100% Recycled Asphalt Mixes

Mix Design

The secret to producing quality 100% recycled mix is to fractionate RAP stockpiles and blend the separate fractions at proportions required for the desired mix. Only three fractions are needed to produce 4.75, 9.5, 12.5 and 19mm SuperpaveTM mixes – ¼” RAP sand, ½” stone and 1” CRAP (coarse RAP). RAP should be free of debris and coal tar sealers and crushed to only 1″ to minimize generating unnecessary fines. The 1” stone is saved for base mixes. Consistency is important to mix quality and fractionated RAP is a surprisingly consistent raw material.

 
 
Stone Fractions

¼” Sand Fraction

A 5/16″ “Z” wire screen removes liberated sand which improves plant operation as will be discussed later. As a rule there is excess RAP sand when Marshall mixes are used to produce Superpave gradations. Often the excess sand is a valuable raw material for conventional plants due to the almost 7% AC content. The ¼” size is critical since it matches most control sieves and enables a higher percent passing the ⅛” sieve. You will have trouble complying with minimum AC content with sand opening less than ¼”.

Mix Design

½” Stone Fraction

½” stone is blended roughly 50/50 with the ¼” RAP sand to produce 12.5 m m surface mixes. This fraction still contains a reasonable quantity of sand which is OK since it tends to stay consistent as long as stockpile gradation doesn’t change due to screening problems.

¾” Coarse RAP

As with surface mixes, ¾” coarse RAP is blended 50/50 with ¼” RAP sand to produce 19 mm binder. This fraction contains all the ½” stone required so two cold feed bins are sufficient to make both base and surface mixes by emptying the stone bin between mix changes. This will be a difficult product to keep on the ground for two reasons, there isn’t a lot of ¾” stone in most millings and binder will be a popular mix due to it’s superior quality.

Asphalt Binder Content

Satisfying minimum binder requirements is as simple as adding enough ¼” RAP sand! After all that’s where the asphalt content (AC) is. As a rule if you have enough ¼” minus in your mix there will be plenty of AC. Remember you started with a mix that had enough AC to coat everything and you add even more liquid (recycling agent) to soften the age hardened RAP binder.

AC vs RAP Sand

Role of recycling agent

Most RAP comes from roads that have deteriorated to the point of needing to be replaced. Since the aggregate is unaffected by time, aging occurs in the asphalt binder. Aged binders are stiff and brittle at cold temperatures. Rejuvenators will soften RAP binders, restoring workability and improving mix longevity.

There are numerous softening agents available that range in price from $2.50 to $10 per gallon. Dosage range is typically ½ % of total mix or 1.5 gallons per ton. Some products are superior at improving workability while others excel at low temperature performance. Care needs to be taken with all products to avoid overdosing which will create flushing and rutting concerns.

Aromatic extracts have traditionally been the rejuvenator of choice due to their compatibility with asphalt cement. Some refineries however are moving away from aromatic products due to health risk concerns but many products are still available marketed as Industrial Process Oils. Only a few can be pumped cold with most heated to 100F to flow freely.

A new generation of organic plant oils shows considerable promise of being excellent rejuvenators. In general they are twice as effective as refinery products and can be waste derived. More work is needed to evaluate mixture properties but waste derived plant oils may offer it all – waste derived, low cost, and great performance.

Binder properties control mixture stiffness and flexibility. A durable material must balance stiffness with flexibility. Recycled binders start out stiff and are further hardened in the dryer. It doesn’t matter whether the hardening is due to the tendency of polar compounds to form asphaltene chains, the evaporation of light oils, or oxidation; aged asphalt is not be durable.

Rejuvenators are added to soften recycled binders and restore youthful properties – flexibility, shear strength, and adhesion. Care must be taken to add enough rejuvenator to achieve low temperature performance without losing high temperature strength.

Extractive analysis is done on finished RAP mixes to determine stiffness and flexibility of recycled asphalt binder and rejuvenator combination. Asphalt cement is washed of the aggregate with solvent and recovered to test its properties. This test enables user to determine the exact dose needed to produce a flexible mix that resists cracking while also stiff enough not rut.

 
 
Recycling Agent Graphs