15 May 2001
improvement of gradation or the use of another aggregate is a matter of engineering judgment involving
an analysis of the available aggregate supplies and cost considerations.
c. Mineral fillers.
(1) General. Some mineral fillers are more desirable in asphalt paving mixtures than others.
For example, fine sands and clays are less suitable fillers than limestone filler or portland cement, and
well-graded materials are more suitable than poorly graded materials. Satisfactory pavements may be
designed using commercial fillers that conform to ASTM specifications. The apparent specific gravity of
the mineral filler is required to perform a void computation except when ASTM D2041 is used. The
specific gravity will be determined following ASTM D 854 or ASTM C 188 procedures (as appropriate),
except when ASTM D 2041 is used, in which case the mineral filler shall be included in the blended
(2) Addition of mineral filler. The filler requirements of each aggregate blend must be
estimated after the blends to be tested in the laboratory have been selected. The quantity of mineral
filler to be added generally depends on the amount of filler naturally present in the aggregate. The
amount of filler that exists naturally in most aggregates is sufficient to produce satisfactory hot-mix
asphalt. Research has indicated that under normal circumstances, the addition of mineral filler reduces
the quantity of asphalt cement required for the paving mixture. The addition of a satisfactory mineral
filler within practical limits also increases the stability of a paving mixture. Excessive amounts of filler,
however, may decrease the durability of the paving mixture because of the decrease in asphalt cement
film thickness. Practical considerations and optimum performance usually will dictate quantities of about
5 percent filler for hot-mix asphalt and 10 percent for sand-asphalt mixtures.
d. Antistrip agents.
(1) General. Several antistrip agents have been successfully used to reduce the probability of
the asphalt stripping from the aggregate. Some antistrip agents are added to the asphalt binder before it
leaves the refinery, while others are added directly into the mixer as mineral filler. The immersion
compression test described in CRD-C 652-95 is used to evaluate the stripping property of a dense-
graded bituminous hot mix.
(2) Recommended procedure. The recommended procedure for improving the resistance of
an aggregate to stripping is to add approximately 1 percent by weight hydrated lime to the mixture. This
1 percent lime must be included in the determination of the aggregate gradation.
e. Antifoam agents. Silicone additives or modifiers can reduce the effects of moisture or other
conditions in asphalt mixtures. Silicone additives have been successfully used to suppress foaming of
asphalt in asphalt plants. The silicone that has been used for this purpose is mixed at a rate of
1 milliliter per 640 liters (1 ounce per 5,000 gallons) of asphalt. The recommended range is also given
as 1 to 2 parts per million. Silicones have been used to reduce the hardening of hot-mix asphalt while in
storage silos. Silicone additives have successfully prevented slumping of mixes in trucks, which
sometimes occurs when the hot-mix gradation is such that the mix traps escaping steam. In addition,
silicones have provided better finishing qualities to pavement mixtures. These qualities include
improved workability, reduced tearing during placement, and a reduction in the amount of effort required
for compaction. Testing by several agencies has revealed no detrimental effects on the properties of
asphalts when silicone is used in the recommended concentrations. Silicones are very persistent
materials and their effects may carry over from one tank of asphalt to another. Proper mixing and
control is best achieved by addition of the silicone at the refinery.