Rubberized Asphalt Concrete (RAC)
The unit costs of RAC products are higher than those of conventional or polymer modified products. Generally RAC-G hot mixes cost about 20-25 percent more than conventional mixes, although this may vary with job size. Added material costs (rubber, asphalt) plus mobilization and set up of the asphalt rubber binder production equipment increases initial unit costs. Large projects may see some reduction in unit costs because mobilization costs can be spread over a greater RAC tonnage. In many cases, the reduced thickness of RAC overlays offsets all or much of the increase in initial unit cost.
Costs of RAC-O and RAC-O (HB) overlays are higher than conventional open-graded mixes due to the higher contents of binder. Since open-graded mixes are not a structural element, there is no reduction in thickness compared to conventional open-graded mixtures. However the higher initial cost may be offset by life cycle costs (mainly due to longevity).
RAC projects may not be cost effective with respect to initial cost for smaller projects. RAC is generally cost effective when used as thin (1.2-2.4 inches) gap- or open-graded surface courses or overlays, or in chip seals and interlayer applications.
The following examples are meant to demonstrate the potential cost effectiveness of RAC when used for specific projects, such as when resurfacing is a viable alternative to reconstruction or resurfacing with conventional asphalt concrete (AC).
These examples take advantage of the 2:1 equivalency factor using RAC in lieu of AC for resurfacing projects. The 2:1 equivalency factor is based on Caltrans testing and assumes a minimum thickness of 1-1/2 inches of RAC. The costs per ton shown here are more typical of costs in Southern California however, depending on the size of the project these costs could be achieved in Northern California.
Project Design Example #1
- Soil/deflectometer testing indicates that a 4-inch overlay of conventional AC is required.
|$158,400||= 1,584 tons @ $100.00 per ton|
|$12,000||= Pavement preparation|
|$94,250||= 754 tons @125.00 per ton|
|Note: RAC weighs 5% less than AC|
Savings Per Lane Mile Using RAC
$170,400 - $94,250 = $76,150
Project Design Example No. 2
- Soil/deflectometer testing indicates that a gravel equivalent (GE) of 26 is required.
- Existing structural section = 4" AC/12" CAB
- Existing GE = (4 x 1.4) + (12 x 1.1) = 5.6 + 13.2 = 18.8
Solution No. 1: Reconstruct Structural Section 4"AC/17"CAB
- GE = 4 x 1.9 + 17 + 1.1 = 7.6 + 18.7 = 26.3
|$164,280||= 4,107 CY @ $40/CY (EXC.)|
|$99,720||= 3,324 CY @ $30/CY (CAB)|
|$158,400||= 1,584 TN @ $100/TN (AC)|
|$422,400||= Cost Per Lane Mile|
- Advantages: New roadway elevation will be the same as existing.
- Disadvantages: High cost, long-term disruption of traffic during construction.
Solution No. 2: Resurface with 4" of Convention AC
- GE = 4 x 1.9 + 18.8 = 26.4
Cost Per Lane Mile (from Project Design Example No. 1) = $170,400
- Advantages: Minimal disruption to traffic.
- Disadvantages: Adding 4 inches to the existing roadway elevation may not be practical.
Solution No. 3: Resurface with Rubberized Asphalt Concrete - Cold Mill 1 inch, Add 2-1/2-inch RAC.
- GE = 2-1/2 x 1.9 x 2 + 18.8 - 1.4 = 26.9
|$25,200||= 63,000 sq. ft. x $.40 (Cold Mill)|
|$117,813||= 942 tons @ $125 per ton (RAC)|
|$143,013||= Cost Per Lane Mile|
- Advantages: Roadway elevation is only raised 1-1/2 inches. Minimum disruption to traffic.
- Disadvantages: None.
Summary of Costs Per Lane Mile
- Solution No. 1 = $422,400
- Solution No. 2 = $170400
- Solution No. 3 = $143,013
Note: If the cost of RAC exceeds the cost of AC, its appropriateness for the project should be evaluated against the advantages/disadvantages of the other solutions.
Using a RAC chip seal as a stress-absorbing membrane interlayer (SAMI) in place of a thicker layer of AC saves material costs and speeds construction, providing additional savings. In cases where reflective cracking governs overlay thickness, a SAMI also provides the benefit of reduced overlay thickness that paving fabric does not. RAC spray applications (chip seals, interlayers) may cost twice as much as similar conventional treatments, but they typically are cheaper than the equivalent amount of conventional AC hot-mix.
Life Cycle Cost Analysis
Cost effectiveness can also be evaluated using Life Cycle Cost Analysis (LCCA). Life cycle cost analysis is recognized by public agencies as an effective tool to assist in the selection of construction, rehabilitation, and maintenance treatments. The Federal Highway Administration (FHWA) has developed a LCCA approach which will likely become the standard in the industry. The approach can be used to evaluate the life cycle costs of paving materials containing asphalt rubber binders as well as alternate treatments. A manual has been developed that describes LCCA procedures to be used to ensure that pavement project alternatives are analyzed objectively and consistently statewide, regardless of who designs, builds, or funds the project.. The manual also provides step-by-step instructions for using RealCost, a computer software program developed by the FHWA. RealCost was chosen by Caltrans as its official software for evaluating the cost effectiveness of alternative pavement designs for new roadways and for existing roadways requiring rehabilitation or reconstruction.
CalRecycle is also funding the development of a life cycle cost analysis for RAC that will include asphalt-rubber and terminal blend materials and different rehabilitation strategies such as: chip seals, multi-layer rehabilitation and thin overlays. The study should be completed by summer 2012.
Durability, Safety, and Noise Reduction
When produced and constructed properly RAC has shown a greater longevity compared to conventional AC. RAC is a proven durable material that demonstrates increased performance in resistance to rutting and fatigue and reflective cracking. In addition to increased skid resistance, which improves traction, the semi-porous surface of RAC-G reduces the amount of spray from wet roadways, which helps improve visibility. The added benefits of improved durability, reduced maintenance demand, and longer service life provided by RAC materials should substantially reduce the overall life cycle costs and help offset the increase in initial cost.
Vehicle-generated noise comes from the power train (engine, exhaust system) and external factors including aerodynamic noise and tire noise. At speeds greater than 50 mph, pavement/tire noise dominates the other sources. Freeway traffic noise is equivalent in level to a household vacuum cleaner, about 70dBA.
Simply reducing the noise level by 3 dBA provides the same effect as doubling the distance between the source of the noise and the person hearing it, or in the case of traffic noise, reducing traffic volume by 50 percent.
The most common method to combat noise in residential areas is to construct sound walls along busy roadways. Sound walls do not reduce noise at the source and may bounce or reflect noise to other locations, and may increase noise to drivers passing between the walls. Sound walls are expensive to build and they often detract from the appearance of a given area, which may lower property values.
It is cheaper to address the noise issue first at the source by placing pavement material which absorbs more sound. Such materials include open-graded friction courses and stone-matrix asphalt, which has an aggregate gradation very similar to RAC-G mixes. Significant reductions in traffic noise, ranging from 40 to 88 percent, have been measured not only for open-graded but also for gap-graded RAC however, the initial noise reductions have shown to decrease over time with all pavement materials, not just with RAC.
The primary environmental benefits of RAC are the value-added reuse of scrap tire rubber and the diversion of the tires from landfill disposal or stockpiles. In addition, RAC typically is used at a reduced material amount compared to convention asphalt concrete, which requires less aggregates (mining) and asphalt (drilling) and less energy to produce.