In December 2011, FHWA’s Office of Pavement Technology and Office of Infrastructure R&D at the Turner-Fairbank Highway Research Center released an important study, Investigation of Low- and High-Temperature Properties of Plant-Produced RAP Mixtures (FHWA-HRT-11-058). Written by Rebecca S. McDaniel, Ayesha Shah and Gerald Huber, this new study analyzes the results of testing plant-produced, hot-mix asphalt containing various levels of RAP and different grades of virgin binder, undertaken initially to examine the effects of RAP on low-temperature properties of mixtures.

For this work, FHWA — in conjunction with the North Central Superpave Center and Heritage Research Group — is investigating and evaluating plant-produced HMA mixtures containing varying percentages of RAP.

“So far, results appear to confirm the current suggested guideline for the use of RAP in the amount of 15 percent or less that states that ‘no change is required in the binder Performance Grade (PG) or in the mixture design,’” the researchers said in December. “However, in the case when higher amounts of RAP (i.e., greater than 25 percent) are used, the stiffness properties of the mixture increase (significantly in some cases) and it may be necessary to lower the PG of the binder or use blending charts to [either] determine the amount of RAP to use based on the virgin binder grade or determine the virgin binder grade, and amount based on the desired amount of RAP.”

The results highlight some challenges in using high amounts of RAP. “A primary concern is the performance of high-RAP mixtures in cold climates,” they say. “The addition of high amounts of RAP to an HMA mixture usually increases the mixture’s stiffness. In cold climates, this may contribute to the occurrence of low-temperature cracking. Another concern is due to the uncertainty of what occurs between the virgin and the RAP binder during mixing in a HMA plant.”

Current activities include expanding to include materials from multiple HMA suppliers in order to get insight on producing HMA mixtures with RAP, and to substantiate the results of the original project now referred to as Phase I.

Among other conclusions, the researchers found:

• Test results on binders extracted and recovered from the plant-produced mixes show that, in general, as the RAP content in the mixture increases, the high-temperature grade of the recovered binder also increases, but only by a few degrees (33.8 to 37.4 °F).

• As the RAP content increases, the low-temperature grade of the recovered RAP binders also increases, but not as much as the high-temperature grade.

• The use of a softer virgin binder grade typically decreases both the high- and low-temperature grades of the recovered binders by half a grade or more.

• Increasing the RAP content to 25 percent changes the recovered binder low-temperature grade by no more than 35.6 degrees F, compared to the binder recovered from the virgin mix (with no RAP).

• RAP content can possibly be increased to 25 percent before changing the virgin binder grade, but a grade change should be incorporated when increasing the RAP content to 40 percent. The findings are considered representative of mixtures produced in hot-mix plants in Indiana using virgin asphalt binders and RAP typically found in Indiana (see the November 2011 Road Science for more on Indiana and RAP).

• And other agencies should review their typical materials, especially RAP stockpiles, to determine their own course of action.

WMA Quells RAP Cracking?

Counties as well as states are active in studying RAP and thermal cracking. In central Minnesota in 2009, Crow Wing County used a warm-mix asphalt (WMA) additive — Evotherm from MWV Asphalt Innovations — to see if it could get from a lower-cost PG 58-28 binder the same cold-weather performance of the more expensive polymer-modified PG 58-34 binder specified in Minnesota for newly constructed low-volume bituminous roadways. One of its mixes used 30-percent RAP.