The payoff may not always be immediate, but spending time on small test projects can be a wise investment. This proved true for Geneva Rock Products Inc., when the Murray, Utah-based producer/contractor supplied concrete for a two-day paving project in 2009.
Students at the University of Utah and the Utah Department of Transportation (UDOT) were comparing two kinds of concrete on a municipal street in Salt Lake City. One was a standard paving mix with Type II/V cement and one was made with Envirocore cement. Envirocore, made by Holcim (US) Inc., incorporates limestone with portland cement clinker to reduce the carbon footprint associated with clinker production.
Geneva Rock was interested in using Envirocore as a low-CO2 replacement for portland cement, but wondered about its performance. “The university took samples during production from both mixes to compare results,” says Cody Preston, concrete paving manager for Geneva Rock. “The mixes were pretty comparable in trial batches and during production in terms of workability and overall performance, including strength.”
Fast forward five years: Preston was asked to provide mix designs for a $34.6 million UDOT repaving project in April 2014. The agency had received fast-tracked funding to replace a 7.5-mile section of Interstate 80, a major commercial route, as the damaged pavement wasn’t expected to withstand another winter.
Preston considered an Envirocore paving mix, remembering the university study results, but didn’t want to rush it. “We had to start trial batches right away to get construction going, and we were already using a new aggregate,” he says, “so I wanted to have a traditional mix baseline before we jumped into using new cement.”
Instead, he used a more traditional portland cement concrete paving (PCCP) mix with Type II/V cement for the first phase and ran side-by-side trial batches of an Envirocore mix. But the concrete overlay was just one part of constructing a more durable highway; the solution went much deeper.
Interstate 80 was originally an asphalt highway built in the 1960s, and the stretch running through Utah’s Silver Creek Canyon had seen a lot of wear and tear. Snow, ice, and frequent freeze/thaw cycles in the mountainous region compromised pavement strength, and heavy traffic led to cracking and rutting.
“I-80 is a critical part of the nation’s infrastructure,” says Matt Zundel, resident engineer for UDOT. “Eighty percent of the country’s east-west truck traffic goes through here.” After several “mill-and-fill” repairs over the last 15 years, I-80 moved to the top of the list for reconstruction.
“We evaluate each of our roadways based on what makes sense in a particular region, and this was no longer a good candidate for asphalt,” he says. UDOT wanted a long-sighted solution for I-80, to avoid more extensive repairs in the future — and because the canyon terrain makes construction access difficult.
The agency specified a 40-year pavement design consisting of 12 inches of portland cement concrete pavement placed over 4 inches of a cement-treated asphalt base (CTAB). This type of base, which combines recycled asphalt pavement and cement powder, creates a concrete-like foundation that adds durability to the concrete overlay.
The specification required a partial depth reclamation to reuse a portion of the existing asphalt. “Typically we’ll grind up the entire asphalt pavement, move it to one side of the road, and then replace part of it — the ground asphalt and its original aggregate base — at an even depth,” says Joe Serre, project manager for Geneva Rock. But with a cliff on one side of the interstate and a mountain on the other, the contractor had to rethink its approach.
The contractor ground only the top few inches of asphalt in place with a profile mill, and redistributed the tailings to create an even 4-inch base of recycled asphalt. “Using only the recycled asphalt was a new process for UDOT,” says Serre, “but it allowed us to produce a superior base material that held up very well during construction.” The technique also reduced waste and fuel costs associated with hauling materials to a landfill.
Although the agency had specified a 9% to 11% cement content for the CTAB, Geneva Rock produced test strips and determined they could reach the required strength (300-500 psi in seven days) using just 7% cement, and reduce materials and costs.
Coughlin Co., a subcontractor based in St. George, Utah, used a profile mill to mix portland cement with the recycled asphalt and added water to complete the road base. Once the base was compacted and shaped, UDOT required a 72-hour wet-cure period.
Because it was difficult for water trucks to reach the jobsite and keep the material hydrated, UDOT allowed Geneva Rock to substitute a spray-on prime coat. The contractor chose an environmentally friendly, water-based alternative to a traditional solvent-based prime coat.
“After we did the reclamation, we needed to keep the project moving and start the paving process as soon as possible,” says Serre. The road could only be accessed from each end and one interchange near the middle, so Geneva Rock prepared to start delivering concrete as soon as the new road base reached the mid-point.
The contractor set up an Erie Strayer plant onsite with a 12-cubic-yard mixing drum and a mixing capacity of 300 cubic yards per hour. “Once the portable plant is set up, we re-certify the scales on the plant and run a mixer uniformity test that determines if the plant is mixing the concrete uniformly,” says Preston. “We also re-certify the plant with NRMCA, and UDOT does their own plant inspection and approval.”
About a week after construction began, Geneva Rock started delivering concrete with a fleet of 10 to 20 trucks in a given shift. On average, the plant ran 14 hours a day during the paving phases, and on an as-needed basis to produce concrete for median barriers.
Geneva Rock watched its inventory closely throughout production to ensure consistent delivery to the remote location. “With a portable plant we’re working with limited space so we are always monitoring our materials on-hand to make sure we have what we need for the next few days,” says Preston.
Between June and November, Geneva Rock reconstructed the two eastbound lanes of I-80 from Silver Creek Junction to Wanship, with traffic rerouted to one lane in each direction on the westbound side. With winter approaching, the project team took a hiatus and UDOT reopened the road with its normal lane configuration. “Cold weather paving is generally cost prohibitive,” says Zundel, “and we were concerned about the safety of head-to-head traffic on the interstate during winter.”
A season of firsts
Geneva Rock took advantage of the break to reevaluate its materials and processes. Preston reviewed the Envirocore concrete test results, and determined the new mix would exceed requirements and improve the project’s sustainability. “We had seen equivalent performance in strength and workability with the Envirocore test mix during the first season, so we planned to use it on the westbound lanes,” he says.
The UDOT specification required 25% fly ash replacement of portland cement for the I-80 concrete pavement. Adding Envirocore limestone cement to the mix reduced its CO2 content by 35%. Preston also switched to a new sand from a supplier closer to the jobsite in order to decrease transportation-related emissions.
“Sometimes when producers are using fly ash in concrete, they know they are already producing a more environmentally friendly product so they may not look at cement as a way to reduce the carbon footprint even further,” says Todd Laker, senior technical service engineer for Holcim.
Envirocore has been widely adopted for paving in some states, including Colorado (see “Travelers Welcome,” TCP January/February 2015). “Envirocore is well-suited for paving, especially where sustainability is a priority,” Laker says.
Geneva Rock also decided to implement newly released 3D paving solutions to improve its paving logistics. The change resulted in some paving firsts for the contractor and its equipment suppliers. (See sidebar, “No-strings-attached Paving.”)
A highway divided
When construction began on the westbound lanes, the project's two phases had little in common except a stunning view of Silver Creek. The materials, paving process, and even the weather had all changed remarkably by the time the project was finished in October 2015.
Geneva Rock had produced and placed 377,500 square yards of concrete with four different mix designs: Type II/V cement in the concrete for paving and barrier mixes on one side, and Envirocore in the other.
More importantly, the contractor implemented sustainable materials and technologies that will change its approach, and possibly DOT specifications, in the future. “We are always looking for ways to improve our pavements, and make our concrete more sustainable in general,” says Zundel, “so we’ll be keeping an eye on this new pavement’s performance.”
In the meantime, Geneva Rock and Holcim are developing test mixes for more upcoming UDOT paving projects.