In this study, four plant-produced mixtures containing zero, 20, 30 and 40 percent RAP were long-term oven-aged in the laboratory to three levels. The dynamic modulus was measured for each aging level and was compared to unaged values to determine if there was a statistical difference. It was found that as RAP content increased, aging had less of an effect on stiffness.

Long-term oven aging to simulate aging in the field can be used to evaluate stiffness changes over time, they say. The impact of the measured increases in stiffness on the fatigue performance of the pavement has been shown to be influenced by the binder type, as well as the pavement structure. Aging has also been shown to reduce the stress relaxation capacity of the binder. The authors found the stiffening effect of long term oven aging on RAP mixtures is less than that of virgin mixtures. This could be due to the inclusion of already aged binder in the RAP mixtures that does not age further under laboratory conditioning.

Spectroscopic Evaluation of RAP

The ignition oven is not the only way to determine the content and chemical condition of residual binder in RAP. Spectroscopic analysis offers the chance of moving the analysis from the lab into the field.

“Spectroscopic investigation of the oxidization age hardening in asphalt products has been the focus of pavement research for more than three decades.”

- Yut and Zofka

The ongoing Second Strategic Highway Research Program (SHRP-2) project titled Evaluating Applications of Field Spectroscopy Devices to Fingerprint Commonly Used Construction Materials targets, among the other objectives, evaluation of oxidation in RAP, say Iliya Yut and Adam Zofka, Ph.D., University of Connecticut, in their 2012 TRB paper, Spectroscopic Evaluation of Recycled Asphalt Pavement Materials.

The study investigates the effect of the RAP content on the concentration of oxidized components of asphalt by using advanced, yet portable, easily interpretable spectroscopic methods. Two types of samples are prepared in the laboratory: binder blends containing 15 to 40 percent weight RAP-binder, and loose HMA samples modified by up to 80 percent weight RAP.

Spectroscopic measurements were performed using a portable attenuated total reflection Fourier transform infrared (ATR FT-IR) spectrometer. Quantitative analysis of the ATR spectra indicates that an increase in RAP content is highly associated with concentration of ketones and sulfoxides in RAP binder. It’s also possible to determine RAP content based on the analysis of extracted RAP binder, the authors write.

“Spectroscopic investigation of the oxidation age hardening in asphalt products has been focus of pavement research for more than three decades,” Yut and Zofka write. Earlier research employed spectroscopy to study long-term aging in asphalt binders. The researchers recognized three major products of oxidation, i.e., benzylic ketones, sulfoxides and free hydroxyl radicals that may interact with ketones and form carboxylic acids. Later studies confirmed that an increase in viscosity of aged binders is related to an increase in their carbonyl content, they write.

They found that a portable ATR spectrometer is capable of detecting main chemical components usually present in both binders and HMAs, namely aliphatic and aromatic hydrocarbons and mineral aggregates, thus allowing timely field tests of pavements.

RCA Needs Analysis

Recycled concrete aggregate (RCA), like RAP, must be crushed, screened and tested, and stored in blended stockpiles to ensure consistency. It should consist of mineral aggregates bonded by a hardened cementitious paste; residual mortar causes processed RCA to have a rougher surface texture, lower specific gravity and higher water absorption than similar virgin aggregates, says FHWA

The properties of recycled concrete aggregate can vary greatly, depending on the original aggregate source, and the production techniques. Therefore it’s necessary to characterize the material so it’s used properly, and if using in new concrete, appropriate adjustments are made in the structural or mix design.

Residual mortar causes processed RCA to have a rougher surface texture, lower specific gravity and higher water absorption than similar virgin aggregates – FHWA