WATERPROOFING ASPHALT PAVEMENTS WITH SEALCOATS

Chip Seals

The application of a seal coat has a number of functions however one of the most important is to waterproof the pavements, protecting them from water damage and oxidation. If pavements were sealed early in their life, e.g. within a year, the pavements would last a lot longer. Chips seals are used especial on highways.

Chip Seal Emulsion. The emulsified asphalt used for chip seals are specially designed to break very fast on contact with aggregate. Emulsions can be either anionic (basic) or cationic (acidic) although the cationic are very popular. With asphalts from some crude oils the amount of emulsifier required for anionic chip seal emulsions is very small, approaching zero as a result naphthenic acids in the asphalt which serve as emulsifiers when neutralized with caustic soda.

Special Seal Emulsion. There is a product called PASS that has the ability to re-seal cracks and regenerate pavements.

Where to Use. A chip seal does an excellent job as a seal. While it can be used in cities, in my opinion a slurry seal would be better, unless it is a Capeseal in which a slurry is placed over the chip. The disadvantage of use in cities is that the chips can spread over lawns, in driveways, etc.

Mix Design. It is very important that a mix design is done, otherwise there can be failures.

Problems. One of the causes of failure is dirty aggregate. The chip seal emulsions are designed to break immediately on a surface thus when it hits the dust it breaks on the dust and not on the surface of the aggregate. An emulsion type called High Float is more tolerant of dust. Not enough emulsion can cause loss of chips while too much emulsion can called bleeding.  Also when used in cities, loss of chips can occur at the centerline as along the centerline there can be less asphalt as a result of less overlap of the spray. For rural roads this isn’t a big problem as there is not that much turning stress on the aggregate, however in the city, there can be turning traffic out of driveways. Also, there is another important problem; it is difficult to skate on chips.

It isn’t a good career move for a director of public works to place a chip seal on streets in expensive neighborhoods, especially if chips end up on the lawns, sidewalks and driveways.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

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