A Changing Industry (HMA)

Changing IndustryFor many decades, procedures used in hot-mix asphalt (HMA) production remained basically unchanged, until the practice of milling asphalt pavements began.  Today, milling highways and roads prior to repaving is a common practice.  As the newly generated milled asphalt, known as "RAP" (Reclaimed Asphalt Pavement) began to become plenteous, awareness grew as to the inherent value of the RAP product. The asphalt industry started trying different methods of blending this valuable RAP product in with the current virgin made HMA. Initially, mechanical, environmental and QC problems were experienced.  With improved techniques, the issues that caused initial production problems were identified and corrected.  Today it is normal for RAP to be blended into new HMA, although the percentage of RAP introduced is generally limited because of limitations with standard processing equipment. In many geographical regions, especially in metropolitan areas, there is now an abundance of the valuable RAP product; in fact, in many areas huge RAP inventory stockpiles have become problematic.

Although the valuable RAP inventories are continuing to grow, HMA today is still primarily produced with virgin asphalt binder and virgin aggregates.   However, in recent years the value of RAP has been realized and industry focus has been placed on increasing the amount of RAP in new HMA production.  This new focus is primarily the result of the increasing costs of petroleum based PG binder and raw aggregates which have a direct impact on HMA productions costs.  This concern comes at a time when many federal and state funding sources are becoming increasingly strained.  

In today's worldwide economy recycled materials are used (whenever and wherever possible) if economic and environmental advantages can be realized.  High quality HMA can be produced with great economic advantage realized using 100% processed RAP.  New HMA  produced with 100% RAP can yield production cost savings of 30% or more vs. HMA produced primarily with virgin ingredients. In addition, high utilization of RAP has an extremely beneficial environmental impact.

Examination as to why there are still limited amounts of RAP used in new HMA production has revealed two primary reasons:

1) Although there may be a few exceptions, drum-mix, batch and double-barrel type asphalt plant technologies generally are incapable of attaining greater than a 30%-40% blend of RAP into the new HMA without experiencing limitations and/or environmental drawbacks.  These limiting factors are basically determined by normal laws of physics.  Superheated virgin material generally acts as the heat transfer medium to the cool RAP bringing the combined blend to the desired temperature. It has been heard that up to 50% RAP can be blended in some plants, but it must be noted that the material must be of extremely low moisture content and usually attainable only on the hottest and driest days of the year.

2) It has been noted that some state agencies have been very slow to embrace change, yet for those agencies that have approved HMA comprised of higher RAP percentages, positive results have been realized.

Green PerspectiveToday, HMA pavements comprised of increased percentages of RAP are now produced regularly throughout the nation. Additionally, various agency and industry studies are revealing very positive results. The FHWA has taken a position of being very pro-recycling.  By following standard recommended RAP stockpiling methods and RAP "fractionating" methods, a consistent high quality RAP product can be produced. Today, new technology specifically designed utilizing indirectly heated drum plants are available for producing high-quality HMA in high production capacities, comprised primarily or entirely from 100% RAP.  

Today, the industry is seeking pavement sustainability, "Green" environmental awareness, as well as energy reduction. HMA produced primarily from RAP provide these benefits and more.  For various significant reasons, great emphasis should be made now to review and modify existing specifications allowing for the acceptance of HMA produced primarily from RAP, provided that the new HMA produced meets or exceeds specification equivalency and test results.