Thanks to decades of advocacy work and code development, fiber-reinforced polymer composites have become a more familiar part of the concrete repair and strengthening toolboxes for civil and structural engineers. But fabric-reinforced cementitious matrix systems, which are built on similar reinforcement principles but using a mineral binder instead of epoxy, remain less widely understood.
Attendees at the American Concrete Institute’s fall convention last week in Baltimore had an opportunity to hear from researchers and practitioners about the connection between the two, showing how new testing, modeling and field applications could shape future updates to ACI 549, ACI 620 and ACI 440 documents.
Refining test methods for code calibration
Christian Carloni from Case Western Reserve University offered a technical look at how FRCM bond performance is evaluated. Current ACI 549 procedures rely on tensile coupon tests to determine design strain and elastic modulus, but Carloni argued that these may not represent real bond behavior.
His team’s research, now informing revisions to ACI 549 and related guidance in ACI 620 and ACI 420, focuses on the single-lap shear test as a more representative way to measure the load transfer between FRCM and concrete or masonry substrates. By controlling the bonded length and tracking how cracks form and propagate, the test reveals how stresses shift between fibers and matrix and how that interaction controls overall capacity.
Carloni showed that small differences in specimen geometry can lead to large variations in measured strain and modulus. For that reason, he said, the current coupon test cannot produce a unique design value “unless we prescribe something more.”
The proposed single-lap procedure, already used in Europe, would provide the bond-dependent coefficient needed for realistic design equations in ACI 549. A new draft version of the guide is now being validated with long-term test data before publication.
Flexible bonding for weak concrete substrates
Moving from code calibration to application, Arkadiusz Kwiecień from Cracow University of Technology described using polyurethane flexible adhesives for FRP and FRCM strengthening where substrate strength falls below the 1.5 MPa pull-off limit normally required by design standards.
Where epoxy creates stress concentrations at the interface, Kwiecień’s flexible adhesive redistributes shear more evenly and allows post-cracking load transfer rather than sudden delamination. Tests with steel, carbon and glass fibers showed smoother load–displacement curves and up to 90 percent recovery of brick strength when using the flexible system versus only 7 percent for cement-based repairs alone.
Although not yet codified, this work suggests a potential addition to ACI 440.2R and related guidance on externally bonded systems—particularly for emergency or seismic repair where ductility and controlled deformation are priorities.
FRCM under fatigue and environmental exposure
John J. Myers from Missouri University of Science & Technology brought the focus back to ACI 549 design practice with a series of fatigue and environmental durability tests on FRCM-strengthened reinforced-concrete beams. His group compared unreinforced control specimens with one- and four-ply FRCM systems exposed to moisture and temperature cycles.
The results confirmed that fatigue was not a critical issue for the tested PBO-fiber FRCM, even after two million load cycles. Environmental exposure produced only small stiffness losses—and in some cases improved post-peak behavior, since moisture in the matrix enhanced bond quality rather than degrading it.
Myers tied these findings to current ACI 549-R guidance, noting that existing equations can under-predict capacity by a factor of up to 2.5, highlighting the need for database expansion before code calibration.
He also examined anchorage details, showing that mechanical or spike anchors are effective only once the failure mode shifts from fiber slip to debonding—a distinction that will help refine design provisions in future ACI documents.
FRCM jacketing for seismic retrofit
Moustafa Mansour from the University of British Columbia presented large-scale cyclic tests on seismically deficient RC columns retrofitted with FRCM jackets. Compared with unretrofitted controls, columns wrapped in one or two FRCM layers showed 30–38 percent gains in lateral load capacity, 10–20 percent higher ductility and up to 30 percent greater energy dissipation.
Mansour observed that the mineral matrix provided compatibility with the concrete substrate and preserved performance under repeated loading. Those are advantages that complement ACI 549’s emphasis on repair durability and reversibility. Fragility-curve analysis of the test data suggested a meaningful shift toward higher earthquake-intensity thresholds before major damage occurs, supporting FRCM’s use in seismic retrofit provisions now being discussed within ACI 620 (seismic repair and rehabilitation).
Scaling up FRP reinforcement in the field
Shifting from cementitious matrices to polymer bars, Steven Nolan from the Florida Department of Transportation described Florida’s first field use of #11 GFRP bars to replace deteriorated steel in a bridge cap on the US-90 Bridge over the Aucilla River. Because post-tensioning was impractical, the team substituted a fully reinforced GFRP design, adapting calculations from ACI 440.1R and AASHTO GFRP bridge specifications.
The project required verifying tensile and modulus properties beyond the current ASTM size range (which stops at #10). Laboratory tests confirmed strengths roughly twice the yield load of Grade 60 steel, while maintaining acceptable stiffness. The presenters noted that Florida DOT is contributing data from these installations to support expansion of ACI 440 and ASTM provisions to include larger bar sizes—an essential step for using FRP as primary reinforcement in substructures.
Hybrid reinforcement for durability and recentering
Alessandro Palermo (University of California), speaking on behalf of himself and Luciano Ombres and Pietro Mazzucca from the University of Calabria), linked structural resilience research to code development. Their collaborative tests examined hybrid FRP-steel reinforcement for improved ductility and post-event recentering.
By varying bar ratios and confinement levels, they found that combining materials can deliver both strength and recoverability—the steel contributing energy dissipation while FRP resists corrosion and maintains elastic confinement. Palermo noted that this concept may guide future design provisions within ACI 440 and emerging international standards, particularly for seismic columns and bridge piers. He also previewed planned large-scale cyclic tests aimed at developing guidelines for hybrid bridge piers, in coordination with Caltrans and University of Calabria researchers.
Anchorage you can see and verify
Concluding the session, Scott F. Arnold from FYFE revisited one of the practical barriers to FRP design acceptance: anchor reliability and inspection. Traditional fiber-fan anchors can achieve high capacities but are difficult to verify once installed. Arnold demonstrated mechanically assisted and hybrid anchors using bolted or plated terminations that deliver comparable strength with clear visual confirmation.
Tests showed more uniform stress transfer and delayed debonding, allowing engineers to reach design capacity with less composite material and simpler detailing. Such inspectable anchorage concepts could inform future updates to ACI 440.2R and field-inspection guidelines under development in ACI Committee 440.
Codes catching up to materials
Across all seven presentations, a single theme tied the work together: Test-validated data are now feeding directly into code language.
Carloni’s bond research is refining ACI 549’s basis for design strain; Myers’s fatigue data and Mansour’s column tests are expanding the range of validated applications; and field demonstrations like Florida DOT’s are pushing ACI 440 beyond supplemental use.
What began as academic exploration of alternative binders and anchorage details is now shaping prescriptive equations and design limits that will soon appear in updated ACI 549, 620 and 440 documents.
For the broader nonmetallic materials community, the session marked a turning point: FRP may be established, but FRCM is entering the same path toward full codification—transforming from promising research to standardized engineering practice.
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This is part 2 of SmartBrief’s recap of the American Concrete Institute’s annual convention. Click here for part 1. For more insight into the latest news and trends affecting the concrete industry, subscribe today to ACI SmartBrief and NEx SmartBrief.