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Pavement Preservation Compendium
The Right Treatment for the Right Road
Type of treatment, road, and timing affect pavement maintenance management
Pavement maintenance is much more complex than most people think. It is not as simple as sealing a pavement surface or joints and cracks every several years. In this column every other month, I will try to explain "The Right Treatment on The Right Road at The Right Time." There has been a lot of thought put into that phrase, and it has been adopted as a battle cry by the Foundation for Pavement Preservation (FP2).
Therefore, in this first article I want to discuss the workhorse of preventive maintenance treatments: chipseals. There are many variations of this technique and the materials available for this purpose. It is good to have an understanding of the reasons for selecting a particular alternative.
First, the most commonly used chipseal consists of a uniform film of asphalt placed on an existing pavement, followed by an evenly distributed, clean aggregate. The asphalt seals the road surface and binds the aggregate in place; and the aggregate provides a skid-resistant wearing surface.
The size and type of aggregate vary from 0.25 inches (6.35 mm) to more than 0.75 inches (19.1 mm) and can be gravel, sandstone, limestone, rock asphalt, expanded shale, trap rock and the list goes on. In any case, it should be durable and skid resistant. The aggregate can be precoated or uncoated. It may be preheated in some cases, but it should always be clean. Single-sized aggregate in most applications is preferred. This is because in a chipseal design, the size and surface area of the aggregate are used to calculate the asphalt rate to achieve a desired level of embedment. If the design calls for 50 percent embedment of a 0.5-inch (12.7-mm) aggregate, then a 0.25-inch (6.35-mm) aggregate would be 100-percent embedded. That's not to say acceptable results cannot be achieved with a varied aggregate, but, ideally, single-sized is best. Another issue with regard to embedment is there are two stages: Initial embedment occurs on the day of placement; and final occurs within the first year. Predicting the final embedment of the aggregate is very tricky because it is almost completely dependent on the surface you are sealing. If the surface is very hard, you may have no further embedment. If the surface is soft or flush with asphalt, subsequent embedment from traffic is a certainty. This is one of the reasons the same aggregate and binder may yield different results on different pavements.
The asphalt in a chipseal performs two major functions. First, it seals the road surface. Second, it bonds the cover stone to the road. The asphalt bond is created chemically and mechanically, and the aggregate, once properly seated, contributes to the bonding by frictionally interlocking. All these things are needed in producing a good chipseal. The other considerations are binder types, equipment, weather, traffic, season of placement, etc. It's not possible to cover all of those aspects in this article. Therefore, I am going to explain chipseal variations and touch on why one may be best for a particular road section.
Single course chipseal
This is the most commonly used treatment and is best suited for a sound asphalt pavement that has very limited defects. This treatment is a single shot of asphalt binder followed by aggregate evenly spread so it is only one stone thick. There is a wide range of asphalt rates and aggregate sizes that will work very well. The larger the stones in a chipseal, the longer they will last. The larger stones are also more skid-resistant and produce more road noise. Keep in mind the amount of asphalt depends on the size of aggregate used. Larger stone requires more asphalt. Smaller stone chipseals are generally used on residential or other lower traffic volume streets.
Racked in is a variation to the single course chipseal. In this type, there is one application of asphalt binder followed by two applications of stone. The first stone is two to three times larger than the second. This allows the larger stone to be well placed in the binder, and the second layer of the smaller stone fills the voids and displaces the binder further upward on the larger stone. The larger stone is mechanically locked in place by the smaller stone and binder.
This type, also called two-course surface treatment, is two applications of asphalt binder and cover stone. The first application is the larger stone. The voids in the newly placed mat allow interlocking of the second application of asphalt binder and smaller stone. The placing of the second course should follow the first within a relatively short period of time, because the two courses interlock and work together as a system. A two-course treatment is often used in new construction over a compacted base.
Sandwich seal is very much like a two-course treatment except the first application of binder is omitted. This treatment works well to combat a flushed surface or to kill bleeding. The coarse stone is placed on the road first with no binder underneath it. The single application of binder is sprayed on the uniformly spread stone, followed by the second application of smaller stone and then rolled. The technique addresses the problems associated with further embedment. The excess asphalt on the existing surface squeezes up through the voids created by this treatment.
Chipsealing has much more potential than most people think. Not only are there variations in techniques that are underutilized, but, with different binders and aggregates, many problems can be solved. One last thought - chipsealing is more an art than a technique, and, in practicing this art, you must know "The Right Treatment on The Right Road at The Right Time."
Bill O'Leary is the president of Prime Materials and Supply Corp., Houston.