In the case of a surface treatment on a granular base, potholes and smaller breaks may develop over poorly bonded areas from which traffic has removed the surface treatment (Figure 1-4). Unless attended immediately by maintenance crews when they first appear and are still small, these holes in the surface treatment may quickly become so numerous and so large that it is no longer economical to attempt to restore the surface by simple patching methods. A poor bond between the binder and the granular base that results in these breaks is usually due to pockets of dust or other fine material, or to areas with excess moisture that existed in the prepared surface, or to low temperatures when the surface treatment was constructed. Attempting to use a binder that is too viscous will also contribute to this type of failure through lack of bond to the road surface.�

The quality of the base finish is critical to the bonding of the surface treatment to the base. Both pneumatic and steel wheel rolers are used. The pneumatic roller is used first, followed by a steel-wheel roller. The kneading action of the pneumatic roller helps the initial rolling to even-out the bladed surface. The steel wheel roller helps to get an even and less rocky surface before the prime coat is applied.

One type of base finishing known as slush rolling is sometimes used and this technique varies depending on the amount of water used. Slush rolling with excessive water can weaken the base, first by trapping water in the base and then by altering the gradation of the base due to the pumping of fines to the top. Slush rolling can build up a layer of fines on the top of the base that will hinder the penetration of the prime coat and the prime can debond from the base easily. Therefore, slush rolling is not recommended.�

The trimming technique uses the subgrade trimmer to finish the base. Excess base is used to compact the base 1-2 inches above the blue-top level, and then the trimmer is used to cut it down to the required finish level. Then the trimmed surface is rolled. This eliminates the need to do slush rolling.�

A prepared road base structure that is to be surfaced using the surface treatment concept should always be primed first. The prime coat plays a very important role by facilitating the bond between the surface treatment and the base layer. A well-applied prime coat can protect the base layer from adverse weather conditions and from wear due to construction and regular traffic until the surface treatment is applied. It can also either prevent or slow down the formation of dust on the surface that will have a serious negative impact on the bonding of the binder to the base.�

There are three types of prime coats used by TxDOT districts: spray-applied, worked-in prime and covered prime. A spray-applied prime (Figure 1-5) utilizes an asphalt distributor to apply between 0.1 and 0.2 gal/sy of either MC-30 or AEP products.�

Figure 1-6 shows a worked-in prime coat application where diluted emulsion is sprayed on the finished base, which is then covered with a thin coating of fine base material dust working the windrow with a motor grader. This process is usually repeated 2-3 times to get a total emulsion application rate of 0.2 gal/sy. The emulsions commonly used are SS-1, CSS-1h and MS-2. This leaves an asphalt-sand layer on the finished base that is approximately 1/8 in. thick.�

Figure 1-7 shows a covered (or inverted) prime applied on the finished base. This covered prime is similar to a surface treatment where RC-250 cutback is applied to the finished base, which is covered by spreading Grade 5 rock. This “priming” technique is particularly useful when traffic has to be let on the primed surface before the other half of the roadway is primed. This type of prime can provide 2-3 months of satisfactory service as a very temporary wearing course under favorable traffic conditions including little or no turning traffic or heavy traffic.�