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51
Coarse Aggregate
PCS
Coarse Portion of Fine Aggregate
SCS
Fine Portion of Fine Aggregate
Figure 16. Overview of the divisions in a continuous gradation
that allows an analysis of gradation (124).
trol sieve (PCS), secondary control sieve (SCS), and tertiary sieve size (2.36-mm or 4.75-mm, depending on the maximum
control sieve (TCS). Table 12 lists the control sieves for var- aggregate size) and the 0.3-mm size. The restricted zone was
ious asphalt mixture sizes. recommended to reduce the incidence of tender or rut-prone
The design and analysis of an aggregate blend using the Bai- mixes. The origin of the Superpave-defined restricted zone is
ley method of gradation selection is built on four parameters: documented in a SHRP report (1, 127).
From a historical perspective, the restricted zone is some-
1. Chosen unit weight, which describes the interlock of thing new: not until the Superpave method was there a for-
the coarse aggregate; mal guideline for aggregate gradation called the "restricted
2. CA Ratio: coarse aggregate ratio, which describes gra- zone." However, the industry has been aware of potential
dation of coarse aggregate; performance problems with gradations that pass through the
3. FAc Ratio: fine aggregate coarse ratio, which describes Superpave-defined restricted zone-region. In 1940, Hveem
gradation of coarse portion of fine aggregate; and (128) described a number of HMA gradations that showed a
4. FAf Ratio: fine aggregate fine ratio, which describes gra- hump between the 0.6-mm and 0.15-mm sieve sizes. Hveem
dation of fine portion of fine aggregate. indicated that the hump was caused by an excessive amount
of sand in this size fraction. He said that the hump is indica-
Changes to any of these parameters will affect the air voids, tive of wind blown sand (smooth-textured, rounded sand)
VMA, constructability, and performance of the resulting within the aggregate blend and that in his experience, the
asphalt mixture. These changes are the same whether the hump resulted in HMA mixes with low stability.
change is made in the laboratory during design or in the field The initial concept of a restricted zone around the maximum
during construction. density line can probably be indirectly traced back to Goode
and Lufsey (123). Based upon work by Nijboer (129) to iden-
tify a maximum density line, Goode and Lufsey presented a
2.8.2 Effect of the Restricted Zone
on HMA Performance 0.45 power grading chart for plotting aggregate gradations. To
utilize the newly developed gradation chart, Goode and Lufsey
During SHRP asphalt research, an Aggregate Expert Task evaluated 24 gradations to observe the effect of sand con-
Group (ETG) was formed to develop recommendations for tent on the stability of HMA mixes. What prompted their study
aggregate properties and gradations for HMA. The final rec- were reported cases in which tender mixes were encountered
ommendations for gradations included a restricted zone that with gradation humps between the 0.6-mm and 0.3-mm sieve
lies along the maximum density line between the intermediate sizes. Goode and Lufsey found that, in general, gradations that
TABLE 12 Control sieves for various asphalt mixes
NMAS, mm 37.5 25.0 19.0 12.5 9.5 4.75
Half Sieve 19.0 12.5 9.5 ** 4.75 2.36
PCS 9.5 4.75 4.75 2.36 2.36 1.18
SCS 2.36 1.18 1.18 0.60 0.60 0.30
TCS 0.60 0.60 0.30 0.30 0.150 0.150 0.075
** The nearest "typical" half sieve for a 12.5-mm NMAS mixture is the 4.75 mm; however, the 6.25-
mm sieve actually serves as the breakpoint. Interpolating the percent passing value for the 6.25-mm
sieve for use in the coarse aggregate ratio will provide a more representative ratio value.
