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OCR for page 203
Design of Open-Graded Mixtures 203 Sample Compaction Specimens should be compacted at the established compaction temperature after laboratory short-term aging. Laboratory samples of PFC are compacted using 50 SGC gyrations. More than 50 gyrations should not be used; PFC is relatively easy to compact in the laboratory and exceeding this compactive effort can cause excessive aggregate breakdown. Step 3--Selection of Optimum Gradation After the samples have been compacted, extruded, and allowed to cool, they are tested to determine their bulk specific gravity, Gmb, using dimensional analysis. Dimensional analysis entails calculating the volume of the sample by obtaining four height measurements with a calibrated caliper, with each measurement being 90 degrees apart. The area of the specimen is then multiplied by the average height to obtain the sample volume: 2 D V = h (11-2) 2 where V = specimen volume, in3 h = specimen height, in D = specimen diameter, in Then Gmb is determined by dividing the dry mass of the sample by the sample volume. Uncom- pacted samples are used to determine the theoretical maximum density, Gmm (AASHTO T 209). Using Gmb, Gmm, and Gca, percent air voids, or voids in the total mixture (VTM) and VCAMIX are calculated. The VTM and VCAMIX are calculated by equations 3 and 4 below. 1 - Gmb VTM = 100 (11-3) Gmm G P VCAMIX = 100 - mb ca (11-4) Gca where Pca = percent of coarse aggregate in the mixture Gmb = combined bulk specific gravity of the total aggregate Gca = bulk specific gravity of the coarse aggregate Once VTM and VCAMIX are determined, each trial blend mixture is compared to the PFC mixture requirements, which are presented in Table 11-7. The trial blend with the highest air void content that meets the 18% minimum and exhibits stone-on-stone contact is considered the design gradation. The Cantabro Abrasion test or draindown test may be required in order to select the design gradation. Step 4--Selection of Optimum Asphalt Binder Content Once the design gradation has been selected, it is necessary to evaluate various asphalt binder contents in order to select the optimum binder content. Additional samples are prepared using the design gradation and at least three asphalt binder contents. Eighteen samples are needed for this procedure. This provides for three compacted (for Gmb and Cantabro Abrasion Loss) and

OCR for page 203
204 A Manual for Design of Hot Mix Asphalt with Commentary Table 11-7. PFC mixture specification for SGC compacted designs. Property Requirement Asphalt Binder, % Table 10-6 Air Void Content, % 1 18 to 22 Cantabro Loss % 15 max. VCAMIX% Less than VCADRC Tensile Strength Ratio 0.70 min. Draindown at Production Temperature, % 0.30 max 1 Air void requirements are provided for PFC mixes but not ACFC mixes. three uncompacted samples (one for determination of theoretical maximum density and two for draindown testing) at each of the three asphalt binder contents. Optimum asphalt binder content is selected as the binder content that meets all of the requirements of Table 11-7. Cantabro Abrasion Loss Test The Cantabro Abrasion test is used as a durability indicator during the design of PFC mix- tures. In this test, three PFC specimens compacted with 50 SGC gyrations are used to evaluate the durability of a PFC mixture at a given asphalt binder content. To begin the test, the mass of each specimen is weighed to the nearest 0.1 gram. A single test specimen is then placed in the Los Angeles Abrasion drum without the charge of steel spheres. The Los Angeles Abrasion machine is operated for 300 gyrations at a speed of 30 to 33 rpm and a test temperature of 255C. After 300 gyrations, the test specimen is removed from the drum and its mass determined to the near- est 0.1 gram. The percentage of abrasion loss is calculated as follows: ( P1 - P2 ) PL = 100 (11-5) P2 where PL = percent loss P1 = mass of specimen prior to test, gram P2 = mass of specimen after 300 gyrations, gram The test is repeated for the remaining two specimens. The average results from the three specimens are reported as the Cantabro Abrasion Loss. Resistance to abrasion generally improves with an increase in asphalt binder content or the use of a stiffer asphalt binder. Figure 11-5 illus- trates a sample after the Cantabro Abrasion Loss test. Draindown Sensitivity The draindown sensitivity of the selected mixture is determined in accordance with AASHTO T 305 except that a 2.36-mm wire mesh basket should be used. Draindown testing is conducted at a temperature of 15C higher than the anticipated production temperature. Permeability Testing A laboratory permeability test is conducted on the selected PFC mixture. Laboratory permeabil- ity values greater than 100 m/day are recommended. Permeability of asphalt concrete mixtures can be measured using the provisional standard ASTM PS 129, Measurement of Permeability of Bituminous Paving Mixtures Using a Flexible Wall Permeameter.