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OCR for page 205
Design of Open-Graded Mixtures 205 Figure 11-5. Illustration of sample after Cantabro Abrasion test. Step 5--Moisture Susceptibility Moisture susceptibility of the selected mixture is determined using the modified Lottman method in accordance with AASHTO T 283 with one freeze-thaw cycle. The AASHTO T 283 method should be modified as follows: (1) PFC specimens should be compacted with SGC 50 gyrations; (2) no specific air void content level is required; (3) a vacuum of 26 inches Hg is applied for 10 minutes to saturate the compacted specimens, with no specific saturation level required; and (4) the specimens are kept submerged in water during the freeze-thaw cycle. Trouble Shooting PFC Mix Designs If the designer cannot produce a mixture that meets all requirements, remedial action will be necessary. Some suggestions to improve mixture properties are provided below. Air Voids The amount of air voids in the mixture can be controlled by the asphalt binder content. How- ever, lowering the asphalt binder content below the minimum to achieve a proper amount of air voids violates the required minimum asphalt binder content (Table 11-6). Instead, the aggregate gradation must be modified to increase the space for additional asphalt binder without decreas- ing the voids below an acceptable level. Decreasing the percent passing the breakpoint sieve will generally increase the air void content at a given asphalt binder content. Voids in the Coarse Aggregate If the VCAmix is higher than that in the dry-rodded condition (VCADRC), then the mixture gra- dation must be modified. This is typically done by decreasing the % passing the breakpoint sieve. Cantabro Abrasion Loss If the Cantabro Abrasion loss is greater than 15%, then either more asphalt binder or a binder with a greater high-temperature stiffness is needed.
OCR for page 205
206 A Manual for Design of Hot Mix Asphalt with Commentary Moisture Susceptibility If the mixture fails to meet the moisture susceptibility requirements, lime or liquid anti-strip additives can be used. If these measures prove ineffective, the aggregate source or asphalt binder source can be changed to obtain better aggregate/asphalt binder compatibility. Draindown Sensitivity Problems with draindown sensitivity can be remedied by increasing the amount of stabilizing additive or by selecting a different stabilizing additive. Fibers have been shown to be very effective in reducing draindown. Bibliography AASHTO Standards AASHTO M 320, Performance-Graded Asphalt Binder AASHTO R 30, Mixture Conditioning of Hot-Mix Asphalt AASHTO T 19, Bulk Density ("Unit Weight") and Voids in Aggregate AASHTO T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine AASHTO T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate AASHTO T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalency Test AASHTO T 209, Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures AASHTO T 245, Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus AASHTO T 283, Resistance of Compacted Asphalt Mixtures to Moisture-Induced Damage AASHTO T 304, Uncompacted Void Content of Fine Aggregate AASHTO T 305, Determination of Draindown Characteristics in Uncompacted Asphalt Mixtures AASHTO T 326, Uncompacted Void Content of Coarse Aggregate (As Influenced by Particle Shape, Surface Texture and Grading) Other Standards ASTM C 612, Mineral Fiber Block and Board Insulation ASTM D 4791, Flat Particles, Elongated Particles or Flat and Elongated Particles in Coarse Aggregate ASTM PS 129, Measurement of Permeability of Bituminous Paving Mixtures Using a Flexible Wall Permeameter Other Publications Cooley, L. A., et al. (2009) NCHRP Report 640: Construction and Maintenance Practices for Permeable Friction Courses, TRB, National Research Council, Washington, DC, 90 pp. Mallick, R. B., et al. (2001) Design Construction and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00-01. NCAT. Auburn University. Auburn, AL. NAPA (2002) Open-Graded Asphalt Friction Courses: Design, Construction and Maintenance (IS-115), Lanham, MD, 22 pp.