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Chapter 4

Production and Laydown of Bituminous Materials

The efforts and skills of a military engineer supervisor are used to the fullest extent during the production and laydown of bituminous materials. His knowledge and surveillance of paving operations can mean the difference between a durable, smooth-riding pavement and a rough, unsound pavement. An engineer supervisor has two primary responsibilities during paving operations—ensuring that the project's specifications are met and providing the equipment operator or ground crew every opportunity to meet the job specifications in the most cost-effective way. Therefore, the supervisor must be familiar with the equipment used during the operation. Since communication is essential for a successful operation, allowing all working parties to participate in a construction meeting before starting an operation minimizes confusion and friction after the operation starts.

Construction

4-1. A bituminous pavement (wearing surface) is a compacted mass of bituminous material and aggregate. It can be constructed in one or more layers, and each layer is usually 1 or more inches thick. Laying a thicker pavement in smaller lifts allows better compaction and greater densities in the pavement mat. The layers distribute the load over the base through the interlocking action of the aggregate, and bituminous pavements depend on the base for their load-carrying capacity. A pavement should be constructed over suitable base and subbase courses that are not subjected to excessive settling or deformation. The three types of pavement most widely used in TO areas are—

  • Penetration macadam.
  • Road mix (mixed in place).
  • Plant mix (hot mix, hot laid, cold mix, cold laid).

4-2. This chapter discusses prime and tack coats, surface treatments, and production and laydown of bituminous pavements. Chapter 5 discusses storing and dedrumming bituminous material and the procedures for the field manufacture of asphalt cutback.

Handling and Stockpiling Aggregate

4-3. Aggregate is transported in trucks from the pit where it is produced and screened to a convenient location near the construction site. ( TM 5-332 addresses the issue of processing aggregate from pits and quarries.) Aggregate is usually stockpiled at the pit and construction site to prevent shortages during an operation. Large stockpiles are usually rectangular with a flat top to ease computation of volumes and built-in layers, to retain uniformity of gradation, and to avoid segregation.

4-4. Maintain separate stockpiles for each aggregate. Aggregate may separate if it is dumped into piles with long, sloping sides. The larger particles may fall down the slope, and the smaller particles will be left near the top of the stockpile. Deposit aggregate in layers that are about 3 feet thick. To minimize segregation, place a truck ramp at one end of the pile so that trucks can back up the ramp and dump aggregate over the top area of the stockpile or use a clamshell bucket without teeth to build up the stockpile. Time limitations may prevent the use of ramps in the TO; therefore, aggregate may be deposited at ground level directly from the trucks.

Traffic Problems

4-5. When a bituminous operation is planned for roads with traffic, use the following precautions:

  • Place SLOW signs or other warning devices 100 yards and 20 yards from each entrance to the project.
  • Ensure that flagmen wear safety vests or other conspicuous attire.
  • Use a small vehicle as a pilot (lead) truck to help control traffic. It helps keep traffic off freshly prepared surfaces and limits travel to a moderate speed when passing the operation.
  • Advise flagmen to organize traffic in convoys for the pilot truck. The driver of the last vehicle in the convoy passes an object, such as a red flag, to the flagman on the far end to indicate that all the traffic in the convoy has passed.

4-6. An airfield will probably have to remain operational during bituminous operations. A preconstruction conference should be held with the airfield safety officer. Discussion areas should include the construction schedule, the equipment routing, and the maximum height of the equipment needed in the operation. Liaison with the air-traffic control office must be established if trucks and other equipment are to cross the runways that are in use.

 

Prime Coat

4-7. Priming operations should be planned so that there is always an adequate amount of cured prime base ahead of the surfacing operations, but not so far ahead that the base becomes dirty. Apply a prime coat, consisting of a low-viscosity liquid bituminous material, to the base before placing a bituminous wearing surface. The prime coat—

  • Penetrates the base about 0.25 inch and fills the voids.
  • Acts as a waterproof barrier to prevent moisture from penetrating the wearing surface and reaching the base.
  • Binds the particles of the base together and bonds the base to the wearing surface.

Base Preparation

4-8. Ensure that the base is well-graded, shaped to the desired cross section, compacted to a specified density, well-drained, clean, and free of excessive moisture but not completely dry. Broom the surface of the base if it contains an appreciable amount of loose material (fine or coarse) or if it is excessively dusty. If the base is not broomed, apply a prime coat and lightly roll it with a pneumatic roller.

4-9. The base can be sprinkled with water, although it is not preferred. If sprinkling is necessary, however, apply water lightly at a rate of about 0.2 to 0.3 gallon per square yard, depending on the condition of the base, the temperature, and the humidity. Completely cover the base with the minimum amount of water and let it dry or nearly dry before applying the prime coat. If the base is too wet, it will not take the prime properly.

4-10. Moisture tends to come out, especially in hot weather, and strip the prime from the base during construction. Rain also tends to strip the prime from a base that was too wet when primed. Heavy rains may strip a properly primed base to some extent, but the tendency will not be so great as on an improperly cured base. In general, the lowest acceptable moisture content for the upper portion of the base course before priming should not exceed one-half of the optimum moisture content. On the contrary, if the base dries out completely, cracks may develop and a heavy rain could cause swelling and density loss. For information on subgrade, subbase, and base preparation, see FM 5-430-00-1 .

Materials

4-11. Table 2-2 lists bituminous materials used for prime coats. Specific materials used depend on the condition of the soil base and the climate. Consider the following:

  • In moderate and warm climates, use MC-70.
  • In cold climates, use RS asphalt cutbacks such as RC-70 or RC-250.
  • In very cold climates, eliminate the prime coat because the curing process may be extremely slow.
  • On densely graded base courses, use MC-30.
  • On loosely bonded, fine-grained soils, such as well-graded sand, use MC-70 or SC-70.
  • On coarse-grained sandy soils, use MC-250 or SC-250.

Application

4-12. Figure 4-1 shows the application sequence for a prime coat applied with a bituminous distributor, and Table 2-1 lists the characteristics of bitumens. Adjust the application rate according to the porosity (density) of the base, the atmospheric temperature, and the temperature and grade of the bituminous material.

    Figure 4-1. Prime-Coat Application Sequence

 

4-13. Use the thinnest possible application that will effectively coat the base. To waterproof the base, extend the prime coat at least 1 foot beyond the edge of the surface on each side to ensure complete coverage. If the base course has a low plastic index (PI), extend the prime coat far enough to include the shoulders. In general, vary the application rate between 0.2 and 0.5 gallon per square yard.

4-14. Soils with several voids require a higher rate of application than dense soils. For example, uniform, open-graded sand may require 0.4 gallon per square yard to fill the voids; whereas, a tightly bonded, fine-grained soil may require about 0.2 gallon per square yard. To establish the application rate, conduct trial runs on a test strip. Adjust the application rate if it does not provide the desired quantity per square yard.

4-15. Use the application chart that comes with the distributor to get the speed of the distributor and the discharge rate for the conditions. Adjust the distributor, as necessary, while applying the prime coat. Always observe safety precautions when using bituminous materials.

4-16. If the base absorbs all of the prime material within 1 to 3 hours or if penetration is too shallow, the base is underprimed. Apply a second coating of the prime material to correct this problem.

4-17. Overpriming is more undesirable than underpriming. An overprimed base may fail to cure and can contribute to pavement failure. A free film of prime material remaining on the base after a 48-hour curing period indicates that the base is overprimed. Correct this condition by spreading a light, uniform layer of clean, dry sand over the prime coat to absorb the excess material. After applying the sand, lightly roll and broom the surface. Correct excess prime that is held in minor depressions by applying clean, dry sand. Lightly broom loose sand away before laying the wearing surface.

Curing Period

4-18. Ensure that the prime is adequately cured before laying the surface course. In general, allow a minimum of 48 hours for complete curing. A proper surface condition is slightly brown instead of shiny black.

Protection

4-19. Set up barricades around the soil base before covering it with a bituminous wearing surface. This prevents traffic from carrying dust or mud onto the surface before and after it is primed. If the primed base course must be open to traffic before it has completely cured, spread light sand over the surface to absorb the excess material.

Tack Coat

4-20. A tack coat is bituminous material that is sprayed onto an existing wearing surface (concrete, brick, bituminous material, or binder course) before the bituminous pavement is placed. A tack coat provides a bond between the existing pavement and the new surface and becomes tacky within a few hours. A tack coat is not required on a primed base unless the prime coat has completely cured and is coated with dust. The application sequence for a tack coat is the same as for a prime coat ( see Figure 4-1 ).

Materials

4-21. See Table 2-1 for the bituminous materials to use for tack coats. RC cutbacks, RS emulsions, and medium ACs are normally suitable for a tack coat. RC-250 or RC-800 is most satisfactory because either become tacky quickly. Because RC cutbacks are highly flammable, follow the safety precautions prescribed for handling bituminous materials. Emulsions can be used in warm weather. If facilities are available, 200-300 penetration AC can be used with excellent results.

Application

4-22. Sweep the existing surface clean so that the tack coat can adhere to it. Apply the tack coat with a bituminous distributor at the temperatures prescribed for spraying. Unlike a prime coat, a tack coat can only be applied over the width of the existing surface. The application rate varies between 0.05 and 0.25 gallon per square yard. On a smooth, dense surface, a minimum rate of 0.05 gallon per square yard should produce a satisfactory bond. If the surface is worn, rough, or cracked, use a maximum rate of 0.25 gallon per square yard. An extremely heavy tack coat may absorb into the surface mixture and cause bleeding, flushing, and stability loss. Roll the surface lightly with a rubber-tired roller or truck tires for uniform distribution of bituminous materials.

Curing Period

4-23. Ensure that the tack coat is completely cured (normally takes a few hours) before laying the surface course. A properly cured surface will feel tacky to the touch; further curing is required if the cutback or emulsion comes off easily under light finger pressure. If the wearing surface is laid before the tack coat becomes tacky, the volatile substances may act as lubricants and prevent bonding. If the tack coat hardens, the heat from a hot bituminous mix or the cutter stock in the asphalt cutback will usually soften the tack coat enough for a bond to develop between the old and new surfaces. Dust must be broomed from the tack coat before it will bond with the wearing surface.

Protection

4-24. Set up barricades around the area to be covered with a bituminous wearing surface. This prevents traffic from carrying dust or mud onto the surface before or after the tack coat is applied. If the surface must be open for traffic, open only one lane. Apply the tack coat, pave, prime, and sand the unused lane. Open the freshly paved lane to traffic and pave the remaining lane. Sweep and reprime the first lane after completing the second lane.

Single Surface Treatment

4-25. A single surface treatment usually consists of a sprayed application of bituminous material and aggregate that is less than 1 inch thick. A surface treatment may also be called a seal coat, an armor coat, or a carpet coat. Surface treatments—

  • Serve as abrasive, weather-resisting mediums that waterproof the base.
  • Are not as durable as bituminous concrete and may require constant maintenance.
  • Are particularly suitable for surfacing aged or worn bituminous pavements that are dry, raveled, or beginning to crack.
  • Should possess a surface texture that does not require surface treatment to fill the voids if they are new, well-designed, and well-constructed.
  • Are used largely on roads.
  • May be used as expedient measures for airfields, although it is not recommended.
  • Are particularly suitable for TO construction because they may be laid quickly with minimum materials and equipment, constructed in multiple layers with little interference to traffic, and used as the first step in stage construction.
  • Will not withstand the action of steel-wheel vehicles, tracked vehicles, or nonskid chains on vehicle wheels.
  • Should not be laid when the temperature is below 50°F.

4-26. The three requirements for a surface treatment are as follows:

  • The quantity of bituminous material must be sufficient to hold the aggregate without submerging it.
  • Sufficient aggregate must be used to cover the bituminous material.
  • The base course must be strong enough to support the anticipated traffic load.

4-27. A single surface treatment consists of an application of bituminous material that is covered with mineral aggregate and rolled to a smooth, even-textured surface. Figure 4-2 shows the sequence of operations for a single surface treatment.

    Figure 4-2. Single Surface Treatment

 

Aggregate

4-28. Use uniformly graded sand, crushed stone, gravel, or slag for surface treatments. The surface treatment determines the size of aggregate to select. For example, to seal a smooth, existing surface, use coarse sand; for a badly broken surface, use 1/2-inch aggregate (maximum). The minimum size aggregate used should pass a number 4 sieve. Table 2-1 shows the gradation specification limits for surface treatments.

4-29. Spread aggregate immediately after applying bituminous material and while it is still fluid. Use an adjustable, mechanical aggregate spreader ( Figure 4-3 ) or spread the aggregate from trucks or by hand. If trucks are used, operate them backward so that their wheels move over the bituminous material that has already been covered with aggregate. If hand-spreading, dump the aggregate in piles adjacent to the areas to be treated and then spread it. If aggregate is spread unevenly, drag-broom the surface to ensure complete, uniform coverage of the bituminous material and a smooth, even surface.

    Figure 4-3. Hopper-Type Aggregate Spreader

 

Bituminous Material

4-30. As shown in Table 2-2, RC cutbacks, MC cutbacks, RS emulsions, and AC can be used for surface treatments. RC cutbacks are most widely used because they evaporate rapidly, and the road can be opened to traffic almost immediately after applying them. The viscosity grade of bituminous material depends on the size of the aggregate used as cover stone. Larger particles of aggregate require bituminous material of higher viscosity so that they will hold the aggregate. For example, RC-70 or RC-250 can be used with coarse sand for a surface treatment to seal cracks in an otherwise satisfactory surface. For resurfacing a badly cracked or rough surface, use RC-800 or RC-3,000 with 3/4-inch aggregate.

4-31. Surface treatments are usually applied to a thoroughly compacted, primed base that has been swept clean. The existing surface or base course must be dry or contain minimal moisture that permits satisfactory bonding of the surface treatment to the base, and the atmospheric temperature should be above 50°F. To ensure uniform distribution, apply bituminous material with a bituminous distributor. The quantity of material required is based on the average particle size of the cover stone as shown in Table 4-1 .

4-32. The amount of bituminous material must be sufficient to hold the aggregate in place without leaving a sticky surface. The aggregate must not be completely submerged in the bituminous material. The viscosity used depends on the size of the cover stone; the larger the cover stone, the higher (or thicker) the viscosity of the bituminous material. Submerge 1/4-inch aggregate about 30 percent; 3/8-inch aggregate, 32 percent; 1/2-inch aggregate, 35 percent; and 3/4-inch aggregate, 43 percent. Use about 1 gallon of bituminous material per 100 pounds of aggregate.

Example: Determine the recommended application rate of bituminous material, in gallons per square yard, if 30 pounds of aggregate are required to cover a 1-square-yard area.

Solution:

 

 

Rolling

4-33. Roll the surface with a 5- to 8-ton roller while drag-brooming. Using a heavier roller could crush the aggregate rather than embed it into the bituminous material, and crushed particles change the selected gradation of the aggregate. When rolling—

  • Roll parallel to the centerline of the roadway to reduce the number of times the roller changes direction.
  • Overlap succeeding passes one-half the roller's wheel width to ensure complete coverage.
  • Complete rolling operations before the bituminous material hardens to ensure that the aggregate is well embedded in the bituminous material.
  • Make succeeding passes from the low side to the high side of the surface to maintain a surface crown and prevent feathering at the edges.
  • Conduct rolling operations at a slow speed.
  • Ensure that rollers are wet to prevent bituminous material from sticking to the wheels.

Final Sweeping

4-34. After rolling and curing, the surface treatment is ready for traffic. If the surface treatment is used on an airfield, sweep excess aggregate from the surface to avoid damaging the aircraft. Although not essential, road surfaces should also be swept.

Multiple Surface Treatment

4-35. When a tougher, more resistant surface is desired than that obtained with a single surface treatment, use a multiple surface treatment. This type of treatment involves two or more successive layers of single surface treatments. Use smaller particles of aggregate and less bituminous material for each successive layer. Although multiple surface treatments are usually more than 1 inch thick, they are still considered surface treatments because each layer is usually less than 1 inch thick and the total surface treatment does not add appreciably to the load-carrying capacity of the base. Use the following guidelines for a multiple surface treatment:

  • First layer. Lay this layer the same as a single surface treatment. Sweep away loose aggregate so that subsequent layers will bond together.
  • Second layer. Reduce the amount of bituminous material by one-third or one-half of the first layer. The aggregate used should be about one-half the diameter of that used in the first application. See Table 4-2 for the size of aggregate and the amount of bituminous material to use.
  • Final layer. Drag-boom the final layer, if necessary, so that an even layer of aggregate remains. Roll the surface with a 5- to 8-ton roller so that the aggregate will embed in the bituminous material. After the surface is rolled and cured, it is ready for traffic. If the multiple surface treatment is on an airfield, sweep away loose aggregate to avoid damaging aircraft. Although not essential, road surfaces should also be swept.

Dust Palliative

4-36. Apply a bituminous dust palliative to prevent a soil wearing surface from becoming excessively dusty. Ensure that the bituminous material is thin enough to soak into the surface. The material must retain fluidity indefinitely so that dust particles raised by traffic will become coated with bituminous material. Use MC-70 or SC-70 cutback as a dust palliative.

Concept

4-37. A penetration macadam—

  • Consists of crushed stone or slag that is bonded with bituminous materials by the penetration method and constructed on a previously prepared base or subbase course.
  • Is used as a wearing surface for a road or as a binder or base course in the stage construction of a road or an airfield.
  • May be used as a surface course on overrun areas of an airfield that are not subject to blast.
  • Is particularly suitable for use in a remote locality or for a small paving area.

4-38. Construction of a penetration macadam is rapid and uses minimal equipment. Because the construction method cannot be accurately controlled, the finished surface is less dense than the surface of a plant-mix pavement. Loose aggregate may be a hazard to traffic, particularly on airfields.

Materials

4-39. Table 2-2 lists the bituminous materials used for penetration-macadam wearing surfaces. Emulsions are sometimes used for penetration-macadam base courses. The three sizes of aggregate used in constructing a penetration macadam are—

  • Macadam aggregate (coarse).
  • Uniformly graded keystone (intermediate).
  • Uniformly graded FA or choke stone (stone chips or clean, coarse sand).

4-40. CA and keystone should consist of clean, tough, durable particles or fragments of stone or crushed slag. Table 2-1 lists the recommended gradations for penetration-macadam wearing surfaces.

Construction

4-41. Construct a penetration-macadam surface course on a dry base course. Ensure that the aggregate is dry, the atmospheric temperature is about 60°F, and the weather is not foggy or rainy. Use the following procedures (see Figure 4-4 ) to construct the surface course:

  • Inspect the base and recondition it as necessary.
  • Prime the base if waterproofing or consolidation of the base top is necessary. Use a tack coat when placing a penetration-macadam base course over an existing pavement.
  • Spread the macadam aggregate.
  • Roll the surface.
  • Apply the first layer of bituminous material.
  • Apply the keystone.
  • Drag-broom and roll the surface.
  • Apply the second layer of bituminous material.
  • Apply choke stone.
  • Drag-broom, roll, and hand-broom the surface.
  • Apply a surface treatment when additional waterproofing is necessary.

 

      Figure 4-4. Procedures for Applying a Penetration-Macadam Surface Course

     

Recondition the Base Course

4-42. Inspect the base for discrepancies and defective areas. A penetration-macadam pavement must be built over a well-compacted, adequate base. Sweep away dust and debris from the base surface so that it will absorb the prime coat. Use a tack coat when constructing a penetration macadam on an existing pavement.

Construct Edges on the Wearing Surface

4-43. If the surface has no permanent edgings, curbs, or gutters, install side forms that are securely staked and backed with earth or other material before spreading the CA. Side forms—

  • Can be made of wood or metal.
  • Must be constructed so that they can be securely and rigidly fastened in place to the specified lines and grades.
  • Should be sufficient in number and length to help control construction.
  • Should be 10 to 12 feet long; however, 5-foot sections are acceptable on curves with a radius of 150 feet or less.
  • Should be 1/4 inch shallower than the depth of the finished pavement.
  • Should be backed with earth or other suitable materials on the berm side to a width of not less than 2 feet. When the material is thoroughly compacted, it should be about the same thickness as the compacted wearing course.

4-44. When constructing well-compacted shoulders of earth or other material before spreading the CA, ensure that—

  • Shoulders are about 2 feet wide and have vertical faces that are equivalent to the depth of the wearing course.
  • Material is free of sod, weeds, and other unsuitable matter.
  • Shoulders are the full width and height of the surface so that they hold the course material in place while it is rolled and spread.

Apply the Macadam Aggregate

4-45. Divide the road or runway into moderate-sized strips. Spread loose CA uniformly with improvised or standard stone spreaders, by hand, or with a modified bituminous finisher at a loose depth that will give the required compacted thickness. Use one strip as a test section to establish the required depth, which should be about 3 1/2 inches of loose aggregate for a compacted thickness of 2 1/2 inches.

4-46. Remove thin, flat, oversized aggregate that appears on the surface at any time during construction. Remove all areas of fine material and replace them with macadam aggregate before rolling. The maximum aggregate size should be about two-thirds the thickness of the compacted layer. If the aggregate is larger, the oversized particles may prevent proper compaction or they may be crushed by compacting equipment.

4-47. To compact and bond the macadam aggregate, dry-roll it with a 10-ton, three-wheel roller. Use the following procedures when rolling the surface:

  • Roll from the sides to the center gradually. On superelevated curves, however, roll from the lower to the upper edge, parallel with the centerline of the lane and overlap the preceding track by one-half the track width. Continue this process until the aggregate does not creep or wave ahead of the roller.
  • Cover equal portions of the spread aggregate and the shoulder with the outside wheel of the roller if permanent edgings are not present. Roll backward and forward until the shoulder is firmly compacted against the pavement edge.
  • Tamp the aggregate thoroughly with mechanical or hand tampers along curbs, headers, walls, and other areas where a roller cannot be used.
  • Remove crushed material that will prevent the bituminous material from penetrating the surface.
  • Remove coated or dirty aggregate, replace it with clean material, and roll it before applying bituminous material.
  • Correct irregularities that are greater than 3/8 inch from the specified lines and grades (determined by prior testing of the surface). Loosen and reshape the aggregate, remove or add aggregate, and reroll the areas. The compacted CA should present a firm, even surface that is true to the specified cross section and parallel to the finished grade. The texture should allow a uniform penetration of the bituminous material.

4-48. Remove the forms after compacting the CA and before applying the bituminous material. Fill the spaces with shoulder material and thoroughly compact the area.

Test the Course-Aggregate Thickness

4-49. After spreading and rolling the CA, test the depth before applying bituminous materials. The test generally involves digging three holes at 200-foot intervals (one test hole at the center and one at each quarter point). Correct deviations in excess of 3/8 inch by loosening the surface, removing or adding macadam aggregate, and rerolling the surface. To control thickness, place wooden blocks (equal in height to the desired thickness of the first course) on the base course at short intervals before spreading the aggregate.

Apply the First Layer of Bituminous Material

4-50. Apply a heavy layer of bituminous material to waterproof the macadam aggregate and hold it in place. Apply the bituminous material uniformly using a bituminous distributor. The application rate is about 0.75 gallon per square yard, per inch of compacted thickness for heavy traffic; for lighter vehicles, increase the rate to about 1.1 gallons per square yard. Apply the bituminous material only when the CA is dry throughout its entire depth and when the recommended spraying temperature has been reached (see Chapter 2 ).

4-51. If the distributor is not equipped to distribute bituminous material evenly at the junctions, spread building paper on the surface. Place the paper far enough back from the ends of each application so that starting and stopping is on the paper. Using paper also ensures that spraying operations treat the entire length of the surface. Remove the paper immediately after use and burn it. Use a narrow-spouted pouring pot or the hose attachment on a distributor to apply bituminous material on areas the distributor missed.

Protect the Area

4-52. When applying bituminous material, protect adjacent structures and trees from being spattered or marred by the material. Remove surplus bituminous material from the shoulders or blot it with fine sand before rolling so that the shoulder can be rolled with the wearing course.

Apply the Keystone

4-53. After applying the first layer of bituminous material, spread clean, dry keystone according to the gradations in Table 2-1 . Spread enough keystone to fill the surface voids of the CA. The size and number of voids to fill determine the aggregate size and the application rate. The normal rate is 25 to 35 pounds per square yard. Spread the keystone with an aggregate spreader on a truck moving backward or use shovels to spread it from stockpiles adjacent to the area to be treated. Spread aggregate ahead of the wheels on the spreader so that the bituminous material is covered before the wheels pass over it. Never dump aggregate on the area to be treated.

Broom and Roll the Aggregate

4-54. Spread the aggregate uniformly with push brooms or drag brooms. Start rolling the surface immediately after spreading and smoothing the keystone, while the bituminous material is still warm. Broom additional keystone in voids during the rolling process until all the surface voids are uniform, leaving the CA uncovered. Continue rolling until the course is firmly bound, is even, and shows no perceptible movement under the roller.

Apply the Second Layer of Bituminous Material

4-55. After rolling the keystone, sweep loose keystone off the surface. Apply a second layer of bituminous material. The choke stone determines the rate of application, which is usually 0.8 to 1 gallon per 100 pounds of aggregate. As the estimated traffic increases, the rate decreases.

Apply the Choke Stone

4-56. After applying the second layer of bituminous material, apply a layer of clean, dry choke stone. Retain a portion of the choke stone to add during the brooming and rolling process as needed and broom and roll as discussed above. Continue this process until all surface voids in the CA are filled and the surface is uniform in texture, is thoroughly bonded, and shows no perceptible movement under the roller. A tandem roller can be used for the first rolling, but do not exceed 3 mph because faster rolling can cause waves that are difficult or impossible to remove.

Apply the Surface Treatment

4-57. If the pavement is a final wearing surface, use a single surface treatment to provide a smooth, waterproof surface. RC bituminous material is normally used, and the grade depends on the surface and the weather conditions. The application rate is about 1 gallon of bituminous material for 100 pounds of aggregate. To complete the surface treatment, spread a one-stone thickness of cover stone over the bituminous material. Drag-broom and roll the surface with a 5- to 8-ton roller. For a more durable surface, place a 1-inch layer of hot mix instead of a surface treatment over the penetration macadam. Ensure that the final surface is true to the specified line, grade, and cross section. Replace or readjust portions of the pavement that are defective in composition or deviate from the requirements.

Protect and Maintain the Pavement

4-58. Before applying the surface treatment, ensure that only necessary traffic travels over the pavement. Drag-broom and roll the surface 10 days after its completion. In addition, correct fat spots and other defective areas that appear during that time.

Concept

4-59. A road-mix pavement—

  • Consists of mineral aggregate and mineral filler that are uniformly mixed in place with bituminous material and compacted on a prepared base or subgrade. It is usually a single layer, about 1 1/2 to 3 inches thick.
  • Is likely to become defective unless it has a sound, well-drained subgrade and is well-mixed, uniformly spread, and compacted.
  • May be used as a wearing surface on a temporary road or airfield or as a bituminous base or binder course in the stage construction of a permanent road or airfield.
  • Is an economical method of surfacing a small area when aggregate can be used from the existing base or when satisfactory aggregate is nearby.

Bitumen

4-60. The grade and type of bituminous material depends on the aggregate, the equipment available, and the weather. Table 2-2 shows the recommended types and viscosity grades of bituminous materials (asphalt cutbacks, asphalt emulsions, road tars) that are suitable for a road mix. Normally, MC cutback is used in moderate climates and SC cutback is used in cold climates.

4-61. Viscosity is controlled by the temperature, the aggregate gradation, and the mixing method. Use the highest viscosity that completely and uniformly coats the aggregate. Normally, open-graded aggregate requires higher viscosity and a gradation containing mineral filler requires lower viscosity.

Aggregate

4-62. Scarify aggregate from the existing subgrade or base or haul it in from a nearby source. A wide range of gradations of CA, FA, and mineral filler can be used. Table 2-1 shows the recommended gradation limits, and Table 4-3 shows the suggested bituminous material content for each gradation. After blending the aggregate in windrows, take samples for a sieve analysis. The gradation of the aggregate might have to be changed to meet specific field conditions. Acceptable deviations are—

  • Aggregate passing the number 4 sieve: ±7 percent.
  • Aggregate passing the number 30 sieve: ±5 percent.
  • Aggregate passing the number 200 sieve: ±2 percent.

4-63. The ideal aggregate for a road-mix pavement is well-graded (dense or open), sandy gravel or clean sand. The maximum size of the aggregate is two-thirds of the compacted thickness of the layer. The loose thickness is about 1 1/4 times the compacted thickness. Test for moisture content before mixing the aggregate with asphalt cutbacks or road tars. If the aggregate is too wet, use a mechanical mixer, a grader, or an improvised plow to allow the excess moisture to evaporate. For cutbacks and tars, the moisture content of CA should not exceed 3 percent and the moisture content of FA should not exceed 2 percent. For emulsions, the moisture content of CA should not exceed 5 percent and the moisture content of FA should not exceed 3 percent.

Mix Proportions

4-64. The quality of a road-mix pavement depends on the control of the mix. The percentage of bituminous material shown in Table 4-3 will vary in relation to the absorptive quality of the aggregate, the rate of evaporation of the volatile substances, and other factors. Although an exact formula is difficult to follow, proportioning must be controlled within very narrow limits to assure the stability and life of the mix. With dense-graded aggregates, do not use too much bituminous material. If equipment is available, use the modified Marshall method to establish a trial bituminous content (see FM 5-428 ). Ensure that all particles of the completed mix are coated and uniform in color. If the mix is too lean, it stands almost vertical in the windrow; if it is too rich, it readily flows back to its previous shape. If the mix is correctly proportioned, a handful squeezed into a ball retains its shape.

Construction

4-65. Construct a road-mix pavement on a dry base when it is not raining and the atmospheric temperature is above 50°F. The initial construction procedure depends on whether the base is a newly constructed base, a scarified existing base, or an existing pavement. Figure 4-5 shows the sequence of operations for a typical road-mix project.

    Figure 4-5. Sequence of Operations for a Road-Mix Pavement

 

4-66. If paving on a newly constructed base—

  • Inspect and recondition the base.
  • Prime the base and allow the prime to cure.
  • Place (or import) and windrow the aggregate at the side of the primed base and allow the aggregate to dry.
  • Spread the aggregate on the cured, primed base.
  • Spread the bituminous material on the aggregate in one-third increments.
  • Mix the bituminous material with the aggregate.
  • Blade and aerate the mix.
  • Spread the mix to the specified uncompacted thickness.
  • Compact the surface.
  • Apply a seal coat if necessary.

4-67. If scarifying the aggregate from the base course—

  • Loosen the aggregate from the base.
  • Dry and break up lumps of material.
  • Blade the material into parallel windrows of uniform size at one side and/or in the center.
  • Inspect and recondition the base.
  • Sweep the base.
  • Prime the base and allow the prime to cure.
  • Spread the aggregate on the cured, primed base.
  • Spread the bituminous material on the aggregate in one-third increments.
  • Mix the bituminous material with the aggregate.
  • Blade and aerate the mix.
  • Spread the mix to the specified uncompacted thickness.
  • Compact the surface.
  • Apply a seal coat if necessary.

4-68. If using an existing pavement as a base—

  • Sweep the base.
  • Apply a tack coat and allow it to cure.
  • Deposit the aggregate in windrows at the side of the cured, tacked base and allow it to dry.
  • Spread the aggregate on the tacked base.
  • Spread the bituminous material on the aggregate in one-third increments.
  • Mix the bituminous material with the aggregate.
  • Blade and aerate the mix.
  • Spread the mix to the specified uncompacted thickness.
  • Compact the surface.
  • Apply a seal coat if necessary.

Prepare the Base

4-69. Inspect the base course, test it according to the criteria in Section II of this chapter, and correct deviations. Correct ruts or soft, yielding spots in the base course by loosening, removing, or adding material and recompacting the base to hue and grade. Maintain the line and the grade with grade stakes or steel pins. Place the line and the grade parallel to the centerline of the area to be paved, and space them so that string lines can be stretched in between. Apply a prime coat to the base.

Place and Windrow the Aggregate

4-70. Aggregate for a road mix on a base or an existing pavement used as a base should be hauled to the paving site in trucks that have suitable spreading devices. Bottom dumps or scrapers can also be used for hauling. The aggregate and the mineral filler should be proportioned by weight or volume, mixed thoroughly by windrowing, and turned with a blade grader. Blade the aggregate into uniform windrows in sufficient quantities and proportions to provide a finished course of the specified thickness. Ensure that the aggregate does not segregate in pockets or mix with the shoulder, subgrade, subbase, or base material.

4-71. When the aggregate is obtained from an existing base, make test bores along the proposed road or runway to determine if a sufficient amount of aggregate is available and if using the aggregate will affect the bearing qualities of the remaining subgrade. Scarify the base to the required depth, mix the loose material using harrows and graders to break up the lumps, and blade the aggregate into parallel windrows of uniform size at the side and/or center of the paving area. If material must be blended with the scarified base to improve gradation or increase thickness, dump the new material and spread it evenly before or after scarifying. If a prime coat is placed on top of the uncovered base, apply it to the spaces between the windrows and let it cure. Blade the windrows to the cured, primed strips and prime the remaining strips of the base.

4-72. Using windrows helps reduce aggregate loss from the base caused by traffic. They also reduce the moisture absorbed by uncompacted aggregate during wet weather. Windrows should have a cross-sectional area of 6 to 10 square feet for ordinary construction equipment and 10 to 14 square feet for a travel plant.

Mix the Materials

4-73. If using cutback asphalt for spraying or mixing, heat it to a temperature of 80° to 200°F. Apply asphalt emulsions at 50° to 120°F and tar at 80° to 225°F.

Travel-Plant Method

4-74. With this method, dump, mix, and blade loose aggregate into uniform windrows. As the bucket loader tows the mixer and elevates the aggregate to the hopper, the mixer meters the aggregate, sprays it with the correct amount of bituminous material, mixes the two materials, and redeposits the mix into another windrow behind the plant. Control the travel rate and the mixing operation so that all aggregate particles are coated and the mix is homogenous. Accuracy in proportioning the mix is extremely important. Correct excess or deficient bituminous material and uneven distribution.

4-75. The travel-plant method usually produces a uniform, high-quality mix. Heavy types of asphalt cutback and tar, which reduce the curing time, and an asphalt finisher can be used with the travel plant. Ensure that the finisher's hopper is directly under the travel plant's output chute; this arrangement greatly reduces the maximum output of the plant.

4-76. Windrows cannot contain more material than the finisher can place. The major advantage of this method is that in-place aggregate can be used in an intermediate-type mix and placed with a finisher without loading and transporting aggregate. Use the finisher with the travel plant when constructing airfields and when surface tolerances are critical.

Blade Method

4-77. After blading the dry aggregate into windrows, flatten the windrows and apply bituminous material at the specified temperature. Use a bituminous distributor, and complete at least two equal applications of 0.3 to 0.5 gallon per square yard. After each application, mix the treated aggregate with spring-tooth or double-disk harrows, graders, and/or rotary tillers until aggregate particles are evenly coated.

4-78. When using motor graders, move the windrow from side to side by successive cuts with the blade. Several graders can operate, one behind the other, to reduce the time required for complete mixing. In hilly terrain, the mix tends to drift downhill and collect at the bottom. After mixing the aggregate, blade the mix into a single windrow at or near the center of the road and turn the mix at least four complete turns from one side of the road to the other. If the bituminous material is deficient or the mix is unevenly distributed, correct the situation by adding the required amount of material and remixing.

4-79. If the materials, the weather conditions, and the equipment are well suited to mixed-in-place paving but the road or airfield must carry traffic quickly, windrow the aggregate and mix and spread the bituminous material somewhere else. Do this on any area of smooth ground that can be compacted for this purpose or on any unused road or airfield surface. Prime or tack the surface and allow it to cure. Pick up the bituminous mix, truck it to the site, dump it at proper intervals, and blade it into windrows that are ready for spreading.

Blade Thickened Edges

4-80. When a thickened edge of surfacing is specified, make a triangular cut that conforms to the specified dimensions. Use a motor-grader blade at each edge of the surfacing. Throw the excavated material in a small windrow to the shoulder where the mixture will be spread. Prime the trench and fill it with the bituminous mixture. Roll the layer thoroughly and spread the surface course.

Spread and Shape the Mix

4-81. Spread the mixed, aerated bituminous mixture and the mineral aggregate from the windrow onto the base course in a layer that is uniform in thickness and conforms to the established line and grade. Do not start spreading until the subgrade or base has been inspected and reconditioned as necessary. Do not spread the mixture on a damp surface or when the mix contains excess moisture.

4-82. Spread the mixed material to the required width in thin, equal layers using a self-propelled blade grader, a finisher, or similar equipment. (If using a finisher, support equipment is required. Split the material into two windrows for an 8- to 12-foot wide pavement.) When spreading the mix from a windrow, do not cut into the underlying subgrade or base. If necessary, leave a layer of mix about 1/2 inch thick at the bottom of the windrow. Roll the material and level it with a blade grader. Spread the remaining material and roll it in thin layers until the entire mix is evenly spread to the specified depth and width. When spreading and compacting, drag or blade the surface to fill ruts and remove corrugations, waves, or other irregularities. Use a multiple-wheel, pneumatic-tired roller when rolling in the early stages.

Roll the Finished Surface

4-83. The rollers should be in good condition, suitable for rolling asphalt, and operated by trained roller operators. After spreading all the layers, roll the surface with a tandem roller. Start at the outside edge of the surface and proceed to the center, overlapping on successive trips at least one-half the width of the roller's wheel. Ensure that alternate trips of the roller are of slightly different lengths and control the roller's speed to avoid displacing the mix. A patrol grader may be necessary for lightly blading (or floating) the surface. Continue rolling until the maximum density is obtained and roller marks are eliminated.

4-84. Moisten the wheels so that the mix does not stick to them. Hand-tamp the mix in places that are inaccessible to the roller. If the surface course becomes rough, corrugated, uneven in texture, or marked by traffic, tear up unsatisfactory areas and rework, relay, or replace them. If any part of the surface becomes water-soaked while being laid, tear it up, place it in a windrow, aerate it until the moisture content is within the specified limits, spread it, and reroll it.

Shape the Edges

4-85. Trim the outside edges if forms were not used. This can be done while the surface is being compacted and finished if time permits.

Take Samples

4-86. Take samples of the mix every 400 tons of mixed aggregate. Test the gradation, the percent of bituminous material, and the thickness and density of the completed wearing surface. If deficiencies in composition, density, or thickness exceed the specified requirements, remove and replace the material.

Treat the Surface

4-87. If the road-mix surface course is open-graded aggregate, a surface treatment may be necessary for waterproofing. A surface treatment is unnecessary on a densely graded, well-compacted, road-mix pavement.

Concept

4-88. A plant mix is a mixture of mineral aggregate and bituminous material that is mixed at a central plant. Proportioning of the aggregate and the bituminous material is closely controlled, and the mineral aggregate is usually dried and heated before mixing. The mixed material is transported to the site, spread, and compacted. A mix plant generally consists of portable units that may be assembled in various combinations. (See Chapter 3 for information on various types of central plants.) Plant-mix pavement is usually superior to mixed-in-place pavement or penetration macadam because a heavier grade of bituminous material can be used; therefore, proportioning and mixing is more effective.

4-89. Different setups are used for central plants, allowing mixes to be produced with the required degrees of accuracy and economy that are compatible with the type of road or airfield under construction. (See TM 5-3895-374-10 for recommended layouts for a drum-mix plant.)

Hot-Mix Bituminous Concrete

4-90. Hot-mix bituminous concrete that is mixed in a batch or drum-mix plant is generally composed of well-graded mineral aggregate, mineral filler, and bituminous cement. It is particularly suitable for surfacing airfields and roads and streets with heavy traffic. It is also used for base and binder courses.

4-91. To prepare hot-mix bituminous concrete, thoroughly dry and heat the aggregates. Then, accurately proportion and mix the aggregates at the specified rate with bituminous material that is 250° to 350°F (liquid form). Spread the mix with an asphalt finisher or by hand where the finisher cannot be used. While the asphalt is still hot (about 250°F), use power rollers to compact it to the required density and thickness. Hot mixes lose their workability when cooled to air temperature.

4-92. The hot-mix method ensures that the aggregate is coated with a uniform film of bituminous material and that the size of the aggregate and the quantity of the bituminous material are accurately controlled. Hot-mix pavements—

  • Do not require a curing period.
  • Can be used as soon as they have been compacted to the required density and cooled.
  • Can be constructed rapidly with minimum probability of damage to unfinished pavements from unfavorable weather conditions.
  • Have a high stability and resistance to moisture-penetration damage immediately after rolling and cooling.
  • Are durable, waterproof, and homogenous in character.
  • Must be laid over a base course that is constructed to provide structural stability and distribute the wheel load over the subgrade.

4-93. The highest quality bituminous pavements are constructed from hot-laid mixes made with AC. Based on the grades of mineral aggregate and bituminous material used, they are also known as asphalt concrete, tar concrete, sheet asphalt, or stone-filled sheet asphalt. Hot mixes are recommended for use whenever equipment, material, and manpower are available. Probable causes of deficiencies in hot mixes observed at the plant are shown in Table 4-4 ; probable causes of imperfections in finished pavement are shown in Table 4-5 .

  

 

Cold-Laid Bituminous Plant Mix

4-94. Cold-laid bituminous plant mixes cover a wide variety of types and grades of bituminous material and aggregate gradations. Cold-laid asphalt-concrete pavements are composed of AC; a well-graded mineral aggregate; and a liquefier, an asphalt cutback, or an asphalt emulsion. The aggregates in a cold mix are usually the same as those in a hot mix. They are heated only for drying, but the bituminous material must also be heated so that it will thoroughly coat the aggregates. A cold-laid plant mix is similar in appearance and general physical characteristics to a hot mix, but it is less durable. Cold mixes can be stockpiled and used for small repairs.

4-95. A curing period is required to allow evaporation of excess moisture and volatile substances contained in the bituminous materials of the liquefier. In cold weather, high density is difficult to secure by rolling and the initial stability is likely to be low. It is especially suitable for patching and for small jobs where the amount used does not justify erecting a hot-mix plant. A cold-laid plant mix can be manufactured at a central plant, shipped by rail or truck to the paving site, and produced in small quantities.

4-96. One type of cold-laid bituminous plant mix is composed of graded mineral aggregate and RC-250 or RC-800. Use the modified Marshall method to determine the quantities of bituminous material needed (see FM 5-472 ). Another type of cold mix is prepared with aggregates containing no more than 2 percent moisture and sprayed with a liquifier and AC of 85 to 100 penetration. Regulating the amount of liquid, using SC asphalt, and using RT-5 through RT-9 reduce the curing period for cold-laid plant mixes. Spread cold-laid plant mixes using blade graders or mechanical spreaders.

Bituminous Materials

4-97. Table 2-2 lists the recommended bituminous materials to use in hot-mix and cold-laid pavements. Table 2-1 lists aggregate gradations for the various types of hot-mix surface and binder courses. Aggregate gradations for cold mixes are the same as for hot mixes and road mixes.

Laying a Hot mix

4-98. The following procedures are recommended for laying hot mix, but other methods may also be effective. Figure 4-6 shows the sequence of operations for placing a hot-mix pavement.

Step 1. Sweep the base.

Step 2. Patch cracks and holes if placing the mix on existing pavement.

Step 3. Apply a prime or tack coat. Use a tack coat over a prime coat if the prime coat has become old and dirty.

Step 4. Allow the coat to cure.

Step 5. Inspect the equipment.

Step 6. Inspect and sample the mix.

Step 7. Place and spread the mix.

Step 8. Hand-rake the mix where necessary.

Step 9. Roll the surface with a breakdown compaction roller and check for conformance to grade requirements.

Step 10. Roll the surface with an intermediate roller.

Step 11. Roll the surface with a finish roller.

 

      Figure 4-6. Sequence of Operations for a Hot-Mix Pavement

     

4-99. Ensure that construction equipment complies with the operating specifications, is adequate for the operation, and is in good operating condition. Adjust, repair, or replace items or pieces of equipment as necessary. Check the rollers and the spreader. Check the screed on the spreader for warping or scoring. Adjust the screed setting to achieve a properly leveled crown in lanes that deviate from the plane surface. For lanes that require a level surface, set the screed about 1/8 inch high in the center, sloping uniformly toward both edges. This prevents the surface from becoming dished. Adjust the screed based on the straightedge depth tests on the completed pavement.

Determining the Placement Temperature

4-100. The optimum hot-mix placing temperature depends on the weather and the rolling characteristics of the mix. Once the placing temperature is reached, maintain it at ± 25°F. Remember to consider the average heat loss between the plant and the spreader.

Inspecting and Rejecting the Mix

4-101. Check each truckload of hot mix when it arrives from the central plant and when it is dumped. Ensure that the mix temperature complies with the requirements. Most mistakes are made during batching, mixing, and heating.

4-102. After inspection, reject and discard all or part of the truckload as necessary. When rejecting a portion of a truckload, remove the unsatisfactory material from the spreader hopper or the grader and discard it. Rejection is caused for the following reasons:

  • Too hot. Blue smoke rising from the spreader hopper usually indicates an overheated batch; check the temperature immediately. Discard the batch if the temperature exceeds the maximum specification requirement (usually 350°F). Do not discard a batch when it exceeds the optimum temperature (325°F) but stays below 350°F. However, check the equipment immediately and adjust it to avoid overheating.
  • Too cold. A stiff appearance or improper coating of large aggregate particles indicates a cold mix; check the temperature immediately. Discard the mix if the temperature is below the specification requirement (usually 250°F), and take immediate steps to correct the condition.
  • Too rich. A mix that has too much bituminous material can be detected immediately. If a load normally arrives at the spreader with the material peaked or domed and a load suddenly appears with the material lying flat, the mix probably contains too much asphalt. Inspect the load immediately. Excess bituminous material is apparent under the screeds by the way the mix slicks off. Sometimes a hot screed causes a slick surface, which indicates excessive bituminous material. This appearance will disappear when the mix cools. If a slick surface appears, check the screed. If the screed is hotter than usual, keep the mix in place until it cools to determine whether the slick surface was caused by a hot screed or an excess of bituminous material. If the slick appearance does not disappear, reject the mix.
  • Too lean. A mix that does not have enough bituminous material has a lean, granular appearance. The aggregate is improperly coated, and the amount of slump in the truck is less than usual. The mix lacks the typical shiny, black luster; and its action under the screeds is abnormal.
  • Miscellaneous. Other reasons for rejection are excess fines, excess coarse material, and nonuniform mixing.

Placing and Spreading the Mix

4-103. Placing and spreading a hot mix consists of uniformly distributing the loose material on a completed base to obtain the specified thickness, line, and grade.

Hand

4-104. Spread the hot mix by hand if an asphalt finisher is unavailable or an asphalt spreader cannot be used (adjacent to curbs, surrounding manholes, and at curved corners of intersections). Dump the mix from the trucks onto dump boards or into conveniently sized piles. Shovel the material onto the prepared base, rake it smooth to grade and contour, and roll it. This method is only used to supplement other spreading methods because of the labor involved and its inability to obtain a smooth, even-textured surface.

Blade

4-105. Occasionally, it is necessary to use a motor grader to spread hot mix. To obtain surface smoothness, position the blade midway between the front and the rear and make multiple passes. Successive passes further reduce base irregularities in the surface. Because blade spreading usually leaves unsatisfactory edges, do not attempt to make joints. Instead, spread the mix across the full width of the road at one time by using an echelon of graders or by making multiple passes with one grader. Blade spreading can cause the mix to segregate. As the blade spreads out the windrow, large particles of coated aggregate fall toward the outside edge of the road. Like road-mix pavements in hilly terrain, blading tends to drift the mix downhill and the excess mix accumulates at the bottom.

Mechanical

4-106. Except when starting at a transverse joint butted against previously placed pavement, place a wooden shim under the screed of the asphalt finisher. The shim's thickness should be equal to the uncompacted thickness of the course being placed. This allows the screed to start at the proper height and prevents dragging the mix, which causes ragged, uneven pavement and depth deficiencies.

4-107. When starting from a transverse joint, use metal strips equal in thickness to the difference between the rolled and the loose thicknesses of the pavement. At the start of a placing operation, heat the screed to the approximate temperature of the mix. Check the screed temperature occasionally during the operation; and when the operation is delayed for more than a few minutes, reheat the screed to the desired temperature. Do not spread the mix too quickly because doing so can tear or pull the surface. Tearing can also result from insufficient heat or the mix adhering to a dirty screed. Pulling can also result from the spreader being operated too fast, a worn-out screed being used, tamping bars being out of adjustment, or a poorly graded mix.

Asphalt Finisher

4-108. Figure 4-7 shows an asphalt finisher spreading hot mix. Trucks dump the mix into a hopper at the front of the finisher. After dumping the mix, the truck driver firmly fits both sets of truck wheels against the finisher's bumper rolls that push the trucks. (If the driver pushes only one set of wheels against the bumper rolls, the finisher tends to turn.) The finisher's operator makes continual corrections to avoid leaving irregular, ragged, or poorly compacted joints. If the mix tends to cling to the sides of the hopper, loosen it continually to promote correct placement. When the mix accumulates, it cools rapidly and a slug of semicold material will eventually reach the grade.

    Figure 4-7. Asphalt Finisher Placing Hot Mix

 

4-109. Compared to blade spreaders, asphalt finishers have a higher capacity, provide a smoother riding surface, and are able to handle stiffer hot mixes. When a load is deposited into the hopper, CA has a tendency to flow to the outside edges of the hopper and cause undesirable joints of coarse material. To prevent this, use shovels to push the coarse material to the center of the hopper.

Computing the Depth

4-110. Experience is important when placing the loose depth required for the specified compacted depth. Loose depth is roughly equal to the compacted depth multiplied by 1.25. Measure the loose depth with a depth gauge at frequent intervals, and take at least three measurements across the lane. At the beginning of the paving operation, take enough measurements to secure a good average of the loose depth.

4-111. When paving operations are well underway and progressing smoothly, conduct yield checks and take depth samples. Cut the depth specimen carefully and remove it from the hole, trying not to disturb the base. When removing the specimen, place it top down on a plane surface. Lay a straightedge across the specimen and measure it to the supporting surface. A quick depth check of loose mix or finished pavement can be achieved by laying a straightedge on the surface, overhanging the lane edge, and measuring from the straightedge to the base or binder course.

4-112. After determining the average loose and compacted thicknesses, establish a ratio between them for correcting the loose depth. The following example shows how to compute for loose depth:

Example: The desired compacted depth is 1.5 inches, the average loose depth is 1.86 inches, and the average compacted depth is 1.44 inches. Determine the corrected loose depth.

Solution:

 

 

 

Adjusting the Screed

4-113. Reset the screeds when starting a paving job or resuming paving operations, such as following a change in course thickness. Depth tests are approximations and represent the pavement thickness only at the points tested. Therefore, do not adjust the screed every time a depth check does not yield the prescribed thickness. The finisher is designed to bridge irregularities in the base; thus, a thin spot in the course could be covering a high point in the base. Frequent screed adjustments to maintain a preset depth will accent irregularities in the base and yield a nonuniform surface. A more accurate check of average pavement thickness can be done by watching the yield. Where the yield is appreciably and consistently high or low and a discrepancy cannot be determined, adjust the screed setting accordingly.

Raking the Mix

4-114. Keep hand-raking to a minimum; use it only to correct nonuniformity of grade or surface texture. Remove coarse material that is segregated in areas, replace it with fresh hot mix from the hopper, and rake the fresh mix smooth. Return small amounts of coarse material to the hopper; if a large quantity goes through the paver, discard it. Do not let it cast over the fresh surface because coarse particles cool rapidly and the rollers merely embed them into the surface, leaving small surrounding pits. Press loose material that is along the edge of the lane firmly against the edge.

Adjusting the Mix

4-115. Change the mix against specifications and the mix-design criteria if placing, rolling, or surface texture are unsatisfactory. Even if the mix is uniform, it may require adjustment. Conditions and corrective measures are shown below:

  • Insufficient fines. Correct this problem by adjusting the job-mix formula to a finer gradation.
  • Mix pulling and checking. If a mix pulls under the finisher, correct it immediately if the small open checks are not definitely closed during rolling. This condition is especially critical in the surface course. It may be caused by unsatisfactory proportioning of the mix, a mix that is too cold, or improper finisher operation. When the total FA is composed of stone screening and sand, using an excessively high proportion of screenings may unduly toughen the mix. A tough mix is stabile, but can pull badly. A small change in the amount of FA (sand) may make an appreciable difference in placing and rolling characteristics. Also, increasing the amount of sand may not necessarily affect the stability. If the condition is limited to surface checking and if checking persists in spite of mix-design changes, correct it by scratching the surface lightly. One method is to fasten stiff-bristled pushbrooms on a light timber (such as a 2- by 4-inch board) the length of the lane width and drag the surface 2 or 3 feet behind the spreader. Incline the brooms slightly to prevent the bristles from gouging the pavement.

Rolling the Pavement

4-116. Ensure that the rollers are wet to prevent the mix from sticking to them. Use clean boards of sufficient length at the end of the lane to support the rollers and prevent contact with the base. The boards should be as thick as the loose depth of the pavement or should meet it. Ensure that the roller's wheels are completely clean before the roller moves from the boards to the mix. Roll freshly laid pavement when the desired density is obtained without raveling or blistering. Time periods required for rolling the pavement after it is laid may vary greatly depending on the thickness of the pavement and the weather conditions.

Techniques

4-117. Roll the surface before it has cooled (breakdown rolling) to avoid surface checking, which is a condition that is difficult to correct. A steel-wheel roller is normally used during the initial rolling. Roll with the power wheel forward or nearest the finisher. This causes the mat to be tucked under the roller wheels instead of being pushed as when the tiller wheel is in front. If the mat is too cold, proper density will not be obtained. The roller should follow the spreader closely.

4-118. If the mix is too hot, the surface may blister as a result of the water boiling off the wheels and the mix sticking to the dry wheels. If this occurs, use rakes to loosen the mix to a depth of about 1/2 inch over an area somewhat larger than the blister and rake the mix smooth. For best results, vary rolling techniques with mix characteristics and other conditions. Hot mix is sensitive material, and few mixes react to the rollers identically. The rear wheels of the steel-wheel roller supply most of the compaction; although when some mixes are very hot, they cannot withstand the rear wheels without undue displacement and checking. To prevent this, hold the roller farther back and allow the mix in the back of the spreader to cool. Siliceous aggregates that have a poor affinity for asphalt can also show this tendency.

4-119. If the temperature is suitable for rolling and the three-wheel roller unduly checks or shoves the pavement, it may be necessary to adjust the mix. Satisfactory mixes should not check or shove under proper rolling with a three-wheel roller. Ensure that the density in the field always meets the requirements. Variations in the surface may occur from the forward and backward movements of the roller.

NOTE: Overpriming will often cause rolling difficulties similar in appearance to those caused by a very hot mix.

Uneven Surfaces

4-120. Ensure that the completed pavement conforms to the requirements for surface smoothness. Test the surface with a straightedge after the first roller passes, while the mix is still hot and the surface can be loosened. If a deficiency exists, check the problem area immediately to determine if the problem is caused by a depression or a high spot. Correct a depression by using rakes to loosen the material to a depth of about 1/2 inch; distribute fresh, hot material by hand; and rake the surface smooth. Correct a high spot by using rakes to loosen the mix to a depth greater than the final desired grade, remove the excess material, and rake the surface smooth. Conduct straightedge tests after the final rolling to check conformity with the smoothness criteria.

Uniformity

4-121. Roll the surface as uniformly as possible to obtain a pavement that is true to grade and compacted to a uniform density. Heavier rolling at the edges of the lane than in the center will produce a crowned lane. Insufficient rolling is far more objectionable than excess rolling. When rolling an outside lane, uniform coverage cannot be obtained with the rear wheels because a roller cannot operate with one of the wheels off the pavement. Complete the rear-wheel coverage after placing the adjoining lane.

4-122. When rolling a fresh lane that joins a completed lane (hot or cold joint), ease the steel-wheel roller from the old lane onto the new one. Make the first pass with one rear wheel centered on the joint and the other rear wheel on the completed pavement. Continue rolling across the fresh lane, progressing by the width of the rear wheel, less a reasonable lap to avoid leaving a ridge. With this rolling system, the rear wheel passes over the half of the lane that is nearest the old pavement twice as many times as the other half. However, if this system is followed through successive lanes, uniform coverage is obtained. Stagger the area at least 5 feet where rolling directions are reversed, thereby eliminating ridges formed by starting and stopping the roller.

Displacement

4-123. Displacement occurs when the pavement moves on the base or binder course while it is being rolled. This movement may be accompanied by transverse or longitudinal cracks. Displacement also results from a very hot mix, an excessive tack or prime coat, or too much rounded FA. If displacement occurs, discontinue rolling immediately, determine the cause, and take corrective action.

Pneumatic-Tired Rollers

4-124. Pneumatic rolling of asphalt-concrete pavement provides a more closely knit surface by seating the larger aggregate particles better and by closing hairline cracks. Conduct pneumatic rolling while the pavement is still warm. Light rollers have little effect when the pavement temperature is lower than 130°F. To get the desired results, work the pneumatic-tired roller continuously during laydown. After rolling the surface with a steel-wheel and/or pneumatic-tired roller, iron out the surface with a two- and/or three-axle tandem roller. Do not leave the rollers on the fresh pavement because they will leave marks that are almost impossible to remove. Mat imperfections and their probable causes are shown in Table 4-6 .

Density Tests

4-125. Conduct a density test as often as the conditions require, but at least once for every 400 tons of mix placed. To get a satisfactory specimen, take a sample early in the morning when the pavement is cool. If extra rolling is required due to the test results, do it during the heat of the day. As a general check, conduct at least one density test daily on a chunk sample (see FM 5-428 ).

Selecting and Marking Samples

4-126. A sample can be taken from any portion of a lane as long as that area is typical of placing and rolling conditions. Cores that are 4 inches in diameter are good samples. These cores are relatively small and errors are multiplied accordingly; therefore, take care when weighing them. Taking samples from a surface course that is placed on a binder course is practically impossible. To make removal easier—

  • Select the spot before laying the surface course.
  • Place an 18-square-inch piece of wrapping paper on the selected spot. Ensure that the mix placed over the paper is the same as the rest of the pavement. To allow for the possibility of multiple density checks, place extra paper between the courses in other locations. Do not use frames or separators to mark these spots.
  • Drive two nails in the base or the pavement an equidistant from the center of the paper on the side of the lane that is subjected to the least amount of construction traffic.

Removing Samples

4-127. Use a coring machine or a concrete saw to cut out samples. Avoid chopping them out or removing them with a jackhammer because it can cause cracks or other problems to occur that lead to erroneous results. Cutting completely through the pavement's thickness, carefully remove 1-foot square samples from the binder or surface course. In hot weather, the area can be chilled with ice for 15 to 30 minutes before cutting out the sample. Ensure that the sides of the hole are vertical or dressed to vertical after the specimen is removed. Paint the edges and the bottom of the hole according to the instructions for a trimmed joint. Fill the hole with fresh hot mix, roll it, and check it for straightedge requirements.

Identifying Samples

4-128. Forward the specimen to the plant laboratory with proper identification. Use a simple numbering system and annotate the—

  • Course (binder, surface).
  • Date the course was placed.
  • Date the sample was taken.
  • Type of structure (runway, taxiway).
  • Lane number (with lane centerline offset).
  • Station number.

Correcting Low Density

4-129. If the density test shows that the minimum specified density has not been obtained, correct the problem by additional rolling or by removing the pavement and replacing it to meet the minimum specified density. If the density problem persists, make a thorough check of the job-mix formula. A slight change in the gradation or the bitumen content may improve the density. Test the pavement's density as often as necessary to ensure that it conforms to specifications.

4-130. If the density is within 0.5 percent of the minimum specified, reroll the pavement thoroughly with a steel-wheel roller, ensuring complete rear-wheel coverage, and follow the steel-wheel roller with a tandem roller. Because rolling is more effective on warm pavement, reroll it during the heat of the day when possible. After rerolling, recheck the density. If it is still below the minimum specifications, reroll the pavement thoroughly and check the density one more time. If the density is more than 1 percent below the minimum specifications, reroll the pavement during the heat of the day with a heavy pneumatic roller whose tires are inflated to 90 psi and check the density. Make turns on a radius that is large enough to prevent scuffing.

4-131. Binder and surface densities can be readily achieved without a great deal of difficulty if the aggregate gradation is within the typical specification band, the mix conforms to the mix-design criteria, and the weather is warm or hot. If the weather is cold, achieving density may be difficult. Check the job mix thoroughly if the density consistently falls below the specifications, and reroll the pavement if necessary.

Longitudinal Joints

4-132. When laying binder and surface courses, offset the joints by at least 1 foot. To avoid unsatisfactory spacing of joints, follow a general paving lane plan. Asphalt finishers usually operate with greater efficiency when laying standard 10-foot lanes; however, adjust them for wider or narrower lanes. Because the surface course is usually more critical than the binder course, lay it in standard 10-foot lanes. (A variation in the width of a lane is usually limited to the binder course.) The following examples show how to place lanes on binder and surface courses, ensuring that the joint is offset by at least 2 feet and the center-crowned runway is 150 feet wide:

  • Binder course. Place the first lane of the binder course to a 10-foot width, with one edge along the centerline of the runway. Place the adjoining four lanes to 10-foot widths, leaving 25 feet between the outer edge of the fifth lane and the runway edge. Place the sixth lane to a 9-foot width, and the seventh and eighth lanes to 8-foot widths. Repeat this procedure on the other half of the runway.
  • Surface course. Center the first lane of the surface course on the centerline of the runway, and adjust the screeds to provide the desired crown and grade. Continue with the adjoining lanes in even, 10-foot widths to the runway's edges.

Construction

4-133. Joints can be laid and rolled while they are hot or cold. When placing an adjacent lane and the mix in the preceding lane is hot, the joint is hot. When the mix in the preceding lane has cooled before the adjacent lane is placed, the joint is cold. Generally, hot joints require less work and are better. However, the only way to place hot joints is to make very short runs that result in a large number of transverse joints. Because smooth transverse joints are difficult to make, laying operations consist of both hot and cold joints.

4-134. Ensure that the vertical face is free of excess loose, coarse material. This eliminates voids and depressions in the joint when it is rolled. Use a combination of hot and cold joints only when the plant breaks down or when delivery of the mix is interrupted. Pave a length, such as 1,000 to 1,500 feet, and then pave the adjoining lane in the opposite direction.

Hot Joints

4-135. To lay hot joints, roll a short paving run on one lane, return to the starting point using a finisher, and pave the adjacent lane in the same direction. Roll the first lane to within 6 inches of the edge, and place the fresh mix against the hot, unrolled material. The length of the run varies with the rolling characteristics of the mix, the temperature of the mix, and the weather conditions. Make the length of the initial lane short enough so that the joint will be hot, but not so short that the operation is merely a series of stops and starts.

4-136. In general, do not let the length of the run exceed the distance required to maintain the mix in the unrolled edge of the initial lane at a minimum of 175°F. Ensure that the finisher is butted tightly to the old lane. Use rakes to smooth off the joint, including any ridge of loose mix in the first lane pushed up by the finisher (Figure 4-8 ), and dress the surface to a uniform texture. The density and uniformity of a hot joint are superior to those of a cold joint. A hot joint is tighter and more homogeneous if the run is short enough to maintain the temperature requirements in the unrolled edge.

    Figure 4-8. Raking a Joint

 

Cold Joints

4-137. To form cold joints, roll the existing lane across its entire width during the original construction. Roll the edge with the roller overhanging the edge about 1 inch. Before placing the new lane, trim the vertical edge of the existing lane to a vertical face so that clean aggregate faces are exposed. Tack or paint the trimmed joints very lightly with RC-70 cutback or an emulsion. If RC-70 cutback is unavailable, use AC cutback mixed in equal parts with white gasoline.

4-138. Remove loose material from the cold edge before laying fresh mix against it. When placing a new lane, ensure that the finisher is as close as possible to the cold edge, with the finisher shoe riding the old lane. The advantage of laying cold joints is that long, continuous runs can be made while keeping one lane open to limited traffic. Disadvantages are that the cold joint must be trimmed and painted, and the loose thickness must be compacted to the same thickness as the previously placed lane.

Alignment

4-139. Place a string line to guide the finisher, place the first lane along the edge of the line, and place succeeding lanes against the edge of the previously laid lane. In airfield construction, place a string line along the outside edge to verify that the pavement conformed after placing the third or fourth lane. Sharp, short, horizontal waves frequently exist in the outside edges of the third or fourth lane. When a finisher tries to follow these waves, they are accentuated in the next lane. Besides being unsightly, the wavy condition is conducive to ragged, nonuniform joints of variable density.

4-140. Before the condition becomes exaggerated, lay a new string line along the outside edge, parallel to the first string line, as a guide for the next lane. Lay the string line by cutting through the sharp protruding areas, which should be cut back to the line; and fill in the minor recesses when the next lane is paved. At or near the ends of the protruding areas, leave only minor portions to be cut back to the line and larger recesses to be filled.

4-141. Fill gaps and recesses that remain between the previous lane and the next lane; rake the surface smooth. Because hand-placed material has a lower density than the material placed by a finisher, dress these areas slightly higher. Test the areas closely with a straightedge after the first roller pass.

Transverse Joints

4-142. A transverse joint is formed in the pavement at the end of a lane. Try to carry a continuous transverse joint throughout as many lanes as possible; however, joints may be staggered.

Terminating a Lane

4-143. When stopping mix placement in a lane, roll the transverse joint to a feathered edge. Cut back the feathered section, which may extend several feet, to the full depth of the pavement. If the portion being removed is directly on the primed base, tear out some of the base material when the edge is removed. Patch the pockets in the base to grade or take other corrective measures to avoid having extra pavement depth along the edge of the joint.

4-144. One preventive measure is to place a strip of heavy wrapping paper, 30 to 36 inches wide, so that one edge is along the joint line and the other edge extends beyond the joint line. Run the finisher to the edge of the paper. When the edge of the joint is rolled, cut back the pavement to the joint and remove it without disturbing the base. This procedure can be modified, as necessary, due to insufficient hot mix from the plant or break downs.

4-145. When the mix runs out of the finisher, take depth measurements and conduct straightedge tests to determine the dividing line between the satisfactory mix, across the entire width of the lane, and the unsatisfactory mix. Mark the dividing line with a string line that is perpendicular to the centerline. Shovel out the unsatisfactory mix down to the base, leaving a vertical edge across the line. Place heavy wrapping paper (30 by 36 inches wide) over the base, flush with the trimmed edge. Ensure that the paper is longer than the width of the lane. Replace the mix on top of the paper, rake it smooth, and roll it out. When operations restart, strip the mix and the paper from the base.

4-146. Regardless of the method used to terminate a lane, neatly cut back the pavement to a vertical edge, at the full depth, and paint the joint. Move the finisher back until the leading edge of the screed is 12 to 18 inches behind the joint, and place a small amount of the mix on the old pavement. Push the mix up to the joint with the back of a rake, and rake the fresh section so that the mix is at the proper depth and uniform in texture.

Rolling a Transverse Joint

4-147. On a single lane, roll the joint once with the tandem roller and check it with a straightedge. If the new section is flush or nearly flush with the old pavement, the new section will be too low after rolling. To adjust the depth, loosen the material with rakes to a depth of about 1/2 inch. Add the desired amount of material, rake it smooth, and roll it with a steel-wheel roller. If the new lane is too high after tandem rolling, loosen the material with rakes, remove the desired amount of material, and roll the pavement.

4-148. On lanes other than single lanes, make the first pass parallel to the centerline from the new surface to the existing surface with a tandem roller. Test the surface with a straightedge, and correct low and high conditions. Roll the pavement three times, parallel to the joint, with the tandem roller centered on the joint. Immediately after tandem rolling, roll the pavement parallel to the joint with the steel-wheel roller, having one rear wheel centered on the joint and the other rear wheel on the old pavement. Do not extend transverse rolling beyond 6 or 8 inches from the outside edge that will be rolled longitudinally.

Constructing Pavement Ends

4-149. Flatten pavement ends by using trucks and other equipment. Add 6 to 8 inches of length to the pavement. When the job is finished, cut back the pavement to a neat line that conforms to the specifications and backfill it.

 

Sand Asphalt and Tar

4-150. In regions such as coastal areas where acceptable-quality sand is the only available aggregate, use sand mixes for economical surface courses that meet minimum requirements. They can be used for—

  • Roads and streets where light, fast-moving traffic is anticipated.
  • Surface and binder courses in pavements designed for low-pressure tires.
  • Nontraffic areas.

4-151. Sand mixes should be well-graded to meet the specified requirements for the type of course being laid and be free from excessive amounts of foreign material. They are not suitable for airfield surface or binder courses designed for high-pressure tires or for industrial-type pavements designed for solid-rubber tires or steel wheels.

4-152. When high stability is required, improve gradation by selecting and blending locally available sand. Add mineral filler to increase the density and stability of the mix. Use AC, asphalt cutback, tar, or asphalt emulsion as a bituminous material. Hot mixes and cold-laid plant mixes are produced in a central plant; road mixes are produced with a travel plant or mixed-in-place equipment.

Sheet Asphalt

4-153. Sheet asphalt is a refined type of sand asphalt where the grading and quality of sand and mineral filler are carefully controlled. The percentage of asphalt required is normally higher than the amount required for sand asphalt. Sheet asphalt provides a smooth, impermeable, homogeneous surface course that can be used in thin layers on an adequate base course. It is usually constructed over a binder course as a surface course in layers that are 1 1/2 to 2 inches thick. The surface may be finished to a fine, gritty, skid-resistant texture with smooth-riding, low-abrasive qualities and high uniformity. Sheet asphalt is satisfactory when subjected to the action of traffic over the entire surface. Like sand asphalt and tar, sheet asphalt is suitable for roads and streets, but it is not suitable for airfields or industrial-type pavements designed for solid-rubber tires or steel wheels.

Stone-Filled Sheet Asphalt

4-154. Stone-filled sheet asphalt consists of CA, well-graded sand, mineral filler, and AC. It is prepared in the same manner as sheet asphalt. The CA passes a 5/8-inch sieve but is retained on a number 8 sieve in varying quantities that do not exceed 40 percent. The percentage of CA varies proportionately when the specific gravities of FA and CA are not uniform. Stone-filled sheet asphalt is normally used as a surface course, and it is constructed in layers that are 1 1/2 to 2 inches thick. It is widely used for surfacing roads and streets; but like sheet asphalt, it is unsuitable for use in industrial areas and airfields.

Rock Asphalt

4-155. Rock asphalt is composed of crushed, natural, and/or asphalt-impregnated stone (limestone, sandstone) that is used alone or mixed with extra asphalt. Depending on the type, rock asphalt can be laid hot or cold. Rock asphalt is used for surface courses only. Natural rock-asphalt deposits are located in Kentucky, Alabama, Texas, New Mexico, Oklahoma, and Utah. The nature of the aggregate and the asphalt varies in different pits. In addition, materials in the same pit can vary and they are blended to produce a uniform mix. Materials in some pits are low in asphalt content and must be enriched with extra asphalt.

4-156. The two general types of rock asphalt are blended and fluxed:

  • Blended rock asphalt. It is prepared by blending crushed, impregnated limestone and/or sandstone in the proper proportions to produce a mix that is properly graded and meets the specified asphalt content. Grading the mix is often subordinate to balancing the asphalt content.
  • Fluxed rock asphalt. It is prepared by blending crushed, impregnated limestone and/or sandstone with AC and/or cutter stock in a pug mill to produce a mix that is properly graded and meets the specified asphalt content. Enrichment is necessary if the rock asphalt contains insufficient asphalt to produce a satisfactory pavement.

4-157. Hot mixes are sometimes produced by heating crushed limestone that is impregnated with relatively hard asphalt (can add sand) and mixed with extra AC in the central plant. This type of mix is essentially a sand asphalt. Rock asphalt is a satisfactory surfacing material for roads and streets subjected to all-over traffic, but it has the same limitations as sand asphalt for use in airfields and industrial areas.



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