Asphalt is the most extensively used material for paving highways, roads, parking lots, home driveways, etc., due to its outstanding binding (for the aggregates) and waterproofing properties. Asphalt, however, has poor resistance to chemicals, salts, oils and sun’s ultraviolet rays. Pavements with low to medium traffic (parking lots, airport aprons, restaurants, gas stations, schools, etc.) deteriorate under the attack from these damaging elements. If not protected, such pavements become useless and have to be repaired or replaced to keep them functional. Highways and major roads have the benefit of continuous rolling traffic, which keeps the pavement functional through the kneading action of the traffic.
Low to medium traffic pavements can be protected and preserved through the use of sealcoatings, which are specialty water-based coatings, especially designed to resist the destructive elements of weather and chemicals. Sealcoating is a protective measure, designed to keep an asphalt pavement lively and attractive. The very term "sealcoating" means keeping the redeeming properties (like waterproofing, flexibility) of asphalt sealed in to extend the pavement life and preserve its functional properties. After all, the pavement is a sizable investment. In this article we will review;
The major strengths of asphalt are its adhesive (gluing), waterproofing and flexibility properties. Asphalt pavements are not what you see on the surface. It goes much deeper. The asphalt mix, which you see, is only the roof, so to speak, of the entire pavement. It covers a bed of graded stone aggregates of varying depths according to ground conditions as well as the traffic requirements. The base of aggregate is what really carries the load of the traffic. The strength of a pavement is directly related to its design from the ground up. Asphalt mix provides a firm resilient surface over the stone base and keeps the pavement bed dry.
Hot asphalt, mixed with stone aggregate and mineral filler is most commonly used paving material. The hot mix contains 4-12% asphaltic binder (AC or asphalt cement) the rest being aggregates and mineral. The mixture is made in a hot mix plant by blending selected aggregates and mineral fillers with hot molten asphalt. The mix is then transported to the paving site, spread in a uniform thickness and compacted with rollers. As the pavement cools it becomes firm and ready to carry traffic loads.
Asphalt has poor resistance to sun’s ultraviolet rays, petrochemicals, salts, etc. due to its chemical make up. Asphalt is a very complex mixture of thousands of chemicals, which are predominantly open chain (aliphatic) in structure, with a considerable degree of unsaturation within their molecular structure. The open chain and unsaturation make asphalt an easy target for deterioration. The first visual sign of this phenomenon is a progressive change in the color of asphalt pavement: rich black to brown to gray.
Asphalt is a by-product of petroleum distillation so are gasoline, mineral solvents, oils, grease, fat, etc. When it comes in contact with these materials it is easily dissolved by them. The treason is quite logical. Asphalt has a natural affinity, after all they existed in nature in form of crude petroleum, separated only through the process of petroleum refining. Additionally, asphalt has poor resistance to deicing salts.
Major roads and highways, having the advantage of continuously rolling traffic do not need protection. The rolling action of the traffic steadily brings up the lower layers, rich in asphalt, to the surface and kneads the oxidized surface layers back into the pavement. In due course, depending on traffic volume and the climatic conditions, all the asphalt binder is eventually exhausted and the aggregates begin to unravel due to the absence of binding cement (asphalt). The next step is the development of minor cracks, which widen and deepen with time. If the cracks are not repaired at this stage, water seeps into the base courses and damages the load bearing capacity. It is evidenced by rutting, shifting and serious alligatoring. The roads must then be either overlaid with fresh asphalt mix or the pavement completely removed and reinstalled, depending upon the condition.
The need to protect asphalt pavement generally applies to “off-street pavements” which are defined as those carrying very low traffic e.g. parking lots, minor streets, airport aprons, runways, service stations, home driveways etc. Off-street pavements do not have the advantage from the "kneading action" unlike the major roads. The surface layers under continuous attack by the weather and other destructive elements develop minor surface cracks. Again, aggregates start unraveling producing minor cracks, which widen and deepen with time. The damage will continue if proper protective actions are not taken.
Based on the foregoing facts, it would be logical to conclude that off-street pavements can be preserved if a "protective coating" is applied over the surface. It will form a barrier and shield the pavement from attack by the damaging elements of weather, chemicals and salts. The sealcoating should be applied as soon as the oils from the surface of the pavement are oxidized and dissipated.
Sealcoatings are stable dispersions of refined tar or asphalt, ball clays, fillers, emulsifiers and specialty chemicals. The dispersion is obtained either in a high-speed disperser (batch process) or a colloid mill (continuous process). The resulting product is thixotropic with heavy paint consistency. The product in wet state is chocolate brown in color that dries to black/dark slate. Contractor grade sealcoatings are supplied either in a concentrated which must be diluted with water and sand/aggregate added, for proper application consistency. For retail customers it is supplied in a ready to use form, packaged in 5-gallon pails.
Sealcoatings based on refined tar (RTS) were introduced in late 50s and have been used extensively to protect off-street pavements. Refined coal tar is also a very complex mixture of thousands of chemicals but quite different in their molecular structure than asphalt. The molecules have a predominantly "closed ring" (aromatic) structure with a minor degree of unsaturation. Being stable in molecular structure, these chemicals do not allow the destructive elements of weather and chemicals to affect the properties of coal tar. The most commonly used sealcoatings are, therefore, based on refined coal tar.
In recent years, sealcoatings based on asphalt emulsions have been introduced with a considerable degree of success. Such coatings are fortified with rubbers, specialty chemicals, additives etc. to overcome some of the basic deficiencies in asphalt binder.
Sealcoatings have the ease of application and clean up of water based systems. Prior to application of sealcoatings, pavement must be properly cleaned, oil spots primed and the damaged areas repaired.
Sealcoating is mixed with 25-30 gallons of water and 200-300 lb. of sand/aggregate per 100 gallons of sealer. This yields a good workable consistency. Commonly two (2) coats are applied at the combined coverage rate of approx. 35-40 square feet/ gallon of the mixture. When applied per recommendations and instructions of the manufacturer, the sealcoating will last two to three years, depending on traffic frequency. Both hand or machine applications are used.
Sealcoatings applied at the recommended rates to asphalt pavements;
Sealcoatings have been successfully used for protection and preservation of asphalt pavements over 40 years for all types of commercial and residential applications. They impart beauty and value while protecting one of your major investments, asphalt pavement.
Girish C. Dubey is the president of STAR, INC. He has 40 years of professional experience in the coatings industry. He holds master’s degrees in polymer and inorganic chemistry and has authored several papers on coatings and related subjects. He also has lectured at many technical conferences. He was the chair of The Pavement Coatings Technology council in 2000.