When its parking lot showed signs of poor condition further disrupted by the addition of a median after asphalt placement, the Rio Verde, Ariz., Community
When its parking lot showed signs of poor condition further disrupted by the addition of a median after asphalt placement, the Rio Verde, Ariz., Community and Welcome Center turned to ultra-thin white topping reinforced by alkali-resistant glass fibers. Community center patrons were prompted by Marlin Knutson, P.E., retired president of the American Concrete Pavement Association and a civil engineer, to consider a concrete solution satisfying both aesthetic and housekeeping concerns. Concrete pavement not only fit the cost allotment, he observes, but its clean surface also prevents tracking of oil into the center.
A 4,000-psi mix, incorporating _-in. top-sized aggregate and 3 lbs. of Saint-Gobain Specialty Reinforcements Group (SRG) alkali-resistant (AR) glass fibers, was designed by Mike Kohout, Engineering Services/Rinker Materials-West Division, Arizona Region. The pavement was placed at a depth of three inches and saw cut in 3-ft. squares. Although milling the top half of distressed asphalt is typically recommended for improved concrete bonding to the surface, as well as to reduce reflective cracking, a decision was made by project managers to omit this step for time and cost savings.
Key to the concrete’s performance is the addition of AR Glass Fibers, SRG representatives affirm. The fibers act as a bonding agent to the cement paste, they explain, absorbing stresses until the concrete has gained adequate strength. AR Glass Fibers reportedly have a tensile strength 2.5 times greater than that of polypropylene. The durability of thin concrete overlays depends also on tight joint spacings. The 3- _ 3-ft. sections address mid-panel stresses and greatly reduce the likelihood of curling and drying shrinkage cracking.
Two of our biggest concerns in using 3 lbs. of 1.5-in. length fibers were the negative effect on the finished surface and loss of slump created by the high addition rate, notes SRG Senior Sales Representative Richard Hartwick. Those concerns proved unwarranted, however, as AR Glass Fibers have a 2.68 specific gravity, which leaves the concrete finish unaffected. Furthermore, Hartwick adds, We experienced a 1.5-in. slump loss Û half of what would be expected from polypropylene fiber at the same dosage and length.
After six months of service, the ultra-thin white topping exhibits performance exceeding design team expectations, says SRG Regional Sales Manager James Patterson. Because concrete was placed directly over distressed asphalt, signs of reflective cracking were fully anticipated. Yet, to date, no visible stress or reflective cracks are evident. Patterson concludes, The performance of the concrete speaks for itself. Many local Rio Verde residents are so impressed that they’re now paving over their asphalt driveways with the same concrete mix and 3-ft. joint configuration. Û www.saint-gobain.com