The pavement community has strict specifications for the design and manufacturing of asphalt mixtures. But like any other process, a strict adherence to the rules costs more money. As a result most local pavements and especially parking lot pavements might not receive the highest quality asphalt mixtures. It is very common that a parking lot is constructed with materials that failed to meet the specifications of a highway project.
Whatever the circumstances that occurred during the construction process, eventually it will be the responsibility of the owner, with the help of a contractor, to keep the pavement looking good and providing its intended service. With a little education on pavement performance, most owners and contractors will be able to make smart decisions. Following are a few simple facts to keep in mind:
- As long as pavements are subjected to traffic loads and environmental factors, they will experience deterioration and eventual failure.
- Pavements must be maintained regularly. National studies have shown that every $1 spent on preventive maintenance saves $5 on major rehabilitation.
- It is wise to keep records on how the pavement is performing on a regular basis. Walk the project every 4-6 months and take pictures to document the conditions of the surface.
- Accept the fact that good pavements also need maintenance, very similar to changing the oil every three months in a brand new car. If preventive maintenance is delayed then its effectiveness is greatly reduced. In most cases, once cracking appears on the pavement surface, it is too late for preventive maintenance.
The Pavement Coating Technology Center (PCTC) is a group of manufacturers and suppliers of pavement sealers that are highly dedicated to the betterment of the entire industry. The PCTC was initiated 10 years ago with the objective of conducting research and development to improve the state of knowledge on pavement maintenance. PCTC's efforts have led to significant progress on several issues confronting the pavement sealer community, ranging from the design of sealers to the environmental concerns of the public.
Sand in the mix design
In general coal tar based sealers are made of the coal tar binder, sand, water, and additive. The overall success of the sealer depends very heavily on the proportioning of its various components. The first thing to recognize is that there is no such thing as one formula fits all. Each sealer should be designed individually to take into consideration the unique characteristics of the coal tar binder and sand that significantly influence the performance of the final product (i.e. the sealer). The PCTC developed guide specifications for the design of coal tar sealers: PCTC01 for un-modified sealers and PCTC02 for polymer-modified sealers. The guides are available at no charge through the PCTC web page "www.Pavtec.com".
The guides present recommendations on the gradation of sand and the amount of sand to be used in the design of the sealer. The amount of sand, commonly referred to as "sand loading," has been a contentious issue in the industry for quite a long time. A recent PCTC field project generated valuable data in support of the sand loading issue. Field test sections were constructed with sand loadings of 3, 5, and 18 pounds. The test sections were monitored for their resistance to wear under normal traffic loading. After four years of field monitoring, it was concluded that the sections with the 3- and 5-pound sand loadings significantly out-lasted the sections with the 18-pound sand loading. The field data showed that the 18-pound sand loading sections totally wore-off the pavement surface after nine months of traffic while the 3- and 5-pound sand loading sections performed successfully for the entire four-year period.
Based on this study, the PCTC strongly recommends that the sealer be designed to account for the unique characteristics of the coal tar binder and the sand that are being used on the job and sealer designs with high sand loadings should not be accepted.
When to seal new pavement
Asphalt mixtures are highly sensitive to age and temperature. They are flexible when they are young and they gain strength and lose flexibility as they age. This gain in strength is necessary to help resist permanent deformation while the loss of flexibility would increase the potential for cracking. Sealing the surface of an asphalt pavement is highly effective in slowing the aging process and protecting the pavement from cracking. However, the critical question becomes; when to seal the pavement? Should the pavement be sealed immediately after construction or should there be a waiting period before the pavement is sealed? Data generated from a PCTC study provided solid information toward the knowledge of when to seal the pavement. In the mid 1990s, the PCTC sealed a parking lot in Reno, NV, within 30 days after construction. Several types of sealers were used side-by-side. One year later the majority of the sealers started showing cracks and two years later the majority of the sections were totally cracked. In 2001, the PCTC sealed another pavement in Reno one year after construction. Again, several types of sealers were used side-by-side. Three years later none of the sealcoat sections have shown any cracking. This data indicates that sealing a brand new pavement would jeopardize the success of the sealcoat while sealing a one-year-old pavement would lead to a successful sealcoat.
In summary, the data indicates that while it is necessary to provide a one-year curing period for asphalt pavements, the degree of success of the sealer is also greatly improved if it is applied after one year from construction. Therefore, the field data support the earlier PCTC recommendation of: "under normal traffic and environmental conditions, a new pavement be sealed after either: a) 90 to 180 days of +70° F weather, or b) one year of normal weather conditions.
Peter E. Sebaaly, Ph.D., P.E., is director, Pavement Coating Technology Center, University of Nevada, Reno.
ASMA Tips for Asphalt-based Sealer
"Dedicated to the advancement of the sealcoating industry," the Asphalt Sealcoat Manufacturers Association (ASMA) is made up of suppliers to and producers of asphalt emulsion-based sealcoating material. An ASMA "Standard Specification" developed to guide the industry calls for:
- Minimum application rates (based on two coats of undiluted material) of 20 gal./sq. ft. for a smooth, dense surface; 30 gal./sq. ft. for a medium surface; 40 gal./sq. ft. for a rough, aged surface; and 50 gal./sq. ft. for an excessively rough, aged surface.
- While aggregate (often in the form of slate) is added to asphalt emulsion sealers by the sealer producers, contractors can add up to 3 lbs. per gal. for the first coat on severely weathered asphalt pavement.
- Two application coats of material by "mechanical means" including broom, squeegee, or distributor bar or wand, or a combination.
- Pretreatment of pavement with a water mist if ambient temperatures are in excess of 80° F. Surface must be damp but without standing water prior to sealcoat application.
- Applying a tack coat when applying asphalt emulsion over a refined coal tar based sealer.