He continues, “The same attention to gradation detail is important when using crushed boiler slag, which is much harder than sand and incapable of absorbing as much binder or water. This means that the aggregate will not be as tightly held in the cured sealer film as sand, making it susceptible to being ‘knocked off’ if the grade is too coarse.”
Adding the Aggregate
Crenson says a sealcoat mix design has to meet several requirements. “Among them, it must be the right consistency to achieve uniform application and flow adequately enough to fully wet the surface, but not so runny that material drains off the aggregate and surface plateaus into crevices. The mix consistency needs to have enough body to envelop the aggregate and keep it coated as the sealant cures to lock it into the coating membrane.”
He advises adding the aggregate last, after water and additives have been mixed in with the concentrated sealer and the consistency and viscosity of the design have been adjusted. “Additives are included in the mix design to improve drying, adhesion, color, and elasticity of the end product, with some having multiple benefits,” Crenson notes.
A mix consistency that keeps the aggregate suspended in the mix tank helps to ensure uniform distribution of aggregate throughout the seal coating job, he adds. “If a thin mix allows aggregate to sink to the bottom of the tank during application, variation in the appearance of the cured coating could be noticeably thin in some areas, and excessively coarse textured in others.”
Dubey encourages contractors to purchase their aggregate in bags to ensure it is dry and can be measured accurately as part of the mix design. “Ideally, one would add in one bag per minute with agitation,” he points out.
Aggregate particle gradation is determined by its mesh size, which represents the number of openings per inch in a screen. Hence, an aggregate with a lower mesh number has particles that are coarser than an aggregate with a higher mesh number.
“The different grades of aggregate are classified according to their average particle size and denoted as AFS. For example, AFS 60 grade has the majority of particles passing through a 60-mesh screen,” Dubey says.
Sealer producers such as Star Inc. and GemSeal/Bonsal American recommend using an aggregate with their products. As Dubey notes, not every contractor is aware of the benefits that an aggregate brings to the sealcoating job or has the right equipment -- a machine with a pump capable of spraying a sealer with aggregate or a tank with agitation.
Both producers offer refined coal tar- and asphalt-based sealers, and Dubey says that despite their different properties aggregates should be used in both sealer types to improve the finished sealcoating job.