The most recycled material in America ? 80% of hot mix asphalt (HMA) removed is recycled back into roads ? reclaimed asphalt pavement (RAP) is a very valuable resource for the HMA producer.  As shown in last week’s article, using 20% RAP in HMA can save you nearly $5.00 per ton!  Despite the economic savings and environmental benefits, as is seen in the figures below, there’s a big difference between “RAP Allowed” vs “RAP Used” in surface mixes.[1]  Why?  Sometimes, it’s availability of RAP, but more often than not specifying agencies limit the amount because of the concern about variability in the RAP stockpile and its effect on long-term performance.

This perception of high variability, not entirely without merit, comes from the notion that all RAP stockpiles are a heterogeneous agglomeration of reclaimed materials from a variety of sources.  However, many contractors maintain separate stockpiles of RAP obtained from different sources.  When a large quantity of millings is obtained from a single project, for example 5000 tons or more, it makes sense to stockpile the RAP separately.  For small quantities it is more common to combine the millings with other sources of RAP, e.g., plant waste, rejected asphalt pavement material, and pavement rubble from complete demolition of roads or parking lots.

What contractors and agency personnel need to know is, “What are the best practices for processing multiple-source RAP stockpiles to ensure consistency in the material?”  The National Center for Asphalt Technology (NCAT), based on a survey of contractors and a review of QC data, suggests the following best practices:[2],[3]

Avoid Contamination ? Keep deleterious materials out of the RAP stockpile. Contamination can occur because an uninformed truck driver dumps general road debris or construction trash on the stockpile.

Mix as You Feed ? As the stockpile material is being excavated, randomly dig into different areas of the pile so that the material going into the crusher is mixed up, i.e., not just from one place in the pile.

Don’t Over-Crush ? Excess dust limits how much of the RAP can be used in a new mix design.

Fractionate the RAP ? The advantage of screening the RAP into two or more sizes is that it provides much greater flexibility in designing mixes.  In general, it is easier to use more total RAP in a mix when it is fractionated compared to a crusher-run RAP.

Stop Processing RAP When it Rains ? Experience at the NCAT Test Track indicated that RAP does not screen as efficiently when it is wet as the material sticks together, clogging the screens.

Blend Again When Moving ? Moving processed RAP should be done so as to further mix and blend the material as it is being loaded and unloaded.

Cover, Slope, and Pave ?  RAP stockpiles tend to retain a lot of moisture which increases the drying and heating cost for superheating the virgin aggregate and limits the mix production rate.  Covering RAP stockpiles to minimize moisture infiltration can help.  As with virgin aggregates, paving under stockpiles provides an even foundation to minimize yard loss and contamination underlying materials.  Sloping the surface under the stockpile allows rainwater to drain away.

Testing and Analyzing ? The best time to test RAP is as the stockpile is being built at its final location.  The recommended frequency for sampling the RAP for QC testing is at least one test per 1000 tons.  At least five samples of each RAP stockpile should be obtained and tested before starting the mix design.

Percentage of Rap Used