The parking lot at the Rio Verde Community and Welcome Center in southern Arizona was cracked and showing signs of age. A later-added median caused further fatigue failures. It was time for a change. The center wanted the clean, bright look of a concrete pavement, but milling the top half of the distressed asphalt, which typically is recommended for reducing reflective cracking and improving the bond between the concrete and asphalt, was going to be expensive.
They turned to alkali resistant glass fibers and an ultra-thin whitetopping to get the desired look and remain on budget.
The 4000-psi whitetopping mix, designed by Mike Kohout Engineering Services and Rinker Materials West Division Arizona Region, used ½-inch top-sized aggregate and 3 pounds of AR Glass Fibers from the Specialty Reinforcements Group of Saint-Gobain Vetrotex America.
The concrete pavement was placed at a depth of 3 inches and sawcut in 3-foot squares. However, a decision was made by the project managers to omit milling the top half of the distressed asphalt to save both time and money.
The key to the concrete's performance is the addition of the glass fibers. Acting as a bonding agent to the cement paste, the fibers absorb the stresses until the concrete has gained adequate strength. AR Glass Fibers have a tensile strength 2½ times greater than that of polypropylene and bond well to the cement matrix versus the poor bonding characteristics of polypropylene.
The use of AR Glass Fibers had no negative affect on the finished surface, said SRG senior sales representative Richard Hartwick. The fibers have a specific gravity of 2.68 and don't affect the concrete's finish like other fibers can. In addition, the high dosage rate of 3 pounds of 1½ inch fibers resulted a slump loss of only 1½ inches, about half of what would be expected from polypropylene fiber at the same dosage and length.
Another secret to the durability of thin concrete overlays is the tight joint spacing. The 3x3-foot sections address midpanel stresses and greatly reduce the likelihood of curling and drying shrinkage cracking.
After six months of service, the ultra-thin whitetopping continues to perform above the expectations, said James Patterson, SRG's regional sales manager. By placing the concrete directly over the distressed asphalt, some reflective cracking might have been expected by this time. To date there is no visible stress or reflective cracks. Many of the local residents in Rio Verde are so impressed that they're now paving over their asphalt driveways with the same concrete mix and 3-foot joint configuration.
For more information on the Specialty Reinforcements Group of Saint-Gobain Vetrotex America call 877-777-8072, or visit www.srg.saint-gobain.com.