15 May 2001
should improve the ability of the asphalt to hold the aggregate in place and reduce oxidation
deterioration in the porous mat. When economically available, the use of a latex rubber modified binder
should be specified.
c. Mixture design.
(1) Proportioning of aggregates. The proper aggregate gradation should be selected from
(2) Asphalt content. The asphalt content of PFC's is expressed as a percentage of the total
mix by weight. A surface area constant, Kc, as described in the centrifuge kerosene equivalent (CKE)
test (ASTM D 5148), is used to determine the optimum asphalt content. The Kc value is used in the
relation 2Kc + 4.0 to determine the Estimate of Asphalt (EOA). This asphalt content is valid for
aggregates with an apparent specific gravity in the range of 2.60 to 2.80 and with a water absorption less
than 2.50 percent when tested by ASTM C 127 for coarse aggregate and ASTM C 128 for fine
aggregate. A slight increase in asphalt content (up to 0.5 percent) is required when the absorption is
greater than 2.50 percent. The EOA is inversely proportional to the specific gravity of the aggregate
used and adjustments must be made when the specific gravity is outside of the 2.60 to 2.80 range.
(a) Kc factor. The Kc factor indicates the relative particle roughness and surface capacity
based on porosity of the aggregate to be used for the PFC. The Kc factor is determined from the percent
of SAE 10 oil retained, which represents the total effect of the coarse aggregate's absorptive properties
and surface roughness. The Kc factor is determined from that portion of the aggregate sample which
passes the 9.5 millimeter (3/8-inch) sieve and is retained on the 4.75 millimeter (No. 4) sieve using the
procedure as described in ASTM D 5148. If the specific gravity for the aggregate is greater than 2.70 or
less than 2.60, apply a correction to oil retained, using the formula given in ASTM D 5148. No correction
need be applied for asphalt viscosity.
(3) Mixing temperature. The mixing temperature shall be chosen to provide an asphalt
viscosity of 275 25 centistokes. To obtain this temperature, the temperature-viscosity relationship
must be evaluated for the type of asphalt selected at a minimum of three temperatures (ASTM D 2170
and ASTM D 2171). Plotting this information on a graph with temperature versus log viscosity will
normally result in a straight-line relationship, and the temperature for the correct viscosity can be chosen
from the graph.
d. Plant control.
(1) Plant laboratory. A plant laboratory is needed to insure that the aggregate is properly
graded and that the mix contains the prescribed percentage of asphalt binder. The laboratory should be
located at the plant to minimize the time between production and testing. If the laboratory is not located
at the plant, testing could fall behind and cause considerable quantities of unsatisfactory mix to be
(2) Sieve analysis. All sieve analyses should be conducted by the method described in ASTM
C 136. Recommended sieve sizes for plant sieve analysis are: 19 millimeter (3/4 inch), 12.5 millimeter
(1/2 inch), 9.5 millimeter (3/8 inch), 4.75 millimeter (No. 4), 2.36 millimeter (No. 8), 600 Fm (No. 30), and
75 Fm (No. 200). For batch-mix plants, sieve analyses shall be made on material from each plant hot
bin. Samples for these sieve analyses shall be obtained after a few tons of aggregate have been
processed through the dryer and screens in order that the sample will be representative. For drum
mixers, the sieve analysis must be made directly from the cold feeds. Final mix proportions may be
determined on the basis of these analyses.