In the time it takes to watch a movie on TV, it is now possible to mix, place, finish, and reopen full-depth slab replacements to highway traffic.
In June, the Virginia Department of Transportation, in conjunction with Transfield Services, Corman Construction, and mobile volume mixer truck manufacturer Heartland Concrete, did just that. It was an opportunity to evaluate a new type of ultra-high performance, non-portland cement. The project also was ideal to leverage the just-in-time flexibility of mobile volumetric mixer trucks.
Just-in-time was key in this case. Road departments are striving for new products and developments that allow them to make repairs and then quickly reopen the thoroughfares to traffic. Motorists don't have the patience to detour or to be held up in traffic while a section of road is being fixed. Quick road repair projects will become more widespread as money from the $787 billion American Recovery and Reinvestment Act, or the stimulus bill, trickles down to localities.
Researchers have developed a non-portland cement to specifically address this need for rapid, large-volume repairs. A liquid admixture is incorporated into the mix of cement and aggregates, as any admixture would be. The result is a slump-adjustable concrete that will, depending on mix design, reach compressive strengths of more than 3500 psi within two hours.
The non-portland cement, Great-White, from Ceratech of Baltimore, mixes, places, and finishes like traditional portland cement concrete. It is also nonhazardous and cleans up easily with water. It is comprised of 88% recycled industrial waste. Using it eliminates one ton of carbon dioxide greenhouse gas for every three yards of concrete produced. The 15 yards placed in this project negate the carbon dioxide produced by a typical SUV in six months.
You can use this cement with most traditional fine and coarse aggregates, plus recycled, crushed and broken glass without regard for alkali-silica reactivity concerns. This further enhances its environmental benefits.
Mobile mixers' advantages
Using mobile volumetric mixing equipment enables rapid, large-volume infrastructure and highway repairs in two to three hours. This minimizes dangerous exposure to traffic by highway maintenance crews, loss of toll revenues, and inconvenience to motorists.
With a nominal mix design ratio of 715 pounds of GreatWhite cement per yard of concrete, compressive strengths of more than 2500 psi are achievable in just two hours, with 28-day strengths reaching more than 9800 psi. Flexural strengths greater than 350 psi are also possible in just two hours.
The full-depth slab replacement on Interstate 295 near Richmond, Va., was ideal for this type of project. The first area to be replaced was 12 x16 feet by 9 inches deep. Failed concrete was removed and a rebar mat placed just before the 11 a.m. arrival of the volumetric mobile mixer containing the cement. Ground temperature was 80° F; ambient air was 82° F. Skies were partly cloudy with a light breeze.
The concrete was initially at about a 6-inch slump visually. This was adjusted to a 4-inch slump by changing the mix water flow rate. It took 17 minutes to place 5.25 yards of concrete. Material was progressively hand-screeded as the placement proceeded. Finishing was aided by using a Sika film as an evaporation retardant. The material had 20 minutes working time with final set occurring at 42 minutes.