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OCR for page 65
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Frequency, Percent
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9 10 12 15 18 20 30
Loss, Percent
Magnesium Sodium
Figure 28. Frequency distribution of sulfate soundness specifications.
freeze-thaw testing. Equipment currently available to easily Of the responding agencies, 25% differentiate between
conduct freeze-thaw testing in the laboratory was not avail- coarse- and fine-graded Superpave mixes. Pavement perme-
able when the soundness test was developed (115). ability has been a concern with some coarse-graded Superpave
The distribution of soundness specifications for coarse mixes. However, only two agencies specify different in-place
aggregate is shown in Figure 28. A maximum allowable loss pavement densities for coarse- and fine-graded Superpave
of less than 12% is specified by 53% of the agencies specify- mixes. In addition, Florida DOT includes permeability speci-
ing sodium sulfate soundness. There is little consensus on the fications for coarse- and fine-graded mixes. Two other states
criteria for magnesium sulfate soundness, with values rang- have permeability specifications for use during design, and
ing from 12% to 30% loss. four states are considering permeability specifications.
3.4.2 Aggregate Specific Gravity
3.4 MIX DESIGN PROPERTIES
The Superpave method specifies the use of the dry bulk
3.4.1 Gradation aggregate specific gravity for the calculation of VMA. Of the
responding agencies, 89% use dry bulk specific gravity to
Superpave gradation control consists of control points calculate VMA. Four agencies (9%) use the aggregate effec-
on four sieve sizes: the maximum aggregate size, NMAS, tive specific gravity to calculate VMA. The effective gravity
2.36-mm (No. 8) sieve, and 0.075-mm (No. 200) sieve. Of the is determined using the HMA maximum specific gravity or
responding agencies, 33% have altered the Superpave grada- rice value, asphalt content, and binder specific gravity. The
tion bands. In some cases, these changes are as simple as effective specific gravity is always larger than the bulk spe-
adding additional control points between the sieves specified cific gravity and, therefore, results in a larger calculated VMA.
by the Superpave method or altering the range for the percent The use of the effective aggregate specific gravity to calcu-
passing the 0.075-mm (No. 200) sieve. In other cases, agen- late VMA includes the volume of absorbed asphalt as part of
cies have tightened the Superpave gradation bands to pro- the void volume between particles. One agency uses the appar-
duce mixes that more closely resemble dense-graded mixes ent specific gravity to calculate VMA. This would result in a
used prior to Superpave. One agency has modified the Super- larger calculated VMA then would be determined using either
pave gradation bands to include the 16.0-mm sieve, used the bulk or effective aggregate gravities. One state does not
prior to the introduction of the Superpave method. calculate VMA.
