Mohammad R. Ehsani

PREDICTION OF FAILURE LOAD OF R/C BEAMS STRENGTHENED WITH FRP PLATE DUE TO STRESS CONCENTRATION AT THE PLATE END

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.

Strength and Ductility of Concrete Columns Externally Reinforced With Fiber Composite Straps

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.

REPAIR OF EARTHQUAKE-DAMAGED RC COLUMNS WITH FRP WRAPS

A study was conducted on the flexural behavior of earthquake-damaged reinforced concrete columns repaired with prefabricated fiber reinforced plastic (FRP) wraps. Four column specimens were tested to failure under reversed inelastic cyclic loading to a level that is considered higher than would occur in a severe earthquake. The columns were repaired with prefabricated FRP wraps and retested under simulated earthquake loading. The test specimens were designed to model single-bent, nonductile concrete columns in existing highway bridges built prior to the modern seismic design provisions were in place. FRP composite wraps were used to repair damaged concrete columns in the critically stressed areas near the column footing joint. The physical and mechanical properties of FRP composite wraps are described. Seismic performance of repaired columns related to their hysteretic response is assessed and compared to those of the original and unretrofitted columns. The results suggest that the proposed repair technique is highly effective. Both flexural strength and displacement ductility of repaired columns were higher than those of the original columns.

SEISMIC STRENGTHENING OF CIRCULAR BRIDGE PIER MODELS WITH FIBER COMPOSITES

An experimental study was conducted to examine the seismic behavior of reinforced concrete columns strengthened with fiber reinforced plastic (FRP) composite straps. Five concrete column-footing assemblages were constructed with a one-fifth--dimensional scale factor. The unidirectional glass fabric straps were impregnated with polyester resin and wrapped around the potential plastic hinge zone of the columns. An epoxy layer was applied to the straps while wrapping for interlaminar bond. All specimens were tested under inelastic reversal loading while simultaneously subjected to a constant axial load. Experimental findings reveal that the seismic resistance of retrofit concrete columns improves significantly due to the confining action of the FRP composite straps. The straps are highly effective in confining the core concrete and preventing the longitudinal reinforcement bars from buckling under cyclic loading.

Elastic Wave Propagation in Circumferential Direction in Anisotropic Cylindrical Curved Plates

Ultrasonic nondestructive inspection of large-diameter pipes is important for health monitoring of ailing infrastructure. Longitudinal stress-corrosion cracks are detected more efficiently by inducing circumferential waves; hence, the study of elastic wave propagation in the circumferential direction in a pipe wall is essential. The current state of knowledge lacks a complete solution of this problem. Only when the pipe material is isotropic a solution of the wave propagation problem in the circumferential direction exists. Ultrasonic inspections of reinforced concrete pipes and pipes retrofitted by fiber composites necessitate the development of a new theoretical solution for elastic wave propagation in anisotropic curved plates in the circumferential direction. Mathematical modeling of the problem to obtain dispersion curves for curved anisotropic plates leads to coupled differential equations. Unlike isotropic materials for which the Stokes-Helmholtz decomposition technique simplifies the problem, in anisotropic case no such general decomposition technique works. These coupled differential equations are solved in this paper. Dispersion curves for anisotropic curved plates of different curvatures have been computed and presented. Some numerical results computed by the new technique have been compared with those available in the literature. ©2002 ASME

Seismic Retrofitting of Rectangular Bridge Columns with Composite Straps

Behavior of typical rectangular bridge columns with substandard design details for seismic forces was investigated. The poor performance of this type of column attested to the need for effective and economical seismic upgrading techniques. A method utilizing fiber reinforced polymer (FRP) composites to retrofit existing bridge columns is investigated in this paper. High-strength FRP straps are wrapped around the column in the potential plastic hinge region to increase confinement and to improve the behavior under seismic forces. Five rectangular columns with different reinforcement details were constructed and tested under reversed cyclic loading. Two columns were not retrofitted and were used as control specimens so that their hysteresis response could be compared with those for retrofitted columns. The results of this study indicated that significant improvement in ductility and energy absorption capacity can be achieved as a result of this retrofitting technique.

Fiber Composite Bar for Reinforced Concrete Construction

Behavior of concrete beams reinforced with Glass-Fiber-Reinforced-Plastic (GFRP) bars was experimentally investigated. Two types of reinforcements were considered; longitudinal or flexural reinforcement and transverse or shear reinforcement. For each of the two types of reinforcements, three concrete beams were tested to failure. The behavior of each beam was characterized by its load-deflection response to failure. The study was mainly focused on experimentally determining the feasibility of GFRP bars as reinforcement for concrete structures. The results indicated that the plastic bars performed reasonably well in the beams tested.

C-scan and L-scan generated images of the concrete/GFRP composite interface

Abstract Glass Fiber Reinforced Plastics (GFRP) are fast becoming a viable new construction material. They are being used for rehabilitating old concrete structures—columns, beams, slabs and walls—by gluing the composite plate to the concrete surface. Proper attachment between the GFRP plate and the concrete surface is necessary for efficient use of GFRP composites to increase the strength of the rehabilitated structures. Delamination between the rehabilitated structure and the GFRP plate significantly reduces the strength of the reinforced structure. Hence, delamination defects, if present at the interface, should be detected as soon as possible. In this paper, two ultrasonic techniques to detect delamination defects between the GFRP plate and the concrete surface are presented. One technique uses longitudinal waves or P-waves and the second technique uses Lamb waves. It is found that both techniques can detect the defect when used properly. However, Lamb waves give a better image quality of the defect than longitudinal waves.

CONFINEMENT EFFECTS ON HIGH-STRENGTH CONCRETE

This paper describes an experimental investigation conducted on the stress-strain characteristics of steel sleeve confined high-strength concrete. The axial load and strains of concrete, and the axial and hoop strains of the confining steel sleeves were measured. From these measurements, accurate stress-strain relations of the concrete core were produced, along with confinement calculations based on von-Mises elastoplastic response of the steel sleeves. Confinements ranging from 5-19 MPa were calculated. This confinement had a profound effect on the concrete strength, as much as tripling its unconfined strength of 70 MPa. The increase in ductility was found to develop slower for low amounts of confining steel due to a lagging development of confining pressure. These findings are discussed.

Experimental Study of Rectangular RC Columns Strengthened with CFRP Composites under Eccentric Loading

This paper presents the results of experimental studies on reinforced concrete columns strengthened with carbon fiber-reinforced polymer (CFRP) composites under the combination of axial load and bending moment. A total of seven large-scale specimens with rectangular cross section ( 200 mm×300 mm ) were prepared and tested under eccentric compressive loading up to failure. The overall length of specimens with two haunched heads was 2,700 mm. Different FRP thicknesses of two, three, and five layers; fiber orientations of 0°, 45°, and 90°; and two eccentricities of 200 and 300 mm were investigated. The effects of these parameters on load-displacement and moment-curvature behaviors of the columns as well as the variation of longitudinal and transverse strains on different faces of the columns were studied. The results of the study demonstrated a significant enhancement on the performance of strengthened columns compared to unstrengthened columns.