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Article "Hazards of Altering Sealer Mix Designs" reprinted from January 1998 issue of "Pavement Magazine". This article explains the problems of mixing and mix designs by contractors who have "altered" the mix to be "low bid." Please contact us if you would like a copy of the article.
HAZARDS OF ALTERING SEALER MIX DESIGNS
Manufacturer's make recommendations for good reason- to produce an optimum sealcoat.
By Girish Dubey
As supplied by the manufacturers, asphalt emulsion or refined coal tar sealers are always supplied in a concentrated form that must be diluted 25% to 30% with water. The sealer then, in most cases, needs to be mixed with sand or aggregates to provide both a proper textured appearance and to give the sealer its non-slip properties.
It's an industry convention that the quantities of water and aggregates are expressed as a percentage or quantity based on the amount of concentrated sealer. For example, 25% or 30% water will denote 25 to 30 gals. of water that should be added to 100 gals. of concentrated sealer (as supplied by the manufacturer). The same is true for the amount of aggregate recommended as well as for the amount of additive used.
What happens when the
contractor alters the manufacturer's recommended mix design? How does the
sealer perform? And of what value is that sealcoat to the property owner.
An improper mix design can
result in reduced flexibility |
First,
be assured that improper mix designs do occur. Whether it's the result of inaccurate
measurements or even a contractor trying to "stretch" the sealer by
overdilution, mix design mistakes are a hazard of the job. When they do
occur, they weaken the sealcoating system, leaving the property manager with
a lower-quality job and the likelihood that she will have to sealcoat again
sooner that she would have to. |
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Silica and aggregates
When contractors increase the amount of silica and aggregates, otherwise known as fillers, in the sealcoating system, they detract from the performance of the coating. Research has found that excessive amounts of silica and aggregates will:
- Reduce flexibility of the cured sealcoating. With the addition of silica and aggregates, the sealcoating film becomes increasingly tougher in terms of tensile strength and fails to flex with the pavement.
- Reduce adhesion. All fillers and extenders have their own specific binder demands. The binder is thus progressively deleted to meet the binder demand of the extenders. At a certain stage, the binder loses its ability to satisfy both, that is, the binder demand of additional fillers and adhesion to the pavement, manifested as adhesion failure.
- Reduce the weathering and chemical resistance. Increasing the percentage of fillers can increase the porosity of the coating itself. The destructive elements get easy access to asphalt in the pavement. It's easy to visualize considering that refined tar or asphalt is the binder portion of the coating which, while performing its primary function of protecting the pavement, also holds the fillers in the coating system. Silica and other fillers absorb the binder to satisfy their own absorption demands, thus reducing the proportion of the refined tar which would otherwise be available for forming a continuous film and bonding to the pavement. Proper care should be exercised in designing sealer mixes and excessive amounts of silica sand should be avoided.
Research and field experience have concluded that the limit is reached when filler volume is almost equal to that of the coal tar volume.; the ratio of coal tar solids by volume, for example, should not be allowed to exceed one to one (1:1). The amount of bitumen solids should be slightly more than the amount of filler solids for continuous film formation as well as proper adhesion to the pavement. Conversely, an excessive amount of refined coal tar might produce tackiness under hot climatic conditions, even after the full cure.
For some applications it's 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 latex (rubber) additives are used that offer satisfactory performance. 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.
Water
The use of excessive amounts of water should always be avoided as they directly reduce the solids by volume of the mix. The reduced solids results in a thinner dried film, which cannot be expected to have the same longevity as properly mixed material. So follow the manufacturer's recommendations where water is concerned . The amount of water can be exceeded slightly if the pavement surface is hot, but in such cases it is better to use extra water for fogging to cool the hot pavement- not to alter the mix design.
Additives
Following the
manufacturer's recom- |
Additives have been used for many decades in the
sealcoating industry, almost coinciding with the introduction of
sealcoatings. The most commonly used additives are based on
Acrylonitrile/Butadiene latex , which offer excellent sand suspension, reinforcement
of sealer films, and improved fuel resistance. These additive types also have
been specified in FAA P-627 and P-628 specifications. There are various
other additives based on other latexes such as acrylics, polyvinyl
acetate/acrylic, neoprene and others, which are designed to offer a variety
of other benefits. It is the responsibility of the sealcoating
professional to determine which additive will best complement the project.
The additive manufacturers can be of immense assistance and their recommendations
- especially concerning the amount of additive to use and when to use it -
should be strictly followed. The limitations of additives and their specific
contribution to overall performance should be evaluated in light of the added
cost and longevity of the sealcoating. Additives, though an invaluable
component of the sealcoat system, should not be expected to overcome
performance deficiencies caused by excessive use of water or aggregates in
the system. Additives will neither enhance the film thickness nor
"create" enough binder to accommodate the excessive filler content.
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