Institute for Transportation

Research suggests macrofibers reduce cracking, support extended joint spacing

Iowa leads the nation in the construction of concrete overlays, with more than 2,000 centerline miles constructed since record-keeping began in the 1970s. Recently, new overlay projects have more often been constructed using fiber-reinforced concrete (FRC).

FRC, where synthetic macrofibers are mixed into fresh concrete and embedded throughout the concrete mass, is more commonly used on thin overlays to improve performance by resisting crack opening and providing additional residual strength.

However, until the National Concrete Pavement Technology Center (CP Tech Center) recently concluded research into the subject, the full scope of the characteristics and behavior of FRC overlays had not been fully understood.

“The impacts of FRC on crack opening and fatigue life are well understood, but a number of other proposed benefits of using fibers in concrete overlays have not been studied as thoroughly,” said CP Tech Center Director Peter Taylor, who was the principal investigator (PI) on the project.

The research included conducting tests on 37 unique overlay sections with variables consisting of overlay type, overlay thickness, transverse joint spacing, fiber dosage rate, and geotextile use, which provided a number of key insights into concrete overlay behavior, which could impact their future design and construction.

“The findings and recommendations of this study can be immediately useful to state and local agencies in Iowa looking to improve and optimize the design of concrete overlays, both with and without fiber reinforcement,” said CP Tech Center Research Engineer Dan King, a co-PI on the project.

Highlights from the findings include the following:

• Fiber-reinforced sections had fewer cracked slabs than sections without fiber reinforcement.
• The use of fibers appeared to prevent mid-slab transverse cracking in overlays with extended joint spacing designs, whereas that type of cracking occurred in nearly all slabs in the corresponding sections without fibers.
• The use of fibers did not appear to demonstrate improvements in terms of joint load transfer efficiency, pavement smoothness, or curling/warping behavior.
• Joint activation rates were 100% for nearly every section with a joint spacing of 9 ft or greater, while no sections with a joint spacing of 6 ft or less achieved 100% joint activation. However, the presence of unactivated joints did not predict any negative performance outcomes.
• Concrete on asphalt (COA) overlays, both with and without fibers, appeared to benefit significantly from bond to the underlying asphalt pavement in terms of structural capacity and load transfer efficiency, even when they were not designed as bonded overlays.

“The results of this study should help agencies in the evaluation of their existing concrete overlays, providing insight into the relative performance of different types of overlay designs in their network and suggesting adjustments that could be made to typical existing designs,” King said.

For more findings and recommendations, visit the research project page. For more on the CP Tech Center’s extensive research into concrete overlays as well as to view webinars on FRC overlays, visit the center’s Concrete Overlays page.