Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures
|Publication Date:||1 July 2008|
Scope and limitations
This document provides guidance for the selection, design, and installation of FRP systems for externally strengthening concrete structures. Information on material properties, design, installation, quality control, and maintenance of FRP systems used as external reinforcement is presented. This information can be used to select an FRP system for increasing the strength and stiffness of reinforced concrete beams or the ductility of columns, and other applications.
A significant body of research serves as the basis for this document. This research, conducted over the past 20 years, includes analytical studies, experimental work, and monitored field applications of FRP strengthening systems. Based on the available research, the design procedures outlined in this document are considered to be conservative. It is important to note, however, that the design procedures have not, in many cases, been thoroughly developed and proven. It is envisioned that over time these procedures will be adapted to be more accurate. For the time being, it is important to specifically point out the areas of the document that do still require research.
The durability and long-term performance of FRP materials have been the subject of much research; however, this research remains ongoing. Long-term field data are not currently available, and it is still difficult to accurately predict the life of FRP strengthening systems. The design guidelines in this document do account for environmental degradation and long-term durability by suggesting reduction factors for various environments. Long-term fatigue and creep are also addressed by stress limitations indicated in this document. These factors and limitations are considered to be conservative. As more research becomes available, however, these factors will be modified and the specific environmental conditions and loading conditions to which they should apply will be better defined. Additionally, the coupling effect of environmental conditions and loading conditions still requires further study. Caution is advised in applications where the FRP system is subjected simultaneously to extreme environmental and stress conditions.
The factors associated with the long-term durability of the FRP system do not affect the tensile modulus of the material used for design. Generally, this is reasonable given that the tensile modulus of FRP materials is not affected by environmental conditions. There may be, however, specific fibers, resins, or fiber/resin combinations for which this is not true. This document currently does not have special provisions for such materials.
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 many different varieties 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 level achieved in the FRP material (and thus the stress level achieved) are imposed to conservatively account for debonding failure modes. It is envisioned that future development of these design procedures will include more thorough methods of predicting debonding.
The document does give 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. Some research has been conducted on various methods of anchoring FRP strengthening systems (by mechanical or other means). It is important to recognize, however, that methods of anchoring these systems are highly problematic due to the brittle, anisotropic nature of composite materials. Any proposed method of anchorage should be heavily scrutinized before field implementation.
The design equations given in this document are the result of research primarily conducted on moderately sized and proportioned members. While FRP systems likely are effective on other members, such as deep beams, this has not been validated through testing. Caution should be given to applications involving strengthening of very large members or strengthening in disturbed regions (D-regions) of structural members. Where warranted, specific limitations on the size of members to be strengthened are given in this document.
This document applies only to FRP strengthening systems used as additional tensile reinforcement. It is currently not recommended to use these systems as compressive reinforcement. While FRP materials can support compressive stresses, there are numerous issues surrounding 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 involved with the use of the material for this purpose, nor does it address the design concerns surrounding such applications. The use of the types of FRP strengthening systems described in this document to resist compressive forces is strongly discouraged.
This document does not specifically address masonry (concrete masonry units, brick, or clay tile) construction, including masonry walls. Research completed to date, however, has shown that FRP systems can be used to strengthen masonry walls, and many of the guidelines contained in this document may be applicable (Triantafillou 1998b; Ehsani et al. 1997; and Marshall et al. 1999).