Since the late 1980s the Arizona Department of Transportation (ADOT) has been placing asphalt rubber-asphalt concrete friction course (AR-ACFC) mixes over existing portland cement concrete pavements (PCCP). The initial intent of the overlay was to restore smoothness and assist in the redevelopment of skid resistance of the riding surface. The AR-ACFC mix proved to be outstanding in retarding reflective cracking of the PCCP jointing, with many mixes serving 10 years or more without significant maintenance costs.
As more of the surfacing was placed, public interest with considerable general approval motivated ADOT in 2003 to begin placing the AR-ACFC as a noise mitigation strategy. Addressing a quality-of-life issue through a paving strategy was innovative, and taxpayers have been very supportive of the program. The ADOT Quiet Pavements Program, as it came to be known, strives for a reduction from roadway noise of 4 to 6 dB in adjacent neighborhoods. As of summer 2006, approximately 100 miles of a total of 120 metropolitan freeway miles have been overlaid in the greater Phoenix urban area, and about an additional 50 miles of metropolitan freeways have been surfaced similarly in other separate projects.
Asking the SMART
With such a dramatic change in the coloration and porosity of a significant portion of the urban fabric, researchers at the Arizona State University’s National Center of Excellence on SMART Innovations for Urban Climate and Energy (www.asuSMART.com) began to ponder the possible unintended consequences of this paving program. Like many modern urbanized areas, the Phoenix metropolitan area is experiencing a growing problem with urban heat island (UHI) effect. Mostly a nighttime phenomenon, UHI is characterized by daily low ambient temperatures elevated by as much as 6°F to 8°F as compared with the surrounding undeveloped lands. These higher temperatures lead to many unfavorable consequences such as increased energy demands, greater greenhouse gas emissions, increased water usage and enhanced health risks.
While some wondered if the conversion of this many lane-miles from a light-colored PCCP surface to a dark asphalt surface might be aggravating UHI, ASU civil engineers assisting in this research also hypothesized there may be unidentified benefits from this paving strategy.
It is well known that differences in temperatures between the surface of the PCCP and the bottom of the PCCP induce daily curling stresses that can be very damaging and diminish the service life of the pavement. As the PCCP curls from the temperature differential, a loss of contact with the subgrade magnifies the damage resulting from traffic loadings.
An asphaltic concrete overlay, even less than 1 in. in thickness, might serve as a thermal blanket for the PCCP and mitigate the daily temperature differentials. Anything that would extend the service life of the pavement could result in substantial long-term savings to the department. Also, it was postulated that the high void structure of the AR-ACFC overlays, nearly 18%, might invest greater efficiency in the dissipation of heat from the pavement surface, actually helping to mitigate the pavements’ contribution to UHI in certain circumstances.
The Ray to go
ASU’s ongoing investigation into UHI has gathered a large amount of data documenting the pavement temperature behavior of roadways in the Phoenix area. During a three-year period, over 100 recording sensors were placed in the metropolitan area as installation opportunities arose. Working with the local road construction industry, sensors originally manufactured by the Transtec Group for maturity metering but suitable for hot-asphalt concrete placement were installed by paving and construction crews in freeways, local streets and even airport runways and taxiways. Although the data was useful in characterizing various aspects of the UHI, side-by-side instrumentations to answer questions about the effects of the AR-ACFC overlays of PCCP were not developed in the early part of the research work.
The early sites were useful in providing assessment of factors influencing UHI, such as grade and elevation of the different road segments and the surrounding landscape or urban development, but a major concern for many of the previous sites evaluated within the UHI study was the lack of equal traffic effects over the sensor areas.
As stated before, it was theorized that the high voids of the AR-ACFC would most efficiently dissipate heat when subjected to aeration by traffic. To capture this important aspect, a new site that addressed this issue and others was identified to be very valuable to the overall research effort.
Interstate 10 at Ray Road in the Phoenix area offered an opportunity to create excellent modeling conditions for quantifying the effects of AR-ACFC overlays of PCCP. The I-10 pavement north of Ray had been overlaid with AR-ACFC as part of the Quiet Pavements Phase 3 project in May 2005. The section to the south of Ray Road remains without the AR-ACFC overlay, although it is scheduled for a future Quiet Pavements overlay phase in 2008. With two test sites within the vicinity of the start of the overlay, one within the AR-ACFC overlay area and one with only PCCP, data could be generated for both conditions with similar, if not identical, traffic. Also, each site would include sensor placements located in the shoulder areas of each condition allowing a matrix that included data from areas with and without traffic.
In May 2006, the Ray Road I-10 test site was constructed. The existing PCCP was cored, and sensors were placed using dowels to ensure top-to-bottom spacing. Saw-cut lines were then made from the cores to the shoulder for the sensor leads so that future data collection could be made without impeding traffic. The cores were filled using a concrete repair material and the saw-cut lines were sealed with joint sealant including the use of backer rod. The costs of the installation were shared by industry, ADOT and ASU, with each donating in-kind services.
By design, the Ray Road test site provided data to isolate various factors that contribute to the thermal behavior of PCCP pavements. The matrix in Table 1 illustrates the data structure where the confounding factors of traffic and an overlay of AR-ACFC over the PCCP can be isolated. Sites 2 and 5 are replicates of 1 and 4, respectively.
The straight line on curling
Data was recovered for the month of June 2006, and two days were examined in-depth. The day of June 21, the summer solstice, provided data for the longest daylight of the year. June 25 recorded a monthly high of 108°. Therefore, data from June 21 and June 25 was selected for a shorter version of the analysis. Figure 1 illustrates a typical thermal signature showing that as the depth in the PCCP increases, the difference between the daily high and low mediates. This is data from June 21 for a PCCP section overlaid with AR-ACFC and subject to traffic. The depth of the sensor embedment in the PCCP is recorded in the respective legend data.
As the data was analyzed, several effects became clear. First, the aeration from traffic is a significant factor in mitigating the temperature cycles the typical PCCP section experiences throughout the day. With the added benefit of the highly porous surface of the AR-ACFC, the PCCP immediately below the AR-ACFC interface was a few degrees cooler than its counterpart in the PCCP section without the AR-ACFC. Also, the overall span of temperature differentials throughout the daily cycle was diminished. Figure 2 shows an example comparison of the temperature differentials (Delta T) for June 25, 2006.
Using established equations for curling stress calculations based on known temperature differentials, it was possible to quantify the curling stresses in each respective section. The section with traffic and without the AR-ACFC overlay experienced daytime-induced stresses on the magnitude of 25% greater than the section with traffic and with the AR-ACFC overlay. Nighttime values for the section without AR-ACFC were about 8% higher. These calculations were valid only for June 25, the hottest day of the month. A similar analysis was done for more moderate days of the month and a 12% to 15% reduction in daytime stress for the section with the AR-ACFC overlay was still noted. The effect of traffic aeration was significant and, without traffic, the section with the AR-ACFC overlay showed nighttime curling stresses 25% higher than the one without the overlay.
Handling the heat
There is no dispute that the darker AR-ACFC surface color increases the surface temperatures of the pavement during daytime. However, the nighttime UHI effect showed a benefit of using the AR-ACFC overlay in reducing the pavement surface temperatures due to the porosity and the lower thermal mass of the layer. An important consideration is subjecting these surfaces to traffic, which provides the necessary aeration effect.
Furthermore, AR-ACFC overlays have been shown to greatly reduce the induced stresses in PCCP due to thermal gradients. When coupled with the aeration effect of traffic, an AR-ACFC overlay can reduce the daily stresses in extreme climates from thermal gradients by as much as 25% during the heat of the day and by about 8% in the nighttime lows. Both can translate to an extension of several years in the service life of the pavement. The effects of traffic are shown to greatly reduce the magnitude of the thermal gradients. The data supported the theory that the high void structure of the AR-ACFC, an open-graded mix, imparts a significant efficiency in dissipating heat from the structure.
The concept of a thermal blanket of asphaltic concrete material over a PCCP shows promise in extending the service life of the pavement as a preservation strategy. Further studies are needed as future data is generated regarding the true economic effect of this strategy, and this aspect should be factored into decision making as well as the original benefits of increased skid resistance, enhanced rideability and roadway noise reduction.