Project Overview
A two-span concrete bridge on a county road in the Southeast had been posted with a 15-ton weight restriction after a routine bridge inspection revealed significant deterioration of the prestressed concrete beams. The weight posting forced heavy vehicles — including school buses, agricultural equipment, and delivery trucks — to use a 12-mile detour, creating significant inconvenience and safety concerns for the rural community. The county engineer needed a solution that could restore the bridge to full legal load rating quickly and within a limited budget.
The Challenge
The bridge, constructed in the early 1970s, carried two lanes of traffic over a creek crossing on a road that served as the primary route for several hundred rural residents. Inspection revealed that 12 of the bridge's prestressed concrete beams showed deterioration including:
- Longitudinal cracking along the bottom flange of several beams, indicating loss of prestress force
- Concrete spalling on beam ends at the bearing areas, exposing prestressing strands to corrosion
- Section loss in prestressing strands at spalled areas, reducing the beams' flexural capacity
- Efflorescence and water staining indicating ongoing moisture infiltration through the deck
Load rating analysis by the county's consulting engineer determined that the bridge's inventory rating had dropped below the legal truck weight threshold, requiring the 15-ton posting. Full bridge replacement was estimated at $1.8–2.2 million and would require 8–12 months of construction with complete road closure. The county's annual bridge budget could not accommodate this expense without deferring other critical bridge maintenance projects.
The CFRP Solution
CFRP Repair proposed a CFRP flexural strengthening system designed to restore the bridge's load rating to the full HL-93 design vehicle standard. The solution involved bonding high-strength carbon fiber laminates to the tension face (bottom) of each deteriorated beam, effectively replacing the lost prestress capacity with external CFRP reinforcement.
Engineering Design
The engineering design followed ACI 440.2R guidelines and was reviewed by the county's consulting structural engineer. Key design parameters included:
Target Load Rating: HL-93 design vehicle per AASHTO LRFD Bridge Design Specifications
CFRP System: High-modulus carbon fiber laminate strips, 4 inches wide × 0.055 inches thick
Application: 2–4 laminate strips per beam depending on degree of deterioration
Adhesive: Structural epoxy paste adhesive with minimum 3,000 psi tensile strength
Anchorage: CFRP U-wraps at beam ends to provide mechanical anchorage and prevent debonding
The design accounted for the existing condition of each beam, with beams showing greater strand loss receiving additional CFRP laminate strips. Environmental reduction factors for exterior bridge exposure were applied per ACI 440.2R Table 9.4.
Installation Process
The entire installation was completed in 5 working days with the bridge open to single-lane alternating traffic throughout the project:
Mobilization and Surface Preparation. Equipment setup, access scaffolding installation beneath the bridge, and concrete surface preparation. Deteriorated concrete was removed, exposed strands were cleaned, and beam surfaces were ground to achieve the required surface profile for adhesive bonding.
Concrete Repair. Spalled areas were patched with high-strength repair mortar. Beam surfaces were primed with epoxy primer to seal the concrete substrate and improve adhesive bond.
CFRP Laminate Installation. Carbon fiber laminate strips were bonded to the bottom face of each beam using structural epoxy adhesive. Laminates were pressed firmly into the adhesive bed and secured until initial cure. U-wrap anchorage strips were installed at beam ends.
Quality Verification and Demobilization. Tap testing and visual inspection confirmed full adhesive bond across all laminate installations. Pull-off adhesion testing at selected locations verified bond strength exceeded minimum requirements. Documentation package was compiled for the county engineer.
Results
HL-93
Full legal load rating restored — weight posting removed
70%
Cost savings versus full bridge replacement
5 Days
Total installation time vs. 8–12 months for replacement
Zero
Days of full road closure during installation
Following CFRP installation, the county's consulting engineer performed updated load rating analysis incorporating the CFRP strengthening. The analysis confirmed that all beams met or exceeded the HL-93 design vehicle load rating, allowing the 15-ton weight posting to be removed. The bridge was returned to unrestricted service within one week of project completion.
Total project cost was approximately $180,000, representing a 70% savings versus the $1.8–2.2 million bridge replacement estimate. The county was able to use the remaining budget to address two additional deficient bridges in the same fiscal year — bridges that would have waited years for funding under a replacement-only approach.
Key Takeaways
This project illustrates the transformative potential of CFRP beam strengthening for the nation's inventory of posted and deficient bridges. The technology provides a cost-effective alternative to full bridge replacement that can restore full load ratings in days rather than months, at a fraction of the cost. For counties and municipalities with limited bridge budgets and growing backlogs of deficient bridges, CFRP strengthening allows more bridges to be addressed per budget dollar, improving safety and reducing detour impacts for rural communities.
Have a bridge that needs strengthening? Request a free assessment or call 661-733-7009.