are set according to the proportions obtained in the
computation of the final trial blend.
Aggregate Blending Limits
Figure 17-7 is an aggregate grading chart (DD Form
1207), showing the specification limits for the mix and
the gradation of the blend when mixed in the proportions
shown in figure 17-6A, trial No. 1.
Specify Gravity of Bituminous Mix
Components
A specific gravity data sheet is shown in figure 17-8
(DD Form 1216). This form is used for computing the
specific gravity of all the bituminous mix components.
If more aggregate fractions are used than are provided
for on the form, additional forms may be used.
Procedures for performing these tests are discussed in
chapter 13 of this TRAMAN.
Marshall Stability Computations
DD Form 1218 (figs. 17-9A and 17-9B) is a data
and computation sheet used in the Marshall stability test.
The specific gravity values of the aggregates and the
aggregate fraction percentages from the trial blending
furnish the information to compute the fractional
weights and to prepare the test specimens. Record the
measurements made on the test specimens in the upper
right-hand corner of the form. Determine, as described
in chapter 13 of this TRAMAN, the stability, flow, unit
weight of total mix, and percentage of voids filled with
binder to complete the form.
Marshall Method Computations
Transfer the values from DD Form 1218 (figs.
17-9A and 17-9B) for each binder content computation
to DD Form 1219 (fig. 17-10). Each graph on the form
represents a different test property. Plot the values for
each property on their respective graph using the binder
contents as ordinates. Draw a smooth curve through the
plotted points.
Marshall Test Criteria
Table 17-7 lists the criteria for determining
optimum asphalt content (OAC). For each test property,
you should consider the type of mix to be used and the
expected load. The optimum bitumen content for each
property is designated as a definite point on the curve
for that property. The bitumen content percentages (one
for each property) are averaged, and the average is used
to read the corresponding value of each test property.
The value, as determined, should be referred to the
criteria portion of table 17-7 to see if it is within the
permissible limits so that the mix will perform
satisfactorily.
Test Variation for Aggregates with
10 Percent or More Larger Than
1-Inch Maximum Size
The procedure described in the Marshall method
and the examples as given in the preceding paragraphs
are applicable to hot-mix design where the amount of
aggregate larger than the 1-inch sieve is less than 10
percent of the total. When the larger than (plus) 1-inch
material exceeds 10 percent of the total, the following
variations are made in the procedure:
1. Mix bitumen at the selected content with the
entire aggregate, including the plus 1-inch portion.
2. Pass the mixed hot batch through a 1-inch sieve.
Discard the plus 1-inch portion.
3. Make compacted specimens from the portion
that passes the 1-inch sieve and perform the Marshall
test, except do not calculate the voids of the compacted
specimens at this time.
4. Determine the bulk specific gravity of the plus
1-inch aggregate, and, with the specific gravity of the
compacted specimens, compute the adjusted specific
gravity (GA) as follows:
where:
A
= weight of dry, plus 1-inch material
expressed as a percentage of the total batch
weight (bitumen plus aggregate)
B
= portion of total batch remaining after the
dry, plus 1-inch portion is removed
(100%-A%)
C =
bulk specific gravity of plus 1-inch
aggregate
D =
actual specific gravity of compacted
specimen
f
= empirical factor = 0.995
17-20
