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21 FIGURE 10 Stresses in overlays. FIGURE 11 Shear and normal stresses near boundary (for normal stresses + = tension). SPECIAL APPLICATIONS tection TPO used to overlay a bridge in Virginia. Two resin systems were used on separate lanes: (1) modified vinyl ester Cathodic Protection and (2) polyester. Each was used with an aggregate consist- ing of 50% calcined coke breeze (an electrically conductive Cathodic protection overlays have been developed and used aggregate) and 50% silica sand. The surface was shot blasted, for bridges subject to or experiencing corrosion activity in the the primary anode system was installed, and the polymer con- reinforcing steel. Fontana et al. (12) describe a cathodic pro- crete was placed and screeded to a 12-mm (0.5-in.) thickness.

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22 FIGURE 12 Shear stresses in overlays for various thickness and modulus ratios. (For assumed values of T = 500 -in./in., Es = 4 106 psi.) FIGURE 13 Normal stresses in overlays for various thickness and modulus ratios. (For assumed values of T = 500 -in./in., Es = 4 106 psi.)

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23 FIGURE 14 Axial stresses in overlays for various thickness and modulus ratios. (For assumed values of T = 500 -in./in., Es = 4 106 psi.) About 25% of the first system had to be replaced because of that the manufacturer states acts like a rigid sponge that delamination. A current of 1 mA/ft2 was applied. stores salt-brine deicing solution, which is released when needed. Silica or basalt aggregates that are typically used It has been shown that cathodic protection overlays can to construct TPOs do not have the required absorption. be sprayed on horizontal and vertical surfaces (12). A PC Four overlays were placed on bridge decks on I-81 in Sep- made of vinyl ester and about 60% by weight of calcined tember and October 2005: two two-layer deicing overlays coke breeze can provide a non-sag material for overhead. and two one-layer VDOT epoxy overlays. The overlays were treated approximately every 2 weeks with deicing salt Wet Surfaces brine at a nominal rate of 70.5 L per lane-kilometer (30 gal per lane-mile) using a spray bar. Four overlays were placed Ahn and Fowler (42) found that the addition of zinc diacry- on Smart Road in September 2006: two two-layer deic- late, in the range of 5% to 15% by weight of MMA, resulted ing overlays and two two-layer epoxy overlays. The two in an increase in tensile bond to wet concrete by up to 50% deicing overlays were pretreated with salt brine at a rate compared with no zinc diacrylate. The addition of the same of 70.5 L per lane-kilometer (30 gal per lane-mile). One of amount to polyester-styrene also produced a very good the epoxy overlays was pretreated at the same rate and the increase in tensile bond, including smooth, wet surfaces. other was untreated (43). Calcium diacrylate worked much better with epoxies, giving up to 20% increase in strength. Although this has not been I-81 Overlays used in TPOs, it is a viable solution. The aggregates used in the deicing overlays had significantly Deicing Overlays higher absorption (1.70%) compared with the quartz (0.72%) and basalt (0.45%) aggregates used in the epoxy overlays. Virginia Department of Transportation (VDOT) conducted The soundness loss for the deicing aggregates was much an evaluation of a patented deicing overlay system designed higher (6.0% to 21.6%) compared with the loss for the epoxy to prevent frost formation and the bonding of ice and snow overlays (1.1% to 1.9%). It can be noted that the report indi- to the deck surface (43). The 9-mm-thick (3/8-in.-thick) cated that the deicing overlay aggregates reported in this overlay consists of epoxy binder and limestone aggregate study are no longer being used.