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Chip Seals

“The increased use of chip seals for maintenance can be a successful, cost-effective way of using preventive maintenance to preserve both low-volume and higher-volume pavements.” - National Cooperative Highway Research Program (NCHRP), Synthesis 342

Effective Pavement Preservation

Chip seals are economical maintenance treatments designed to correct minor surface deficiencies and to protect and prolong the lives of pavements. Long term pavement performance studies have shown that chip seals are one of the most cost-effective methods of pavement preservation. They are used on all types of roads, from gravel county roads to high volume interstate pavements, which are structurally sound but are beginning to age and may have some minor surface distress or have lost their skid resistance. Chip seals are also commonly used as a first bituminous treatment over compacted bases on low volume roads. Polymer modified asphalts, tighter specifications on the use of clean, single-size aggregate, and automated application equipment have all been recently developed to improve the construction and reliability of high performance chip seals, especially on high traffic volume roads. The graphic shows where chip seals can be effective.

Material Selection & Design

A chip seal design should be done in the laboratory using the project aggregate. The selection and application rates of both binder and aggregate are very important. Rapid setting emulsions are normally the binder of choice, although medium-setting emulsions can be used with fine aggregates and under other special circumstances. The choice of anionic or cationic depends upon the project. Typically, anionics have more affinity for limestone aggregates. Cutback asphalts or hot applied asphalt cements are also often used. The best aggregates are one-sized (about 3/8” or 9.5 mm) and cubic in shape to provide good stability and maximum contact with tires. Hard aggregates with good resistance to abrasion and degradation give the best results in resisting traffic wear and impact. Dust prevents the asphalt from coating the aggregates, so the chips should be clean whenever possible. High float emulsions and cutbacks seem to be more forgiving for dusty aggregates. If an emulsion is used, it should be formulated to break quickly with the chip application. Polymer modified emulsions usually give excellent early chip retention. The binder shot rate should be calculated to achieve optimal embedment of the chip in the residual asphalt. In the field, the shot rate should be adjusted for the surface condition of the existing pavement, with higher rates for aged, porous surfaces. There should be enough asphalt to hold the aggregate without shelling, but not so much that it will bleed through the surface.

Martin Asphalt Products for Chip Seal

Martin Asphalt Product	Description
CRS-2	Cationic rapid setting emulsion is the most commonly used, and it sets up quickly.
CRS-2P	Polymer modified cationic rapid setting emulsion. The polymer improves the durability, and polymer emulsions tend to have earlier and better chip retention. This higher quality and cost material is generally used on higher traffic volume roads where durability, traffic delays and loose chips are concerns.
CRS-2h	Cationic rapid setting emulsion with a stiffer base asphalt for higher temperature climates.
CRS-1P	A cationic rapid setting emulsion specially formulated with polymer and emulsifier that sets up quickly at lower temperatures, so the construction season can be extended.
HFRS-2	Anionic high float rapid setting emulsion. The high float gel residue resists bleeding at high temperatures.
RS-2	Anionic rapid setting emulsion for general use.
HFRS-2P	Polymer modified high float rapid setting emulsion. The polymer improves the durability, and polymer emulsions tend to have earlier chip retention. The high float gel residue resists bleeding at high temperatures. This higher quality and cost material is generally used on higher traffic volume roads.
RC-250	A cutback asphalt binder for chip seals
AC-15P	A polymer modified, hot applied binder for chip seals, designed especially for quick traffic return and durability on high volume roads

Chip Seal Benefits

Seals small cracks and surface imperfections
Waterproofs pavement surface
Improves skid resistance and rideability
Rejuvenates surfaces
One of the most cost-effective pavement preservation treatments

Surface Preparation & Construction

Any moderate severity cracks or other distresses should be sealed or repaired, and the surface should be cleaned prior to the treatment. The single most common cause for failed chip seals is the temperature during and after construction. The air temperature should be 60°F and rising, with the expectation of higher temperatures to allow full curing. In a single chip seal, an asphalt emulsion is sprayed on the pavement with a distributor, then immediately covered by a single layer of uniformly sized chips from a chip spreader.

The equipment must yield uniform and accurate application rates of the materials; uneven binder application will result in uneven retained chips. Calibration is critical. Nozzles should be clean, unplugged, and set at an angle so the spray fans not only cover the entire area, but also do not interfere with each other. The spray bar should be set at the proper height. A triple overlap, where each part of the surface is sprayed by three different nozzles normally gives excellent results. The angle of the nozzles and height of the spray bar determines the pattern.

It is similarly important to calibrate the chip spreader. Excess chips are a waste of money, detrimental to the seal and can be a traffic hazard. Normally, you should be able to see 10 to 15 percent asphalt around the chips on a freshly applied chip seal before it is rolled. For larger chips, the amount should be slightly higher, and it should be slightly less for smaller chips. The chips should be spread as quickly as possible after the asphalt is sprayed. This gives the asphalt the maximum opportunity to embed and bond the aggregate to the existing surface. The asphalt layer reaches the temperature of the surface within 60 seconds of application. As it cools, the asphalt loses some of its ability to bond, resulting in potential chip loss.

The new surface treatment is then rolled to seat the aggregate; immediately rolling the chips insures maximum embedment. Pneumatic tired rollers are normally used. The pneumatic tired roller, while essential for rolling, is not the first piece of equipment producing embedment. The aggregate truck is the first roller, and, if used properly, can provide early embedment of the aggregate. The drivers are encouraged to stagger the trucks backing pattern to provide a full rolling coverage prior to the roller. The chip spreader also has the best view of the distributor nozzles and can alert the distributor driver of any problems.

Once the seal has cured, it is broomed to remove any loose chips. Traffic should be controlled so that the new surface is not disturbed until after the final sweeping. The use of a pilot car at reduced speed is recommended until the seal is fully cured.

Variations on chip seals include double seals, where successively smaller aggregates and lighter binder applications are used, and sand seals where sand is used in place of larger aggregates. A Cape Seal is a single chip seal covered with a slurry seal.

Martin Asphalt Makes It Easy

There are a variety of treatments available for effectively keeping “good pavements good.” Choosing the right treatment can make the best use of funds and give taxpayers smooth and safe roads while avoiding costly reconstruction and extended road closures. Through Martin Asphalt, you get Everything Asphalt—a full range of products for your pavements. In addition, you receive technical assistance in selecting the right materials and application. The company's AASHTO Certified Laboratory makes sure the products meet your specifications. And your products are delivered both on spec and on time via Martin's Gulf Coast network of production plants, storage facilities and transportation fleet including ocean-going and inland barges, rail cars and tanker trucks.