CONCRETE PROGRESS: Overlay protective
ABC deck covers prove to be tough enough
Steel reinforcement in concrete bridge decks is prone to corrosion caused by chloride ions from deicing materials. Various overlays are used throughout the country to delay or prevent this corrosion.
The Utah Department of Transportation has recently changed how it constructs bridges to implement the accelerated bridge construction (ABC) method. This construction process includes half-depth deck panels, full-depth precast deck panels, a self-propelled modular transport deck and superstructure, and slide-in bridges. ABC bridge decks undergo additional load conditions, which may cause deflections due to installation prior to standard traffic loading. The purpose of the research was to compare overlay systems for ABC bridge decks.
The primary focus of the research was to evaluate different bridge deck overlay systems under initial static deflection as well as cyclic loading and recommend acceptable overlay systems and procedures for ABC bridge decks. Two different criteria are used to compare two overlay systems: (1) bond strength between the overlay system and the bridge deck and (2) amount of chloride penetration of the bridge deck through the overlays. These comparisons were made through field and laboratory tests.
Two bridges using different deck overlays were examined. The bridge deck overlay systems were applied the previous year. Four locations were selected for performing bond tests on each bridge. Bond tests were performed adjacent to a precast deck panel joint at each bridge site. The location of the pull-off tests was near the longitudinal mid-span of the bridge and transversely between the first and second girders at approximately the center of the truck lane.
Two different laboratory testing procedures were designed for this research: Test Type I and Test Type II. Test Type I simulates the effects of the application of the bridge deck overlay after lifting and placement of the bridge deck. Test Type II simulates the effects of the application of the overlay prior to lifting and placement of the bridge deck. Both test types simulate actual ABC methods.
During Test Type I, two 1-ft 6-in. x 8-ft x 8-ft 3⁄4-in. concrete deck pieces were turned upside down and deflected to induce initial cracking on the top face of the deck. This simulates initial cracking during lifting and placement in the negative moment regions. The two pieces were turned back over and joined through a grouted joint to construct a single 3-ft x 8-ft x 8-ft 3⁄4-in. specimen, as shown in Figure 1. The deck overlay system was then applied per manufacturer’s specifications on the top face of the single specimen where initial cracking had been imposed. Initial bond tests were performed on the combined specimen. The specimens underwent a five-day cyclic vertical displacement-based loading protocol with varying vertical displacement between days. The applied test loading was on one side of the grouted connection, simulating load transfer through the grout as seen in the field. After each day of cyclic loading, two pull-off tests were performed next to the joint. During cyclic loading the specimen was post-tensioned with two 3⁄8-in.-diam. carbon-fiber rods.
During Test Type II, the deck overlay was first applied on the top of each of the two concrete deck pieces to simulate the application of the overlay system prior to the deck being moved and placed under ABC methods, as shown in Figure 2. Subsequently, each piece was turned upside down and subjected to the same initial deflection as Test Type I specimens to induce initial cracking. The two pieces were then turned back over and joined through a grouted joint to construct a single 3-ft x 8-ft x 8-ft 3⁄4-in. specimen. A second application of the overlay system was then carried out as a splice over the grouted pocket, and the specimens underwent the same cyclic testing (half-sine downward cycles) as Test Type I. During cyclic loading the specimen was post-tensioned with two 3⁄8-in.-diam. carbon-fiber rods. After cyclic loading the post-tensioning was removed, and a chloride ponding test was preformed for 90 days over the grout key for each lab specimen.
Bond strength was recorded for both the lab and bridge specimens. The chloride content was only recorded for lab specimens. A concrete specimen without any overlay underwent the same chloride ponding test as the Type I and Type II specimens and was used as a comparison between overlay and no overlay conditions for the concrete. This specimen was cast with the laboratory specimens.
Three different failure modes occurred during the pull-off tests: epoxy failure, overlay failure and concrete failure. Epoxy failure corresponds to failure of the glue used for the pull test and is not used as a comparison between bond strengths. Overlay failure corresponds to failure within the overlay or the bond between the overlay and the concrete. Concrete failure corresponds to tensile failure of the concrete and is considered preferable because the overlay has a higher bond strength than concrete tensile failure.
Table 1 shows the average valid concrete and overlay pull-off stress values for two overlay systems. The results of these tests show significantly higher bond strength values in the laboratory than the actual bridges tested in the field. The specimens in the laboratory performed better than those in the field. This difference in strength and failure type is due to the difference of laboratory and field preparation conditions for the application of the overlay.
Ponding results showed an average chloride content of 2.96 lb/cu yd for the first 1⁄8 in. below the concrete surface for concrete specimens with no overlay application. An average chloride content of 2.51 lb/cu yd was found at a depth between 1⁄8 in. and 1⁄4 in. for the same specimens. No chlorides were detected in the concrete specimens with any of the overlay systems.
It was determined in this research that the overlay systems preformed properly for the laboratory testing protocol used.
As a result of this research it was found that several overlay systems could be used to protect precast bridge decks. Regardless of the overlay system selected, it should be applied strictly per manufacturer’s specifications. It also was recommended that bridge decks should be properly cleaned and prepared prior to the installation of the overlay to ensure an adequate bond between the overlay and the concrete.
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