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Example 18.5 Determining a Suitable Binder Grade
- Correct for slow moving traffic: Move binder one grade higher. For
minimum 50 percent reliability, binder if PG 64-16. For minimum of 98
percent reliability, binder is PG 70-16.
- Correct for ESAL > 30 x 108 . Move binder one grade higher. For minimum
50 percent reliability, binder is PG 70-16. for minimum 98 percent reliability,
binder is PG 76-16.
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Table 18.13 Section of Asphalt Binder Performance Grades on the Basis of
Climate, Traffic Speed, and Traffic Volume
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1. Select the types of loading.
2. Move horizontally to the high pavement design temperature.
3. Move down the low pavement design temperature.
4. Identify the Binder Grade.
5. ESALS > 107consider increase of one high temperature grade. ESALS > 3 x 102
increase one high temperature grade.
Example:
Standing load, high design temperature
Low design temperature = - 25oC
Grade = PG 70-28
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Table 18.14 Coarse Aggregate Angularity Criteria
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The clay content is defined as the percentage of clayey material in the
portion of aggregate passing through the 4.75-mm sieve. It is obtained by
conducting the AASHTO T176 designated test Plastic Fines in Graded Aggregates
and Soils by Use of Sand Equivalent Test.
Tapping Bottom of
Cylinder
ApparatusManually-Operated Shaker
Irrigation Clay Reading
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Table 18.17 Clay Content Criteria
Table 18.17 gives minimum allowable values for sand equivalent for
different traffic loads.
Traffic, Million ESALs Sand Equivalent
Minimum, Percent
< 0.3 40
< 1 40
< 3 40
< 10 45
< 30 45< 100 50
100 50
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Gradation. The distribution of aggregate particle sizes for a given blend of
aggregate mixture is known as the design aggregate structure. The gradation
system used for Superpave is based on the 0.45 gradation plot. In order to
understand the gradation system used, it is first necessary to define certain
gradation terms that the Superpave system uses. These are maximum sizes,
nominal maximum sizes and maximum density gradation.
Maximum size is defined is one sieve larger than the nominal maximum size, and
the nominal maximum size is one sieve larger than the first sieve that retains more
than 10 percent of the aggregate. Five mixture gradations are specified in the
Superpave system, as shown in Table 18.18.
Maximum density gradationis obtained when the aggregate particles fit together
in their densest form. An important characteristic of the 0.45 power plot is that
the maximum density gradation for a sample of soil is given by the straight line
joining the maximum size and the origin (Figure 18.16).
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An acceptable aggregate gradation is defined by specifying control points on the
maximum density gradation chart for the smallest sieve size (0.075 mm), the
nominal maximum size, and an intermediate size sieve (2.36 mm). An acceptable
soil blend therefore should have maximum density gradation line that lies within
these control points.
Table 18.18 Superpave Mixture Gradation
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18.6.2 Volumetric Trial Mixture Design
This consists of:
Selection of design aggregate structure
Determining trial percentage of asphalt binder for each trial aggregate blend
Evaluating trial mix designs
Obtaining design asphalt binder content
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Determining Trial Percentage of
Asphalt Binder for each Trial Aggregate Blend
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