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33 A binder system is required that has a viscosity suffi- seal coat to help bind the aggregate that is seeded over the ciently low to spread easily, and relatively thinly, over the surface for additional skid resistance. Epoxies and methac- deck, bonding well to the deck and to the aggregates that are rylates are the recommended resins for slurry overlays. dispersed into the surface. Additionally, the binder system must be low enough in solvents and nonpolymerizing chemi- Cured Properties cals to preclude pinholes or permeability. Finally, the cure system for the binder must provide adequate working time to The cured binders need to possess certain properties to apply the binder wherever needed and to adequately broad- perform well in TPOs. These properties include high bond cast and bond the aggregate into the binder before becoming strength to concrete substrates and the embedded TPO too viscous, and then curing relatively quickly. The recom- aggregates, high tensile elongation, and a very low modulus mended binder for multiple-layer overlays is epoxy. of elasticity (compared with the concrete substrate) to off- set the higher coefficients of thermal expansion. They must Premixed Polymer Overlay also exhibit very low permeability to water, as well as good resistance to tire abrasion, acid rain, concrete alkalinity, and The premixed TPO is typically specified for thicker over- ultraviolet exposure. lays, typically 75 mm (0.75 in.) and up, used for accommo- dating uneven or rough riding surfaces, or where the thicker, Commercially available binder systems for the TPOs lower modulus polymer concrete can resist the stress and include several types of polymers, including epoxies (modi- impact of tire chains. Epoxy and polyester-styrene are the fied with copolymers in some cases), styrenated unsaturated recommended resins for premixed overlays. polyesters, vinyl esters, and polyurethanes. Table 2 lists properties for several of the binder systems that are used on The requirements of the binder for the premixed TPO are bridges (15). not necessarily different from the binders used in the mul- tiple-layer system. However, because the premixed system incorporates the aggregates into the resin before placement AGGREGATES on the deck, resin viscosity and rheology would permit a low percentage of resin while permitting good workability. TPOs require clean, dry, hard aggregates, including angu- lar silica sand, basalt, trap rock, or flint. Most contractors The premixed systems usually require a primer to ensure a use prebagged aggregates supplied by the overlay material good durable bond between the cohesive polymer matrix and supplier to ensure that the aggregates will be free of dirt, the deck. For polyester-styrene resins, the primer is typically dust, oils, and moisture, and will have the correct grading for a high-molecular-weight methacrylate that penetrates into the specific application. Known standard bag weights also the concrete surface and provides for excellent mechanical make it easier to keep track of the aggregate application rate, bond to the concrete while providing chemical bonding to the although large bags can result in segregation and the possi- overlay matrix. Additionally, the primer prevents long-term bility of the fines collecting in the bottom of the bags. deterioration of the polyester at the concrete interface owing to alkaline attack when wet. Premixed systems are cohesive Multiple-Layer Aggregates enough to be tined or screeded to provide more skid resis- tance. Aggregates are sometimes broadcast over the screeded The aggregates typically specified for multiple-layer TPOs surface to enhance the surface friction of the overlay. are hard (6 or higher for basalt and 7 or higher for other min- eralogies on Mohs scale), angular, and tough (nonbrittle), Slurry Overlays and they are typically either single-sized or a gap-graded blend of several complementary sizes. Basalts (containing at Like the premixed system, the slurry incorporates aggre- least 10% aluminum oxide), calcined bauxite, some natural gates into the binder before placement on the deck, but the granites, and angular grained silica sand are all commonly lower viscosities of their binders, such as epoxies and meth- used. Their size is usually very near a no. 8 sieve to keep the acrylate, require a well-graded fine filler component to help overlay thin, yet skid resistant. support and more uniformly disperse the larger sand par- ticles at the desired depth. They are normally thinner than Premixed Aggregates premixes and slightly thicker than multiple-layer systems, somewhere between 6 and 12 mm (0.25 in. and 0.5 in.). They The aggregates for a premixed system require a few smaller are frequently applied with a gauge rake and sprinkled with sizes to uniformly distribute, embed, and support the largest an angular aggregate. aggregates that provide durable surface friction. The aggre- gates used in the premixed systems tend to be well graded Slurries also rely on a primer of the substrate before the and more regularly shaped, like siliceous river pea gravel placement of the overlay. Slurry systems usually require a and natural river sand, because they pack more easily and