Better Roads Staff
New Jersey DOT has established “tiers” describing increasing levels of RAP in mixes.
• Tier 1: 10- to 15-percent RAP. Asphalt binder grade for the mixture is selected for the environmental and traffic conditions the same as required for a mixture with all virgin materials. With RAP limited to 15 percent or less, it is not necessary to determine the properties of the RAP binder. No [PG] grade adjustment is made to compensate for the stiffness of the asphalt binder in the RAP.
• Tier 2: 16- to 25-percent RAP. The asphalt binder grade can also be selected using an appropriate blending chart if the supplier chooses to adjust the binder selection to compensate for the stiffness of the reclaimed asphalt binder. Extraction and recovery of RAP binder with binder testing is required. Regardless of the method used to select binder grade, adjust binder grade as necessary to meet mixture performance requirements
• Tier 3: 26- to 40-percent RAP. The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperatures. Extraction and recovery of RAP binder and testing is required on a minimum of five samples. Adjust binder grade as necessary to meet mixture performance requirements.
Indiana Justifies More RAP
Use of larger amounts of RAP – and thus changes in a state’s RAP acceptance spec – can be justified by adjusting the type of PG grading used for a Superpave mix, say Matthew Beeson and Michael Prather, Indiana DOT, and Gerry Huber, Heritage Research Group, in their 2011 TRB paper, Characterization of Reclaimed Asphalt Pavement in Indiana: Changing INDOT Specification for RAP.
But doing so requires intensive study of the binder properties in the existing storehouse of RAP across the state, and comparing them to the properties of new asphalt binders being used in the state.
“Interest has been growing to increase reclaimed asphalt pavement usage by increasing the allowable percentage that can be used in hot-mix asphalt,” Beeson, Prather and Huber say. “[Indiana DOT] undertook a detailed evaluation of asphalt binder properties in RAP and the properties of new asphalt binders being supplied to INDOT.”
Asphalt binder was recovered from 33 RAP samples taken across the state, and characterized for low temperature and high temperature grade [for Superpave mixes], they say. “New asphalt binder properties were obtained from more than 200 quality acceptance samples that covered three minus-22 grades and three minus-28 grades,” they said in January. “The data were analyzed to determine the maximum amount of RAP that could be added. Up to 22-percent RAP could be added without changing the original grade of binder. By shifting to a minus-28 grade, the allowable RAP increased to 38 percent. These findings supported the findings of a North Central Superpave Center study on five asphalt plants.”
The result was enough confidence in the existing RAP and its interaction with virgin PG binders that Indiana DOT was able to change its RAP acceptance specs upward. “On the basis of this information, INDOT changed the specification for RAP to allow up to 25-percent binder substitution without changing the normal binder grade of minus-22, and up to 40 percent if the binder was changed to a minus-28 grade.”
WMA and RAP
As WMA is an unfolding technology, its interaction with RAP is being studied in an ongoing investigation. The Effect of Warm Mix Asphalt on RAP in Hot Mix Asphalt is being undertaken by Rutgers’ Bennert and Nazhat Aboobaker of the New Jersey DOT.
The project is intended to determine how RAP can be used at typical and higher percentages in warm-mix asphalt. “One of the critical issues facing warm-mix asphalt is the lack of a formal mixture design procedure,” the researchers say. “If warm-mix asphalt is to replace or be used in conjunction with hot-mix asphalt in the future, a laboratory mixture design procedure for warm-mix asphalt must be established.”
Key issues that will be addressed during the research project are: the appropriate RAP percentages due to potential of decreased RAP and virgin asphalt binder blending with WMA; possible mixture design modifications for WMA technologies and additives, including foamed asphalt; possible recommendations for minimum production temperature and storage times of warm-mix asphalt; and acceptance procedures for allowing the use of current and new WMA technologies and additives. The project began Mar. 1, 2010, and is set to end April 30, 2012.
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