4. After soaking the sample, place it in the bucket,
which is filled with water. Then turn the bucket and
aggregate sharply back and forth to help remove any air.
5. Suspend the bucket from the brass hanger and
bring the water level up to the overflow pipe.
6. Determine the submerged weight using weights
placed in the scoop on the right-hand pan. Record the
weights in the appropriate spaces on the data sheet. The
calculations required for the determination of the
apparent specific gravity of coarse aggregate are shown
on the data sheet and are self-explanatory.
MIX DESIGN TESTS
Mixture design tests for bituminous pavement are
carried out on samples mixed and compacted in the
laboratory to determine the optimum bitumen content,
the optimum aggregate content, and gradation required
to produce a pavement that will meet given quality
specifications. Mixes with various bitumen and
aggregate contents and gradations are prepared,
compacted to specified density, and tested. From the test
results, design engineers determine optimum values.
Mix design test procedures vary considerably. this
course can give only a general description of typical
procedures. Mixture design is more fully discussed in
NAVFAC MO-330 and at the EA1 level in Part 2 of this
TRAMAN.
Selection of Sample Bitumen Contents
Bitumen content for laboratory test mixes must be
estimated to get the tests started. Tests are made with a
minimum of five contents: two above, two below, and
one at a content estimated to be about right. Bitumen
content is expressed in terms of percentage of bitumen
by weight to the total weight of the mix. Percentages
commonly run from 3 to 7, depending upon the type of
binder used and the specification requirements.
Preparation of Aggregate
A quantity of aggregate of the selected blend
sufficient to make the required number of test samples
is dried at 230°F. The dry aggregate is separated into
several size ranges by sieving, and a sieve analysis is
then made of each range.
From this procedure, design engineers can
determine trial percentages for test blends. Test blends
are then made with these trial percentages. Again, a
sieve analysis is made, this time to determine a blending
gradation, such as shown in figure 13-36.
The explanation of figure 13-36 is as follows. The
aggregate here was first sifted into four categories:
coarse, fine, fine river bar sand (FRBS), and limestone
dust (LSD)-the last being a commonly used mineral
filler. All of the coarse aggregate consisted of material
that would not pass the No. 8 sieve; 89.5 percent of fine
aggregate consisted of material that would not pass the
No. 80 sieve; and 90 percent of the FRBS consisted of
material that would not pass the No. 200 sieve. These
three sieves, then, were the ones used to make the
original broad separation. Limestone dust was added to
the extent of 2 percent.
After the sample was broadly divided, a sieve
analysis was made of each broad division category, as
shown. This analysis was studied by experts, who
estimated, among other things, the probable void
percentages that would exist in pavements made with
the aggregate used in various combinations.
Percentages that would minimize void percentages were
estimated.
For trial blend No. 1, these percentages are listed
under “percent used.” The percentages are 27 coarse, 63
fine, 8 FRD, and 2 LSD. A blend containing these
percentages was made, and again the material in each
category was sieve-analyzed, as shown. From these
individual analyses, the blend analysis (that is, the sieve
analysis for the mixed blend) was determined by adding
together the percentages in each column.
Thus an aggregate gradation for the blend was
obtained. If tests showed that this particular gradation
produced a mix that met the specifications for the
pavement, this gradation would be specified for the
aggregate used in the highway.
Specimen Mixing and Compacting for Testing
To prepare an aggregate blend for testing,
thoroughly mix and heat enough blend for two
specimens (about 3,000 grams) to the desired mixing
temperature. Trough the aggregate blend; then heat the
test amount of bitumen to mixing temperature and pour
it into the trough. Mix the aggregate and bitumen
together thoroughly with a mechanical mixer if one is
available. Then place the mix in a compaction mold and
compact it with a tamper. Give the number of blows
required to produce the density that will be attained
under the traffic for which the pavement is being
designed. NAVFAC DM-5.4, Civil Engineering,
specifies 50 blows for secondary roads and 75 blows for
13-48
