Article based on a webinar originally presented during the International Grooving & Grinding Association (IGGA) Technical Sessions, Dec. 2020. To watch the full presentation titled “Concrete Pavement Preservation – The Phoenix Diamond Grinding Story”, please click on image below, or go to: www.forconstructionpros.com/concrete/article/21389918/igga-international-grooving-grinding-association-the-phoenix-highway-concrete-pavement-preservation-diamond-grinding-pilot-program#Phoenix_Diamond
Click to view VIDEO on YouTube
Diamond Grinding Pilot Projects Show Promise as Cost-Effective Fix for Phoenix Highways
Returning Arizona highways to Portland Cement Concrete (PCC), and finishing them with diamond-ground surfaces, is a cost-effective alternative to the standard mill and fill. The outlook suggests a $3.9 billion reduction in maintenance costs over a 30-year period. Phoenix is the fifth most populous city in the U.S., and it is growing. All 435 miles of the city’s road and bridge network, most of which consists of four-lane highways with additional high-occupancy vehicle (HOV) lanes, is maintained by the Arizona Department of Transportation (ADOT)-Central District. Phoenix-area residents expect smooth, beautiful, quiet roads, but due to explosive population growth in the region and insufficient funding, pavement conditions across the area have been declining over the last decade. ADOT is examining creative solutions for maintaining its highway network and delivering the kind of roads the public expects. With a $500-million-per-year construction budget, the district typically conducts 20 to 25 construction projects per year on the high-volume road network.
Arizona’s Pavement History
Click to enlarge: PHOTOS by Randy Everett, Senior Division Administrator-ADOT, Central District
& the ADOT:
1: IRI 2018 2: After testing with diamond grinding, NGCG, and finishing concrete surfaces
with a skid abrader or micro-milling 3: Not all pilot projects have been completed
Arizona has used concrete pavement since the 1920s. For Arizona, 40 years of success with concrete pavements led to its use when the state’s main highways were being built in the 1960s (Typical concrete lasts for decades and requires maintenance only every 20-25 years). In the 1960s, paving with concrete meant making trade-offs when it came to road noise, as the concrete surface was finished by tining (pulling a metal rake across the concrete surface perpendicular to traffic flow), so tires over tined concrete generated a noisy, tonal whine. In 2003, to address the noise, a 1-in asphaltic rubber friction course (AR-ACFC) was applied on top of the PCC which delivered a consistent look, and quiet and comfortable travel. However, preserving the AR-ACFC surfaces has proved to be a challenge.
Since AR-ACFC has a design life of approximately 10 years, Arizona’s highways have exceeded that design life and are showing their age. Related issues of a 1-in depth of delimitation (asphalt separated from the concrete underneath) are regarded as serious problems for drivers and ADOT—such as perceived potholes by the traveling public.
Arizona measures pavement condition by the International Roughness Index (IRI), and numbers have been trending down. (The IRI is a roughness statistic that is almost universally used by state agencies as a performance metric for their pavement management systems, with lower numbers, and therefore less roughness, being desirable). According to IRI measurements taken in 2010, 72% of the area’s interstates were in good condition. By 2018, that percentage dropped to 53%. National Highway System routes are also trending down, with 2010 measurements showing 68% of the highways in good condition and the number falling to 35% by 2018.
In April 2019, with costs mounting and budgets not keeping up, ADOT started to look for better preservation options. It is increasingly important for the department to keep existing pavement in good condition, and pavement within the 10-to-15-year design life window was still in good condition. Options included:
• Crack sealing the pavement
• Fog sealing (applying a light coat of diluted, slow-setting asphalt emulsion)
• Repairing individual potholes
For pavement past its design life, one option was to mill off the deteriorated asphalt and put down a new rubberized asphalt overlay. This, however deterrents to any solution requiring repeated milling of the asphalt surface were:
• Cost-prohibitive due to the extent of the area needing repair
• Milling equipment inevitably impacts and damages the underlying concrete
• Joints experienced particularly severe loss of concrete material
SOLUTION? What if Arizona went back to its original PCC surfaces? ADOT knew the state’s concrete highways were designed to offer a long life of structural stability. The department realized that if they could use the underlying PCC, making improvements to its original tined surface that would result in:
• A quiet roadway
• A rideable roadway
• May last a lot longer
• Require much less maintenance than asphalt
A Return to Concrete Pavements: Pilot Studies
An AR-ACFC layer had already been removed as part of a project being conducted on SR 101 in the northern part of Phoenix, so ADOT installed 4 test sections to explore future solutions that would take advantage of the existing concrete while ensuring a smooth, quiet ride and good pavement appearance. The department reviewed:
• Diamond grinding
• Next Generation Concrete Surface (NGCS)—a texture with a controlled land profile and an overall negative or downward texture developed to be a particularly low-noise concrete pavement surface
• Finishing concrete surfaces with a skid abrader—process in which a machine directs ball bearings at the pavement to improve friction characteristics, or with micro-milling
After testing each method, ADOT determined that the longitudinal texture achieved with diamond grinding offered:
• A pleasing corduroy appearance
• A nice ride (as indicated by low IRI numbers)
• Low sound emissions.
• Gentle enough to protect areas of the concrete, especially around joints, that had previously been damaged by milling
• A cost-effective solution!
In May 2019, ADOT decided to diamond grind a short section of SR 202 in southern Phoenix. After project was complete, ADOT representatives witnessed drivers moving from the AR-ACFC lanes to the diamond-ground concrete lanes to experience the improved ride and sound characteristics.
During the same May 2019 timeframe, a Maricopa Association of Governments (MAG) report came out assessing the noise and rideability of local freeways. The report:
• Acknowledged the difficulty of maintaining the road network
• Focused on the noise characteristics of the road
• Key takeaway was since the noise benefits of AR-ACFC vanish so quickly, “consideration should be given to replacement of the rubberized asphalt overlay with a diamond ground treatment.”
• Concurrent development was a maintenance procurement contract that allowed diamond-grinding contractors to be brought in for maintenance as well as for construction
In February 2020, ADOT took the next step by scheduling a bigger diamond-grinding project on SR 202: This project covered a 4-mile section four lanes wide with ramp sections included. The area was too big to remove asphalt using a loader, so a milling machine was used. The department cut strips in the rubberized asphalt to be used by the milling contractor as a guide during the milling process. By allowing the operator to more easily see the PCC surface below the overlay, milling equipment could be adjusted and damage to underlying concrete was minimized. The final diamond-ground surface of SR 202 met all ADOT’s criteria:
• Quiet
• Smooth
• Attractive
• IRI numbers in the 20s and 30s—comparing very favorably to asphalt surfaces. These comparable noise characteristics were achieved because, while new AR-ACFC pavement is about 5 dB quieter than diamond-ground surfaces, by the time AR-ACFC pavement is 5 to 9 years old it is measuring at comparable or higher dB levels. Noise levels on the new SR 202 diamond-ground surface were low for drivers, and the pavements generated less noise in nearby neighborhoods.
The success of their early projects prompted ADOT to begin 3 other diamond grinding pilot projects. ADOT will be tracking the performance of all projects to see how joints are holding up, whether the concrete spalls and how well sound and ride quality are maintained. Even though not all pilot projects have been completed, data collected so far justifies consideration of diamond grinding as an alternative to the standard mill and fill.
By using existing concrete pavement in Phoenix, not only will maintenance budgets stretch farther and allow more roads to be kept in good condition, but concrete’s durability ensures that interruptions associated with road repair will be minimized. Most importantly, the public will be able to enjoy driving surfaces that are smooth and quiet.
For the details of the May 2019 ADOT southern Phoenix diamond-grinding project; the 3 post-2020 diamond-grinding pilots; all costs details; and the entire original article in “ForConstrucitonPros.com” article titled “Diamond Grinding Pilot Projects Show Promise as Cost-Effective Fix for Phoenix Highways”, please go to: www.forconstructionpros.com/concrete/article/21389918/igga-international-grooving-grinding-association-the-phoenix-highway-concrete-pavement-preservation-diamond-grinding-pilot-program#Phoenix_Diamond