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       A sealant without filler is like apple pie without cheese. It’s doable, but the end result would probably be much better if it were added. Yes, the metaphor is a bit of a stretch, but adding filler to the sealant mix makes a huge difference. The combined benefits include improved traction, filling micro cracks and hiding minor surface imperfections, and extending the life of the sealant job.
       There are two types of aggregates available: silica sand and ground boiler slag. US Silica is one of several producers of high-purity silica sand. High-purity silica sand is available nationwide, according to George Dodson, the company’s marketing manager. “The geologic environments that produce high-purity silica sand are typically those where the sand is exposed to high energy levels for long periods of time,” he says. “Beaches, wind-blown dunes, or tidal areas with strong currents provide the right geologic environment for the concentration and washing of silica sand particles.” US Silica has 12 silica sand manufacturing plants in the eastern and central United States.
       Harsco Minerals (formerly Reed Minerals) produces ground boiler slag under the registered trademark BLACK BEAUTY® Abrasive. BLACK BEAUTY® slag is an inert byproduct of coal-fired power plants that is cleaned, ground and classified to produce uniform, angular particles with color and hardness characteristics that make it attractive to the sealcoating and paving industries. Harsco Minerals has 15 facilities throughout the eastern and midwestern United States.
       Both types of aggregate have their applications. Rounded sand provides excellent adhesion for sealants. With its shiny color, BLACK BEAUTY® abrasive imparts a shine to the pavement. Because the particles are more angular than sand, this aggregate is often used where greater adhesion is required or where the surface is particularly abrasive. Whichever aggregate is selected, it must be clean and dry, free of dust, clay, organic matter or other contaminants, and meet the grading requirements of the sealant manufacturer’s specifications, from 50 to 75 mesh AFS.
       There are four elements to a sealant mix: sealant concentrate, water, additives, and aggregate. Contractors typically find a mix that works for their application and stick with it, says Jeff Crenson, technical manager for GemSeal/Bonsal America. Exceptions are possible if a property manager or other client specifies something different. “In any case, it’s critical to have the right size aggregate and use the right amount,” Crenson says.
       STAR President Girish Dubey agrees. “For normal applications, the recommended dosage is 2 to 2.5 pounds of filler per gallon of undiluted sealant. For heavier coatings, the rule of thumb is to use 4 to 5 pounds per gallon, never exceeding the 5 pounds per gallon limit. If there is too much filler, the sealant will become brittle and lose its flexibility and resistance to chemicals, fuels, salts, etc.
       “When selecting aggregate size, it is important to ensure that the average particle size is very close to the cured film thickness of the sealant. The particles should be at least two-thirds submerged in the cured film thickness of the sealant. For example, two coats of sealant dry to approximately 11 to 13 mils. Therefore, the aggregate particles should be in the same size range. Aggregates with an AFS of 50 to 75 mesh typically meet this requirement.”
       Dubey notes that using too fine an aggregate, such as sand, can absorb too much of the sealant’s binder, reducing its performance. Conversely, large particles will not be able to bond and anchor in the cured sealant film and will likely fall off under the impact of traffic. “Loose sand,” he explains, “acts like sandpaper on the cured sealant film and can cause premature failure.”
       He continues: “When using crushed boiler slag, the same attention to detail in grading is required because boiler slag is much harder than sand and cannot absorb as much binder or moisture. This means that the aggregate will not adhere to the cured sealing membrane as tightly as sand and can be easily ‘knocked off’ if the grading is too coarse.”
       Crenson adds that the sealcoat formulation must meet several requirements. “It must have the right consistency for uniform application and be fluid enough to properly wet the surface, but not so thin that the material flows out of the aggregate, surface plateaus and into cracks. The consistency of the mixture must have enough volume to coat the aggregate and hold the coating while the sealant cures to anchor it into the pavement membrane.”
       He recommends adding filler last, after adding water and additives to the sealant concentrate and adjusting the design consistency and viscosity. “Additives are included in mixtures to improve drying, adhesion, color, and flexibility of the final product, and some of these additives have multiple benefits,” Crenson said.
       He adds that maintaining the consistency of the mix while the aggregate is suspended in the mixing tank helps ensure that the aggregate is evenly distributed throughout the sealant application. “If the aggregate sinks to the bottom of the tank when applying a liquid mix, the appearance of the cured coating can change, and it may be noticeably thinner in some areas and too rough in others.”
       STAR’s Dubey encourages contractors to buy aggregate in bags to ensure it is dry and can be accurately measured when designing the mix. “Ideally, you want to mix one bag per minute,” he says.
       The grade of aggregate particles is determined by their mesh size, which refers to the number of holes in the screen per inch. Therefore, aggregate particles with a lower mesh number are coarser than those with a higher mesh number. “Different grades of aggregate are classified according to their average particle size and are designated by an AFS (American Foundry Specification) number. For example, most AFS 60 grade particles will pass through a 60 mesh screen,” Dubey said.
       Sealant manufacturers such as STAR, Inc. and GemSeal/Bonsal American recommend using filler in their products. However, as Dubey points out, not every contractor understands the benefits that fillers such as sand bring to sealing or does not have the necessary equipment, such as a pump or agitator tank, to spray the filler-filled sealant.
       Both manufacturers offer resin-based and asphalt-based sealants. Dubay says, “While their properties are different (resin-based sealants are made from purified resin, asphalt sealants are made from refined crude oil), the applications for both are similar, including the use of aggregates. Dubay and Crenson recommend applying two coats of sealant for best results, including a machined look and concealment of minor surface imperfections, improved adhesion, and increased wear resistance.