*Co-chairs of the subcommittee that prepared this document. Note: The committee acknowledges the contribution of associate member Paul Kelley. ACI encourages the development and appropriate use of new and emerging technologies through the publication of the Emerging Technology Series. This series presents information and recommendations based on available test data, technical reports, limited experience with field applications, and the opinions of committee members. The presented information and recommendations, and their basis, may be less fully developed and tested than those for more mature technologies. This report identifies areas in which information is believed to be less fully developed, and describes research needs. The professional using this document should understand the limitations of this document and exercise judgment as to the appropriate application of this emerging technology.

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Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures

Design-oriented stress–strain model for FRP-confined concrete

External bonding of steel plates has been used to strengthen deficient reinforced-concrete structures since the 1960s. In recent years, fiber-reinforced polymer (FRP) plates have been increasingly used to replace steel plates due to their superior properties. A key issue in the design of an effective retrofitting solution using externally bonded plates is the end anchorage strength. This paper first presents a review of current anchorage strength models for both FRP-to-concrete and steel-to-concrete bonded joints under shear. These models are then assessed with experimental data collected from the literature, revealing the deficiencies of all existing models. Finally, a new simple and rational model is proposed based on an existing fracture mechanics analysis and experimental observations. This new model not only matches experimental observations of bond strength closely, but also correctly predicts the effective bond length. The new model is thus suitable for practical application in the design of FRP-to-concrete as well as steel-to-concrete bonded joints.

Anchorage strength models for FRP and steel plates bonded to concrete

A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties

Performance and application of a design-oriented stress-strain model for FRP-confined concrete

The static strength of reinforced concrete beams strengthened by gluing glassfiberreinforcedplastic GFRP plates to their tension flanges is experimentally investigated. Five rectangular beams and o...

RC Beams Strengthened with GFRP Plates. I: Experimental Study

Bridge failures in recent earthquakes such as the 1989 Loma Prieta earthquake have attracted the attention of the bridge engineering community to the large number of bridges with substandard seismic design details. Many concrete columns in bridges designed before the new seismic design provisions were adopted have low flexural ductility, low shear strength, and inadequate lap length for starter bars. These problems, compounded by flaws in the design of structural systems, have contributed to the catastrophic bridge failures in recent earthquakes. In this paper, a new technique for seismic strengthening of concrete columns is presented. The technique requires wrapping thin, flexible high-strength fiber composite straps around the column to improve the confinement and, thereby, its ductility and strength. Analytical models are presented that quantify the gain in strength and ductility of concrete columns externally confined by means of high-strength fiber composite straps. A parametric study is conducted to examine the effects of various design parameters such as concrete compressive strength, thickness and spacing of straps, and type of strap. The results indicate that the strength and ductility of concrete columns can be significantly increased by wrapping high-strength fiber composite straps around the columns.

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Strength and ductility of concrete columns externally reinforced with fiber composite straps

Epoxy-bonding a composite plate to the tension face is an effective technique for repair and retrofit of reinforced concrete beams. Experiments have indicated local failure of the concrete layer between the plate and longitudinal reinforcement in retrofitted beams. This mode of failure is caused by local stress concentration at the plate end as well as at the flexural cracks. This paper presents a method for calculating shear and normal stress concentration at the cutoff point of the plate. This method has been developed based on linear elastic behavior of the materials. The effect of the large flexural cracks along the beam has also been investigated. The model has been used to find the shear stress concentration at these cracks. The predicted results have been compared with both the finite element method and experimental results. The analytical models provide closed form solutions for calculating stresses at the plate ends that can easily be incorporated into design equations.

Prediction of Failure Load of R/C Beams Strengthened with FRP Plate Due to Stress Concentration at the Plate End

The results of an experimental and analytical study of the behavior of damaged or understrength concrete beams retrofitted with thin carbon fiber reinforced plastic (CFRP) sheets are presented. The CFRP sheets are epoxy bonded to the tension face and web of concrete beams to enhance their flexural and shear strengths. The effect of CFRP sheets on strength and stiffness of the beams is considered for various orientations of the fibers with respect to the axis of the beam. Nineteen beams were fabricated, loaded beyond concrete cracking strength, and retrofitted with three different CFRP systems. The beams were subsequently loaded to failure. Different modes of failure and gain in the ultimate strength were observed, depending on the orientation of the fibers.

Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets

Analytical models based on the compatibility of deformations and equilibrium of forces are presented to predict the stresses and deformations in concrete beams strengthened with fiber composite plates epoxy-bonded to the tension face of the beams. The models are given for beams having rectangular and T cross sections. A parametric study is conducted to investigate the effects of design variables such as plate area, plate stiffness and strength, concrete compressive strength, and steel reinforcement ratio. The moment versus curvature diagrams for various combinations of these variables are plotted and compared. The results indicate that bonding composite plate to a concrete beam can increase the stiffness, yield moment, and flexural strength of the beam. The method is particularly effective for beams with a relatively low steel reinforcement ratio.

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Mohammad R. Ehsani

Papers

RC Beams Strengthened with GFRP Plates. I: Experimental Study

Strength and ductility of concrete columns externally reinforced with fiber composite straps

Prediction of Failure Load of R/C Beams Strengthened with FRP Plate Due to Stress Concentration at the Plate End

Shear and Flexural Strengthening of R/C Beams with Carbon Fiber Sheets

RC Beams Strengthened with FRP Plates. II: Analysis and Parametric Study