15 May 2001
Surface Treatment Asphalt Materials
(c) Historically, rapid curing cutback asphalts were used for surface treatments. RC-250
was used when cooler temperatures were anticipated, and RC-3000 when very warm temperatures
were anticipated. Currently, environmental requirements limit the availability of cutback asphalts, and as
a result, emulsified asphalt binders are the most widely used binder.
(d) Emulsions require some special handling and storage considerations to prevent
freezing, settling, and premature breaking, but they can be applied with little or no additional heating. In
selecting the type of emulsion, the compatibility of the aggregate and emulsion must be considered. As
a general rule, anionic emulsions adhere better to limestones and other aggregates composed of
predominantly calcium minerals. Cationic emulsions generally adhere better to aggregates high in silica,
such as chert and quartz gravels. Cationic and anionic emulsions both adhere well to damp aggregates.
(e) Asphalt cements harden quickly so that the cover aggregate is held in place better
than other binders provided the asphalt cement does not chill before the cover aggregate is applied.
Chilling of the binder before applying the aggregate is one major disadvantage with asphalt cement
binders. To insure good bond, the aggregates are often heated when asphalt cements are used.
Another disadvantage with the use of asphalt cements is the high amount of heat required for spraying.
Because of the difficulties encountered with asphalt cements, cutbacks or emulsions instead of asphalt
cement should be carefully considered.
(a) The aggregate will have an effect on the degree of wear resistance, riding quality, and
skid resistance of the surface treatment. Only clean, dry aggregate fragments, free from dust or dried
films of harmful material, should be used. The aggregate should have a single-size (uniform) gradation
and it should be composed of hard, angular, polish-resistant material. Flat and elongated aggregate
particles and wet or dusty aggregates are not used. Small quantities of moisture up to about 1 percent
do not create a problem, especially in warm weather, but dust can prevent the adhesion of the binder to
the aggregate. When an emulsion is used as the binder, aggregate with up to 3 percent moisture may