scope:

This document provides guidance for the selection, design, and installation of externally bonded and near-surfacemounted (NSM) FRP systems for strengthening concrete structures. Information on material properties, design, installation, quality control, and maintenance of FRP systems used as external or NSM reinforcement is presented. This information can be used to select an FRP system for increasing the strength, stiffness, or both, of reinforced concrete members or the ductility of columns.

A significant body of research serves as the basis for this guide. This research, conducted since the 1980s, includes analytical studies, experimental work, and monitored field applications of FRP strengthening systems. Based on the available research, the design procedures outlined herein are considered conservative.

The durability and long-term performance of FRP systems have been the subject of much research which remains ongoing. The design guidelines in this document account for environmental degradation and long-term durability by providing reduction factors for various environments. Fatigue and creep are also addressed by stress limitations indicated in this document. As more research becomes available, these factors may be modified, and the specific environmental and loading conditions to which they apply will be better defined. Additionally, the coupling effect of environmental exposure and loading conditions requires further study. Caution is advised in applications where the FRP system is subjected simultaneously to extreme environmental and stress conditions.

Many issues regarding bond of the FRP system to the substrate remain the focus of a great deal of research. For both flexural and shear strengthening, there are different modes of debonding failure that can govern the strength of an FRP-strengthened member. While most of the debonding modes have been identified by researchers, more accurate methods of predicting debonding are still needed. Throughout the design procedures, significant limitations on the strain permitted in the FRP system (and thus, the permitted stress) are imposed to conservatively account for debonding failure modes. Future development of these design procedures should include more thorough methods of predicting debonding.

This document gives guidance on proper detailing and installation of FRP systems to prevent many types of debonding failure modes. Steps related to the surface preparation and proper termination of the FRP system are vital in achieving the levels of strength predicted by the procedures in this document. Research has been conducted on various methods of anchoring FRP strengthening systems, such as U-wraps, mechanical fasteners, fiber anchors, and U-anchors. This document contains design provisions for the use of fiber anchors for shear strengthening with FRP U-wraps. For other anchorage systems and applications, few design guidelines are currently available. The performance of any such anchorage system should be substantiated through representative physical testing that includes the specific anchorage system, installation procedure, surface preparation, and expected environmental conditions.

The design equations given in this document are the result of research primarily conducted on moderately sized and proportioned members fabricated of normalweight concrete. Caution should be given to applications involving strengthening of very large or lightweight concrete members or strengthening in disturbed regions (D-regions) of structural members such as deep beams, corbels, and dapped beam ends. When warranted, specific limitations on the size of members and the state of stress are given herein.

This guide applies only to FRP strengthening systems used as additional tensile reinforcement. These systems should not be used as compressive reinforcement. While FRP systems can support compressive stresses, there are numerous issues associated with the use of FRP for compression. Microbuckling of fibers can occur if any resin voids are present in the laminate. Laminates themselves can buckle if not properly adhered or anchored to the substrate, and highly unreliable compressive strengths result from misaligning fibers in the field. This document does not address the construction, quality control, and maintenance issues that would be necessary for the use of FRP systems for compressive reinforcement, nor does it address design for such applications.

This document does not specifically address masonry (concrete masonry units, brick, or clay tile) construction. Information on the strengthening of masonry structures using FRP systems can be found in ACI PRC-440.7.

Applications and use-FRP systems can be used to rehabilitate or restore the strength of a deteriorated structural member, retrofit or strengthen a sound structural member to resist increased loads due to changes in use of the structure, or address design or construction errors. The licensed design professional should determine if an FRP system is a suitable strengthening technique before selecting the type of FRP system.

To assess the suitability of an FRP system for a particular application, the licensed design professional should perform a condition assessment of the existing structure that includes establishing its existing load-carrying capacity, identifying deficiencies and their causes, and determining the condition of the concrete substrate. The overall evaluation should include a thorough field inspection, a review of existing design or as-built documents, and a structural analysis in accordance with ACI 364.1R. Existing construction documents for the structure should be reviewed, including the design drawings, project specifications, as-built information, field test reports, past repair documentation, and maintenance history documentation. The licensed design professional should conduct a thorough field investigation of the existing structure in accordance with ACI 437R, ACI CODE-562, ACI 369R, and other applicable documents. As a minimum, the field investigation should determine the following:

(a) Existing dimensions of the structural members

(b) Location, size, and cause of cracks and spalls

(c) Quantity and location of existing reinforcing steel

(d) Location and extent of corrosion of reinforcing steel

(e) Presence of active corrosion

(f) In-place compressive strength of concrete

g) Soundness of the concrete, especially the concrete cover, in all areas where the FRP system is to be bonded to the concrete

The tensile strength of the concrete on surfaces where the FRP system may be installed should be determined by conducting a pulloff adhesion test in accordance with ICRI 210.3R or ASTM C1583/C1583M. The in-place compressive strength of concrete should be determined using cores in accordance with ACI CODE-562 requirements. The load-carrying capacity of the existing structure should be based on the information gathered in the field investigation, the review of design calculations and drawings, and as determined by analytical methods. Load tests or other methods can be incorporated into the overall evaluation process if deemed appropriate.

Fiber-reinforced polymer systems used to increase the strength of an existing member should be designed in accordance with Chapters 9 through 15, which include a comprehensive discussion of load limitations, rational load paths, effects of temperature and environment on FRP systems, loading considerations, and effects of reinforcing steel corrosion on FRP system integrity.

Strengthening limits-In general, to prevent sudden failure of the member in case the FRP system is damaged, strengthening limits are imposed such that the increase in the load-carrying capacity of a member strengthened with an FRP system is limited. The philosophy is that a loss of FRP reinforcement should not cause member failure. Specific guidance, including load combinations for assessing member integrity after loss of the FRP system, is provided in Chapter 9.

Fire and life safety-FRP-strengthened structures should comply with applicable building and fire codes. Smoke generation and flame spread ratings in accordance with ASTM E84 should be satisfied for the installation according to applicable building codes, depending on the classification of the building. Coatings (Apicella and Imbrogno 1999) and insulation systems (Williams et al. 2006) can be used to limit smoke and flame spread.

Because of the degradation of most FRP systems at high temperature, the strength of externally bonded FRP systems is assumed to be lost completely in a fire, unless it can be demonstrated that the FRP will remain effective for the required duration of the fire. The fire resistance of FRP-strengthened concrete members may be improved through the use of certain resins, coatings, insulation systems, or other methods of fire protection (Bisby et al. 2005b). Specific guidance, including load combinations and a rational approach to calculating structural fire resistance, is given in 9.2.1 and in ACI PRC-440.10.

Maximum service temperature-The physical and mechanical properties of the resin components of FRP systems are influenced by temperature and degrade at temperatures close to or above their glass-transition temperature T_g (Bisby et al. 2005b). The T_g for commercially available, ambient temperature-cured FRP systems typically ranges from 140 to 180°F (60 to 82°C). The T_g for a particular FRP system can be obtained from the system manufacturer or through testing by dynamic mechanical analysis (DMA) according to ASTM E1640. Reported T_g values should be accompanied by descriptions of the test configuration, sample preparation, sample curing conditions (time, temperature, and humidity), size, heating rate, and frequency used. The T_g defined by this method represents the extrapolated onset temperature for the sigmoidal change in the storage modulus observed in going from a hard and brittle state to a soft and rubbery state of the material under test. This transition occurs over a temperature range of approximately 54°F (30°C) centered on the T_g. This change in state will adversely affect the mechanical and bond properties of the cured laminates. In a dry environment, it is generally recommended that the anticipated service temperature of an FRP system not exceed T_g - 27°F (T_g - 15°C) (Xian and Karbhari 2007), where T_g is taken as the lowest T_g of the components of the system comprising the load path. This recommendation is for elevated service temperatures such as those found in hot climatic regions or certain industrial environments. In cases where the FRP will be exposed to a moist environment, the wet glass-transition temperature T_gw should be used (Luo and Wong 2002). Testing may be required to determine the critical service temperature for FRP in other environments. The specific case of fire is described in more detail in 9.2.1.

Minimum concrete substrate strength-FRP systems need to be bonded to a sound concrete substrate and should not be considered for applications on structural members containing corroded reinforcing steel or deteriorated concrete unless the substrate is repaired using the recommendations in 6.4. Concrete distress, deterioration, and corrosion of existing reinforcing steel should be evaluated and addressed before the application of the FRP system. Concrete deterioration concerns include, but are not limited to, alkali-silica reactions, delayed ettringite formation, carbonation, longitudinal cracking around corroded reinforcing steel, and laminar cracking at the location of the steel reinforcement.

The strength of the existing concrete substrate is an important parameter for bond-critical applications, including flexure or shear strengthening. The substrate should possess the necessary strength to develop the design stresses of the FRP system through bond. The substrate, including all bond surfaces between repaired areas and the original concrete, should have sufficient direct tensile and shear strength to transfer force to the FRP system. For bondcritical applications, the tensile strength should be at least 200 psi (1.4 MPa), determined by using a pulloff type adhesion test per ICRI 210.3R or ASTM C1583/C1583M. FRP systems should not be used when the concrete substrate has a compressive strength fc′ less than 2500 psi (17 MPa). Contact-critical applications, such as column wrapping for confinement that rely only on intimate contact between the FRP system and the concrete, are not governed by these minimum values. In contact-critical applications, stresses in the FRP system are developed due to lateral expansion, also called dilation, of the concrete section, primarily under compressive stresses.

The application of FRP systems will not stop ongoing corrosion of existing reinforcing steel (El-Maaddawy et al. 2006). If steel corrosion is evident or is degrading the concrete substrate, placement of FRP reinforcement is not recommended without arresting the ongoing corrosion and repairing any degradation of the substrate.