concrete overlays | Crossroads

Lab testing has demonstrated that structural synthetic fibers in thin concrete overlays keep cracks tight and help transfer loads across pavement slabs. A recently released research study, co-funded by the Minnesota Department of Transportation and the Minnesota Local Road Research Board, provides recommendations for selecting fiber types and dosages in pavement design.

What Was the Need?

Concrete pavements usually measure 8 to 15 inches thick. For many of these pavements, designers recommend placing dowel bars at the joints during the pour to assist the transfer of wheel load from heavy commercial and agricultural vehicles across concrete slab joints.

MnDOT has found that dowel bars are not effective in a thin concrete overlay, a 4- to 6-inch layer of concrete over an older pavement. These slabs fracture prematurely around the dowels. Adding structural fibers to concrete offers a potential solution. Used primarily to keep cracks from widening, these fibers consist of pieces of thin synthetic material—polymers, carbon fabric, even steel—mixed into the concrete batch.

Many states do not have formal standards for fiber types or characteristics, dosage rates or other specifications for their use. MnDOT currently uses the approved products list created by Illinois Department of Transportation.

Minnesota road engineers agree that fibers work well in concrete, but how well was unknown. Research was needed to determine the optimal physical characteristics of fibers, the amount that should be mixed in to the concrete, and products currently not on the approved products list that may be effective.

What Was Our Goal?

MnDOT wanted to investigate fiber performance in thin concrete overlays, specifically to help identify fibers that are most appropriate in these overlays and recommend acceptable dosage rates for mixing and placing the thin concrete. MnDOT also needed a test procedure and design recommendations or specifications for using fibers.

“This research looked at fiber performance in terms of load transfer to see if fibers can provide an alternative to dowels in thinner concrete pavements,” Maria Masten, Concrete Engineer, MnDOT Office of Materials and Road Research.

What Did We Do?

Research began with a literature search and a survey of state transportation agencies identified by the American Concrete Pavement Association as leading users of fiber-
reinforced concrete overlays.

Laboratory testing first focused on post-crack performance, relying on ASTM C1609, the nationally recognized testing standard. Investigators tested 10 fibers of various lengths, geometries and stiffness in three dosage levels in concrete, evaluating the impact of fiber properties on post-crack performance.

cracked concrete beams with fiber reinforcement — Post-crack performance testing of fiber-reinforced concrete beams shows that after cracking, fibers work to keep cracks from widening.

Testing then turned to joint performance. Researchers used four fibers from the previous lab examination and added a fifth fiber, a synthetic fiber used in MnROAD test cells in 2017, to test load transfer across cracks between sections of fiber-reinforced concrete. Together, the two lab phases tested 11 fibers in 43 concrete mixtures in over 400 samples 10 beams and 10 cylinders each of 30 fiber-reinforced concrete samples for post-crack performance, one plain concrete mix and 12 additional fiber-reinforced mixtures in joint performance testing. Analysis considered post-crack performance, crack width, fiber geometry, dosage, load transfer efficiency and residual strength.

In the final step, researchers analyzed the collected data and developed recommendations for MnDOT.

What Did We Learn?

Results confirmed that fibers help keep cracks and joints tight and improve load transfer across cracks and joints in thin concrete overlays. This research indicated synthetic fibers provide equal or better performance than steel fibers, which are expensive, heavy and difficult to mix. Dosages less than 0.25 percent fiber volume fraction of concrete mixture did not improve post-crack flexural or load transfer efficiency across the joint.

In lab mixing, longer and stiffer fibers tended to ball and mat with greater frequency than shorter fibers, though researchers developed a mixing method that reduces balling and matting. Fiber dosage, stiffness and shape significantly influenced strength. Embossed, twisted and crimped fibers outperformed straight, flat synthetic fibers; longer fibers with larger diameters outperformed shorter, smaller diameter fibers that inhibit workability.

“We studied many varieties of fibers before writing a specification for using fibers in concrete overlays. This is one step forward in understanding fiber’s contribution in concrete pavements or overlays,” Manik Barman, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering.

Fiber shape had moderate influence on load transfer and displacement in joint performance testing. Dosage levels and crack width strongly affected joint performance. Overall, it was found that fibers can increase the load transfer by 30 percent and can reduce the slab displacement by 50 percent.

Researchers suggest designers use trial batches of mixtures, submitting samples to ASTM C1609 testing and selecting fibers based on joint performance results from this study. Graphs and tables from this study correlate fiber properties with post-crack flexural strength and joint performance to help guide selection and dosage.

What’s Next?

Researchers recommend fibers with high lateral stiffness and irregular cross sections in lengths between 1.5 to 2.5 inches and at dosage levels no greater than 1 percent fiber volume fraction to avoid balling, matting and unworkability of concrete mixtures. MnDOT will issue fiber requirements so manufacturers can then submit products and test results for evaluation by MnDOT in developing a new approved products list for fibers in concrete pavements.

Future research could focus on validating design recommendations in the field; establishing fresh fiber-reinforced concrete mixture parameters by running slump, air content and other tests of fresh mixes; and analyzing life-cycle costs and benefits.

This post pertains to the MNDOT and LRRB-produced Report 2018-29, “Comparison of
Performances of Structural Fibers and Development of a Specification for Using Them in
Thin Concrete Overlays,” published August 2018.

Soaring construction costs and a rapidly aging infrastructure will require states to revolutionize how they maintain their roadways — but without each other’s help, they won’t be successful.

That was a key message from pavement researchers last week at a MnDOT-hosted peer exchange event, where pavement experts from around North America shared their ideas and research experiences.

“You’ve got to partner with other states, the FHWA and industry,” said Research Engineer Steve Bower of the Michigan Department of Transportation. “We can’t go it alone anymore.”

Researchers at the event reviewed recent pooled-fund studies conducted at MnROAD, MnDOT’s innovative pavement testing center, to review successful implementation strategies, develop common practices to calculate benefits and help prioritize research topics for MnROAD’s core 2016 research and reconstruction.

The pavement engineers gathered for the event face similar problems in their home states, as demonstrated by the seven pooled fund projects that were discussed. These included developing a better understanding of pavement damage caused by oversized farm equipment, knowing when to chip seal a roadway, developing a test to predict asphalt cracking , creating a national design method for concrete overlays of asphalt roadways and improvements in diamond grinding of concrete pavements.

MnROAD leading the way

State research departments often lack the time or resources to focus on innovations that could reduce future maintenance costs. If not for Minnesota leading the effort on many of these topics and providing a top-notch research facility, the peer exchange attendees said much of this research just wouldn’t happen.

“We don’t have a closed-loop facility with all these different test sections that MnROAD has; no one does,” said Larry Wiser of the Federal Highway Administration’s Turner-Fairbank Highway Research Center.

Researchers came from Missouri, Maine, Texas, Illinois, Michigan, California, Ontario, Wisconsin, Indiana and Washington for the three-day workshop.

WisDOT Chief Materials Management Engineer Steven Krebs said the research done at MNROAD on the impact of modern farm implements on pavement was invaluable in drafting new state legislation. WisDOT was able to quantify the amount of damage done to the pavement and use the data to dispute mistruths and misinformation. The state is now working with counties on possible remedies and weight-limit enforcement techniques.

Whereas Minnesota has taken the lead on studying such issues, it is now asking fellow states to not only participate in future such studies, but to also partner in the operations at MnROAD. At the peer exchange, the response to this idea — especially from states closest to Minnesota — was positive, despite everyone’s lean budgets.

Peer exchange participants said more effort and funding is needed to implement research findings, which FHWA officials said costs significantly more than the research itself.

Past research also needs to be more accessible and there should be better sharing of information, particularly online, they said.

“This (peer exchange) gave us ideas to take back. Our research budget is getting tighter. It’s nice to be able to say, ‘You do a part of it and we’ll do a part of it,’ ” said California transportation researcher Joe Holland.