For the Cook County Highway Department, finding a durable, cost-effective solution for heavily traveled bus stops along Chicago's Western Avenue required creative thinking. Like other stops, these are constantly subjected to pushing, shoving, and rutting from the stress created by the starting and stopping of hundreds of buses every day. Sprinkle in tons of roadway salt and deep freezes in the winter, bake in searing heat in the summer, and you end up with asphalt bus stops that are in need of serious repair.
Much of the roadway consisted of asphalt layered over thick interlocking granite pavers or old paving bricks. As a result, the contractor couldn't count on an existing platform under the asphalt to provide uniform support for a concrete pavement. While replacing the bus stops with full-depth concrete pavement would provide a long-lasting solution, the cost of replacement was simply too high. Installing full-depth concrete bus pads would have cost around 20% of the total cost of some sections of roadway, while representing just a tiny fraction of the roadway surface itself. A better solution was needed. Fortunately. Wally Koss, superintendent of highways for Cook County, was willing to think outside the box to implement one.
Pavement repaired with ultra-thin concrete whitetopping typically offers a 15- to 20-year service life with virtually no maintenance, said Jimie Wheeler of the Illinois Chapter of the American Concrete Pavement Association (ACPA). The technique places a thin layer of concrete over existing pavement and results in a roadway with a service life greater than that of a new asphalt pavement.
Based on analysis of stresses created by the traffic on concrete and the recommendation of the ACPA, the Cook County Highway Department opted for this solution, but required added strength, due to the unusual demands of the project. They incorporated an innovative fiber reinforcement technology, along with more-closely spaced joints to minimize cracking. The project commenced in 2003.
Rather than having to excavate to place 10 or more inches of concrete, crews instead milled the existing asphalt to allow for placement of 4 inches of white topping at bus pad locations. The result was a simple, efficient process that streamlined the job and minimized cost and traffic disruption. Also, rather than paying up to $95 per square yard for the new bus pads, the job came in at $32 per square yard.
When heavy bus tires roll across the slab surface, heavy shifting loads are placed on the concrete. As a result, the 10x 100- foot bus pads were cut with 48-inch transverse by 40-inch longitudinal joint spacing to minimize uncontrolled cracking.
In place of conventional steel reinforcement or fibers, the crews turned to STRUX 90/40 synthetic macro fiber reinforcement, manufactured by Grace Construction Products, Columbia, Md. The product replaces welded-wire mesh, light rebar, steel fibers, and other secondary reinforcement methods in concrete slab-on-ground and pavement applications. The synthetic fibers offer savings over wire mesh and rebar, are not prone to corrosion, and require no additional labor to install because the fibers are mixed in and uniformly dispersed during batching and are therefore properly placed automatically.
In addition, handling and placing the fibers is safer than handling steel fibers or wire mesh. The product improves the durability and expected longevity of concrete. A high-strength, high-modulus, 3-D reinforcement, STRUX 90/40 allows for tighter crack control and better performance than other solutions.
Because the fibers were added at relatively high dosage rates to deliver the required strength, the addition of the fibers initially-raised questions for both the contractor and the producer on the job about the ease of placement and workability. Due to the unique performance characteristics of the synthetic fiber, however the job proceeded smoothly from producing to placing to finishing the concrete. The approach even enabled the simple process of using vibrating screeds to finish the concrete, rather than a paving machine as required in larger jobs.
The results were impressive in testing performed under the direction of Jeff Roesler, P.E., at the Department of Civil Engineering, University of Illinois at Urbana-Champaign. The effects of the repair have stood the test of time. Wheeler feels there are many more applications for this approach, including any intersections or road areas subject to turning, stopping, starting, and heavy loads; feeder and frontage roads; arteries carrying trucks; and any other high-traffic areas subject to heavy loads.
For the Cook County Highway Department, this innovative solution to repairing asphalt pavement in bus pads was just the approach they needed, saving both money and time—today and down the road.
— Mark Kennedy is with Grace Construction Products.