When Geneva Rock Products Inc. in Murray, Utah, won the bid for a $34.6 million Utah Department of Transportation (UDOT) design-bid-build project in April 2014, the clock was already ticking. The state agency had determined that a 7.5-mile stretch of Interstate 80, between Silver Creek Junction and Wanship, wouldn’t make it through another winter.
The original asphalt road through Silver Creek Canyon was built in the 1960s and had been repaved many times, including several “mill-and-fills” in the past 15 years. Much of the rutting and cracking on I-80 was caused by the heavy-duty trucks that make up about half of the traffic on the state’s major east-west commercial corridor.
Climate also played a part in the pavement’s surface failures, with extreme temperatures and frequent freeze/thaw cycles occurring at the 6,400-foot elevation. “We select the type of pavement for each section of roadway based on what makes the most sense in a particular area, and this was no longer a good candidate for asphalt,” says Matt Zundel, resident engineer for UDOT.
Aside from the cost of constant repairs, the mountain canyon terrain is not conducive to construction. “Limited access in the canyon makes it extremely difficult to work on, so it was important to design a new pavement with at least a 40-year lifespan,” says Zundel.
With this specification, and a deadline to complete the eastbound lanes before winter, Geneva Rock began work in mid-June. “We got the mix design and trial batches going right away,” says Cody Preston, concrete paving manager.
Preston had been testing a new, sustainable concrete mix using Envirocore cement from Holcim (US) Inc. With uncalcined limestone ground in with portland cement clinker, Envirocore has a lower carbon footprint than traditional cement.
But Preston hesitated to try a new mix on a tight timeline. Instead, he selected a more traditional portland cement concrete paving (PCCP) mix with Type II/V cement for the project’s first phase and ran side-by-side trial batches of the Envirocore mix to test its performance.
Necessity leads to innovation
Geneva Rock began work in mid-June, closing the eastbound lanes of I-80 and leaving one lane open in each direction on the westbound side. The project specified 12 inches of portland cement concrete pavement to be placed over 4 inches of cement-treated asphalt base (CTAB).
UDOT required a partial depth reclamation to reuse a portion of the existing asphalt. But with a cliff on one side and a mountain on the other, the jobsite configuration was limiting.
“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. Here, the contractor ground only the top few inches of asphalt using a profile mill, and spread the tailings to create a 4-inch base of recycled asphalt material. “This was a new process for UDOT, using only the recycled asphalt,” says Serre, “but it allowed us to produce a superior base material that held up very well during construction.”
Coughlin Co., a subcontractor based in St. George, Utah, applied portland cement and water and mixed it in with the milling machine to create the CTAB. The concrete-like material was specified by UDOT to provide more structural support and durability than rubblized material alone. Although the agency had specified a 9% to 11% cement content, Geneva Rock produced test strips and determined it could reach the required strength (300-500 psi in seven days) using just 7% cement at a significant cost savings.
Once the base was compacted and shaped, UDOT required a 72-hour wet curing period. But since access was difficult for water trucks, the agency allowed Geneva Rock to substitute a prime coat as a curing agent. The fast-curing ePrime primer was applied several times a day.
The jobsite could only be accessed from each end and one interchange near the middle, and concrete could not be placed until the CTAB was completed to the access point. “After we did the reclamation, we needed to get in and keep the project moving so the concrete trucks and pavers could start the paving process as soon as possible,” says Serre. They determined the cement-treated base gained enough strength before 72 hours that light truck traffic was not detrimental.
After about a week of working on the CTAB, the contractor began delivering concrete from its portable plant at the west end of the project. The 12-cubic-yard Erie Strayer plant produced up to 300 cubic yards of concrete an hour, delivered to pavers by 10 to 20 trucks at a given time.
With mild weather on their side, Geneva Rock paved the two 12-foot eastbound lanes to complete the first phase in November 2014.
“The cost of cold weather paving is prohibitive, so we generally don’t do it,” says Zundel. “We were also concerned about the safety of having head-to-head traffic on the interstate during winter, so we switched back to a normal lane configuration until spring.”
In the months before the next phase, Geneva Rock focused on ways to improve sustainability and productivity. “We had seen equivalent performance in strength and workability with the Envirocore test mix during the first season,” says Preston, “so we planned to use it on the westbound lanes.”
The portland-limestone cement, combined with 25% fly ash replacement of portland cement per state requirements, reduced the overall CO2 footprint by 35%. Preston also switched to a new sand from a supplier closer to the jobsite to reduce hauling.
Before the I-80 project began, Geneva Rock had considered adding wireless controls to its pavers. But with the accelerated schedule of the first phase, they didn’t have time to install the new technology and train employees how to use it.
After working in Silver Creek Canyon, it became clear this was the ideal job for paving without string lines. “The roadway was about 40 feet wide, and the pavers took up about 26 to 30 feet, including wire guidelines for the sensors,” says Preston. “This made it difficult for the concrete trucks to drive up and turn around before delivering their material.”
Before spring, the contractor added Trimble PCS900 paving controls to two of its four Gomaco slipform pavers, and employees learned how to remotely steer the pavers and use the SPS930 Universal Total Station to control the pan for proper alignment, design, and slope of the pavement. “This was the first time Trimble and Gomaco had used this system with a zero clearance paving setup,” says Preston. “We worked together to do it successfully on our tight shoulder paving.”
Without guide wires, workers didn’t have to worry about tripping or breaking the stringline and stopping the machines. Concrete trucks could also get closer to the pavers before turning around to unload, instead of having to back all the way down the grade.
A sustainable approach
Geneva Rock rebuilt the westbound lanes between April and October 2015, in what seemed like an entirely different project: better weather, a new concrete mix, and a groundbreaking paving process. In the end, it had placed 377,500 square yards of concrete.
Although UDOT doesn’t plan to replace this stretch of I-80 again soon, the agency may consider adopting some elements of the project in the future. “We are always looking for ways to make our concrete more sustainable, so we’ll be keeping an eye on this new pavement’s performance,” says Zundel.
Shelby O. Mitchell is a Berwyn, Ill.-based freelance writer. You can e-mail her at [email protected].