Asphalt Cement

2-2. AC is usually used in hot-mix pavements and is solid at 77°F. To make AC fluid enough for mixing with aggregate or for spraying, heat it to a temperature of 250° to 350°F. A disadvantage of AC is that adequate heating equipment may not always be available. The various penetration grades of AC are suitable for plant mixes, penetration macadams, and surface treatments.

Asphalt Cutback

2-3. Different types and grades of asphalt cutback are used during various climatic conditions. When the cutter stock evaporates, it leaves AC as an active bonding and waterproofing agent. Consider the prevailing atmospheric temperature during construction when selecting the grade of asphalt cutback. Light grades are usually used in cool weather. If the preferred grade and type of asphalt cutback is unavailable, use a comparable grade of another type. For example, using RC-70 instead of MC-70 or using RC-300 instead of MC-300 will not seriously affect the finished structure. In the field, light grades of asphalt cutback can be made by adding solvents to AC or to heavier grades of asphalt cutback. (See Chapter 5 for more information on the field manufacture of asphalt cutbacks.)

Asphalt Emulsion

2-4. The mixing grades of asphalt emulsion can be combined with damp aggregate with little or no heating. The recommended use depends on the setting rate and the mixing ability. Use emulsions for surface treatments, road and plant mixes, and crack and joint fillings.

Tar

2-5. Since tar does not dissolve in petroleum distillates, use it in areas where asphalt is unsuitable, such as refueling aprons at airfields where petroleum distillates are likely to be spilled. Because tar has greater penetrating qualities than asphalt, it is preferred when penetration is critical. Use hot tar for plant mixes, surface treatments, penetration macadams, and crack fillers. Use RTCB for patch mixes, surface treatments, and road mixes. Do not use open flames near storage tanks or drums of RTCB because it is highly flammable. Tar is more susceptible to temperature changes than asphalt, and tar becomes soft at high temperatures and brittle at low temperatures.

Mix Gradation

2-7. Aggregates and mineral fillers can be combined to produce a dense-graded or an open-graded mix:

• Dense-graded mix. A dense-graded mix contains a continuous gradation of particle sizes that are coarse to very fine. Fine particles fill the voids between large particles, thereby increasing the mix's weight. A dense-graded mix is more impervious to water and provides more protection to the base and the subgrade. It usually requires less maintenance, and the asphalt life is prolonged. However, a dense-graded mix requires precise determination and control of the bitumen-aggregate ratio.
• Open-graded mix. An open-graded mix also contains coarse and fine particles in a continuous gradation. However, it does not contain enough fine particles to fill the voids between large particles so it is less dense. Because of the increased voids in an open-graded mix, the bitumen-aggregate ratio is less critical.

2-8. The allowable variation in gradation of some low and intermediate mixes is great enough that natural or man-made aggregates can be used without any special preparation. High mixes and some intermediate mixes have such a small allowable variation in gradation that two or more aggregates must be blended to obtain the desired gradation.

2-9. The following example shows how to determine specific requirements for a surface course:

Example: You are using an asphalt finisher to place a 2-inch surface course on a main supply route (MSR). The maximum aggregate size available is 1 inch, and a limited supply of limestone dust is available to use as a mineral filler. The bitumen is AC, so an aggregate gradation must be selected.

Solution:

• Surface course thickness: 2 inches.
• Bitumen: AC.
• Tire pressure: Low (truck tires have <100 pounds per square inch [psi]).
• Maximum aggregate size available: 1 inch.
• Method of placement: Machine (2 inches x 0.5 = 1 inch).
• Gradation type: Open-graded (conserves limestone dust).

Aggregate Blending

2-10. Aggregate blending is proportionately mixing several aggregate gradations to obtain one desired aggregate gradation. The normal procedure is to mix three or four aggregates from various sources. Record the results of gradation tests for each source on Department of Defense (DD) Form 1207 ( Figure 2-1 ) or present them in tabular form.

2-11. Use DD Form 1217 ( Figure 2-2 ) as a work sheet to calculate the aggregate blend. There are several methods for determining the correct blend; however, use the test-strip method because other methods require complicated graphs and formulas.

 Figure 2-1. Sample DD Form 1207 (continued)

 Figure 2-2. Sample DD Form 1217

 Figure 2-2. Sample DD Form 1217 (continued)

Example of Aggregate Blending

2-12. This example shows how to determine aggregate blending. Use gradation 2A as the specification limit, and refer to Figure 2-1 for the gradations of stockpile samples. Complete a DD Form 1217 ( Figure 2-2 ) for a bituminous mix design.

Reporting

2-19. Allow an independent party to check the calculations completed for the aggregate blending and record the information on the following forms:

• DD Form 1217 ( Figure 2-2 ). This is a reporting and computational form. Record the gradation of material and trial computations. Enter the contents of the final trial in the block following the last numbered trial.
• DD Form 1207 ( Figure 2-1 ). This is a graphical record of the aggregate blend. Plot the upper and lower limits of the specified gradation, the mean of the specified gradation, and the calculated final blend on the same graph (the mean is sometimes omitted).

Reporting Test Results

2-21. Report the test results on one of the following forms:

• DD Form 1218 ( Figure 2-4 ). This form is the laboratory work sheet for the Marshall test. If the form is used in lieu of bituminous mix curves, plot the curves to evaluate the data.

 Figure 2-4. Sample DD Form 1218

 Figure 2-4. Sample DD Form 1218 (continued)

• DD Form 1219 ( Figure 2-3 ). This form presents the results of the Marshall test in graphic form. Ensure that the curves are smooth and without deviations, including extraneous values.

Evaluating Test Results

2-22. Use the information in Table 2-4 to evaluate the results of the Marshall test and to obtain an OBC.

2-23. Obtain the percent of bitumen for stability, the unit weight, the percent of voids in the total mix, and the percent of voids filled with bitumen from Table 2-3 and Figure 2-3 . Average the bitumen-content percentages and note the result as the trial OBC.

Example: The bituminous mix curves shown in Figure 2-3 are for an airfield AC surface course (high-pressure tires). Determine the trial OBC.

Solution:

• Unit weight: 4.5.
• Stability: 4.3.
• Percent of voids in the total mix: 4.9.
• Percent of voids filled with bitumen: 5.
• Total: 18.7.
• Average: 18.7 4 = 4.7 percent AC (trial OBC).

2-24. Evaluate the curves for stability, flow, percent of voids in the total mix, and percent of voids filled with bitumen at the trial OBC. Compare these values with the criteria in Table 2-36 . If all the values meet the criteria, the trial OBC becomes the final OBC. If one or more of the criteria fails to meet the specifications—

• Recheck computations.
• Determine if any of the curves yields a bitumen that is out of line with the others. If so, rerun the Marshall test.
• Recheck the aggregate for conformance to the specified blend.
• Rerun the Marshall test (check apparatus first).
• Change the aggregate blend.

2-25. The test properties of the sample mix ( Table 2-5 ) are within the specification limits, so designate the trial OBC as the final OBC.