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JOB-MIX FORMULA (AC MIXES) When the mix has proven itself to be satisfactory, the percentages by weight of the aggregate and the averaged optimum bitumen content should be combined to establish the job-mix formula. Figure 17-6A lists the final percentages of the aggregate for a given job mix. By plotting the test results (figs. 17-9A and 17-9B) on DD Form 1219 (fig. 17-10) and applying the Marshall test criteria for determining optimum bitumen content, you make the determination that the mix requires 4.7 percent of asphalt cement. Accordingly, the aggregates must be 95.3 percent of the total mix. The selected blend contained 45-percent coarse aggregate (CA), 30-percent fine aggregate (FA), 20-percent fine river bar sand (FRBS), and a 5-percent limestone dust (LSD) mineral filler. The job-mix formula is computed as follows:
MODIFIED TEST FOR COLD-MIX PAVEMENTS This method is used as an aid in determining the asphalt content for cold-mix design of light-duty pavement. It can be used where asphalt cutbacks will be the binder. The procedures follow those used for hot-mix design (Marshall method), in general, with the following modifications: . Aggregates. Aggregates should be dried to a moisture content expected during construction (up to a maximum of 2 percent, by weight).. Asphalt. Mix selected bitumen with the aggregates, but at the temperature recommended for field application. The aggregates remain at room temperature.. Curing. Before compaction, cure the mixture for at least 12 hours in an oven set at 140F ( + 5).. Cooling. After molding, cool the specimens to room temperature in the molds. You must take care to remove the specimens, undisturbed and undamaged, from the molds.. Testing. Heat the specimens in an oven to 100 ( + 2) and test them in the Marshall machine. Heating will normally take about 2 hours.. Selection of the design amount of asphalt. The asphalt contents at maximum density and maximum stability, after averaging, are used as the design amount. SURFACE AREA METHOD OF MIX DESIGN When laboratory equipment, except for sieve analysis, is not available, the following formulas may be used in place of laboratory procedures to determine the necessary asphalt content: 1. For asphalt cement: P = 0.02a + 0.07b + 0.15c + 0.20d where: P= percent (expressed as a whole number) of by weight of dry aggregate a = percent (expressed as a whole number) of b= percent (expressed as a whole number) of c= percent (expressed as a whole number) of d = percent (expressed as a whole number) of mineral aggregate passing the No. 200 sieveAbsorptive aggregates, such as slag, limerock, vesicular lava, and coral, will require additional asphalt. 2. For asphalt emulsion: P = 0.05 A + 0.1 B + 0.5 C where: P= percent (expressed as a whole number) by weight of asphalt emulsion, based on weight of graded mineral aggregate A= percent (expressed as a whole number) of B= percent (expressed as a whole number) of C= percent (expressed as a whole number) of mineral aggregate passing the No. 200 sieve |
 
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