Washington State has approximately 700 bridge decks that may require a latex modified concrete (LMC) overlay in the next 12 to 15 years. The microsilica concrete overlay is seen as a possible alternative to the increasingly expensive latex modified concrete.
A microsilica modified concrete overlay was placed on Bridge 5/718W in Skagit County. The bridge is a concrete box girder structure with 8,100 square feet of deck area. The ADT on this bridge is 9,150 vehicles per day.
The minimum overlay thickness was 1½ inches. Superplasticizer was added to the mix. The concrete was mixed in a conventional batch plant with the technical representative from the microsilica supplier providing assistance. The microsilica modified concrete overlay was finished and cured as prescribed by WSDOT's specification for LMC overlays.
The microsilica was supplied as a slurry. Force 10,000, produced by W. R. Grace and Company, Cambridge, Massachusetts, was the source of the microsilica.
The long-term performance evaluation of the microsilica-modified concrete will be based on a direct comparison with the LMC overlay to be constructed on bridge 5/718. Both bridges are of similar construction, have the same deck area and ADT, and have existing decks with similar levels of chloride contamination. Both overlays were constructed under the same contract.