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NS766: Effect of using RAP on Gravel Roads

Problem and objectives

Recycled Asphalt Pavement (RAP) is a waste material that local agencies use to reduce costs and environmental impacts. One use of RAP is as a blending material with virgin gravel for roads. While this approach does support the reuse of the material, questions have been raised about the maintained functionality of the gravel roads, specifically regarding water permeability. There are also concerns about the resulting long-term performance when the oils dry out. 
To address these questions, this study will help agencies understand the positive and negative impacts of using RAP as a blending mix for gravel roads.  The research team will conduct the following:

  • Synthesize existing research on the use of RAP in gravel roads and shoulders.
  • Survey local agencies in Minnesota to understand who has used RAP for gravel roads and shoulders, the lifespan for existing gravel roads and shoulders, best practices on blend composition, and feedback on level of success.
  • Identify case studies and conduct drainage and plasticity tests on gravel roads with RAP to empirically showcase effectiveness, and identify results, for different oil contents tested over time to understand depreciated oil content.
  • Conduct a lifecycle cost analysis to address the expected benefits of material reuse in the short-term and any long-term maintenance costs related to this treatment. Address whether agencies should stockpile RAP (if they aren't already) and in what quantity to support a given number of miles of this treatment.
  • Identify other use cases by local agencies for RAP including but not limited to use as part of the aggregate base, reclamation, and use as backfill for culvert installations.
  • Detail findings and provide written guidance.

Suggested deliverables

  • Research report detailing study findings including drainage, oil content, and plasticity case studies
  • A brief guidebook detailing the performance of various RAP blends for gravel roads

Expected benefits

The numbers 1 and 2 indicate whether the source of the benefit measurement is from: 

  1. A specific research task in your project that supports measuring this particular benefit, or
  2. Implementation of the research findings (anticipating positive results)
  • Construction Saving: (1)
    • Determine if construction costs including materials, trucking, and placement decrease from this treatment.
  • Decrease Lifecycle Costs: (1)
    • LCA should consider unique costs with obtaining and placing recycled materials and any associated long-term maintenance costs.
  • Environmental Aspect: (2)
    • Consider this treatment relative to material re-use, trucking, and other potential maintenance treatments with environmental impacts.

Possible members for technical advisory panel

  • Michael Suska, Nicollet County
  • Andrew Enge, McLeod County
  • Nick Klisch, Cottonwood County
  • Jason DiPiazza, Lake County
  • Kory Johnson, Itasca County
  • Matt Stordahl, Stantec
  • Kyle Hoegh, MnDOT OMRR
  • Ed Johnson, MnDOT OMRR
  • Taylor Amiot, Red Lake County

Previous research

Selected Results from the TRID Database

Summary: Results are compiled from the databases named above.  Links are provided for full text, if applicable, or to the full record citation.  I completed my searches using the following terminology: RAP, Recycled Asphalt Pavement, Gravel.

Title: The influence of residual asphalt material in the mechanical behaviour of soil-RAP mixtures for use in paving.

International Journal of Pavement Engineering, Volume 25, Issue 1, 2024, 2407901
https://trid.trb.org/view/2487696
Abstract: The evaluation of Reclaimed Asphalt Pavement's (RAP’s) suitability for pavement layers, optimises resource use, reduces environmental impact, and supports cost-effective practices. In this sense this study aimed to characterise soil-RAP mixtures to ensure they meet the quality standards required for pavement applications. The study examined two types of soil mixtures: one soil-RAP aggregates and the other soil-virgin aggregates (VAM) to understand the impact of residual asphalt binder on aggregate particles. Various mass contents (30%, 50%, 70%, and 90%) were tested alongside conventional soil-Gravel mixtures with equivalent proportions and gradation of RAP and VAM. Additionally, the study investigated the effect of adding 2% cement on the performance of both recycled soil-RAP mixtures and conventional soil-Gravel mixtures. The analysis includes granulometry, Atterberg Limits, MCT (Miniature, Compacted, Tropical) classification, soil chemical characterisation and microscopic images, compaction in Modified Proctor Energy, CBR, Resilient Modulus tests and stress–strain analysis. The results have revealed that the residual asphalt binder, surrounding RAP aggregates, has reduced the soil mixtures’ strength for different RAP contents. However, the study demonstrated that the addition of cement rendered the soil-RAP mixture feasible, enabling the application of the studied mixtures of RAP to constitute subbase and base layers.

Title: Permeability of Granular Materials, and Blended Salvaged Granular Materials. [Project]. 

South Dakota Department of Transportation. Start date: 16 Oct. 2023.
https://trid.trb.org/view/2379666
Description: The Department does not currently have a clear understanding of how drainable a granular base material needs to be in a roadway section to optimize long-term pavement performance.  Furthermore, the Department does not fully understand how permeable the virgin granular materials used on projects are with our dense-graded material granular base sections.  There is also a lack of understanding of how permeability is affected by adding salvaged granular material once it has been blended into the virgin base course, Gravel cushion, or salvaged granular material. Based on the previous research project SD97-03 completed by the Department, our current practice is to blend Recycled Asphalt Pavement (RAP) into virgin granular material with a blend ratio of no more than 50% RAP. Recycled Concrete Aggregate (RCA) can be used in lieu of virgin aggregates if it meets the specified gradation for the aggregate for which it replaces. The expectation with this practice is that both materials (virgin and blended) will have similar drainage characteristics.  With a better understanding of the drainage characteristics of RAP produced by various methods (process-in-place, cold-milling, micro-milling, and crushing) and RCA, the Department could better optimize the use of reclaimed materials in roadway sections and be assured that proper drainage has been achieved in the design in the most cost-effective manner possible.  Currently, that clear understanding is missing from our granular base material designs and this research opportunity could provide that clarity and assure that the best use of tax-payer funds is being utilized.

Title: Effect of Temperature on Interface Characteristics between Compacted Recycled Asphalt Pavement and Geogrid.

Advances in Civil Engineering, Volume 2023, 2023, Article ID 4596750
https://trid.trb.org/view/2259947
Abstract: This study presents an experimental investigation of the interface behaviors of Recycled Asphalt Pavement (RAP) reinforced with biaxial geogrid. A series of direct shear tests were conducted on compacted RAP specimens with or without geogrid under different normal stresses (50, 75, and 100 kPa) and test temperatures (0, 20, and 35°C) using an improved temperature-controlled direct shear apparatus. The effect of test temperature on interface shear strength and strength parameters was systematically examined. Test results showed that shear stress versus shear displacement curves of RAP specimens with or without geogrid show strain-softening characteristics. Under the same normal stress, the curve of the reinforced RAP sample with a high temperature is always below that with a low temperature, which indicates the test temperature has an adverse effect on the strength of the reinforced RAP sample. The shear strength consistently increases linearly with the increase of applied normal stress for all RAP samples. The shear strength with geogrid is greater than pure RAP under a given test temperature and normal stress. The shear strength of RAP samples at lower test temperatures is higher than that at a higher temperature. Both apparent adhesion intercept of RAP-geogrid and cohesion of RAP show a decreasing trend with the increase of the test temperature and tend to be stable with increasing temperature. Both the interface friction angle of RAP-geogrid and the internal friction angle of RAP slightly decrease with the increase of test temperature. For RAP-geogrid, the value of the apparent adhesion intercept is higher than that for sand and Gravel with geosynthetics. The interface friction angle is close to that of sand and Gravel with geosynthetics. Therefore, RAP material can be well used as alternative backfill materials in lieu of natural aggregates such as Gravel and sand in geotechnical applications.

Title: Hydraulic and environmental impacts of using Recycled Asphalt Pavement on highway shoulders.

Construction and Building Materials, Volume 234, Issue 0, 2020, https://trid.trb.org/view/1664129
Abstract: An experimental program was carried out to evaluate hydraulic and environmental behavior of Recycled Asphalt Pavement (RAP) obtained from seven different roadways within the state of Maryland for their potential utilization in construction of highway shoulder edge drop-offs. Due to their common use in these edge drop-offs, graded aggregate base material, Gravel, and topsoil were included as control materials in the testing program. Hydraulic conductivities of RAPs were evaluated through a series of constant-head tests, while their leaching potential was determined through batch and column leach tests. Laboratory test results indicated that the hydraulic conductivity of Recycled Asphalt Pavement was comparable to that of natural aggregates with the gradation of a clean sand-Gravel mixture. The stabilized concentrations of all metals released from the RAPs during the column leach tests were below the water quality limits. The numerical analysis results revealed that, with increasing distance from highway shoulder, the metal concentrations in nearby surface waters decreased further.