Prithvi S. (Ken) Kandhal, P.E. / December 28, 2000

The use of reclaimed asphalt pavement (RAP) material in hot-mix asphalt (HMA) recycling is very common. Numerous studies have shown that recycled HMA pavements perform as good as 100% conventional HMA pavements.

However, we encounter the following question quite often: What is the efficiency and effectiveness of blending the old, aged asphalt binder around the aggregate in the RAP with recycling agents, including soft asphalt cements, in hot-mix recycling operations? In one study, Lee, Terrel and Mahoney (T.C. Lee, R.L. Terrel and J.P Mahoney, Measurement of Mixing Efficiency in Pavement Recycling), used a chemical dye technique for determining the mixing efficiency of recycling agents in HMA recycling operations. The dye was added to the recycling agent or soft asphalt cement, if used.

The HMA mixture was sampled and compacted in the field laboratory. The compacted specimens were sawed and placed against a chemically treated piece of fabric to obtain dye prints showing spots where the dye chemical and recycling agent is located in the HMA mix. Most of the dye prints showed that there was thorough blending.

In a second study, Noureldin and Wood (S. Noureldin and L.E. Wood, Rejuvenator Diffusion in Binder for Hot-Mix Recycled Asphalt Pavement) used a stage extraction technique on both RAP and recycled HMA. This technique consisted of removing the asphalt binder around the aggregate in three microlayers by three successive incomplete extractions. The stage extraction of aged asphalt binder in the RAP particles indicated a nonuniform consistency or viscosity distribution. The outer microlayer was severely hardened due to weathering, the second microlayer was less hardened and the third layer appeared to retain its original consistency. When an AC-2.5 asphalt cement was added to the RAP, it softened the two outer microlayers of the asphalt binder film and restored them to the AC-20 asphalt cement specification range.

Easy answers

These two studies indicate that recycling agents included soft asphalt cements are effective in rejuvenating old, aged asphalt binders when recycled in present-day HMA facilities. One factor often overlooked, is that a substantial portion of the asphalt binder in the RAP is contained in the fine aggregate and is readily available for rejuvenation as soon as the RAP is heated and becomes loose.

About the Author

Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.

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