United States Department of Transportation
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Abstract:Many of these microsilica-modified concrete or silica fume concrete (SFC) bridge deck overlays across the State of Wyoming are suffering from premature distress that includes random cracking, loss of bond and delaminations. To determine the most likely cause of the premature overlay failures and to develop a mitigation scheme, this author with the assistance of Dr. David Rothstein, PhD, PG, FACI with DRP Consulting, Inc. (DRP) and Mr. Larry Mott, PE with GES Tech Group, Inc. (GES) undertook a multi-task research study. Tasks consisted of measuring bond strengths of existing and new SFC overlays representing different surface preparation techniques and measuring both plastic and hardened properties including drying shrinkages for typical SFC overlay mixtures. DRP performed petrographic examinations of bond lines and failure zones of core samples. GES modeled the overlay and substrate and performed non-linear finite element analyses focusing on the cracking potential of the overlay and the bond-line stresses created by varying the strength and drying shrinkage of the overlay. Results show bond strengths for most of the existing and new SFC overlays was insufficient. In general, bond strength failures occurred superficially in the concrete substrate and the petrographic examinations revealed cracks and microcracks located within the top ΒΌ inch of the substrate, most likely caused by an external applied stress after initial bonding. The finite element analyses showed significant tensile stresses in the overlay material, shrinkage cracks and high bond-line stresses occurred when the drying shrinkage of the SFC overlay exceeding about 0.03 percent. Results indicate the magnitude of the drying shrinkage of the existing SFC overlays was too large. Using the 0.03 percent as the upper limit for the field shrinkage, maximum 28-day allowable shrinkages as determined by a standard laboratory procedure were computed using a concrete shrinkage model for different overlay thickness and relative humidities. By limiting the 28- day laboratory shrinkages, SFC overlay shrinkages should not exceed 0.03 percent; thereby, limiting the cracking potential of the overlay and maintaining acceptable bond line stresses.
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