15 May 2001
(r) List of all modifiers and amount.
The JMF may be adjusted when field conditions warrant a change. The JMF should only be adjusted
when changes in materials or procedures occur. The JMF should only be adjusted with the approval of
the contracting officer.
(2) General procedure. Laboratory tests are conducted on laboratory-compacted samples with
densities equal to densities anticipated in the in-place hot-mix asphalt after being subjected to traffic. A
final selection of aggregate blend and asphalt content will be based on these data with due
consideration to relative costs of the various mixes. The procedures set forth in the following paragraph
are directly applicable to all mixes containing not more than 10 percent by weight of total aggregate
retained on the 25 millimeter (1-inch) sieve.
(3) Preparation of test specimens. The selection of materials for use in designing the paving
mix has been discussed earlier. As an example, suppose that an aggregate gradation for a hot-mix
design shall be the median of the 19 millimeter (3/4 inch) maximum (high pressure) aggregate gradation
given in table 2-1. Design data are required on this blend. The initial mix design tests will usually be
conducted in a central testing laboratory on samples of stockpile materials submitted by the contractor.
The procedure for proportioning stockpile samples to produce a blend of materials to meet a specified
gradation is outlined below. The final mix design will be based on samples taken from the asphalt plant
and will usually be conducted in a field laboratory near the plant.
(a) Proportioning of stockpile samples. As a preliminary step in mixture design and
manufacture, it is necessary to determine the approximate proportions of the different available
stockpiled materials required to produce the desired gradation of aggregate. This step is necessary to
determine whether a suitable blend can be produced and, if so, the approximate proportion of each
aggregate to be fed from the cold feeder bins into the dryer. Sieve analyses are conducted on material
from each of the stockpiles, and the data are shown graphically in figure 2-5. Another method of plotting
or graphically illustrating the data is through the use of the 0.45 power curve. This was developed in the
early 1960's by the Federal Highway Administration using formula developed in a study by Fuller and
Thompson. The equation developed by Fuller for maximum density was:
P ' 100 (d/D)n
where d is the diameter of the sieve size in question, P is the total percent passing or finer than the
sieve, D is the maximum size of the aggregate, and n is equal to 0.45 for maximum density. The FHWA
recommends this chart be used as part of the hot-mix design process. The four aggregate fractions
must be combined to produce the desired blend. The estimated percentage of each fraction needed to
produce this blend is determined by trial-and-error calculations. Two or three trials are normally required
to obtain the desired blended gradation.
(b) Proportioning of bin samples from batch plants. Once it is demonstrated that a
suitable blend can be prepared from the available materials, samples of these materials can then be
processed through the asphalt plant for verification of mix design. Sieve analyses must be conducted
for each batch of processed aggregate. The data are shown graphically in figure 2-6. The hot-bin
aggregates should be blended to produce the same gradation as that produced at the cold feeders. The
percentage of each bin is estimated and calculations are made to determine the gradation produced
from these estimated percentages. The gradation of this recombined blend is then checked against the
desired gradation. Two or three trials are usually sufficient to produce a combined mixture having a
gradation within the allowable tolerances.