Coverage rates dictate the quantity of sealer applied on the pavement. It is imperative that the right quantity of sealer is applied for contractors and their clients to get its optimum performance.
Sealer manufacturers recommend appropriate coverage rates for their product, but coverage rates also are recommended by local, state, and federal agencies. Any deviation from the coverage rate recommendation will alter the amount of the sealer (refined coal tar or asphalt) applied on the pavement. Too little sealer applied might result in premature wear and failure, while too much will drive the job cost up and might even result in slow drying and tracking.
Coverage rate is most commonly specified in terms of gallons per square yard (gal./sq. yd.), but occasionally it is expressed in terms of square feet per gallon (sq. ft./gal.). In this article we will work through some calculations, so it will help to have a calculator on hand to work through the calculations as you read.
Note the examples and calculations made in this article are for educational purposes only. Actual calculations and figures, such as film thickness, will vary according to mix designs. Also, calculations used in this article apply to both refined coal tar and asphalt-based sealers.
Determining wet and dry film thickness
Sealcoating contractors know that sealer, after it's applied, dries to a tenacious film after full cure. The film thickness of the wet and the dry sealer depends directly on the coverage rate and it is expressed in “mils.” One (1) mil is 1/1000th of an inch. Film thickness is generally referred to in mils simply because it is much easier than referring to hundredths of an inch.
At the nominal application rate of 0.18 gal./sq. yard, the wet film thickness of the film is 32 mils (or 0.032 inch). After full cure (after release of water and other volatiles) the film shrinks to about 13 mils, which is about the same as the thickness of three sheets of 20-pound paper.
Now let's convert the coverage rate in film thicknesses, both wet and dry. To calculate the film thickness, we have to know the coverage rate in terms of sq. ft./gallon.
But before we can start we need to know how many square feet of pavement are covered by one gallon of coating in one (1) mil thickness. The answer is 1,604 square feet; one gallon of sealer will cover 1,604 square feet of pavement with a coating 1 mil thick. If the film thickness is 4 mils, the area covered will be 1,604 ÷ 4, or roughly 400 sq. ft./gallon.
Conversely, if you divide 1,604 by the square foot coverage rate you will get the film thickness in mils (1,604 ÷ 400 = 4 mils). But if the coverage rate is specified in gal./sq. yd., you need to convert it into sq. ft./gal.
To convert, divide 1 by the number of gallons per square yard, then multiply the answer by 9 (the number of feet in a square yard) to reach the number of square feet per gallon.
To illustrate, let's convert 0.18 gallons per square yard to square feet per gallon. First, divide 0.18 into 1 and the answer is 5.55. Multiply 5.55 by 9 and you get 50, which is sq. ft./gal. Wet film thickness, then, is: 1,604 sq. ft. ÷ 50 sq. ft./gal. = 32 mils.
The next step is determining the dry film thickness. To do that multiply the wet film thickness by the solids content (by volume) of the mix.
Suppose 1 gallon of sealer mixture (sealer, water, aggregate, etc.) contains 40% by volume of solids. Once sealer has been applied to the pavement, and the water and volatiles have evaporated from the sealer, the sealer film, after full cure, will be reduced to 40% of its original wet film thickness.
Why is it reduced in thickness only, when geometry teaches us that volume is a multiple of the area and thickness? The answer is that the sealer film can shrink only in thickness because the sealer is bonded to the other dimension, which is the area of the pavement. The sealer will not pull away from the pavement edges like oil does when it's dropped on a wet surface.