INNOVATING PRACTICES ON THE JOBSITE
Cliff Swenson, project manager and estimator for North Country Concrete, Ramsey, Minn., says his company has five years of pervious concrete installation experience. In terms of placement, they think problems mostly relate to how well the fresh concrete is compacted and how fast curing operations proceed. To address this, North Country Concrete purchased a ride-on roller screed that spanned the entire 21-foot-wide street, rolling on the curb and gutters that were constructed several days earlier. Two rollers guided the machine and screeded the concrete, while the third roller in the machine's rear could be adjusted up or down to compact the concrete at ½ inch as the machine passed by — enough to provide the inter-aggregate connections needed.
To solve the curing problem, North Country Concrete fabricated a moving platform with a water dip-tank to follow immediately behind the screed to perform the wet-curing operation. Workers pulled the curing membrane off a roll, passed it through the dip-tank full of water, and placed the wet material on the pavement. Swenson says when the curing membrane was removed, the pavement was still wet after seven days.
Working with a 12-person crew, North Country placed as much as 200 cubic yards of concrete per day. Two ready-mix trucks arrived every 20 to 25 minutes, backing into position side by side to place concrete along the 22-foot width of the street. The material flowed down the shoots without assistance. Despite the additional admixtures and enhanced design, Swanson thinks the price of the concrete was in line with other pervious mixes North Country has worked with in the past.
It's a little unusual for all parties in a contract to adopt a leading-edge philosophy and work together to solve problems. The City of Shoreview had realistic expectations for the project but also wanted a state-of-the-art result. This was ensured by involving all of the players in early problem-solving discussions.
City engineers also recognized that the pavement would only be as good as its maintenance system so they periodically employ a truck-mounted vacuum to remove debris and dirt from the pores of the concrete throughout the nonwinter season. During the winter months, the city avoids using sand or salts to remove snow and ice as much as possible.
Every good project studies the outcomes, so the city is monitoring the water in Lake Owasso and the wells in the area. At some locations, they can tell how much water is residing in the open-graded stone layer below the pavement. Due to unusual weather conditions this past winter, a layer of snow and ice residing on the surface of the pavement made any damage unclear. But during spring, project participants walked the street to determine how it survived its first winter.
“The concrete held up well,” says Wesolowski. “There were a few isolated areas where spalling of the first layer of rock occurred, but that accounted for only approximately 2% of the total surface area.” The affected areas are mainly located on curves where the city was forced to place salt due to freezing rain and extremely cold temperatures in December.
“We are speculating that the salt combined with the turning movements on the curves caused the spalling.”
As anticipated by the engineering department, the improved technology provided better pavement compared to the first pervious project a couple of years ago. According to Wesolowski, the pavement in the alley has many more pores at the surface and looks more like the typical pictures you see of pervious concrete. But the combination of the mix design and tri-wheel roller on the street project produced a much tighter surface.
“It looks like it is almost too tight for water to flow through it,” he says. “Also, if you look at a cross-section of the concrete you can see that the top 1 to 2 inches is more tightly compacted than the lower portion, which is directly related to the pressure the roller paver placed on the surface of the concrete. That should make the surface more durable.”
Shoreview will evaluate this project for the next few years to ensure it meets performance expectations.
— Nasvik is senior editor of CONCRETE CONSTRUCTION, a sister magazine of PUBLIC WORKS.