Final Report: Evaluation of the 100% Recyclability of Superpave Hot Mix Asphalt

EPA Contract Number: EPD11056
Title: Evaluation of the 100% Recyclability of Superpave Hot Mix Asphalt
Investigators: Ali, Hesham
Small Business: H.I.P. Hot-In-Place Paving LLC
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: March 1, 2011 through August 31, 2011
Project Amount: $80,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2011) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Green Buildings

Description:

At 2.7 million miles of paved roads, the United States has the largest network of roads in the world. The mainstream approach to pavement rehabilitation has been to mill the deteriorated asphalt and replace with new asphalt mix. The hot-in-place pavement recycling process has the potential of replacing the milling and resurfacing process with substantial financial and environmental savings. Hot-in-place pavement recycling restores the pavement to its original condition. The process was improved and refined over the past several years to produce the following outstanding benefits:

  1. About 50% cost savings over the conventional milling and resurfacing, as indicated in FDOT’s published report on SR 471 HIP recycled pavement. This is valued at $18 billion savings a year for the United States.
  2. Eliminates 90% of the emissions resulting from pavement milling and resurfacing process.
  3. Eliminates the growing piles of asphalt millings that threaten to pollute water.
  4. Eliminates the need to haul materials from the plant to the road and haul asphalt millings from the road to the plant. This reduces traffic at construction sites and resulting pollution.
  5. Eliminates the need to mine new materials – an energy-intense operation that involves explosives.
  6. Significantly cuts the needs for new Asphalt Cement and reduces reliance on foreign oil. Approximately saves 15 million tons of asphalt cement, valued at $8 billion a year.
  7. Test results of recycled mix showed that the recycled asphalt had more than double the required stability – excellent indication of strength and durability.
  8. Reduces down-time of pavements being restored.

In 2004, the Florida Department of Transportation evaluated the hot-in-place pavement recycling and performed a side-by-side comparison with conventional milling and resurfacing. FDOT concluded that the process was a viable option, but expressed concern that the process may not yield a Superpave equivalent mix and suggested a further evaluation. This proposal develops an experimental design of three factors (nominal mix size, aggregate type and mix function) and conducts laboratory tests to evaluate the hypothesis that a deteriorated Superpave mix can be successfully recycled to meet a long-term performance similar to that of the virgin Superpave mix. DOTs started using Superpave mixes in the late ‘90s. Many of those roads are now eligible for resurfacing. If the evaluation is successful, it would allow DOTs to proceed with trying the process on Superpave mixes.

Summary/Accomplishments (Outputs/Outcomes):

The experiment conducted in this study was aimed at evaluating a variety of old Superpave mixes for 100 percent hot in-place recycling. To that end, four mixes were obtained, and the in-place properties of the mixes were determined. The optimum amount of Rejuvenator and screenings were determined. The volumetric analysis was performed to ensure compliance with Superpave requirements. The four mixes then were tested and evaluated for rutting, moisture damage, and cracking performance. A life-cycle cost analysis and emissions analysis were performed to evaluate the monetary and environmental savings of hot in-place recycling. The results of the study are summarized below.

 
1.   In general, there is a good match between the gradation of the sample and the average quality control
     data. The exception to this is Mix FC 12.5, which showed significant difference in gradation while mostly  
     meeting the spec requirement. It is noted that the variability of the QC data also was large. Another factor
     that can explain this variability is contamination while sampling. The FC 12.5 was sampled after exposing
     the FC 5 layer on top of it. At the interface of the two layers, fines (dirt) had accumulated. FC 5 friction
     course is used to drain water and has a tendency to catch fines as water evaporates.
2.  All mixes substantially met the gradation requirement with the exception of one or two sieves. On project-
     level design, changes can be made by mixing two layers or adding aggregate to fill in the gap. It is noted
     that in this experiment, no attempt was made to correct gradation. Blending aggregate to correct 
     gradation is a mathematical blending process that commonly is used in mix design and not a research 
     issue.
3.  With respect to FC 5, no attempt was made to recycle this mix to FC 5 due to the small thickness.  
     HIP Paving advocates blending the FC 5 (0.75 inch) with underlying SP 12.5 (0.75 inch) mix to
     produce a hybrid mix.
4.  The Fine Aggregate Angularity (FAA) test was conducted to quantify the change in aggregate shape
     during the hot in-place recycling process. The change in the FAA values is very minor and less than
     1 percent.
5.  Two types of Rejuvenators were used. One was more effective than the other in softening the asphalt.
     The more effective Rejuvenator was used in the rest of the study. This underscores the importance of
     Rejuvenator-binder compatibility.
 6. The recycled mix met the Superpave Performance Grade. The amount of Rejuvenator was adjusted to
     achieve the binder PG requirements. The binder also met the penetration range specified in FDOT
     Specs for reworked Asphalt Concrete.
7.  Inclusion of lightweight screenings was effective in increasing the air voids in all four mixes. All mixes had
     air voids near or at the 4 percent requirement of Superpave mixes.
8.  The Hamburg test showed that all four mixes had a good rut performance with maximum rut depth of
     4.5mm at 20,000 passes. Compared to a Texas study, these results exceed the performance of
     conventional mixes.
9.  The Hamburg test showed that all four mixes did not exhibit a stripping inflection point. This is a very
     good result, indicating that all recycled mixes were not susceptible to moisture damage. 
10.The Indirect Tensile Test and the Dissipated Creep Strain Energy criteria were used to evaluate mix
     cracking performance. The Energy Ratio ranged from 2.6 and 3.95, clearly larger than the critical value
     of 1.0. The DCSEHMA ranged between 5.2 KJ/m3 and 7.9 KJ/m3. This is clearly a very good crack
     performance for all recycled mixes.
11.The Life Cycle Cost Analysis based on SR-471 in Florida showed a life cycle savings of 40 percent over
      conventional milling and resurfacing. Analysis of emissions and petroleum usage showed that the hot
      in-place recycling process saves 72 percent of the emissions and 74 percent of the petroleum material
      used in the conventional process.

Implications

HIP Paving is encouraged by the outstanding results established in this study. Efforts should be made to promote the process for the significant cost and environmental savings it produces: 40 percent cost savings, 72 percent emissions savings, 74 percent petroleum material savings, and 100 percent reuse of existing material.
 
These savings are significant and can annually save the United States $18 billion in highway maintenance costs, and thousands of tons of pollutants. Further, it can reduce reliance on fossil fuels by eliminating the need to import 15 million tons of asphalt cement annually. These savings are not coming at the expense of quality. In fact, some of the test results showed superior results compared to virgin mixes.
 
Among the obstacles to use of this process are the localized failures resulting from:  equipment failure to replicate laboratory success and applying the process without proper mix design. HIP Paving established the following as urgently needed efforts to promote the process:
 
1. Refine the design method to optimize the air voids and binder PG grade for best performance. In
    this study, the Superpave criteria were used. It is believed that the design approach for virgin
    mixes may not be optimized for recycled ones. For instance, the addition of screenings to   
    increase air voids to 4 percent may have the side effect of diluting the asphalt content and
    reducing the mix durability. Would 2.5 percent air voids be better?
2. Quantify the environmental aging of the recycled mix. Although accelerated load testing is done to
    evaluate load effects, the aging process happens over time and under the effect of the
    elements, leading to the binder aging. More work is needed to evaluate the effect of the
    Rejuvenator on binder aging. Use of field testing facilities such as the NCAT and West Track
    facilities should be attempted. Asphalt aging equipment, at PRI and other advanced labs, should
    be used to establish aging curves, allowing the designer to more accurately estimate the design
    life of hot in-place recycled mixes.
3. Refine the process with the inclusion of fibers and binder additives to improve crack resistance,
    and slow aging.
4. Refine the equipment to replicate the laboratory success. This includes an efficient heating
    system, a precise rejuvenator applicator, and a thorough mixing system to get the job done.