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OCR for page 189
Design of Gap-Graded HMA Mixtures 189 Table 10-12. GGHMA mixture specification for SGC compacted designs. Property Requirement Asphalt Binder, % Table 10-12 Air Void Content, % 4.0 0.5 VMA, % 17 min. VCAMIX, % Less than VCADRC Tensile Strength Ratio 0.80 min. Draindown at Production Temperature, % 0.30 max where Ps = percent of aggregate in the mixture Pca = percent of coarse aggregate in the mixture Gsb = combined bulk specific gravity of the total aggregate Gca = bulk specific gravity of the coarse aggregate (coarser than break point sieve) Once the VTM, VMA, and VCAMIX are determined, each trial blend mixture is compared to the GGHMA mixture requirements. Table 10-12 presents the requirements for GGHMA designs. The trial blend mix that meets or exceeds the minimum VMA requirement, has an air void content between 3.5 and 4.5%, and has a VCAMIX less than VCADRC is selected as the design gradation. If none meet these requirements, additional aggregate blends should be evaluated. If one of the trial blends is very close to meeting these requirements, with the air void content and VMA just outside their acceptable ranges, an adjustment in the binder content might provide an acceptable mix design, as discussed below. Step 4--Refine Design Asphalt Binder Content Once the design gradation of the mixture is chosen, it may be necessary to raise or lower the asphalt binder content to obtain the proper amount of air voids in the mixture. In this case, additional samples are prepared using the selected gradation and varying the asphalt binder content. The optimum asphalt binder content is chosen to produce 4.0% air voids in the mixture; because of typical error in volumetric analysis, air void contents within 0.5% of this target are acceptable. The optimum asphalt binder content should meet the minimum asphalt content requirements in Table 10-11. The number of samples needed for this portion of the procedure is again twelve, with three compacted and one uncompacted sample at each of three asphalt binder contents. The mixture properties are again determined, and the optimum asphalt binder content is selected. The designed GGHMA mixture at optimum asphalt content selected should have properties meeting the criteria shown in Table 10-12. If these criteria are not met, the mixture must be modified so that all criteria are met. Step 5--Conduct Performance Testing Performance testing of GGHMA mixtures consists of three tests: (1) evaluation of moisture susceptibility; (2) evaluation of draindown sensitivity; and (3) evaluation of rut resistance. Evaluation of Moisture Susceptibility Moisture susceptibility of the selected mixture is determined using AASHTO T 283. One minor change to AASHTO T 283 is that GGHMA samples should be compacted to 61% air voids instead

OCR for page 189
190 A Manual for Design of Hot Mix Asphalt with Commentary of 71%. This air void content approximates the recommended higher level of compaction in the field of 94 to 95% of Gmm. The mixture should have a minimum tensile strength ratio of 80%. Evaluation of Draindown Sensitivity Draindown sensitivity of the selected mixture is determined in accordance with AASHTO T 305. Draindown sensitivity is determined at the anticipated plant production temperature and should not exceed 0.3%. Evaluation of Rut Resistance The final step in the design of a GGHMA mixture is the evaluation of rut resistance, also called performance testing. Chapter 6 presents a general discussion of performance evaluation of HMA mixtures and discusses specific tests. Chapter 8 provides a more practical discussion of how, during the HMA mix design process, rut resistance is evaluated using one from among the following tests: (1) the flow number test; (2) the flow time test; (3) the asphalt pavement analyzer (APA) test; (4) the Hamburg wheel-track test; (5) the repeated shear at constant height (RSCH) test as performed using the Superpave shear tester (SST); or the high-temperature indirect tension (IDT) strength test. Detailed discussions of these tests can be found in Chapters 6 and 8. A summary of performance testing requirements and their application to GGHMA mixtures is given below. The design procedures set forth in this manual--including that for GGHMA--are structured to provide HMA mix designs that exhibit a high level of rut resistance. The level of reliability against excessive rutting--even without performance testing--ranges from 90 to over 99%, with a typical level of about 95% reliability for design traffic levels of 3 million ESALs and higher. The purpose of rut resistance testing is to increase this level of reliability. For three of the rut resistance tests recom- mended in this manual--the flow number from the asphalt mixture performance test (AMPT), the repeated shear at constant height (RSCH) test, and the high-temperature indirect tension (HT-IDT) strength test--the suggested minimum or maximum test values were determined specifically to increase the level of reliability against excessive rutting from about 95 to 98% and higher. It must be emphasized that the reliability achieved through the recommended performance tests is a result of applying both the suggested mix design procedure and the selected performance test together. If the given guidelines for performance test results are applied to mixtures designed following some other procedure, the resulting level of reliability will not necessarily be the same. It might be similar, or it might be lower or higher. It should also be noted that the specified test values have, in most cases, been selected so that if the procedures given in this manual are followed, most of the result- ing HMA mixtures will pass the selected performance test. It is estimated that only about 10 to 20% of properly designed mixtures will fail. Thus, the suggested rutting performance tests not only increase reliability against excessive rutting to a very high level, they do so in a relatively efficient way. The suggested maximum rut depths for the asphalt pavement analyzer (APA) and the Hamburg Table 10-13. wheel-track (HWT) tests were taken from specifications already in place in a number of states Recommended using these tests. In this case, implementation of these performance tests will certainly increase minimum flow number requirements. the reliability against excessive rutting, but the specific amount of improvement is unknown as is the percentage of mixes likely to fail the tests. However, because these tests with the stated maximum Traffic rut depths have been implemented in several states, it is likely that the increase in reliability and Level Million Minimum the rejection rate will both be reasonable. ESAL s Flow Number Guidelines for interpreting the various rut resistance tests are given in Tables 8-21 through 8-25 <3 --- 3 to < 10 200 in Chapter 8. As an example, Table 10-13 gives recommended minimum values for flow number 10 to < 30 320 as determined using the AMPT. This test was initially called the simple performance test or SPT. 30 580 Details of the latest equipment specification and test procedure are given in AASHTO TP 79-09.