Material Investigation of the Full-depth, Precast Concrete Deck Panels of the Old Woodrow Wilson Bridge

About the report:

The Woodrow Wilson Memorial Bridge crossing the Potomac River near Washington, D.C., was replaced after more than 45 years of service. Researchers examined the full-depth, precast lightweight concrete deck panels that were installed on this structure in 1983. This report covers the visual survey and concrete material tests from this investigation. The concrete deck appeared to be in good condition overall, with no discernible cracks or signs of impending spalls on the top surface, except for a few signs of distress evidenced by asphalt patches. From below the deck, there were some indications of efflorescence and some panel joints exhibited rust staining, efflorescence, and small pop-out spalls. Closure pours for the expansion joints had more severe corrosion and efflorescence. Steel bearing plates and hold-down rods used for panel-to-deck connections were generally in good condition, although there were the occasional elements that rated poorly. The concrete sampled from the lightweight precast deck panels had an average compressive strength of 7.01 ksi (48.3 MPa), which represented little increase over the average 28-day strength. The average elastic modulus was 2,960 ksi (20.4 GPa), which is on the low end for typical modern concrete mixtures. The average splitting tensile strength was within a typical strength range at 535 psi (3.67 MPa). The average equilibrium unit weight of the plain concrete was 116.5 lb./ft3 (1866 kg/m3). The concrete was sound with no evidence of cracking or other deleterious reactions. The results of absorption, permeability, and chloride tests indicated a material matrix with the capability of absorbing moisture and other contaminants. An epoxy concrete surface layer, an asphaltic concrete wearing surface, and cover depths greater than 2 in seemed to have limited harmful chloride exposure to the reinforcing steel, which appeared to be in good condition. The full-depth, precast lightweight concrete panels appeared to have performed well, with few maintenance issues observed. Reports of similar, more recent, projects have noted additional direct costs associated with precast deck systems on the order of 26 to 30 dollars per square foot. However, anecdotal information from those projects, as well as an analysis of the construction alternatives presented herein, demonstrates that use of precast deck systems for deck replacement of existing bridges can shorten construction time by several weeks or months and induce far less disruption to travel than the conventional cast-in-place alternative, resulting in a dramatic reduction in user costs. When total life-cycle costs, including those associated with road user costs, construction time, construction safety, and maintenance, are taken into account full-depth precast concrete deck panels are the more economical alternative. The costs and benefits assessment demonstrated a clear advantage to using precast bridge deck technology for select deck rehabilitation projects. However, the nature of the estimates and the infrequency with which this sort of repair is implemented make it unreasonable to attribute a direct value in annual savings.

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