A bituminous surface treatment (BST), also known as a seal coat or chip seal, is a thin protective wearing surface that is applied to a pavement or base course. BSTs can provide all of the following:

Historically, BSTs have been in use since the 1920s, primarily on low volume gravel roads. BSTs are increasingly in use as a preventative maintenance procedure on flexible pavements of good structure.

A BST offers preventive maintenance from the effects of sun and water, both of which may deteriorate the pavement structure. BSTs create a new wearing course, as well as a waterproof covering for the existing pavement. A BST makes it more difficult for water to enter the base material, and preventing freeze thaw damagefor those locations with below freezing temperatures.

BSTs also increase the surface friction of the pavement, due to the addition of the cover aggregate. This combats the effects of raveling, which can make the pavement slippery and stopping difficult. A BST gives good, gripping texture to the pavement surface.

BSTs should be applied to a distress-free to moderately distressed pavement surface. A two to four year service life is common, while five year service lives are achievable.

BSTs are created using two main materials: asphalt and a cover aggregate.

Asphalt (as asphalt binder, cutback asphalt or asphalt emulsion) and aggregate (uniformly graded).

The asphalt is typically an emulsion. Care must be taken with the weather on the day of construction – ideally, a warm day with low humidity is preferred. BSTs should never be constructed on rainy days or when rain is predicted. Rain can dilute the asphalt binder if it has not yet cured, bringing the binder to the top of the cover aggregate; after the water evaporates, tires can pick up the loose aggregate or track binder across the surface.

While cutback asphalt is historically an option for BSTs, the solvent used (usually gasoline or kerosene) is expensive and potentially dangerous. A cutback is asphalt dissolved in a solvent, allowing the asphalt to be pumped and sprayed without heating it to high temperatures. The solvent evaporates into the surrounding air, leaving the asphalt binder. Once the solvent has completely evaporated, the cutback has cured. Fast setting time cutbacks use gasoline, while kerosene is used for longer curing times.
Cutback asphalt usage has declined in recent years due to pollution and health concerns.

Today, asphalt emulsions are more commonly used. An emulsion consists of an oil suspended in water. In this case, the asphalt cement is the oil component. A surfactant (also called an emulsifying agent) needs to be added in order to make the oil and water mix. Having the asphalt suspended in water allows the placing temperature for a BST to be significantly lower than for a hot mix asphalt.

A surfactant offers two benefits – one, it causes asphalt to form tiny droplets that will suspend in water by lowering the surface tension between the asphalt and the water. Two, a surfactant determines the electrical charge of the emulsion. Aggregate will have an electrical charge, usually negative. Since opposite charges will attract, it is important to choose an emulsion with the opposite charge, which will enhance the binding of the asphalt to the aggregate. Cationic (having a positive charge) emulsions are typically used.

read more : http://www.pavementinteractive.org/article/bituminous-surface-treatments/