MIXED-IN-PLACE CONSTRUCTION

Emulsified asphalt and many cutback asphalts (although the use of cutbacks is declining) are fluid enough to be sprayed onto and mixed into aggregate at moderate to warm weather temperatures. When this is done on the area to be paved, it is called mixed-in-place construction. Although mixed-in-place is the more general term and is applicable whether the construction is on a roadway, parking area, or airfield, the term road mix is often used when construction is on a roadway.

Mixed-in-place construction can be used for surface, base, or subgrade courses. As a surface or wearing course, it usually is satisfactory for light and medium traffic, rather than heavy traffic. However, mixed-in-place layers, covered by a high-quality asphalt plant-mix surface course, make a pavement suitable for heavy traffic service. The advantages of mixed-in-place construction include the following:

1. Utilization of aggregate already on the roadbed or available from nearby sources and usable without extensive processing.

2. Elimination of the need for an asphalt mixing plant. Construction can be accomplished with a variety of machinery often more readily available, such as motor graders, rotary mixer with revolving tines, or traveling mixing plants.

ROAD-MIX PAVEMENTS

Road-mix pavements consist of mineral aggregate and mineral filler uniformly mixed in place with a bituminous material and compacted on a prepared base course or subgrade. A single layer, about 1 1/2 inches to 3 inches thick, is generally used. This type of pavement is likely to become defective unless it has a sound, well-drained subgrade and is well-mixed, uniformly spread, and properly compacted. Road-mix pavements may be used as a wearing surface on temporary roads and airfields and as a bituminous base or binder course in construction of more permanent types of roads and airfields. Road mix is an economical method of surfacing small areas when aggregate can be used from the existing base or when satisfactory aggregate is nearby.

For road-mix pavements, the grade and type of bituminous material depend upon the aggregate and equipment available as well as weather conditions and time required to complete the project. Good weather is important to the success of a road-mix project. Where possible, road-mixing operations should be scheduled when weather conditions are likely to be hot and dry during, and for some time after, the project. Recommended types of bituminous materials suitable for road mix are asphalt cutbacks, asphalt emulsions, and road tars. A medium-curing cutback is generally used in a moderate climate, and a rapid-curing cutback is used in a cold climate. Viscosity required is determined by the temperature, aggregate gradation, and method of mixing. The highest viscosity that will completely and uniformly coat the particles of aggregate should be used. In general, open-graded aggregate requires a high viscosity; a gradation, containing mineral filler, requires a less viscous grade.

Aggregate, used in road mix, may be scarified from the existing subgrade or hauled in from a nearby source. A wide range of coarse and fine aggregate and mineral filler may be used. The ideal aggregate for road-mix pavement is a well-graded (dense or open) sandy gravel _{or clean sand.} Maximum size of the aggregate, in general, is limited to two thirds of the compacted thickness of the layer. Loose thickness is approximately 1 1/4 times the desired compacted thickness.

"Surface moisture" is defined as the film of water around each particle of stone or sand. The amount present is determined by heating a weighed sample of aggregate at 212F in an open pan and stirring it with a rod until the surface water disappears (3 to 10 minutes). The difference between the original and final weights is considered to be moisture lost during drying. The loss in weight, expressed as a percent of the final or dry weight, is the moisture content, allowed before the aggregate is mixed with asphalt cutbacks or road tars. When the aggregate is too wet, it should be worked with mechanical mixers, graders, or improvised plows to allow the excess moisture to evaporate. For cutbacks and tars, moisture content of coarse-graded aggregate should not exceed 3 percent, and of fine-graded aggregate, 2 percent. For emulsions, moisture content of coarse-graded aggregate should not exceed 5 percent, and of fine-graded aggregate, 3 percent.

The quality of the road-mix pavement depends largely upon the control of the mix. The percentage of bitumen will vary in relation to the absorptive quality of the aggregate, rate of evaporation of the volatile substances, and other factors. Although an exact formula is difficult to follow, proportioning must be controlled within narrow limits to assure the stability and life of the mix. With dense-graded aggregates especially, too much bitumen should not be used. All particles of the completed mix should be coated and uniform in color. When the mix is too lean, the aggregate in the windrow will stand almost vertically and have a dull look; and when the mix is too rich, it will ooze or slip out of shape. When the mix is correctly proportioned, a handful, squeezed into a ball, will retain its shape when the hand is opened.

Road-mix pavements should be constructed only on a dry base when the weather is not rainy. Atmospheric temperature should be above 50F. Mixing should take place at the temperature of the aggregate, but not below 50F or above the recommended temperature of the liquid asphalt being used. The construction procedure depends upon whether the base is a newly constructed base, a scarified existing base, or an existing pavement.

When a newly constructed base is used, perform the following procedure:

1. Inspect and condition the base.

2. Prime the base and allow the prime to cure.

3. Haul in and windrow the aggregate at the side of the primed base. (Allow the aggregate to dry or aerate with a blade when wet.)

4. Spread the aggregate on the cured prime base one half of the roadbed width.

5. Spray the bitumen on the aggregate in increments of about one third of the total amount required.

6. Mix the bitumen with the aggregate; blade back and forth until a uniform mix is obtained.

7. Repeat as directed in (5) and (6) until thoroughly mixed.

8. Spread the mix to the specified thickness.

^9. Compact the surface.

10. Apply a seal coat when necessary.

For a scarified base, the aggregate is scarified when it is not available from other sources. The construction procedures are as follows:

1. Loosen the aggregate from the base.

2. Dry and breakup all lumps of material.

3. Blade into parallel windrows of uniform size at one side or in the center.

4. Sweep the base, when needed

5. Prime the base and allow time to cure.

6. Continue as directed in (4) through (10) in the above procedures for a newly constructed base.

When an existing pavement is to be used as a base, the construction procedures are as follows:

1. Sweep the base.

2. Apply a tack coat and allow it to cure.

3. Bring in the aggregate and deposit in windrows at the side of the cured, tacked base.

4. Aerate the aggregate.

5. Spread the aggregate on one half of the tacked base.

6. Spray bitumen on the aggregate in increments of about one third of the total amount required

7. Mix the bitumen with the aggregate by blade.

8. Spread the mix to specified uncompacted thickness.

9. Compact the surface.

10. Apply a seal coat when necessary.

When you are mixing in place (road mix), here are some helpful hints:

1. Do not try to buck nature; stop operations when you are working under adverse weather conditions.

2. Keep the mixture or aggregate in a well-packed windrow for better water shedding and control.

3. Provide drainage cuts through the windrow during heavy rains.

4. When a grader comes to the end of a section with a full blade, lift the blade rapidly to avoid carrying materials into the next section.

5. The distributor spray must be cut sharply at sectional joints; carry-over to the next section will cause undesirable fat joints.

6. Plan the work to avoid inconvenience to traffic.

7. Apply the asphalt at the recommended spraying viscosity to ensure uniform application.

8. Using a shoe on the outer end of the grader blade or moldboard helps obtain a good edge during spreading operations.

9. Aggregate in shaded areas usually requires extra aeration.