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6 Special Mixture Design Considerations and Methods for Warm Mix Asphalt
Table 5. Recommended nominal maximum aggregate sizes
for dense-graded HMA mixtures.
Application Recommended Minimum Lift Thickness, mm
NMAS, mm Fine-Graded Coarse-Graded
Mixtures Mixtures
4.75 15 to 25 20 to 25
Leveling course mixtures
9.5 30 to 50 40 to 50
4.75 15 to 25 20 to 25
Wearing course mixtures 9.5 30 to 50 40 to 50
12.5 40 to 65 50 to 65
19.0 60 to 100 75 to 100
Intermediate course mixtures
25.0 75 to 125 100 to 125
19.0 60 to 100 75 to 100
Base course mixtures 25.0 75 to 125 100 to 125
37.5 115 to 150 150
9.5 30 to 50 40 to 50
Rich base course mixtures
12.5 40 to 65 50 to 65
Table 6. VMA requirements for
dense-graded mixtures.
Aggregate Minimum Maximum Target
NMAS VMAA VMAA VMA
(mm) (%) (%) (%)
4.75 16.0 18.0 17.0
9.5 15.0 17.0 16.0
12.5 14.0 16.0 15.0
19.0 13.0 15.0 14.0
25.0 12.0 14.0 13.0
37.5 11.0 13.0 12.0
A
The specifying agency may increase the minimum and maximum
values for VMA by up to 1.0% to obtain mixtures with increased
asphalt binder content, which can improve field compaction, fatigue
resistance, and general durability. Care should be taken to ensure that
the resulting HMA mixtures maintain adequate rut resistance for their
intended application.
design binder content may also be necessary to account for differences between the initially
assumed binder absorption and the actual absorption in the trial mixtures.
For the design of WMA mixtures, the same minimum, maximum, and target VMA values
discussed in Chapter 8 for HMA should be used. These values are reproduced in Table 6. Higher
design VMA will increase the binder content of the mixture, thereby improving compactability,
durability, and resistance to fatigue damage, but decreasing the resistance to rutting. Decreas-
ing the design VMA will have the opposite effect on compactability, durability, resistance to
fatigue damage, and resistance to rutting. The target air void content for WMA mixtures
should be 4.0% with an acceptable range of 3.5 to 4.5%. Lower design air voids will increase
the design binder content of the mixture, thereby improving compactability, durability, and
resistance to fatigue damage, but decreasing the resistance to rutting. Higher design air voids
will have the opposite effect on compactability, durability, resistance to fatigue damage, and
resistance to rutting.
Step 6. Calculate Target Binder Content
The target binder content by volume for WMA is calculated in the same manner as described
in Chapter 8 for HMA: target VMA minus design air voids plus volume of binder absorbed. The
lower temperatures for WMA mixtures result in less binder absorption compared with HMA. In
NCHRP Project 9-43, the binder absorption for WMA mixtures was about 90% of that for HMA
