AC321 - Fiber-Reinforced Polymer Systems
ICC-ES Acceptance Criteria for Fiber-Reinforced Polymer (FRP) Systems
Last updated: September 21, 2025
Overview
Fiber-reinforced polymer (FRP) systems in construction use strong fibers (such as carbon or glass fibers) embedded in a polymer resin to strengthen structural elements. These systems are often used to retrofit or reinforce existing buildings – for instance, by wrapping fiberglass or carbon fiber sheets around a concrete column or beam and saturating them with epoxy, the member's load capacity and ductility can be greatly increased once the resin hardens.
FRP materials are lightweight and non-corroding, yet have very high tensile strength thanks to the fibers. They provide a way to add reinforcement without heavy steel or bulky additions. Acceptance criteria make sure that FRP retrofit systems provide reliable strength improvement and that the materials (fibers and resins) perform well together under real-world conditions. For structural wood applications, see composite wood products (AC265) standards.
Purpose
This acceptance criteria provides a basis for evaluating fiber-reinforced polymer retrofit systems that may not be directly covered by existing codes. It establishes testing protocols, structural enhancement requirements, and quality standards needed for code approval of these advanced reinforcement systems.
Benefits of Compliance
- Nationwide acceptance by building officials for FRP retrofit systems
- Streamlined approval process for carbon and glass fiber reinforcement
- Third-party verification of structural enhancement performance
- Enhanced marketability for structural retrofitting applications
Resources
Frequently Asked Questions
What types of FRP systems does AC321 cover?
AC321 covers fiber-reinforced polymer systems used to strengthen existing concrete and masonry structures. This includes carbon fiber, glass fiber, and aramid fiber sheets or fabrics applied with epoxy or other polymer resins to increase load capacity and ductility.
Is AC321 only for retrofit applications?
AC321 primarily addresses FRP systems used to strengthen existing structures, such as seismic retrofits, load upgrades, or damage repairs. For FRP products used as primary structural members in new construction, different evaluation criteria may apply.
Do FRP retrofit systems require engineering design?
Yes. FRP strengthening systems must be designed by a licensed professional engineer following the manufacturer's ESR and ACI 440.2R guidelines. The design must account for existing structural capacity, load requirements, and environmental exposure conditions.
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