So determine the mix design for a two-coat application on each job as follows: Concentrated sealer needed for:
- 1st coat @ 0.12 gal./sq. yd. = 0.12 x 10,000 = 1,200 gals.
- 2nd coat @ 0.08 gal/sq. yd = 0.08 x 10,000 = 800 gals.
- Total = 0.12 + 0.08 = 0.20 gal. concentrated sealer/sq. yd.
- Total concentrated sealer needed = 1,200 + 800 = 2,000 gals.
Mix Design: 30% water dilution and 4 lb. of aggregate/gal. of the concentrated sealer.:
- Sealer = 2000 gals.
- Water = 600 gals. (30% of the sealer)
- Aggregate = 8,000 lbs. (4 lbs./gal. based on sealer concentrate = 8,000 lbs. x .0462 = 370 gal. of sand
Total mix design volume:
2000 gal. sealer concentrate + 600 gal. water + 370 gal. sand = 2,970 gal. total sealer mix
For Job A, the contractor does it right and applies 2,970 gallons of the mixed sealer to 10,000 sq. ft.
For Job B, the contractor misreads the specifications, confusing the concentrated sealer application rate for the mixture and applies only a total of 2000 gallons of the mixture. It does happen and now let's look at what happens and how the film thickness is affected, if you don't realize the basis for the recommended coverage rates.
Job A: The correct result
The contractor on Job A has produced 2,970 gallons of final sealer mix. This means that once all material is added to produce the final mixture of sealer, 0.20 gallons of concentrated sealer produces 0.297 gallons of sealer mixture. So the coverage rate of the sealer mixture is: 2,970 gal. ÷ 10,000 sq. yds. = 0.297 gal./sq. yd.
The job specifies that 0.2 gallon of the concentrated sealer /sq. yd. be applied to the pavement. To put down 0.2 gallon of concentrated sealer, you will have to use 0.297 gallon of the mixture. Now you need to determine a new coverage rate based on the final mixture. To do that you divide the application rate of the mixture by the specified application rate: 0.297 ÷0.20 = 1.49 giving you the ratio between the mixture and concentrated application rates. To determine the new application rates of the mixture, multiply the specified application rate by the ratio (1.49).
- Apply 1st coat of the mixture @ 1.49 x 0.12 = 0.18 gal./sq. yd.
- Apply 2nd coat of the mixture @ 1.49 x 0.08 = 0.120 gal./sq. yd.
- Total of the two coats of the mixture is 0.18 + 0.12 = 0.30 gal./sq. yd.
Film Thickness of the wet film. To determine how thick the wet film will be, 1st Coat: first convert 0.18 gal./sq. yd. to sq. ft./gal. following the formula introduced earlier.
1 ÷ 0.18 = 5.56 sq. yard/gallon, 5.56 x 9 = 50.04 sq. ft/gallon
Wet film thickness of the 1st coat = 0.18 gal./sq. yd. = 50.04 sq. ft./gal. = (1604 ÷ 50.04) or 32.05 mils;
For the second coat, 1 ÷ 0.12 = 8.33; 8.33 x 9 = 74.97 (call it 75) 2nd coat = 0.12 gal/sq. yd. = 75 sq. ft./gal. = (1604 ÷ 75) or 21.39 mils.
Total wet film thickness of two coats = 32.05 + 21.39 = 53.44 mils.
Film thickness of the dry film.
Multiply the wet film thickness by percentage of solids by volume of the mixture (see Chart 2).
- 1st coat = 32.05 mils. x 39.4% = 12.63 mils,
- 2nd coat = 21.39 mils. x 39.4% = 8.43 mils,
- Total wet film thickness of two coats = 12.63 + 8.43 = 21.06 mils.
This (21.06 mils) is essentially the correct film thickness, when the sealer was applied per specifications.
Job B: Too little sealer
Now let's see what will happen to the total film thickness on the same 10,000-sq.-ft. job with the same mix design but the contractor misread and applied the mixture at the coverage rate that was specified for the concentrated sealer. The contractor applies two coats:
- 1st coat of the mixture @ = 0.12 gal./sq. yd.,
- 2nd coat of the mixture @ = 0.08 gal./sq. yd.,
- Total of the two coats of the mixture = 0.12 + 0.08 = 0.20 gal./sq. yd.
Note: Because the contractor has made a mistake in reading the specifications, there is no need to determine the ratio between the concentrated sealer and the final mixture, as we did on Job A. The contractor applies application rates directly to the total sealer volume.