Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/221550
Photo courtesy of Arkansas SH&TD RoadScience In Fayetteville Shale Play region in north-central Arkansas, Arkansas SH&TD is studying the use of roller-compacted concrete to accommodate heavy truck traffic at a substantial cost savings. Typically, RCC is produced in a pug mill or central batch plant and is transported by dump trucks. It's often placed by a robust, oversized European-style paver, or sometimes by a conventional asphalt paver. It's compacted by smooth-wheeled rollers and cured with water or curing compound, Luhr says. RCC is placed at a 4-inch minimum and 8-inch maximum depth, but sometimes 10 inches when used with Eurostyle heavy-duty pavers. "Adjacent lanes are placed within 60 minutes to maintain a 'fresh joint,' and multiple lifts [are] placed within 60 minutes for bonding," he says. Production should match paver capacity and, like all pavements, a continuous forward motion should be maintained for optimum smoothness. RCC is seeing use in more than just pavements and industrial parking and storage areas. Recently, RCC was used for a Catholic grade school parking lot overlay in New York City. The Wayne Companies of northwestern Pennsylvania and southwestern New York completed a 50,000-square-foot unbonded overlay of 4 inches of RCC directly on the existing asphalt, which makes it one of the few, if not the only, unbonded overlay projects using RCC in the country, notes the National Ready Mixed Concrete Association (NRMCA). By using a conventional asphalt paver and hauling the RCC in dump trucks, Wayne convinced the owners RCC could solve their situation more economically than tearing up existing asphalt and replacing with either new asphalt or conventional concrete. Despite RCC's low rate of shrinkage cracking, the decision was made to joint the pavement similarly to a conventional overlay to reduce the risk of reflective cracking from the old asphalt. The new RCC parking lot came in under the asphalt bid and should provide years of service due to the durability 6 December 2013 Better Roads of concrete, NRMCA says. Use of roller-compacted concrete in pavement construction is increasing, say Yoon-moon Chun, Tarun R. Naik and Rudolph N. Kraus of the Department of Civil Engineering and Mechanics, College of Engineering and Applied Science at the University of Wisconsin-Milwaukee, in their paper, RollerCompacted Concrete Pavements. "Roller-compacted concrete is a zero-slump, highly compacted concrete that is placed by equipment similar to that used in asphalt pavement construction," Chun, Naik and Kraus say. "RCC pavements may contain fly ash or other powder materials to increase the fines content and to fill voids between aggregate particles in RCC." Normally, use of air-entraining admixture provides a proper air-void system to prevent damage to the concrete due to freezing and thawing, they say. "Air-entraining admixtures have been added to RCC mixtures in laboratory tests and such concrete mixtures have performed satisfactorily," they write. "However, due to the nature of RCC, it is very difficult to provide a sufficient amount of entrained air in RCC mixtures." Instead, Chun, Naik and Kraus say the most common method of providing sufficient durability for RCC pavement against freezing and thawing is by judicious selection of mixture proportions, including a low water-cementitious materials ratio, a free draining base course material and achieving a high degree of RCC compaction (96 to 98 percent of maximum density), with the use of fly ash or other supplementary cementitious materials, or other materials that add fines to the RCC mixture. "Although the rate of strength gain of RCC is lower than that for conventional concrete pavement, the final strength is higher," the authors say. Compressive strength, at the age of 28 days, as high as 40 MPa (6,000 psi) is common, and the 28-day flexural strength can be 5.5 MPa (800 psi) or greater. It's been shown that RCC pavement compacted to 96 percent of maximum density is more than twice as strong as the same mix compacted to 86 percent. Because RCC pavement is much stronger and durable than asphalt pavement, Chun, Naik and Kraus add, RCC will not rut from high-axle loads, or shove or tear from turning or braking of operating equipment. It will not soften from heat generated by hot summer sun or material stored on RCC floors (for example, compost). "RCC pavement offers a substantial cost savings over conventional portland cement concrete and asphaltic concrete pavements when used in heavy wheel load applications," they