RAP Detection and Quantity Determination in US Asphalt Pavements

Introduction

Reclaimed asphalt pavement (RAP) in asphalt mixtures provides the means to recycle the existing roadway and use that material, in a limited amount, in the construction of new pavements.  This is an environmentally friendly method to recycle the existing pavement and provide a new pavement surface. The use of RAP presents an opportunity for the contractor to use a lower price ingredient as a significant percentage of the new material.  Therefore, there is a possibility that increased percentages of RAP could be included in the mix as a means to decrease cost of the final product.  Increasing the percentage of RAP in the mix can have detrimental effects on the pavement performance.  Increased percentages of RAP may lead to premature pavement deterioration from distresses such as thermal or block cracking, raveling, and weathering (Solaimanian and Kennedy, 1995).

RAP can considerably alter the properties of the existing mixture, generally resulting in a stiffer, more brittle mixture unless compensated for in mixture design and control.  A careful RAP mixture design will achieve the target binder properties in the blend by considering aged binder properties, virgin binder properties, and the proportions of these two binder components to be present in the final blend.  For mixtures with RAP contents greater than 15% of total mixture weight, a softer virgin binder grade is generally required to arrive at the target binder grade for the mixture containing RAP, Federal Highway of Administration (FHWA, 1997). Once the design is established, it is also necessary to ensure that the target RAP amount is not significantly deviated from during mixture production.  Too much RAP can lead to a brittle mixture, and too little RAP may lead to an excessively soft mixture (when a softer virgin binder grade is utilized).  In any case, deviations from target RAP amounts may also cause other deviations from mixture design targets, such as gradation, voids, and asphalt content, which can decrease pavement performance.

Therefore it is important to be able to quickly and accurately determine the amount of RAP used in a mixture. As part of a larger research project to devise a test that can identify the presence and quantity of RAP in Hot-Mix Asphalt pavements, this paper presents the results of a survey conducted to investigate the availability of a RAP detection and quantity determination method used by the various states. Before presenting the results of the survey, a brief literature review will be presented next.

Literature Review

An initial literature search for recycled or reclaimed asphalt pavements revealed close to 1,500 articles. In addition to the Reclaimed Asphalt Pavement (RAP) search, literature related to field management of Hot-Mix Asphalt (HMA) in general was searched for issues of importance to mix variability and reconciling differences between mix design and production. Agencies are constantly seeking to reap the benefits of utilizing RAP. Some of the advantages of utilizing RAP include the preservation of the existing profile, conservation of asphalt and aggregate resources, conservation of energy, and reduction in life-cycle cost.

Therefore, it is no surprise that state highway agencies have been moving toward increasing the percentages of RAP in their hot-mix asphalt pavements. While up to 80% RAP has been used in some hot-mix asphalt pavements (FHWA, 1993), 20-50% RAP is typically used (Flynn, 1992; Solaimanian and Tahmoressi, 1996). It should be noted that high percentages of RAP are not used in normal practice. Most highway agencies have noticed a significant reduction in project cost when RAP is used. The Florida Department of Transportation project showed a savings of 15–30% compared with the cost of conventional paving using all virgin materials (Page, 1988). These savings are due to lower bids by contractors who pass the savings on to state highway agencies and, ultimately, to the taxpayer. Rehabilitation of asphalt pavements sometimes requires the removal of old asphalt layers. If agencies and industry had not developed techniques to use RAP, asphalt pavement rehabilitation costs would significantly increase to account for the cost of disposing of nonrecycled materials. Thus, the effective use of RAP solves a larger societal problem in that the milled material is recycled rather than disposed of in landfills.

In addition to the economic benefits of using RAP, rehabilitation options that create RAP may have substantial engineering benefits. For example, the ability to mill and remove old, distressed pavements allows for more effective rehabilitation techniques. Severely cracked or rutted layers can be removed so that their damage is not reflected through a new surface layer (Page and Murphy, 1987).

The highway industry is placing more and more emphasis on pavement performance. Performance is the measurement of quality with time (or age). There is current concern that since RAP contains aged asphalt binder; it may not perform as well as mixes with virgin Binder. While this a valid concern, it has been shown in multiple studies that the structural performance of recycled mixes is equal and in some instances better than that of the conventional mixes  (Little and Epps, 1980; Little et al (Little et al., 1981; Brown, 1984; Meyers et al, 1983; and Kandhal et al., 1989).

Performance is not the only concern when using RAP. Material variability is probably the biggest setback when using RAP. Because RAP is removed from an old roadway, it may include the original pavement materials, plus patches, chip seals, and other maintenance treatments. Base, intermediate, and surface courses from the old roadway may all be mixed together in the RAP. Reclaimed material from several projects is sometimes mixed in a single stockpile, although this mixing is not encouraged. Mixed stockpiles may also include materials from private work that may not have been built to the same original standards. Because of variability concerns, some states limit the amount of RAP that can be included in new mixtures.  Fourteen states allow blending RAP material into one stockpile. This is mainly due to insufficient space in asphalt plants to store multiple RAP stockpiles. Some states allow the use of higher percentages of RAP if the material is milled off the same project where the new mix will be placed. However, some research has shown that the variability of RAP can be controlled and may not be as great as expected (Nady, 1997). Others show the high variability in RAP material greatly affects the variability of the asphalt content and gradation of the production mixture, especially at higher percentages of RAP (Solaimanian and Kennedy, 1995).

It has been demonstrated by Federal Highway Administration project No. 74, Field Management of Asphalt Mixes, that the volumetric properties of laboratory design and plant produced HMA are seldom alike (D’Angelo, 1991). The study also identified Voids in Mineral Aggregate (VMA) and Voids in Total Mix (VTM) as the two independent parameters that need to be studied. Kandhal et. al. used the data obtained from this study to provide practical guidelines for reconciling differences between volumetric properties of mix design and production (Kandhal et. al., 1996). They concluded that VTM is affected most by asphalt content, the percent passing No. 200 sieve, and the relative proportions of coarse and fine aggregates. VTM can be increased by reducing the asphalt content or the percent passing No. 200 or both. This conclusion, although it did not address RAP material, is valuable in addressing what the contractor might do to accommodate the additional RAP aggregate used. The use of a bigger percentage of RAP in the mix will require altering the proportions of the other aggregates and the asphalt content to meet the requirements for the Job-Mix Formula (JMF).

In summary the literature review has shown that all the answers are not yet available for mix design of RAP within the SuperPaveTM system. While the concepts appear to be sound, verification of the ideas need to occur. The literature also shows that the addition of the RAP aggregate in an asphalt mix affects the gradation, the binder grade and the binder content of the final mixture. However, most of these studies were performed in the laboratory. A state survey, described below, of the RAP practices also shows the same conclusion.

Table 1, State Survey Summary

States Survey

A state survey of RAP usage was conducted by Illinois Department of Transportation, IDOT, and the results are summaries in Table 1 as well as figure 1. The survey was conducted via e-mail and the response to five questions was collected. Survey was sent to the states and 23 surveys where returned, translating into a response rate of 47%.  The results show that although the majority of the state allow the use of RAP and do limit the amount of RAP used, most of the states do not have a method for checking the amount of RAP or for insuring that mix production is secure. A follow up phone survey was conducted to get more information about developing a possible test to detect RAP and to understand what are the different state practices regarding mix security.

The following conclusions can be drawn from the survey regarding RAP detection:

The following conclusions can also be drawn regarding mix security:

Figure 1, State Survey Summary

Summary and Conclusions

The main objective of this paper was to investigate any RAP detection, and RAP quantity determination methods.  It was found little has been done directly in these areas.  However, several other important points can be summarized from the literature and the conducted survey. It is apparent from the literature review that the effect of the aggregate gradation has to be separated from the effects of the blended binder. Furthermore, if more RAP is added to the HMA the proportions of the other aggregates will have to be altered to meet the Job-Mix Formula. It was shown that the binder in the RAP material blends completely with the virgin binder. Accordingly, accurate quantification of the effect of aggregate gradation and binder type and content will have to be performed in the developed testing methodology.

References

D. H. Little and J. A. Epps. Evaluation of Certain Structural Characteristics of Recycled Pavement Materials. AAPT, Vol 49, 1980, pp 219-251.

D. H. Little, R. J. Holmgreen, Jr. and J. A. Epps. Effect of Recycling Agents on the Structural Performance of Recycled Asphalt Concrete Materials. AAPT, Vol. 50, Feb 1981, pp 32-63.

FHWA, (1997) Guidelines for the Design of SuperPave^TM Mixtures Containing Reclaimed Asphalt Pavements, SuperPave^TM Mixture Expert Task Group. Federal Highway Administration, Washington, D.C.

E.R. Brown. Evaluation of Properties of Recycled Asphalt Concrete Hot Mix. U.S. Army Engineer Waterways Experiment Station, Final Report # CL-84-2, Feb 1984.

Frank Meyers, G. R. Tessier, Ralph Haas and T. W. Kennedy. Study of Hot Mix Recycling of Asphalt Pavements. Roads and Transportation Association of Canada, Report # TP 2964 E, 1983, Ottawa, Ontario.

Nady, R.M. “The Quality of Random RAP: Separating Fact from Supposition,” Hot-Mix Asphalt Technology. National Asphalt Pavement Association: Lanham, MD (1997).

D’Angelo, J. A. and Ferragut, T. (1991) Summary of Simulation Studies from Demspostration Project No. 74: Field Management of Asphalt Mixes. Asphalt Paving Technology Vol.60.

Kandhal, P. S., Foo, K. Y., and D’Angelo, J. A. “Field Management of Hot Mix Asphalt Volumetric Properties, ASTM Special Technical Publication 1209, 1996.

FHWA, Recycling of Asphalt Pavements Using at Least 80 Percent Recycled Asphalt Pavement (RAP), Engineering and Environmental Aspects of Recycled Materials for Highway Construction, Report No. FHWA-RD-93-088, Environmental Protection Agency and Federal Highway Administration, June 1993.

Solaimanian, M., and M. Tahmoressi, “Variability Analysis of Hot-Mixed Asphalt Concrete Containing High Percent Reclaimed Asphalt Pavements,” Presented at the 75th annual meeting of Transportation Research Board, Transportation Research Record, No. 1543, National Research Council, Washington, D.C., January 1996, pp. 89-96.

Solaimanian, M. and Kennedy, T. W., (1995) Production Variability Analysis of Hot-Mix Asphalt Concrete Containing Reclaimed Asphalt Pavement, Center for Transportation Research, Bureau of Engineering Research, The University of Texas, Austin, TX.

Page, G. C., “Florida’s Experience in Hot Mix Asphalt Recycling,” Hot Mix Asphalt Technology, Spring 1988.

Page, G. C., and K. H. Murphy, “Hot-Mix Recycling Saves Florida DOT $38 Million,” Asphalt, Vol. 1, No. 1, Spring 1987.