So now you know all the elements for calculating film thicknesses: factor 1604, coverage rate in sq. ft./gal., wet film thickness, and solids by volume of the sealer mix. So the question that should pop into your mind is, how do you calculate that percentage of solids (by volume)? That brings us to the sealer composition.
You will note later that it is crucial to know whether the recommended coverage rate is based on the concentrated sealer (before water and aggregate are added), or the final sealer mix (which includes the water and aggregate). Huge and costly errors have been made by contractors not knowing these coverage rate details. So please read the job specifications very carefully and your supplier will be delighted to assist you, if you have questions.
Sealers are stable dispersions of refined coal tar or asphalt in water, with clay or other fillers, and additives, such as emulsifiers and specialty chemicals. Chart 1 shows the typical composition of a contractor-grade sealer (ASTM 5727-00, what in the past had been a P355e type sealer), given both by weight and by volume.
It is important to note that the concentrated sealer contains about 60% water by volume and 50% by weight. After application, the sealer dries and cures releasing the water (and other volatiles). What is left behind as the film is the solids portion (coal tar or asphalt, clay/fillers, and additives), after the sealer is fully cured.
It is the percentage of solids by volume that determines the film thickness, not the solids by weight. So from this point on the article will deal only with volume – not the weight – of the sealer and its components.
Material requirements and mix design
Concentrated sealer is seldom applied as such on the pavement. Virtually all applications require the contractor to mix the concentrate with a specified amount of water, often aggregate (silica sand or boiler slag for coal tar sealers) and sometimes additives. Sealer manufacturers or government agencies also often specify the number of coats and the coverage rate for each coat.
Most of time the manufacturers specify coverage rate in terms of the concentrated sealer and that is strictly to calculate the quantity of the sealer needed for a job. Some governmental specifications (such as the Federal Aviation Administration) denote the coverage rate in terms of the final mixture. So it is imperative that the contractor make sure he is mixing the components in the proper amounts according to the manufacturer's recommended proportions and that he applies the sealer to abide by the coverage rate requirements.
Now let us look at some examples:
JOB A. This is a 10,000-sq. ft. parking lot. The recommended coverage rate is based on the concentrated sealer. The final mixture is applied at the mixture coverage rate, so the cured sealer film will be according to the specification, resulting in optimum performance of the sealer.
JOB B. This is also a 10,000 sq.-ft. parking lot, but the contractor misreads the specification and instead of applying sealer at a coverage rate based on the mixture, he applies the final mixture at the rate of the concentrated sealer. The result is the cured sealer film will be much thinner than intended and might result in a job that wears poorly or looks bad.
See Chart 2 for mix components. For both examples we will use a general recommendation of:
- Water for dilution – 30% by volume of the concentrated sealer.
- Aggregate loading – 4 lb./gal. based on the concentrated sealer.
The percentage of sealer in Chart 2 composition is 67.3% (100 gallons of sealer ÷ 148.5 total gallons = 67.3%).
The percentage of solids (by volume) in the above mix is 58.5 (40 sealer solid gallons + 18.5 aggregate gallons) x 100 ÷ 148.5 or 39.4%. The sealer will shrink to 39.4 % of its wet film thickness.
For both Job A and Job B the specified coverage rate is based on concentrated sealer – without water, additives, or aggregate added. Specified material coverage rates, in a two-coat application, are:
- 1st coat @ 0.12 gals. of concentrated sealer/sq. yd.,
- 2nd coat @ 0.08 gals. of concentrated sealer/sq. yd.,
- That's a total of 0.20 gal. of concentrated sealer/sq. yd. for two coats.