Asphalt is a highly durable paving material, one that can easily deliver lifespans of twenty years or more. Yet despite its inherent strength and stability, asphalt still remains vulnerable to damage if not protected. Constant exposure to water and ultraviolet rays from the sun eventually causes the surface to oxidize, making it much more prone to structural issues.

Fortunately, you can minimize such problems by periodically having your asphalt treated with protective coating. Depending on the exact state of your asphalt, several different coating options exist. Distinguishing between them can be confusing, especially since many terms are used interchangeably. This article breaks down the differences between two types of coating — sealcoat and slurry coat.

Sealcoat

Sealcoat is specifically designed to rejuvenate an asphalt surface. To understand why this is necessary, it first helps to understand a bit more about the nature of asphalt, as well as what happens as asphalt ages.

An asphalt surface consists of two main ingredients: crushed gravel aggregate and a sticky asphalt binder made from petroleum derivatives. When hot, the asphalt binder has a semi-fluid nature that allows it to be more easily spread. As the paving cures, however, the binder thickens up, essentially locking the aggregate chunks into a stable matrix.

As time goes on, asphalt binder continues to change. Oxidation as the result of environmental exposure weakens the binder, making it more susceptible to damage. In particular, the asphalt loses much of its resistance to water. Serious problems like cracks, potholes, raveling, and shoving soon ensue. 

Benefits of Sealcoat 

On a structural level, this interaction has three key benefits. First, the sealcoat renews the asphalt's natural resistance to water, both waterproofing the top layer of asphalt and also making it more difficult for water to penetrate to the vulnerable deeper layers.

Second, sealcoat protects against oxidation caused by UV exposure. Third, sealcoat increases asphalt's protection from oil and other potentially destructive chemicals. Finally, it should be noted that seal coat also improves asphalt on an aesthetic level, by restoring its lustrous black appearance.

Composition of Sealcoat

There are two main types of sealcoat available on the market today: those made from coal tar, and those made using asphalt emulsion. Most contractors agree that sealcoats made from coal tar tend to exhibit more consistent results. The quality of sealcoats made from asphalt emulsion, by contrast, tend to vary more depending on the producer — and even the particular batch.

That said, differences in cost and availability often make asphalt emulsion-based sealers more economically viable. As a result, asphalt emulsion sealcoats are becoming more and more common — and more and more stable. Many manufacturers have also begun to offer blended sealcoats made from various proportions of both coal tar and asphalt emulsion.

In either case, all sealcoats contain numerous other ingredients in common. Water is used to dilute the sealcoat to a thin enough consistency that it can be easily applied to the asphalt using sprayers. Likewise, most sealcoats contain mineral fillers, polymers, and other secondary additives meant to improve attributes like consistency, toughness, UV-resistance, color uniformity, and chemical resistance.

Application and Curing

Depending on the size of the asphalt surface being treated, sealcoat can be applied either by hand, or through the use of truck-mounted pressurized sprayers. First, however, the surface must be thoroughly cleaned to remove all traces of dirt, oil, and debris. Any foreign matter present on the paving will seriously undermine the sealcoat's ability to bind with the asphalt.

After sealcoat has been applied, it must be allowed to fully cure before cars can pass over the pavement again. During this time, the water in the sealcoat will be evaporating, leaving behind a film of sealer. Curing may take between several hours and a couple of days, depending on environmental conditions such as temperature and humidity. For optimal curing rates, most contractors recommend sealcoat asphalt during warmer, drier months.

Slurry Seals

Slurry seals are very closely related to sealcoats. In fact, many paving contractors use the two terms interchangeably. That said, on a technical level, there is one compositional difference between the two protective coats. Unlike sealcoats as strictly defined, slurry seals contain one extra ingredient: aggregate.

As asphalt ages, and the binder weakens, chunks of aggregate are often stripped from the surface. A sealcoat is unable to compensate for such areas of loss. As a result, small depressions may remain across the surface, giving water a place to pool up and begin wreaking havoc. The aggregate in a slurry seal, by contrast, helps to fill in those small cracks and gaps and promote a more even surface.

Types of Slurry Seal

To make things more complicated, slurry seals can be broken down into three different categories depending on the size of the aggregate they contain. Type I slurry seals contain fine aggregates, with diameters less than 2.36mm. This type of slurry seal is commonly used to treat lower traffic areas such as parking lots. 

Type II slurry seals contain aggregate particles with maximum diameters of 6.4mm. This larger aggregate allows the seal to repair raveling and other more advanced states of wear usually found on roads. Contractors usually avoid using these slurry seals on parking lots, since the larger aggregate pieces can cause tire scuffing as cars turn in and out of parking spots.

Finally, Type III slurry seals contain coarse aggregate chunks. These seals are used to compensate for more severe types of surface defects. The large pieces of aggregate can also fill depressions in the surface, thus reducing the amount of water ponding.

Slurry Seal vs. Sealcoat

The aggregate particles found in all types of slurry seals tend to improve overall adhesion and integration with the existing pavement. As a result, slurry seals tend to last longer than sealcoats. Whereas contractors recommend sealcoating asphalt roughly every two years, slurry seals only need to be applied every five to seven years.

That said, there is no clear cut winner between the two restorative techniques. Rather, a contractor must consider not only the type of pavement being treated, but also its overall condition. To learn more about which type of seal would make the best choice for your asphalt, contact the industry experts at Empire Parking Lot Services. Please check out our "Anatomy of a Parking Lot" blog for a more in-depth view of what makes up a parking lot.