Over the last two years, the Indiana Department of Transportation (INDOT) has been testing the effectiveness and value of a new pavement overlay. Structural Fiber Reinforced Concrete (SFRC) evolved from traditional concrete overlays.
“The change was making them bond to the old pavement – adding to the structure – and the advancement of high-strength structural fiber technology,” said Mike Byers, Executive Director of the Indiana chapter of the American Concrete Pavement Association (ACPA). “Adding macro-fiber reinforcement provides additional residual strength, and as a result the overlays can be designed thinner.”
The Indiana ACPA chapter worked with INDOT to provide technical support for implementing 4- to 6-inch, bonded concrete overlays. According to Patrick Long, the organization’s Director of Marketing and Government Affairs, “You can go back to Biblical times and the pyramids built using mud with straw mixed in; this is a modern-day version of that. If the concrete slab starts to crack, the fibers tie it together.”
Although the use of structural fibers evolved in the last five years, bonded concrete overlays have been used in other states for the past 15 years.
“Data from projects across the country indicate that although initial costs can be 25 to 50 percent more than a typical thin asphalt overlay, the concrete overlay has two to three times the life of an asphalt overlay,” Byers said. “In areas where you don’t want to go back in a few years and interrupt traffic or disturb residents and businesses, a thin concrete overlay is a good option. Plus, it offers enhanced load-carrying capacity so it’s a good way to gain additional service to carry traffic without having to rebuild the road.”
After a 2010 test project, INDOT began a formal evaluation in 2015. “Our intent is to conduct a thorough, critically reasoned assessment of thin concrete overlay technology at the programmatic use level,” said David Holtz, P.E., Pavement Director for INDOT’s Pavement-Geotechnical Division.
INDOT let eight projects with SFRC overlays in 2017 and two more in 2018. “The preliminary construction results witnessed so far clearly indicate that when done well, with effective and meaningful partnerships between the contractor and the department in specific projects, the results exceed expectations,” Holtz said.
For contractors, compared to typical concrete placement, “The thin overlays use the same crews, same product, same paving equipment, and same placement techniques – we just pay a little more attention to sawing, curing, and traffic management,” Byers said.
Various contractors completed INDOT’s SFRC projects across the state. E&B Paving, Inc., headquartered in Anderson, Indiana, paved three of the 10 projects with 4.5-inch overlays:State Road 161 Phase II, a two-lane highway from Holland, Indiana, to State Road 68, completed in 2017; State Road 3, a four-lane, divided highway from Muncie to New Castle, Indiana, completed in two halves starting in July 2017 and finished in September 2018; and State Road 9, a four-lane highway south of Huntington, Indiana, begun last August and finished in October 2018.
The mix for SFRC contains common concrete components, with four to five pounds of high-strength, macro synthetic fibers added per cubic yard. To handle the fiber content on their projects, E&B attached a conveyor to their concrete plant.
“It took an extra person and a little extra coordination,” said Tony Korba, E&B’s Concrete Operations Manager. “There are thousands of fibers in each 5-pound bag, so you have to coat that as you would your aggregate. We added a little more cementitious content to be sure we had the workability we needed for the paver to meet the strength requirements.”
With SFRC, no dowels or tie bars are required unless it ties into existing concrete pavement.
Joints are saw-cut, not formed, at 6 feet by 6 feet. The tighter joint spacing results in additional sawing, so contractors need to pay close attention to the pace of their paving operations.
“We paved at an average rate of 10 feet per minute with the 4.5-inch overlay,” Korba said. “With the six-by-six panels, we were initial-cutting upwards of 20,000 linear feet a day behind the paver. It’s very easy to outrun the sawing.”
To keep up, “We ran up to six saws a night,” Korba added. “We normally have our own saw crews, but this was the first time we subbed out the sawing. My crew chalked the joints and did joint layouts while our subcontractor performed the sawing.”
The speed of laying the thin concrete also made weather a more critical factor. “We were more apt to call off on a day with a 50 percent chance of rain than if we were paving a thicker piece of concrete,” Korba said. “At 4.5 inches, we’re paving 6,000 feet in a 10-hour timespan and it’s impossible to protect that much pavement from rain.”
After paving, the SFRC requires two applications of curing compound. “INDOT normally requires one gallon per 150 square feet of pavement; on the thin overlays they require two passes at one gallon per 100 square feet, so you’re more than doubling the curing compound,” Korba said. “On our projects, we had a very good guy and had him spray going backwards and forward.”
To prevent curling and cracking, “It’s important to get the curing compound on as quickly as possible,” Byers added. “SFRC is fairly thin with a very large surface exposed, so you don’t want the moisture evaporating.”
Keeping Traffic at Bay
For Milestone Contractors LP, headquartered in Indianapolis, traffic management posed the biggest challenge as they paved 9 miles of State Road 9 south of Shelbyville, Indiana, with SFRC through multiple phases in 2018.
Unlike asphalt, where drivers can proceed over it immediately, the concrete overlays require some time before they’re ready to carry full loads. To speed the process, “We used a little tougher, quicker-setting mix design,” said Steve Friess, Milestone’s Concrete Operations Division Manager. “In the middle of summer when it was hot, we were getting 16-hour cures; in the spring and fall we got a little slower breaks.”
Since Milestone needed to keep one of the two highway lanes open throughout their project, “We had to pave one half at a time,” Friess said. “To manage traffic, we used portable traffic lights and pilot cars to run traffic through the site and make sure cars didn’t stray.”
The rest of the job proved fairly routine, thanks in part to Milestone’s recent purchase. “We had just bought a Rexcon Mobile 12 self-erecting central mix concrete batch plant,” Friess said. “We tossed the fibers on the conveyor, which is at ground level, and it brought up the bags and threw them into the mixer.”
For sawing, Milestone also used a subcontractor. “We had them do 16,000 to 25,000 linear feet of sawing a day and they kept up without any problem,” Friess said.
The project used a 6-inch concrete overlay for added strength. “There are a bunch of quarries at the south end so there’s a lot of trucking over the top of it, plus they felt the existing roadway wasn’t as stable,” Friess said. Milestone started the job last April and finished at the end of October, using 34,000 cubic yards of concrete.
As INDOT continues to collect and analyze data from the 10 initial SFRC projects, “It appears at this time to be an effective engineering approach and cost-effective financial approach to maintaining select INDOT roadways at the least cost of ownership while simultaneously providing good levels of service over time,” Holtz said. “The approach appears to work well when done on the right road, at the right time, with a full recognition of the road distresses needing to be addressed.”
SFRC on Display
To familiarize more agencies, contractors, and practitioners with SFRC technology, the Indiana chapter of ACPA worked with INDOT to conduct an educational open house at INDOT’s Greenfield District office last August. Attendees learned about design, construction, and maintenance of traffic options, then toured Milestone’s SR 9 project and E&B’s SR 3 project. (Photos courtesy of Jim Reid/Anchor Media)
Our newsletter right to your inbox.
See stories from other regions.