It may not be rocket science, but today’s longer-lasting, more durable roads are the result of better science from improved concrete mixes to advanced construction materials and methods.
Industry professionals and academic researchers became concerned between 2000 and 2003 when isolated cases of early concrete deterioration were found in a few pavements that were less than 10 years old. After a thorough analysis conducted by the National Concrete Pavement Technology Center in 2005, the Michigan Concrete Association (MCA) published new guide specifications designed to address inconsistencies and deficient practices determined to have contributed to the premature breakup of a few pavements.
As a result of the research and the new guidelines, concrete pavements are now constructed with tighter process controls and with a better understanding of how the individual components in concrete work together. The changes have contributed to improved quality and concrete durability and even broadened construction uses for concrete.
Fighting with fabric
The resurfacing of 2 miles of Little Mack Avenue in St. Clair Shores, Mich., was an innovative project utilizing many emerging products and construction methods. The project demonstrates a cost-effective way to repair an existing pavement. The 3-mile segment of the arterial road was originally constructed in 1995 at a cost of $12 million. The original design was for a five-lane roadway with 9 in. of nonreinforced concrete constructed over 6 in. of a slagcrete stabilized base.
Within five years the pavement began to show minor stress, especially at joints where spalling became prevalent. Some repair work was initiated in 2005. This was accomplished mostly with partial-depth repairs and some surface patching. The roadway continued to deteriorate, and city engineers began looking for a long-term fix.
A complete rebuild and reconstruct was considered, but testing of the existing pavement showed that the bottom 6 in. of concrete was sound and the stabilized base was still performing well. Core samples of the 16-year-old pavement taken by the Michigan Department of Transportation (MDOT) showed the upper portion of the original pavement was deficient in air-content and air-bubble distribution. Premature cracking at the joints was caused by freeze-thaw damage and then exacerbated as additional moisture penetrated the concrete surface. City engineers and their consultant determined that resurfacing with a concrete overlay on a 2-mile portion between 10 Mile Road and 12 Mile Road was necessary. The strategy would include the use of a new nonwoven geotextile fabric interlayer to mitigate reflective cracking and to provide a drainage path for moisture that may get in through the joints.
“The mill-and-fill design we ultimately chose was the result of research and consultation with the Michigan Concrete Association, the consultant and MDOT,” said St. Clair Shores Department of Public Works and Water Director Bryan Babcock. “This heavily traveled section of highway was experiencing accelerated deterioration—we needed a long-lasting, cost-effective fix that would hold up under heavy traffic with minimal maintenance. After consultation, we were confident the design we selected would give us a durable product that would serve our community for many years.
The $2.3 million project began in mid-April 2011 and was completed and opened to traffic in September. The construction process called for the temporary removal of the inlet and manhole castings, milling 4 in. of the existing concrete surface, installing a 1?4-in.-thick fabric interlayer and placing a 4-in. dense-graded concrete overlay. The project was completed in two phases. Two lanes of traffic remained open during construction. Though only 2 miles of the original 3 miles of the 1995 pavement was reconstructed, it was accomplished at a fraction of the cost of the $12 million 1995 construction.
Milling in twos
Milling and filling has been used for years in the asphalt industry to extend the life of roads where a total reconstruction is not feasible. Recent studies and the early success of concrete overlay projects has confirmed the effectiveness of this strategy for concrete roads as well. However, because of the hardness of the concrete, milling techniques for concrete are somewhat different than asphalt. An ample water supply is needed to lubricate and cool the milling teeth in addition to controlling dust. Tooth wear is usually accelerated with concrete milling, and the hardness can lead to broken or uneven wear; typically the milling crew will replace individual teeth during each shift. Often a complete change of teeth may be required, even before they are completely worn out, to maintain a uniform surface texture.
Initially, on Little Mack, the approach was to mill the 4 in. of concrete in a single pass. While this approach worked, the milling crews found that by milling the 4 in. in two separate passes they could achieve a more effective result. This innovative approach resulted in a uniformly milled surface essential for an unbonded overlay.
The key is to separate the new concrete from the old and prevent any mechanical interlock between the two; the required thickness of the interlayer is dependent upon the uniformity of the old pavement surface. New teeth were installed prior to milling the second pass to assure a clean cut. When the teeth on the mill started to become worn, the surface displayed more irregularities. Rather than replacing teeth at this time, the mill moved ahead and started making the initial 2-in. cut on an adjacent section of pavement. When the teeth finally needed to be replaced, the mill moved back to making the final cut with its new set of teeth. This process produced an excellent surface finish and reduced construction costs by maximizing the life of the carbide milling teeth.
Following the milling process, the fabric was installed in segments staying just ahead of the paver. The fabric was kept wet to prevent it from drawing water out of the freshly placed concrete. The use of the fabric interlayer proved to be a cost-saving innovation for this project.
The milling of the pavement surface allowed the contractor to reconstruct the road to the existing grade. Since there was no appreciable increase to the pavement height, the side streets and driveways did not have to be completely reconstructed. The curb was replaced simultaneously as an integral curb placed along with the concrete overlay. This feature alone resulted in major savings and reduced construction time for the city.
The third step of the project was placing the 4-in. concrete overlay directly onto the fabric interlayer. The contractor on the Little Mack project used an optimized well-graded concrete mix design allowing the maximum use of aggregate and minimizing paste content. This created a dense concrete. In addition, 30% of the cement was replaced with slag cement. This improved mix provided a significant reduction in permeability compared with the existing concrete and a better air matrix for enhanced freeze-thaw durability. The overlay was unbonded, so the joints did not have to match up with the joints on the existing pavement. Joints were sawn into the fresh concrete in 5-ft panels and were sealed with hot-pour joint filler. Since the fabric underlayer will serve to drain water that finds its way into the pavement and because the concrete mix is much denser, the new concrete will be resistant to environmental damage.
The project was completed and opened to traffic with minimal disruption to residents and businesses. The completed segment is expected to last 20 years or more, and the cost savings in improved durability and minimal maintenance was a welcome feature for the city of St. Clair Shores.
MDOT participated in the Little Mack project as both observer and advisor. The first use of a fabric interlayer on a Michigan road drew the interest of the department because of its potential use on state-controlled roads.
“The Federal Highway Administration has shown increased interest in the use of the fabric interlayer,” said Andy Bennett, MDOT engineering technician in the Construction and Technology Division, Materials Section. “We are considering using this product on some of our composite jobs where a concrete overlay will be placed over existing concrete or a composite pavement. We were eager to see how well this material performed.”