Forensic Investigation of Two Voided Slab Bridges in the Virginia Department of Transportation's Richmond District

About the report:

The precast prestressed concrete voided slab structure is a popular bridge design because of its rapid construction and cost savings in terms of eliminating formwork at the jobsite. However, the longitudinal shear transfer mechanism often fails, leading to leakage of salt-laden runoff water between individual beams and increased corrosion of prestressing strands in the beams.

Two such slab bridges in Virginia, the Qualla Road Bridge and the Adkins Road Bridge, had delaminations, spalls, and broken prestressing strands. These bridges provided an excellent opportunity to conduct both destructive and non-destructive evaluations of beams after more than 50 years in service. This study was designed to conduct a forensics investigation of voided slab bridges to understand the reasons for deterioration or failure of these bridges and to find ways to identify their deterioration states while the bridges are in service.

Tests included material sampling, corrosion and concrete condition assessments, and live load testing of the overall structure. Both structures were found to be in fairly good condition with less corrosion in the strands and a relatively stiffer superstructure than expected.

Deck drainage patterns were found to be closely related to the deterioration mechanisms of superstructures with scaling and pop-outs in the outer sides of the fascia beams. Reflective longitudinal cracks formed through the asphalt riding surface allowed chloride-laden water to drain through the joints and wet the top, sides, and bottom of the beams. Accumulated dirt and vegetation growing at the scuppers obstructed the free drainage of runoff, allowing chloride-laden water penetration in these locations. In addition, clear concrete cover thicknesses were generally less than specified at a considerable number of locations, especially at the bottom of the slabs, providing a weaker defense against corrosive chemicals for the strands.

However, the analysis showed that the concrete in the Qualla Road Bridge showed moderate chloride ion penetrability in permeability testing and that the Adkins Road Bridge still retained adequate flexural strength to support service loads. The AASHTO-calculated girder distribution factor for interior adjacent members, where there is no vertical displacement at the interface between beams, was found to be sufficient.

The results suggest that evaluating bridges considered for replacement with the use of nondestructive techniques, such as determining material and structural conditions, might delay replacement for a number of years and thus free up resources for other needed projects. The study provides a recommendation with regard to how the Virginia Department of Transportation’s Structure and Bridge Division could revise its guidance on design, construction, and maintenance of adjacent member prestressed structures such as voided slabs to address issues identified in this study.

Authors