant cracking due to thermally induced restraint moments. The reflective cracking from the strain data was confirmed by observed cracks in the core specimens near the locations of the strain gauges.;Besides the detailed field investigations, a parametric study of PCSSS designs was conducted to determine whether there was an economic benefit of continuous system design. In particular, design implications of time-dependent and thermal gradient restraint moments and their effects on continuity were studied. Because the PCSSS is a simple-span system made continuous with a CIP deck, the effects of restraint moments must be considered in the design of continuous systems. (Abstract shortened by UMI.).