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Use of Recycled Pavements Feasibility Study

The report concludes that the use of recycled materials is both financially and environmentally feasible. Download full study (pdf 3.1 mb)

2007 Callander Associates Landscape Architecture, Inc.

Background

Given the use of asphalt concrete and base rock in the construction of City trails, the trail network provides a potentially significant consumption market for recycled materials.

This study analyzes:

1. The feasibility of using recycled AC, PCC, crumb rubber, and base aggregate in the construction of the City's trail network, including a consideration of the following factors:

a. Current legislation related to the use of recycled materials.

b. Types of recycled materials.

c. Performance of recycled materials.

d. Potential effects on the local recycled materials market.

e. Regulatory agency policies regarding the use of recycled materials near riparian corridors.

2. The trail network's potential contribution to meeting the goals of the City's Construction and Demolition Recycling Program.

This study is intended to:

1. Help encourage the market growth of recycled materials production and consumption in San Jose.

2. Encourage City programs to participate in the City's goal of creating zero waste within the City of San Jose.

3. Inform Public Works (PW) and Park, Recreation, and Neighborhood Services (PRNS) Department Project Managers of the feasibility of using recycled materials for trails.

4. Act as a supporting document to grant applications that require demonstrations of "green" policies by PW and PRNS.

5. Support PW and PRNS efforts to use recycled materials for trails.

Findings

The use of recycled AC, PCC, crumb rubber, and base aggregate for construction of the

City trail network is feasible, but with some qualifications. These materials must be evaluated independently because they are used differently; vary in cost, availability, and in levels of performance; and have different potential environmental impacts.

Benefits to the Trail Program from the use of recycled materials include:

1. Potentially lower construction costs, depending on type and proportion of recycled material used.

2. A positive contribution to the environment through the reduction of materials transferred to landfills.

3. A positive contribution to the City's goal of eliminating waste by providing demand for recycled materials.

Benefits to the City's Construction and Demolition Recycling Program would be derived from the Trail Program's participation as a recycled material consumer. In this capacity, the Trail Program would help support the consumption market for these recycled materials.

For Further Study

Two of the broader environmental issues not addressed in this report but which warrant further study include:

1. Additional negative impacts of using virgin base aggregates.

2. Additional benefits of rubberized AC. Negative impacts of using virgin base aggregates that are not addressed in this study include: mining-related energy consumption, increased runoff, scarring of the landscape, and other disturbances. An additional benefit of rubberized AC that is not specifically addressed in this study is its role in the partial diversion of over 32 million scrap tires produced each year in California alone. The stockpiles of tires take up landfill space and create breeding grounds for mosquitoes which are sources of diseases that endanger public health such as West Nile Virus.

Types of Recycled Asphalt

"Recycled asphalt" refers to a variety of materials and methods, all of which contain some portion of recycled content derived from the re-processing of AC or PCC paving.These materials and methods are appropriate for two categories of work: rehabilitation and new construction.

Rehabilitation

Materials and methods used to rehabilitate existing projects, such as cold-in-place recycling or full depth reclamation, are best suited to roadway projects. On trails, the narrower pavement widths, sharper turning radii, steeper inclines, and proximity to sensitive habitats preclude the use of rehabilitation machinery; however, there are several types of recycled asphalt that can be implemented in new trail construction.

New Construction

1. Recycled Base aggregate

Recycled base aggregate uses reclaimed materials from demolition jobs as the base layer in an asphalt pour. The reclaimed materials are either PCC or AC. The PCC and AC are pulverized and sifted by size. Base aggregate can be completely virgin material, completely recycled materials, or a blend of the two. Caltrans currently specifies that a maximum of 50% of the base aggregates be recycled aggregate (Caltrans, 2006).

a. Costs

  • Virgin base material: $14/ton.
  • 50/50 blend: $12.50/ton.
  • 100% recycled base material: $11.50/ton.

b. Availability

It is becoming more and more difficult to find virgin material in the South Bay region and as such, suppliers are selling increasing amounts of recycled base aggregates.

c. Performance

There is a perception that recycled aggregate may contain unknown impurities which will compromise the performance of the material. A study by the University of California, Berkeley's Pavement Research Center shows that recycled base aggregate is stronger and stiffer than virgin material (Caltrans, 2001). This resulted in asphalt concrete paving constructed using the recycled aggregate as its base layer to have reduced elastic deflections, which helped to reduce fatigue cracking in the paving. The same study also found that the recycled base had a slightly higher shear resistance than Class II base, providing a more stable base for the asphalt paving.

d. Special Requirements

There are no special requirements for installing recycled base aggregate.

2. Recycled Hot-Mix Asphalt Concrete

Recycled hot-mix asphalt incorporates recycled AC with virgin AC at an asphalt plant. Typically the mixture uses a maximum of 30% (Calkins, 2006) recycled material. The incorporation of the recycled AC creates a considerable amount of air pollution. The more recycled content that is incorporated, the more air pollutants are released. As such, asphalt plants within the South Bay Area keep their mixes at a maximum of 15% of total weight as recycled content. Some plants prefer to keep even lower thresholds so they are not penalized for air pollution. Raisch Products typically keeps their mix at 8% recycled content.

a. Costs

There are generally increased energy costs when producing recycled hot-mix asphalt because it requires more heat to incorporate the recycled material (Calkins, 2006).

b. Availability

It is becoming more available, but is still not a common material on the market. Because air quality around the plant is reduced when high percentages of recycled content are mixed, significant production of hot-mix asphalt is unlikely to occur until this problem can be effectively addressed.

c. Performance - No information available.

d. Special Requirements - No information available.

3. Rubberized Asphalt Concrete

Rubberized asphalt incorporates rubber crumbs into the AC layer of the pavement. These rubber crumbs can be manufactured from recycled car tires. The rubberized asphalt is created in one of two different processes:

  • Wet Process: The tire crumbs are blended with hot asphalt cement before the cement is mixed with the aggregates.
  • Dry Process: The tire crumbs are blended with the aggregates before they are mixed with the hot asphalt cement.

a. Costs

Costs are dependent on the amount of rubberized asphalt being ordered. The small amounts required by a trail project would mean higher material costs.

b. Availability

Rubberized AC has been used successfully in the Bay Area; however, it is not yet used frequently enough to warrant local asphalt plants to produce a readily available supply. An asphalt plant could produce rubberized AC if a project's demand was large enough to make the production feasible for the plant. An individual City trail project would not create a large enough demand to make production feasible; however, if a separate construction project created enough demand, trail construction could be timed to occur simultaneously and the asphalt plant could produce rubberized AC for both projects. This method of timing has already been implemented by the City's Department of Transportation for a road project, Toyon Road from Penitencia to McKee.

c. Performance

Many studies indicate that rubberized AC lasts significantly longer than conventional AC. There is also agreement that it takes less rubberized AC to perform equally well as conventional AC. The Caltrans Highway Design Manual states that, for their construction projects, maximum thickness of rubberized AC is 2.5" and at minimum should be 1".

d. Special Requirements

There are several roadway projects around the South Bay Area which have installed rubberized AC. Contractors who have worked on these projects note that working with rubberized AC is different than conventional AC. The incorporated rubber increases the viscosity of the material making it "gummier". This "gummy" quality makes it more difficult to work with than conventional AC.

Summary of Findings

Recycled Base Aggregate

Cost: $14/ton

Availability: Readily available and increasing in availability

Performance: At least as well as virgin base aggregate Special Requirements: There are no special requirements for installation.

Recycled Hot-Mix Asphalt

Cost: No information available

Availability: Not readily available, but is increasing in availability.

Performance: Similar to conventional AC.

Special Requirements: There are no special requirements for installation.

Rubberized AC

Cost: $29 more per ton than conventional asphalt

Availability: Not readily available, but is increasing in availability.

Performance: Many studies have indicated that rubberized AC lasts significantly longer than conventional AC. AC layers do not need to be as thick as conventional AC.

Special Requirements: Difficult to install by hand because of the gummy nature of the material.

 

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