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32 CHAPTER FOUR PROVEN PRACTICES Based on the findings from the literature and the agency OVERLAY TYPES surveys, the following practices have been found to be useful. These are not intended to be specifications; rather, The types and recommended uses for each are as follows: they summarize the important preservation methods that agencies/owners have found to produce long-lasting, Multiple-Layer Overlays durable TPOs. Multiple-layer overlays are best used on decks that have good ride quality because the overlays follow the contours CANDIDATES FOR OVERLAYS of the deck surface (15). Sprinkel (15) states that the bridges that are the most likely Slurry and Premixed candidates "(1) are those that are in need of a skid-resistant wearing and protective surface but have peak-hour traffic Decks that have many surface irregularities are the best can- volumes that are so high that it is not practical to close a lane didates for slurry and premixed TPOs (15). to apply the surface, except during off-peak traffic periods" and (2) "are those in which increases in dead load, reduc- tions in overhead clearance, and modifications to joints and MATERIALS drains must be held to a minimum." Binders Harper (7 ) concludes, "Epoxy polymer overlays are not a `repair' for bridge decks. They are only a means of protect- Many polymer-based binder systems have been developed ing a deck that is in fairly good condition but is at risk for for protecting bridge decks. Epoxies (including modified chloride and water penetration. Decks that have more than 5 epoxy urethanes), polyester-styrenes, and MMAs have been to 10% of area that is unsound have been found to continue the most widely used. Earliest attempts to seal decks from to experience problems after the overlay is placed." ingress of water and waterborne deicing chlorides with poly- meric membranes emphasized the need for including aggre- Sprinkel (15) states that projections suggest that, with gates to provide skid resistance in wet or freezing weather. the exception of the methacrylate slurry and the multiple- To accommodate this need, three overlay methods evolved. layer polyester overlays, TPOs constructed in accordance The first, the multiple-layer method, emulated asphaltic seal with AASHTO specifications (6 ) should have a service life coats, and the second, the premixed, as well as the third, of 25 years. slurries, emulated asphalt concrete overlays. Carter (8) states that (1) TPOs properly applied can pro- Multiple-Layer Overlay vide service lives of up to 20 years, but that maintenance will be required if the surface is intended to remain free of The multiple-layer PC overlay is thinly applied and often defects. (2) TPOs can be used in high-salt environments to used to seal the bridge deck while masking any unsightly extend the lives of existing bridges containing noncoated repairs under a well-bonded, uniform, durable, skid-resis- steel, even if some corrosion has begun prior to repair. (3) tant cover. This method is often referred to as "broom-and- TPOs are economically competitive with other repairs, espe- seed" because the method involves spreading the somewhat cially when a minimum of repairs to the deck are required viscous resin system over the deck and then immediately and a minimum of resin is used; that is, the overlay thickness seeding the surface with the aggregate. After the first appli- is less than 10 mm (0.40 in.). (4) TPOs are more suited for cation has set, a second broom-and-seed operation is per- preventive maintenance than for rehabilitation. Resins are formed and allowed to cure. It may be repeated again as too expensive to be used on excessively rough or deterio- needed or done only one time if adequate skid resistance or rated concrete surfaces. waterproofing can be achieved.