ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 385

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9.13.3 Rubber

Rubber has been used in asphalt concrete mixture in the form of natural
rubber, styrene-butadiene (SBR), styrene-butadiene-styrene (SBS), or recy-
cled tire rubber. Rubber increases elasticity and stiffness of the mix and in-
creases the bond between asphalt and aggregates. Scrap rubber tires can be
added to the asphalt cement (wet method) or added as crumb rubber to the
aggregates (dry method).

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9.13.4 Plastics

Plastics have been used to improve certain properties of asphalt. Plastics
used include polyethylene, polypropylene, ethyl-vinyl-acetate (EVA), and
polyvinyl chloride (PVC). They increase the stiffness of the mix, thus re-
ducing the rutting potential. Plastics also may reduce the temperature sus-
ceptibility of asphalt and improve its performance at low temperatures.

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9.13.5 Antistripping Agents

Antistripping agents are used to improve the bond between asphalt cement
and aggregates, especially for water susceptible mixtures. Lime is the most
commonly used antistripping agent and can be added as a filler or a lime
slurry and mixed with the aggregates. Portland cement can be used as an al-
ternative to lime.

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9.13.6 Others

Other additives, such as fibers, oxidants, antioxidants, and hydrocarbons, have
been used to modify certain asphalt properties' tensile strength and stiffness.

SUMMARY

Asphalt produced from crude oil is a primary road-building material. The
civil engineer is directly involved with the specification and requirements
for both the asphalt cement binder and the asphalt concrete mixtures. There
are several methods for grading asphalt cements. The current trend is to-
ward the use of the performance-grading method, used in the Superpave
process developed through the Strategic Highway Research Program. This
grading method directly ties the binder properties to pavement-performance
parameters. Similarly, the Superpave mix design method uses performance