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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.
OCR for page 44
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
OCR for page 45
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.
