ROAD-MIXING METHODS

Two methods of road mixing are travel plant mixing and blade mixing.

Travel Plant Mixing

When a travel plant is used for mixing (fig. 16-7), the loose aggregate is dumped, mixed, and bladed into uniform windrows, and evened when necessary. The windrow should be sufficient to cover the section of the area to be paved with enough loose material to give the desired compacted depth and width. As the bucket loader tows the mixer and elevates the aggregate to the mixer hopper, the mixer meters the aggregate, sprays it with the correct amount of bitumen, mixes these two uniformly, and redeposits the mix into another windrow behind the plant. The rate of travel and the mixing operation should be controlled so that all particles of the aggregate are coated and the mix is uniform. Accuracy in proportioning the mix is extremely important. The travel plant method usually produces a more uniform mix of higher quality than blade mixing.

Figure 16-7.-Schematic layout of a travel plant.

Heavier types of asphalt cutback and tar maybe used in the travel plant method, which reduces the time required for curing. The asphalt finisher may be used concurrently with the travel plant. The hopper of the finisher is kept directly under the travel plant output chute. This arrangement reduces the maximum output of the plant, although it does provide uniform thickness of the mat being laid.

Windrows must contain no more material than the finisher can place. The major advantage of this setup is that in-place aggregate may be used in an intermediate mix and placed with a finisher without the necessity of loading and transporting aggregate. The finisher must be used with the travel plant for construction of some airfields when surface tolerances are critical.

Blade Mixing

In blade mixing, the aggregate is dried and bladed into windrows (fig. 16-8). The windrows are then flattened and the bitumen of the specific temperature is

Figure 16-8.-Blade-mix construction.

applied with a bituminous distributor in three equal _{applications.} Each application is one-third of the amount required.

Immediately following each application of the bituminous material, the treated aggregate should be mixed with spring-tooth or double-disk harrows, graders, rotary tillers, or a combination of this equipment until all the particles of the aggregate are evenly coated. When a grader is used, the windrow is moved from side-to-side by successive cuts with the blade.

Several graders can operate, one behind the other, to reduce the total time required for complete mixing. In hilly terrain, blading should be from the bottom to the top, as the mix tends to migrate down. After all the aggregate has been mixed, the mix should be bladed into a single windrow at or near the center of the road and turned not less than four complete turns from one side of the road to the other. Excess bitumen, a deficiency of bitumen, or uneven mix should be corrected by the addition of aggregate or bituminous material, followed _{by remixing.} Mixing should continue until it is complete and satisfactory; remember, mix will set up if mixed too long.

Suppose that materials, weather conditions, and equipment are well-suited to mixed-in-place paving, but the road or airfield must carry traffic during _{construction.} In such cases, the windrowing of aggregate and the mixing and spreading of bitumen may be done elsewhere-on any area of smooth ground which can be compacted for the purpose or on any unused road or airfield surface. The road or airfield surface, base, or subgrade to be paved is then primed or tack-coated as required to complete construction and to keep portions of the road or airfield open to traffic. As soon as the prime or tack coat cures, the mix is picked up, trucked to the jobsite, dumped, and then bladed into windrows for spreading.

The bituminous mix should not be spread when the surface is damp or when the mix itself contains an excess of moisture. The mixed material should be spread to the required width in thin, equal layers by a grader or finisher. (When a finisher is used, additional support equipment is required, and the material must be split into two windrows for an 8- to 12-foot-wide pavement.) When spreading the mix from a windrow, you should take care to prevent cutting into the underlying subgrade or base course. To prevent such cutting, you should leave a layer of mix, approximately one-half inch thick, at the bottom of the windrow.

The material being spread should be rolled once and then leveled with a grader to remove irregularities. The remaining material should be spread and rolled in thin layers until the entire mix is evenly spread to the depth and width specified. During the spreading and compacting, the surface should be dragged or bladed, as necessary, to fill any ruts and to remove corrugations, waves, or other irregularities. Both pneumatic-tired and steel-wheeled rollers may be used for rolling all surface treatment jobs; however, the pneumatic-tired roller is the preferred type.

After all layers have been satisfactorily spread, the surface should be rolled with two-axle tandem rollers. Rolling should begin at the outside edge of the surface and proceed to the center, overlapping on successive trips at least one half of the width of the wheel of the roller. Alternate trips of the roller should be of different lengths. The speed of the roller at all times should be controlled to avoid displacement of the mix. Light blading (or floating) of the surface with the grader during rolling may be required. Rolling should be continued until all roller marks are eliminated and maximum density is obtained. To prevent adhesion of the mix to the roller, you should keep the roller wheel moist with water; use only enough water to avoid picking up the material. At places not accessible to the roller, the mix should be thoroughly compacted with hand tampers. When the surface course becomes rough, corrugated, uneven in texture, water soaked, or traffic marked, unsatisfactory portions should be tom up and reworked, relaid, or replaced. When forms are not used and while the surface is being compacted and finished, the outside edges should be trimmed neatly in line.

When the road-mix pavement surface course is constructed from an open-graded aggregate, a surfs treatment may be required to waterproof the surface. A surface treatment is unnecessary on a dense-graded, well-compacted, road-mix pavement.

When possible, traffic should be kept off freshly sprayed asphalt or mixed materials. When it is necessary to route traffic over the new work speed must be restricted to 25 mph or less until rolling is completed and the asphalt mixture is firm enough to withstand high-speed traffic.