When 16 to 25% RAP is used: “The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt.No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP”. When 15% or less RAP is used: “The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix.These recommendations are summarized below. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. A few restrict its use in wearing courses even fewer (one or two) prohibit its use completely. Almost all state highway departments now allow the use of RAP. The specifying agency or owner will set the limit for RAP content. RAP has a history of positive performance. The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. If either of these cautions is violated, there is a good chance that upper layer will not bond to the under layer and a slip plane will develop. The emulsion will break, but the water and emulsifying agents will freeze and remain in the layer that has been tack coated. Caution #2: Never apply an emulsion tack coat to a cold pavement (below the freezing point).In marginal weather it may take several minutes. In good paving weather, it will take only a few minutes. The length of time required for this to happen will depend on the weather. Caution #1: Once the tack coat is applied, time must be allowed for emulsion to break (turn from brown to black) prior to placing hot mix on it.Therefore, an application rate of 0.10 to 0.15 gals/sy of the diluted material will give you the 0.03 to 0.05 gals/sy. Slow-setting emulsions generally have a residual asphalt content of about 2/3. You want to accomplish a very uniform application of about 0.03 to 0.05 gal/sy of residual asphalt on the layer to be tacked (a paint job, so to speak). Then relay the mixed material and compact. Apply about 0.20 gal/sy 2 of straight emulsion (undiluted) and blade mix it with the scarified material.
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