By Dwight Walker
Chip seals are one of the most commonly used pavement preservation techniques. Sometimes also called seal coats or surface treatments, chip seals are an asphalt binder sprayed on a prepared pavement surface, followed immediately by an application of aggregate (chips). Rolling then seats the chips in the binder. After curing, the surface is broomed to remove any loose aggregate.
Chip seals are used in a variety of applications, including single chip seals, double chip seals, cape seals, and stress absorbing membrane interlayers (SAMI).
Chip seal basics
Different types of binders can be used in a chip seal. The binder may be cold or hot applied. Cold applied binders include modified and unmodified medium and rapid setting emulsions. Hot applied binders can be polymer-modified asphalt cements, asphalt rubber and polymer/crumb rubber blends, as well as unmodified asphalt cements. The appropriate binder type is selected based on pavement condition, climate, aggregate properties, desired service life and cost considerations.
A chip seal is designed to seal and protect a pavement from oxidation and weathering by sealing the small cracks in a pavement surface and preventing the intrusion of water and air. A chip seal can also be used to address raveling (loss of aggregate), correct bleeding and flushing problems, improve skid resistance, and generally extend the pavement life. Chip seals are not suited for all pavements; the existing structure should be sound with only minor surface defects. They are not appropriate for rutted, potholed, or severely distressed surfaces.
For preservation purposes, chip seals should be applied to pavements that are in relatively good condition with no structural damage. Once structural damage has occurred, preventive maintenance treatments are no longer applicable and more involved corrective strategies are required. Thus, project selection is a critical factor in the success of chip seals as a preservation technique.
When a chip seal is applied to a pavement in good condition, six to eight additional years of service is a typical expectation. For more distressed pavements, less service extension can be expected (four to six years on pavements in fair condition and three to four years for poor pavements).
Many agencies rely on experience and judgment rather than engineering for the design and construction of chip seals, considering these procedures to be an “art,” rather than science. NCHRP Synthesis 342, Chip Seal Best Practices, concluded that “… the experience of highway agency personnel appears to be the major factor for achieving chip seal success.”
With this approach, the use of “best practices” is critical. The following considerations are important.
• Weather conditions during construction have a direct influence on performance. The weather should be warm and dry to promote proper binder setting and curing. Monitor the weather forecast to avoid potential weather problems and avoid possible chip loss.
• Before beginning the sealing work, prepare and clean the existing surface. Fill any potholes, level the ruts and seal large cracks.
• Traffic control is important. The traffic speed should be reduced/controlled until the binder sets, the rolling has been completed and the first brooming has occurred. Take particular care to avoid scuffing and hard braking on the new surface.
• Application rates must be selected to fit site conditions. Proper binder and aggregate application rates result in durable, long-lasting chip seals. The binder rate should be adjusted if the existing surface is absorbent, weathered or flushed and should take into consideration the traffic count. Too little binder can result in aggregate loss; too much binder can result in bleeding. The aggregate chips should be embedded in the binder to about 50 to 70 percent of their thickness after rolling and traffic has completely fully seated the aggregate particles. The chip spread rate should result in one stone deep, uniformly covering the existing surface without excess aggregate. Too little aggregate leads to insufficient cover and asphalt tracking. Too much aggregate is a waste of money and the extra chips can cause aggregate to be dislodged.
• The condition of the aggregate chips is important. Clean aggregate is necessary; the liquid will not adhere to aggregate that is too dusty or too wet. The chips should be crushed and durable to provide a long lasting chip seal. The chip spreader should follow the binder distributor as closely as possible. The chips should be spread on the binder immediately after it is applied. The rolling procedure is critical to embed the chips in the liquid and promote good bonding. The chips should be rolled immediately after spreading with a pneumatic tire roller.
• A properly adjusted distributor spray bar is critical to a uniform application. Adjust the spray bar height and the nozzle sizes and angles to assure a uniform distribution. “Fat” streaks and bleeding can develop when the binder is applied too heavily. The chips can be lost in the lean areas.
• Light brooming with minimum downward pressure is necessary to remove loose aggregate without dislodging or rolling embedded chips. Timing is important. If the sweeping operation starts before the binder sets, it will tear away the cover aggregate.
Because of the importance of chip seals as a preservation technique, a great deal of work has been done recently to move to a more engineered, technical approach. Advancements have been made in materials, design and testing procedures and application equipment. Materials improvements include the use of polymer modified emulsions, tighter specifications on the use of one-size chips, using fibers, crumb rubber additives, etc. Computerized equipment has been developed that makes placing a uniform application of materials easier and allows for simpler adjustments in progress.
Additionally, significant research has been done. One effort, led by Dr. Scott Shuler of Colorado State University, has resulted in several recommendations and potential advancements. This work was done under NCHRP Project 14-17 and is reported in NCHRP Report 680, “Manual for Emulsion-Based Chip Seals for Pavement Preservation.”
Five products to improve design and construction of chip seals were identified in this research. These were:
• Modified sweep test and critical moisture contents A lab test simulating rotary sweeping was developed. The test determines the timing for brooming and opening to traffic by monitoring the moisture content which corresponds to adhesion needed for chip retention.
• Field consistency test This test uses a Wagner cup viscometer, a device used to measure the consistency of paints, to measure the consistency of emulsions.
• Pavement texture A correlation was developed between the sand patch test and the CT (circular texture) meter. The resulting texture measurement can be used to adjust emulsion spray rates during construction.
• Residue recovery and desirable properties The Stirred Can Emulsion Residue Recovery (SCERR) method is recommended for obtaining residue for measuring physical properties of the emulsion.
• Measuring aggregate embedment in the field Two methods of chip embedment were developed. The constant volume is simple to perform but loses accuracy at more than 50 percent embedment. The constant diameter method can be used to estimate embedment up to 80 percent.
Additionally, five standardized testing procedures were drafted for consideration and recommendations were offered for additional study.
Traditionally, chip seals have provided a quick, reliable and economical surface treatment that seals out water, improves rideability and preserves a pavement for years. Recent technological advancements in testing and design procedures, materials and equipment have significantly reduced the dependence on experience and judgment associated with chip seals of the past. Today’s chip seal can be reliably placed on a variety of pavements under a wide variety of conditions.
Dwight Walker is a contributing editor for Asphalt magazine and is a consulting engineer specializing in asphalt materials.