|
Transportation Technology e-Transfer |
|
WRI Looks at RAP–Binder Compatibility to Optimize Pavement Performance
Since RAP, recycled asphalt pavement, is aggregate coated with aged binder, its interaction with new binder produces binder properties―and possibly PG grades―that are different from those of either the old or new asphalt materials selected for a pavement construction project. A new National Cooperative Highway Research Program (NCHRP 9-43, “Mix Designs for Warm Mix Asphalt Technologies” ) conducted by Advanced Asphalt Technologies includes a study by WRI that could have far-reaching application, as it provides a foundation for identifying RAP and new binder blending compatibility.
RAP is the “black rock” material recovered from a roto-milled roadway. Combined with new binder and aggregate, it serves as an aggregate filler, with the new asphalt wetting the aged-hardened asphalt–aggregate surface (which relates to the physical properties of surface free energy and therefore, performance). Wetting and mixing are affected by the solubility, temperature and viscosity properties of both the recycled and new materials. And, as higher concentrations of RAP are used, the greater the differences in physical properties of the resulting binder.
Warm mix asphalt (WMA) is a relatively new paving practice in which processing aids, such as high-temperature melting point hydrocarbon, water-releasing and -absorbing zeolite and emulsifying agents, are used to reduce viscosity so hot-plant temperature can be reduced.
While hot-mix asphalt (HMA) plant temperatures are generally high enough to produce nearly complete mixing in HMA–RAP applications, in warm-mix asphalt–RAP applications, mix plant temperatures are lower and the mixing potential is decreased. With WMA, wetting of the new binder onto the RAP surface becomes more critical, as do mixing time and cure rates, all of which affect quality of the resulting binder. Because RAP and new binder mixing and wetting potentials are temperature- and viscosity-dependent, the use of WMA and higher concentration of RAP requires a better understanding of mixing compatibility and wetting of these materials.
Knowledge of asphalt compatibility should lead directly to prediction of pavement performance. In research at WRI, asphalts derived from different crude sources have been modeled as colloidal nano-emulsions and their material flow properties characterized.
“More compatible” (sol-type) asphalts exhibit more Newtonian-like flow properties, are less sensitive to variation in viscosity caused by temperature change, and are generally more ductile than less compatible asphalts. In contrast, “less compatible” (gel-type) asphalts are often more elastic and ductile than compatible asphalts, possibly because of the presence of more asphaltenes.
WRI has developed an automated flocculation titrimetry test that characterizes asphalt compatibility in terms of Heithaus compatibility parameters. This approach relates to the colloidal stability of a suspension-type material such as asphalt in the bulk phase. Molecular compatibility may also be related to the solvation parameters defined by the Pal-Rhodes model of colloidal suspensions.
WRI’s reversible automated flocculation titrimetry test (REV-AFT) is performed as an “all inclusive” titration; that is, the concentration of a single test solution is varied in-situ. The process of “back titration” effectively changes (decreases) the concentration of the solution by re-dissolving the flocculated material that has just been detected to flocculate, setting the stage for detection of a “second” flocculation onset point measured for the same solution.
Together, the Pal-Rhodes model and Reverse AFT provide tools that make Heithaus compatibility parameters easy to measure for starting materials and blended materials (e.g., RAP asphalt + new binder). With these parameters, blending protocols may be determined for binder selection.
As the price of crude oil rises and federal agencies increasingly specify energy-saving approaches such as the use of warm mix asphalt and RAP, transportation departments and the road paving industry will continue to explore these applications. Greater insight into blending compatibility will lead to improved practices to select optimum material combinations for long pavement life.
Warm-mix asphalt thin-film coating spin cast onto a spin-cast thin-film coating of RAP-representative aged asphalt after annealing of both in a 130 °C oven for 60 minutes (bottom right). AFM scans show the interfacial contact line (top left), the WMA surface (top right), and the RAP film toward the edge (lower-left).
The material toward the center of the WMA film has the appearance of naturally waxy (2-4%) asphalt, as indicated by the “bumble-bee” shaped microstructures (top right). The more subtle microstructures depicted in the bottom left image are indicative of a moderately waxy (1-3%) asphalt, in this case AAD-1. Finally, the image of the thermally mixed interfacial contact line (top left) shows a transition in structuring between that of the WMA-type film surface and the RAP-type film surface. |
|
|