Sprayed Asphalt Surface Treatment

Sprayed asphalt treatments contain no aggregates. They are simply applications of different types of asphaltic materials to a prepared surface. The categories include fog seals, dust laying, and road oiling. Prime and tack coats are also considered as a sprayed asphalt treatment.

FOG SEAL.- A fog seal is a light application of diluted slow-setting asphalt emulsion, used to renew old asphalt surfaces and seal small cracks and surface voids. Fog seals are especially useful for pavements carrying a low volume of traffic. A fog seal may also be used for the following:

1. To seal surface voids in new asphalt plant mixes

2. To prevent dust on sprayed asphalt with cover aggregate surface treatments

3. To increase aggregate retention

4. To provide a uniform dark color

The asphalt emulsion is diluted with an equal amount of water, and the diluted material is sprayed at the ROA of 0.1 to 0.2 gallon per square yard, depending on the texture and dryness of the old pavement. In normal conditions, the separation and evaporation of the water is rapid, permitting traffic within 1 or 2 hours.

DUST LAYING.- Dust laying consists of spraying an untreated surface with a low-viscosity liquid asphalt, such as SC-70, MC-30, MC-70, or a diluted slow-setting asphalt emulsion. The asphalt and dilutant penetrate and coat the fine particles and temporarily relieve the nuisance of dust.

The material is sprayed at a ROA of 0.1 to 0.5 gallon per square yard. When emulsion is used, it should be diluted with 5 or more parts of water by volume. Diluted emulsion dust-laying treatments usually require several applications. The dust stirred by traffic between applications eventually conglomerates and no longer rises. This is an effective treatment in a very dusty environment where one application of asphalt is insufficient.

ROAD OILING.- Road oiling differs from dust laying in that it is usually accomplished as part of a planned buildup of low-cost road surfaces over several years. Each application may be mechanically mixed with the material being treated, or it maybe allowed to penetrate. The light oils in the road oil penetrate into the subgrade and tend to repel moisture absorption. The objective in all road oiling work is to form a dustless ^{wearing surface,} combined with a strong water-repelling subgrade.

Because soils vary widely, procedures for oiling are a matter for local trial and error, rather than scientific analysis. The amount of road oil, required in the first year of work will vary from 0.75 to 1.0 gallon per square yard. The first application is applied at the ROA of about one half of the total; succeeding applications are made in equal amounts.

Road oiling treatments are placed several weeks apart, depending upon the character of the asphalt soil mat. If some breakup occurs after the first winter, light scarifying and retreatment the second year will produce a thicker and stronger surface.

WARNING

Before planning any road oiling work your supervisor should check with local authorities concerning environmental protection restrictions.

Aggregate Surface Treatment

The sprayed asphalt with aggregate cover surface treatments are applications of liquid asphalt, followed by an application of aggregate. This can be done in one or more layers of construction. Two types of sprayed asphalt with covered aggregate surface treatments are in use today: single- and multiple-surface treatments.

SINGLE-SURFACE TREATMENT.- Singlesurface treatments are thin, bituminous-aggregate toppings, applied to existing bases or surfaces, such as concrete or asphalt. Construction involves applying a bituminous prime or tack coat to the base or surface. This coating is followed by an application of bitumen and small-sized aggregate. Single-surface treatments are sometimes called seal coats, because they seal the surface of the road or runway.

Sequence of Operations.- Figure 16-28 shows the sequence of operations for the application of a single-surface treatment. The first steps, such as sweeping, priming or tacking, and curing, are the same as those used for applying a prime coat. The binder (bituminous material) is applied over the prime coat with an asphalt distributor. The aggregate is then spread over the binder by use of aggregate spreaders. The aggregate cover is spread uniformly immediately behind the distributor. As soon as the aggregate is spread, it is pushed into the soft asphalt by rolling it with a pneumatic-tired roller.

^{Binder Application.}- When you are applying the binder, it should be hot enough to spray properly and

Figure 16-28.-Sequence of operations for the application of a single-surface treatment.

cover the surface uniformly. After the binder cools and cures, it should bind the aggregate tightly to prevent _{dislodgement by traffic.} Individual aggregate stones should be pressed into the binder but must not be covered by the binder. Approximately one half of the individual aggregate stones should be exposed to traffic. The ROA for the binder material should be between 0.25 and 0.30 gallon of aspalt per square yard.

For a single-surface treatment, the bitumen must be heated and applied to the surface while hot. The aggregate must be spread and rolled before the bitumen cools. Under no circumstances is traffic permitted to travel upon uncovered fresh bitumen. The distributor should NOT apply bitumen until the aggregate is on hand and ready for application. When the distributor moves forward to spray the asphalt, the aggregate spreader should start right behind it. The bitumen should be covered within 1 minute if possible; otherwise, the increase in asphalt viscosity may prevent good binding of aggregate.

Aggregate Application.- The size and amount of aggregate, used for surface treatments, are important. You must use a size that matches the bitumen application rate. For a single-surface treatment, one-half inch to sieve number 4 is needed. The amount of aggregate should be 25-30 pounds per square yard.

When aggregate is distributed properly, very little hand work is required. At longitudinal joints, the aggregate cover is stopped 8 inches from the edge of the bitumen to ensure ample overlap of the bitumen coat. All bare spots should be covered by hand spreading, and any irregularities of the distribution should be corrected with hand brooms. Excess aggregate in limited areas should be removed immediately with square-pointed shovels. When the aggregate spreader is properly set and operated, handwork is reduced to a minimum.

Rolling.- The aggregate is usually rolled by pneumatic-tired rollers. Steel-wheeled rollers are not recommended by themselves. If used, they should make only one pass (one trip in each direction). The rolling operation should then be completed with the ^{pneumatic-tired rolls.} Steel-wheeled rollers produce maximum compaction but must be used with care to prevent excessive crushing of the aggregate particles. Also, these rollers will bridge over smaller size particles and small depressions in the surface and will fail to press the aggregate in these places in the asphalt.

Faulty rolling can be eliminated or minimized if you adhere to the following procedures:

1. Rolling should be parallel to the center line of the roadway to reduce the number of times the roller must change direction.

2. Succeeding passes should overlap one half of the wheel width of the roller. This action ensures that the aggregate becomes well embedded in the bitumen.

3. Rolling should be completed before the bitumen hardens. This will ensure that the aggregate becomes well embedded in the bitumen.

4. Succeeding passes should be made from the low side to the high side of the surface. This operation maintains the surface crown and prevents feathering at the edges.

5. Rolling should be done at a slow speed.

6. Rollers should be only wet enough to prevent bitumen from sticking to the wheels.

7. The power wheel of the roller should pass over the unrolled surface before the steering wheel(s) of the rollers.

After rolling and curing, the surface is ready for traffic. When the surface is used as an airfield, excess aggregate must be swept from the surface to avoid damage to aircraft. This practice is also recommended for roads.