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Recommended Procedures for Verifying Material Quantities at Asphalt Mix Plants

Description:

Asphalt mixtures produced in the United States in 2018 contained over 82 million tons of reclaimed asphalt pavement (RAP) and over 1 million tons of reclaimed asphalt shingles (RAS). Public transportation agencies typically specify maximum percentages of RAP and RAS that can be used to produce asphalt mixes, based on factors such as mix type and application. These maximums are needed to ensure good performance of the pavement. These maximum values can be based on total percentage of the RAP or RAS by weight, or on a calculated value such as reclaimed binder ratio. Regardless of the specification method, it is necessary to verify the percentage of RAP or RAS being introduced into the asphalt mix plant in order to determine specification compliance. This is further complicated with the fact that RAP and RAS contain both asphalt binder and aggregate, so, the verification process must include the quantity added as well as the percentages of asphalt binder and aggregate comprising each material. Verification of RAS content can be especially challenging because it often does not flow freely when using traditional cold feed bins, so is often mixed with another material (such as RAP or sand) before introduction into the plant. Excess RAP or RAS in a pavement can lead to insufficient new binder and increased stiffness of the mastic, potentially resulting in reduced mixture durability.

States have used various approaches to verify RAP and RAS percentages using techniques such as stockpile measurements or plant recording. Also, plant manufacturers have introduced new equipment to monitor flow from individual cold feeds or improved plant calibration equipment, but there are no national guidelines in the use of these approaches.

In addition to RAP and RAS, materials introduced at asphalt plants include asphalt binder, aggregate, and many additives such as latex, ground tire rubber, warm mix additives, anti-strip additives, fibers, and mineral fillers. Addition of these materials is done with many different types of equipment and control systems. The additions must be accurately controlled and interlocked in order to meet job mix formula requirements and agency specifications. Often, these additives are introduced using systems that do not provide good means to verify the quantity being added.

Transportation agencies and asphalt mix producers would benefit from guidelines recommending best practices for verifying quantities of materials including RAP, RAS and other additives into asphalt mixtures. Improved information will lead to better specification compliance and potentially improved pavement performance.

Objective:

Develop recommended practices to verify quantities of materials being introduced into asphalt mixtures at production facilities.

Benefits:

Inability to accurately verify the percentage of RAP, RAS and other materials during asphalt mix production can lead to production of mixes that do not meet the job mix requirements or material specifications. Current test methods cannot identify the RAP, RAS or additive content; these must be verified through monitoring of plant operations during production. Excessive RAP or RAS in a pavement can lead to decreased durability and premature failure. Incorrect percentages of additives such as hydrated lime, warm mix chemicals or fibers can also lead to performance problems. Given that 94 percent of the nation’s highway network is paved with asphalt, even a small increase in pavement life will result in millions of dollars in savings.

Related Research:

Literature Search Summary

Several published reports and articles discuss production of asphalt mixes containing RAP and RAS. However, none provide specific guidance on verifying RAP/RAS or additive content during mix production.

William R. Vavrik, Samuel H. Carpenter, Steve Gillen, Jay Behnke, Fred Garrott: “Evaluation of Field-produced Hot Mix Asphalt (HMA) Mixtures with Fractionated Recycled Asphalt Pavement (RAP): 2007 Illinois Tollway Field Mix Trials” - Research Report ICT-08-030, Illinois Center for Transportation, Oct. 2008.

Mansour Solaimanian and Maghsoud Tahmoressi: “Variability Analysis of Hot-Mix Asphalt Concrete Containing High Percentage of Reclaimed Asphalt Pavement” – Transportation Research Record 1543, 1996.

Randy West, Fabricio Leiva, Grant Julian, Adam Taylor, Elton Brown, James R. Willis: “Using Recycled Asphalt Shingles with Warm Mix Asphalt Technologies” – NCHRP Research Report 890, 2018.

Fujie Zhou: Best Practices for Using RAP/RAS in HMA/WMA Mixes Including Workshop and Workshop Materials” – Texas A&M Transportation Institute, 2013

Tasks:

Possible tasks include:

Task 1 – Literature review. Include public transportation agency specifications and inspection procedures related to asphalt mix plant and production requirements and AASHTO M 156, Standard Specification for Requirements for Mixing Plants for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures.

Task 2 – Survey of public transportation agencies asphalt mix producers and plant manufacturers to determine current procedures for verifying raw material quantities including RAP, RAS and other additives during mix production.

Task 3 - Identify best practices for verifying raw material quantities used including RAP, RAS and additive, as well as any knowledge gaps in this area.

Task 4 – Develop suggested specification language and inspection procedures. Include plant calibrations and procedures, accuracy requirements for various additive types, and documentation/recordation procedures for both batch plants and drum mix plants. Staffing, timing and frequency of these activates should be addressed. There may not be one approach. There may be a variety of approaches from which a state could choose.

Task 5 – Publish a guide for agencies to use to develop specifications and inspection procedures for verification of materials including RAP, RAS and additive content of asphalt mixes. Develop proposed updates to AASHTO M 156.

Implementation:

Implementation of the guidelines and proposed revisions to AASHTO M 156 will be accomplished through presentations at the AASHTO Committee on Materials and Pavements, AASHTO Committee on Construction, and through presentation at various regional and national meetings and conferences (including the TRB annual meeting), as well as a TRB webinar. Successful implementation will require working with agency materials and construction engineers, and with asphalt mix producers through various industry associations, including the National Asphalt Pavement Association and the Asphalt Institute. Potential challenges will be resistance by mix producers, lack of physical controls on existing asphalt mix plants, as well as reluctance by agencies to implement new requirements which may increase costs. Agency staffing will be an important aspect effecting implementation.

Sponsoring Committee:AKC30, Quality Assurance Management
Research Period:12 - 24 months
Research Priority:High
RNS Developer:Rick Bradbury, Director of Materials Testing and Exploration, Maine Department of Transportation Tel: (207) 624-3482 Richard.bradbury@maine.gov
Date Posted:07/03/2020
Date Modified:07/09/2020
Index Terms:Asphalt mixtures, Asphalt plants, Reclaimed asphalt pavements, Recycled materials, Quality assurance, Asphalt additives,
Cosponsoring Committees: 
Subjects    
Highways
Materials
Pavements

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