Early-Strength Concrete

Story by Carol Carder, Photos by Lafarge North America U.S. Western Region | September 28, 2010

Denver Water has found a solution to reopening main arterials to traffic quickly after a water main repair. The solution is a high-early-strength concrete called Chronolia, a proprietary mix developed by Lafarge.

“When we use this mix we can pull the barricades in five hours when the concrete has attained 2,500-pounds-per-square-inch strength,” explained Sarah Davis, Denver Water paving and permit coordinator.

Davis credits David Neighbour, Lafarge Western U.S. Region concrete representative, with introducing her to the characteristics of Chronolia at a meeting in December. Davis had her concrete contractor and the people from the city and county of Denver lab who test the concrete at the meeting. After testing, the city and county of Denver decided to allow Denver Water to place Chronolia for street repairs.

“Early-strength concrete can arrive hot in the drum and be a problem to place,” said Stuart Ponting, Lafarge front range south value-added products specialist. “Chronolia retains its workability for two hours, then in another two hours after placement will support its own weight. In an application using forms, the forms can be stripped within four hours.”

Denver Water started using Chronolia to reopen busy intersections in June of this year with the “maiden voyage” placement at 15th and Champa streets in downtown Denver. Since then, Denver Water's patching contractor, Colorado Asphalt Services Inc., of Denver, has used it on three more projects: East 1st Avenue and Lincoln Street, East Evans and South Clayton, and West Mississippi at Seneca Street.

Much of the maintenance work facing Denver Water is reactive with anywhere from five to 10 emergency water main repairs in its 2,700-mile system, according to Davis. Normally during repairs a hot-mix asphalt truck stands by to place a 2-inch-deep patch as soon as the water main is repaired. When the pavement is concrete, this means a contractor has to come back another time to dig out the asphalt and replace it with concrete. When coming back to replace an asphalt patch with concrete, the contractor had to replace the concrete, for example, on a Friday night, and then come back Sunday to open the roadway to traffic. With the early-strength concrete, repairs can be made during normal Monday-though-Friday work hours.

“Although this proprietary concrete mix is more expensive than conventional concrete, the client saves in the long run by not having to mobilize and control traffic twice,” Ponting added.

“We are working to fast-track the system so the next time we have a water main repair at an arterial location such as 3rd and Santa Fe Drive, we can call Stuart's people and our concrete people to button up the hole with Chronolia,” Davis observed.

The plan is to use the early-strength concrete on main arterials and downtown street locations where opening to traffic by rush hour is critical. For example, Denver Water plans to use Chronolia on a big job coming up at Broadway and Belleview. A neighborhood repair site would still be patched with asphalt or conventional concrete depending on the pavement surface.

Other Applications

Another application for Chronolia is crane pads. According to Ponting, when using Chronolia the contractor can start building the pad in 24 hours after the pour instead of waiting the standard three to seven days depending on the mix and psi needed.

To date, a couple of crane pads using the early-strength concrete have been placed in Denver and one in Boulder. The pad for Saunders at 1900 16th street hit 5,700 psi in 24 hours, according to Ponting. A crane pad can require up to 60 cubic yards of concrete and attain strength of up to 9,000 psi in 28 days.

Chronolia has fast-tracked vertical building in Canada: “You can pour a footing and form a wall the same day,” Ponting claimed.

Ponting's job is to educate the engineers, architects and contractors on the possibilities of this early-strength workable concrete. He stressed that it's the construction experts out there who will come up with great applications he has not thought of yet.

Currently, the proprietary mix is proving its worth in Colorado primarily with street or driveway repair where reopening time, such as a drive-up for a bank or fast food restaurant, is critical.