Placing Plant-Mix Materials

The material that arrives at the construction site from the plant must be spread. It must cover the entire width of the road being paved. It is then struck off to the desired shape and thickness and compacted. Three general methods of spreading and shaping the material are in use today: hand spreading, blade spreading, and mechanical spreading.

HAND SPREADING.- Hand spreading is the oldest method used to spread and shape the mixed material. For this method, the mix is dumped from the trucks onto dump boards from which the material is shoveled onto the road or runway. After placement, it is raked smooth to grade and contour and compacted with a roller.

WARNING

Asphalt and bituminous materials contain coal tars, benzene, and other components which are suspected or known carcinogens. Workers should avoid inhalation of the vapors and prolonged skin contact with these materials. Review the Materials Safety Data Sheet (MSDS) for specific hazards and precautions.

Because of the high cost of labor and the inability to obtain a smooth and even-textured surface, hand spreading is not used to any great extent. It is used primarily to supplement the other spreading methods. For example, hand spreading is used effectively for adjacent curbing and around manholes.

When placing the material by hand, you should be extremely careful to prevent segregation of the mix. Do NOT throw the material a long distance and do NOT dump it from too great a height. Dump the material in small piles and level the material with shovels, rakes, and lutes. Use the shovel to move the excess material and the lute and rakes to level it. The material should be as level as possible before compacting it.

BLADE SPREADING.- Blade spreading is done with a grader by a skilled operator. The grader blade can obtain reasonably good surface smoothness. Each successive pass of the grader blade reduces the irregularities in the surface. Often, blade spreading is used in areas too large for hand spreading and inaccessible to mechanical spreading.

MECHANICAL SPREADING.- Specialized machines have been developed to spread bituminous paving materials. Self-propelled, these machines have crawler, wheels, or rollers which run on the base course foundation or surface. The mix from the plant is dumped into a hopper on the front of the paver. The paver places the mix evenly on the road itself. Figure 16-6 shows a bituminous paver that can handle any type of asphaltic mix.

Compacting Plant-Mix Materials

The most important phase of flexible pavement construction is compaction. When the specified density of asphalt pavement mix is not obtained during construction, subsequent traffic will further consolidate the pavement. This consolidation occurs principally in the wheel paths and appears as channels in the pavement surface.

Most mixtures compact quite readily when they are spread and rolled at temperatures that assure proper asphalt viscosity. Rolling should start as soon as possible after the material has been spread by the paver but should be done with care to prevent unduly roughening of the surface.

Mix temperature is a principal factor affecting compaction. Compaction can only occur while the asphalt binder is fluid enough to act as a lubricant. When it cools enough to act as an adhesive, further compaction is extremely difficult to achieve. The best time to roll an asphalt mixture is when its resistance to compaction is the least, while at the same time, it is capable of supporting the roller without excessive shoving of the asphalt material. The best rolling temperature is influenced by the interparticle friction of the aggregates, the gradation of the mix, and the viscosity of the asphalt; therefore, it can change if any of these factors change. The critical mix temperature in

Figure 16-6.-Bituminous paver.

an asphalt-paving project is the temperature at the time of compaction.

During rolling, the roller wheels must be kept moist with only enough water to avoid picking up material. Rollers move at a slow, but uniform, speed with the drive wheels nearest the paver. The speed should not exceed 3 mph for steel-wheeled rollers or 5 mph for pneumatic-tired rollers. A roller must be maintained in good condition, capable of being reversed without backlash. The line of rolling should not be suddenly changed or the direction of rolling suddenly reversed, because these actions will displace the mix. Any pronounced change in direction should be made on stable material.

When rolling causes material displacement, the affected areas should be loosened at once with lutes or rakes and restored to their original grade with loose material before being rerolled. Heavy equipment, including rollers, should not be permitted to stand on the finished surface before it has thoroughly cooled or set.

Rolling freshly placed asphalt mix is done in the following order:

1. Transverse joints

^2. Longitudinal joints

3. Breakdown or initial rolling

4. Intermediate or second rolling

_5. Finish rolling

The five steps in rolling freshly placed bituminous or asphalt mix are covered in chapter 11 of this TRAMAN.