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Aggregate Blending Limits Figure 17-7 is an aggregate grading chart (DD Form 1207), showing the specification limits for the mix and the gradation of the blend when mixed in the proportions shown in figure 17-6A, trial No. 1. Specify Gravity of Bituminous Mix Components A specific gravity data sheet is shown in figure 17-8 (DD Form 1216). This form is used for computing the specific gravity of all the bituminous mix components. If more aggregate fractions are used than are provided for on the form, additional forms may be used. Procedures for performing these tests are discussed in chapter 13 of this TRAMAN. Marshall Stability Computations DD Form 1218 (figs. 17-9A and 17-9B) is a data and computation sheet used in the Marshall stability test. The specific gravity values of the aggregates and the aggregate fraction percentages from the trial blending furnish the information to compute the fractional weights and to prepare the test specimens. Record the measurements made on the test specimens in the upper right-hand corner of the form. Determine, as described in chapter 13 of this TRAMAN, the stability, flow, unit weight of total mix, and percentage of voids filled with binder to complete the form. Marshall Method Computations Transfer the values from DD Form 1218 (figs. 17-9A and 17-9B) for each binder content computation to DD Form 1219 (fig. 17-10). Each graph on the form represents a different test property. Plot the values for each property on their respective graph using the binder contents as ordinates. Draw a smooth curve through the plotted points. Marshall Test Criteria Table 17-7 lists the criteria for determining optimum asphalt content (OAC). For each test property, you should consider the type of mix to be used and the expected load. The optimum bitumen content for each property is designated as a definite point on the curve for that property. The bitumen content percentages (one for each property) are averaged, and the average is used to read the corresponding value of each test property. The value, as determined, should be referred to the criteria portion of table 17-7 to see if it is within the permissible limits so that the mix will perform satisfactorily. Test Variation for Aggregates with 10 Percent or More Larger Than 1-Inch Maximum SizeThe procedure described in the Marshall method and the examples as given in the preceding paragraphs are applicable to hot-mix design where the amount of aggregate larger than the 1-inch sieve is less than 10 percent of the total. When the larger than (plus) 1-inch material exceeds 10 percent of the total, the following variations are made in the procedure: 1. Mix bitumen at the selected content with the entire aggregate, including the plus 1-inch portion.2. Pass the mixed hot batch through a 1-inch sieve. Discard the plus 1-inch portion. 3. Make compacted specimens from the portion that passes the 1-inch sieve and perform the Marshall test, except do not calculate the voids of the compacted specimens at this time. 4. Determine the bulk specific gravity of the plus 1-inch aggregate, and, with the specific gravity of the compacted specimens, compute the adjusted specific gravity (GA) as follows:
where: A = weight of dry, plus 1-inch material expressed as a percentage of the total batch weight (bitumen plus aggregate)B = portion of total batch remaining after the dry, plus 1-inch portion is removed (100%-A%)C= bulk specific gravity of plus 1-inch aggregateD= actual specific gravity of compacted specimenf = empirical factor = 0.995
Table 17-7.Marshall Test Specifications and Determination of Optimum Asphalt Content 5. Calculate the voids by using the adjusted specific gravity, and apply the design criteria for this value. 6. Use stability and flow values as measured on the compacted specimens. |
 
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