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OCR for page 43
Aggregates 43 graduated cylinder sand plus clay reading flocculating solution suspended clay sand reading settled aggregate Figure 4-10. Sand equivalent test. Source Aggregate Properties Some aggregate properties were identified by the expert group as important, but about which a consensus could not be reached on specification limits. These aggregate properties were called "Source Properties." Test methods were recommended; however, development of specification limits was left to local agencies that had experience with area materials. These properties are generally used during source approval and, therefore, requirements are not applied to the aggregate blend as with the then consensus properties. Source properties deemed important include Toughness, Soundness, and Deleterious materials. Toughness The term toughness is used to describe the ability of an aggregate to withstand the abrasion and degradation that occurs during handling, production, construction, and in-service use. Toughness is measured using the Los Angeles Abrasion Test, described in AASHTO T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine. In performing the Los Angeles Abrasion test, a graded sample of aggregate is placed in a large steel drum (Figure 4-11). Six to twelve steel charges (depending on gradation of the aggregate stockpile) are placed within the drum in addition to the aggregate sample. The drum is then rotated which subjects the aggregates to impact and abrasion by the steel balls. Results from the test are reported as a percent loss, which is the mass percentage of aggregate lost during the test Table 4-9. Clay content requirements. Design ESALs (million) Minimum Sand Equivalency Value < 0.30 40 0.3 to < 3 40 3 to < 10 45 10 to < 30 45 30 or more 50 Criteria are presented as Sand Equivalent Value.

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44 A Manual for Design of Hot Mix Asphalt with Commentary Figure 4-11. Los Angeles abrasion drum. due to degradation and abrasion. Low Los Angeles Abrasion loss values are desirable, since this indicates that an aggregate is tough and resistant to abrasion. Typical values for Los Angeles Abrasion loss are listed in Table 4-10. Soundness Soundness is used to describe the ability of an aggregate to withstand the effects of weathering. To evaluate the soundness of aggregates, AASHTO T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate, is used. As stated in the title of the test method, either sodium sulfate or magnesium sulfate is used to subject an aggregate sample to the effects of freezing and thawing. This test method can be used to evaluate the soundness of both coarse and fine aggregates. To perform the test, an aggregate sample is washed and dried to a constant mass and then separated into specified size fractions. The test is performed by alternately exposing an aggregate sample to repeated immersions in the prescribed sulfate solution followed by oven drying. During the period of immersion, the sulfate solution is absorbed into the permeable voids of the aggregates and rehydrates creating forces that simulate the expansive forces of water freezing. During the drying phase, the sulfate solution precipitates similar to the action of thawing. One immersion and drying is considered a soundness cycle. Typically, five soundness cycles are specified by agencies. Results from the soundness testing are the percent loss of material after the five cycles. Low values of soundness loss are desirable since this suggests that an aggregate is not susceptible to weathering. Soundness test results obtained using sodium sulfate and magnesium sulfate solutions are not interchangeable, since the expansive forces generated by these salt solutions are Table 4-10. Typical values for Los Angeles abrasion test. Typical Los Angeles Aggregate Mineralogy Abrasion Loss Values, % Basalt 10 to 20 Dolomite 15 to 30 Gneiss 30 to 60 Granite 25 to 50 Limestone 20 to 30 Quartzite 20 to 35

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Aggregates 45 different. Generally, use of magnesium sulfate solution will result in slightly higher loss values than use of sodium sulfate solution. As such, typical specification limits are a maximum of 10% loss when sodium sulfate is used and a maximum of 15% when magnesium sulfate is used, though specification limits can vary by agency. Deleterious Materials Deleterious materials are those materials within an aggregate stockpile that are weak, reactive, or unsound. Examples of materials that can be considered deleterious include clay lumps, friable particles, shale, coal, free mica, and vegetation. The test method for evaluating deleterious materials is AASHTO T 112, Clay Lumps and Friable Particles in Aggregate. In this test, fractions of aggregates are wet sieved over prescribed sieves. The mass percentage of material lost as a result of the wet sieving is reported as the percent of clay lumps and friable particles. High mass percentages of clay lumps and friable particles are detrimental to an HMA mixture; therefore, maximum values are generally specified. A wide range of permissible percentages of clay lumps and friable particles are specified by different agencies. Bibliography AASHTO Standards M 29, Fine Aggregate for Bituminous Paving Mixtures M 43, Standard Specification for Sizes of Aggregate for Road and Bridge Construction M 323, Superpave Volumetric Mix Design R 35, Superpave Volumetric Design for Hot-Mix Asphalt T 2, Sampling of Aggregates T 11, Materials Finer than 75-m (No. 200) Sieve in Mineral Aggregates by Washing T 19M/T 19, Bulk Density ("Unit Weight") and Voids in Aggregate T 27, Sieve Analysis of Fine and Coarse Aggregate T 30, Mechanical Analysis of Extracted Aggregates T 84, Specific Gravity and Absorption of Fine Aggregate T 85, Specific Gravity and Absorption of Coarse Aggregate T 96, Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine T 104, Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate T 112, Clay Lumps and Friable Particles in Aggregate T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test T 248, Reducing Samples of Aggregate to Testing Size T 304, Uncompacted Void Content of Fine Aggregate Other Standards ASTM D 4791, Flat Particles, Elongated Particles, or Flat and Elongated Particles ASTM D 5821, Standard Test Method for Determining the Percentage of Fractured Particles in Coarse Aggregate Other Publications Cominsky, R. J., R. B. Leahy, and E. T. Harrigan (1994) Level One Mix Design: Materials Selection, Compaction and Conditions. Report SHRP-A-408, TRB, National Research Council, Washington, DC. Kandhal, P. S., and F. Parker, Jr. (1998) NCHRP Report 405: Aggregate Tests Related to Asphalt Concrete Performance in Pavements, TRB, National Research Council, Washington, DC. McGennis, R. B., et al. (1994) Background of SUPERPAVE Asphalt Mixture Design & Analysis. National Asphalt Training Center Demonstration Project 101, FHWA-SA-95-003, Washington, DC, FHWA, November. Prowell, B. D., J. Zhang, and E. R. Brown (2005) NCHRP Report 539: Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt, TRB, National Research Council, Washington, DC, 101 pp. White, T. D., J. E. Haddock, and E. Rismantojo (2006) NCHRP Report 557: Aggregate Tests for Hot-Mix Asphalt Mixtures Used in Pavements, TRB, National Research Council, Washington, DC, 48 pp.