courtesy of the Pavement Pro's of the National Pavement Contractors Association

By Girish Dubey 

Research has demonstrated that properly applied (and properly timed) sealcoat can save the owner of a property $100,000 or more — over the life of a hot mix asphalt pavement. But what does seal-coat actually do that results in this substantial savings?

Two types of sealcoat

Currently there are two primary types of sealcoating materials on the market: Those made from refined coal tar and those made from asphalt. Refined coal tar —a byproduct of the coking process— is a very complex mixture of thousands of chemicals and quite different in its molecular structure than asphalt. The coal tar molecules have a predominantly closed ring (aromatic) structure with a minor degree of un-saturation. Being stable in molecular structure, these chemicals do not allow the destructive elements of weather and chemicals to affect the property of coal tar. So for a variety of reasons, the most commonly used sealcoatings have, until recently, been based on refined coal tar. These sealcoatings act as "barrier coats" to protect asphalt surfaces. Sealcoatings based on refined coal tar were introduced in the 1950s and have been used extensively to protect off-street pavements. These often are referred to as C.T.P.E. (Coal Tar Pitch Emulsions), denoting that these coatings are water based, obtained by dispersing refined coal tar in a matrix of clay and water. The finished product is a stable, homogeneous composition that is applied with ease and safety in handling. The mineral fillers serve to extend the softening range of the refined coal tar so that the coating will be functional at normal pavement temperatures. The CTPE incorporates the protective features of refined coal tar and the reinforcing effect of mineral fillers, to offer a "barrier coating" that will flex with the pavement’s movements while protecting the asphalt from the destructive elements of weather, gas, oil, fat, chemicals etc. In recent years asphalt emulsion based coatings have been introduced with varying degrees of success. In fact, many sealer manufacturers that previously produced only refined coal tar sealers now also produce asphalt-based sealers or even asphalt/refined coal tar blends. The asphalt emulsions deliver most of the same properties as refined coal tar-based coatings —except for the resistance to color fading due to ultraviolet degradation, salts, and petrochemicals like oils, fats, grease and solvents. These deficiencies are inherent in the asphalt binder itself. Being a petroleum derivative, asphalt has a natural affinity for petrochemicals, so it is easily dissolved by them. Asphalt emulsion-based coatings are made using either a soap emulsion (SS-1-H, for example) or clay stabilized emulsions. Both types are suitable but the properties of clay stabilized emulsions can be controlled better through selection of the right ingredients.
In recent years sealcoat manufacturers have been quite successful refining the performance of asphalt emulsion sealers through the use of specialty chemicals and pigments. But asphalt emulsion’s resistance to petrochemicals and solvents — while improved — has yet to be overcome. In spite of this deficiency, sealcoaters have recognized some definite advantages of asphalt emulsion over refined coal tar sealers: Asphalt emulsion sealcoats are more "user friendly," practically odorless, and do not irritate and burn the skin (features especially important for people with respiratory conditions or sensitive skin). It is noteworthy, however, that the Federal Aviation Administration Specifications (P-625, 627 and 628) accept only refined coal tar-based sealcoatings for use on airport projects, because the refined coal tar based sealcoatings offer greater resistance to jet fuel.

Sealer manufacturers always supply sealer — whether asphalt or refined coal tar — in a concentrated form that has to be diluted 25% to 30% by volume with water and mixed with sand or aggregate for proper textured appearance and non-slip properties. The quantities of water and sand or aggregate are conventionally expressed as a percentage or quantity based on the amount of concentrated sealer. For example, 25%-30% water will denote 25-30 gallons of water added to 100 gallons of concentrated sealer (as supplied by the manufacturer). Similarly, 2-2.5 lbs. of sand per gallon will mean 200-250 lbs. of sand added to 100 gallons of concentrated sealer. The sand must be clean, hard, angular and fall within a specified range of particle size gradation. Too many fine or coarse particles will detract from performance. The relative quantities of binder (asphalt emulsion or refined coal tar), clay, and fillers are crucial to the performance of the sealer. Excessive amounts of clay and fillers in the sealer formulation will produce porous cured films due to insufficient binder, and thus poor performance. Such sealcoat films tend to lack flexibility and wear pre-maturely. Similarly, excessive amounts of sand or aggregate in the mix design degrade the performance in the same manner. Conversely, an excessive amount of binder (asphalt or refined coal tar) might produce tackiness under hot climatic conditions, even after the full cure. Using standard mix designs, both asphalt emulsion sealers and refined coal tar sealers are capable of suspending sand, holding it in wet film, and keeping it bound  in the cured film. However, when stretched beyond its capability, the sealer might not suspend the large quantities (more than 5 lbs.) of sand and definitely will not keep large quantities of sand bound in the cured film. Sand and aggregates, like any other filler, have their own binder requirements (the surface of the sand will absorb the binder -refined coal tar or asphalt- from the sealer). Used in excessive amounts, sand will rob enough binder from the sealer film which would have been otherwise available to form a continuous film on the pavement. But for some jobs it is necessary to add larger amounts of sand to fill in the profile of badly weathered pavements and produce a uniform textured appearance. In such instances special mix designs using specialty rubber additives are used that offer satisfactory performance.

Sealcoating can save real dollars for pavement owners. Unsealed pavements will require repairs starting with the second year and could require a one-inch overlay as often as every seven years. Cost savings will be a substantial 65% if the pavement is maintained regularly. Estimated savings for a 10,000-sq.-yd. asphalt pavement are $127,000 over15 years.