Chip seals, also known as oil and screenings, aggregate seal coats, and armor coats, are surface treatments which are placed on an existing asphalt pavement. They are most valuable as a preventive maintenance tool, especially for roads which receive light to medium traffic.
Chip seals are placed by first spraying the pavement with a binder, often an asphalt emulsion, from an oil distributor truck. Then a uniform application of cover aggregate (chips or screenings) is immediately applied, usually by a self-propelled chip spreader. The aggregate is rolled as soon as possible, preferably with a rubber-tired roller, to ensure the adhesion of the aggregate to the binder and pavement surface.
In the early days of road construction, many roads and highways were first paved with chip seals. They provided good all-weather surfaces for the motor car, and were constructed using hot asphalt cements and cutback liquid asphalts. As cars and trucks became more popular and traffic loads increased, many of the chip-sealed roads were paved over with hot mix asphalt. Difficulties developed with the use of hot asphalt, such as rapid cooling and low adhesion qualities with dirty or wet aggregates. Slow curing became a problem with cutback liquid asphalts, and environmental and energy concerns also developed. Asphalt emulsions were created to solve these problems.
Asphalt emulsions are a mixture of asphalt cement, water, and an emulsifying agent. Emulsions are easy to use, require low heating temperatures for spray applications, and most asphalt emulsions do not contain any petroleum distillates, which helps to alleviate environmental and energy concerns. After emulsions are sprayed, water starts to evaporate from the emulsion. After curing, the remaining asphalt has all the qualities of the original asphalt cement—adhesion, durability, and water resistance—all important qualities for successful chip seal products.
Today, chip seals are primarily constructed with asphalt emulsions and polymer-modified asphalt emulsions. Also being used are polymer-modified asphalt cements and hot asphalt rubber compounds utilizing crumb rubber materials and asphalt cement.
Chip seals can be placed in single or multiple surface treatments, usually using ¼ to ½ inch aggregates. On multiple surface treatments, the oil spray and aggregate application process is repeated, with the aggregate size declining with each application (3⁄8 inch chip first application, ¼ inch chip second application). Only clean and well-crushed aggregates should be used on chip seals. Chip seals should only be placed in warm weather, and traffic needs to be controlled until the chip seal is well adhered to the pavement surface. Excess loose aggregate should be swept after placement in order to avoid windshield and vehicle damage.
When properly constructed, chip seals are the single most important low cost maintenance tool for asphalt pavement. Although they do not add structural strength to the roadway, chip seals produce an ideal all-weather surface, renew weathered pavement, improve skid resistance, and seal the old pavement. When used for the correct purpose, they provide an excellent pavement surface.