Hamid Saadatmanesh

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.

Ultimate Shear Capacity of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber-Reinforced Plastic Plates

The ultimate shear capacity of reinforced concrete beams can be increased by epoxy-bonding fiber-reinforced plastic (FRP) plates to the web of the beam. The shear crack inclination angle is changed as a result of bonding of the plate. In this paper, truss analogy and compression field theory are used to determine the effect of the FRP plate on the shear capacity and crack inclination angle of reinforced concrete beams at ultimate state. Following the calculation of the crack inclination angle, the equilibrium and compatibility equations are used to obtain the shear force resisted by the plate. A parametric study was carried out to reveal the effect of important parameters such as plate thickness and fiber orientation on the crack inclination angle and shear capacity. The upper bound value of crack inclination angle found in this study is suggested as a conservative value for determining the shear capacity of the retrofitted beam. Knowing the inclination angle of cracks, the shear force in the composite plate and concrete beam can be calculated and used for the design of this type of beam. The results of this method have shown close agreement to experimental results.

Fiber Composites for New and Existing Structures

Fiber composite materials have been used in a variety of industries, such as aerospace, automotive, shipbuilding, chemical processing, etc., for many years. Their application in civil engineering, however, has been very limited. Their high strength-to-weight ratio and excellent resistance to electrochemical corrosion make them attractive materials for structural applications. This paper presents an overview of some of the applications of nonmetallic, fiber composites in concrete construction.

Analytical Study of Reinforced Concrete Beams Strengthened with Web-Bonded Fiber Reinforced Plastic Plates or Fabrics

Bonding fiber reinforced plastic (FRP) plates or fabrics to the web of reinforced concrete beams can increase the shear and flexural capacity of the beam. This paper presents analytical models to calculate the stresses in the strengthened beam, and the shear force resisted by the composite plate before cracking and after formation of flexural cracks. The anisotropic (orthotropic) behavior of the composite plate or fabric has been considered in the analytical models. The companion paper extends this discussion into post cracking behavior at the ultimate load, where the diagonal shear cracks are formed. The method has been developed assuming perfect bond between FRP and concrete (no slip) and using compatibility of the strains in the FRP and the concrete beam. The validity of the assumptions used in this method has been verified by comparing the results to the finite element method. A parametric study has been performed to reveal the effect of important variable parameters, such as fiber orientation angle on the shear force resisted by the FRP plate. The method has been developed for both uncracked and cracked beams, and it can be used for stress analysis of these types of beams.

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.

Environmental Effects on the Mechanical Properties of E-Glass FRP Rebars

Because of their unique properties, fiber reinforced plastics (FRPs) are becoming increasingly popular among researchers and engineers in the construction industry. FRP rebars in particular present an attractive alternative to steel rebars in reinforced concrete. Among the features of this type of rebar are the high strength-to-weight ratio and potential resistance to aggressive environmental factors. This paper presents the results of an experimental and analytical study on the durability of E-glass FRP rebars. A total of 160 rebar samples were placed in corrosive chemical solutions that simulated exposure in the field. Tests were performed at temperatures of 25 deg C and 60 deg C. Test variables included one type of fiber (E-glass), two matrix materials (polyester and vinylester), seven chemical solutions, and ultraviolet radiation. Rebar specimens were constructed from E-glass fibers embedded in polyester or vinylester resin matrix. Rebar sizes were 10 mm (3/8 in) and 19.5 mm (3/4 in) in diameter. Changes in weight and physical appearance were recorded over a one-year period. In addition, 10 beams each reinforced with two 10-mm (3/8-in) E-glass/polyester or E-glass/vinylester FRP rebars were subjected to deicing salt solutions. They were tested in flexure to failure after one-year and 2-year periods, and the load versus mid-span deflection relationships were recorded. Test results of rebars and beams indicate that significant loss of strength can result from the exposure of E-glass FRP rebars.

Extending service life of concrete and masonry structures with fiber composites

Abstract Considering the deteriorating state of the infrastructure worldwide and the limited resources available for repair and rehabilitation of constructed facilities, it is imperative to find effective and economical techniques to revive the aging infrastructure. In this paper, the utilization of modern materials such as fiber reinforced plastics (FRP)for strengthening and rehabilitation of concrete and masonry structures is presented. Three different applications of FRPs will be discussed. In one application, composite plates are epoxy bonded to the soffit of girders to increase their ultimate strength. The results of an analytical and experimental study on ten 4.88 m long girders indicate that significant strength gains can be achieved with this retrofitting technique. In the second study, FRP straps are used as external confinement for enhancing seismic response of concrete bridge columns. Ten columns were subjected to simulated earthquake loading. The results of the hysteresis loops of retrofitted columns, when compared to those for unretrofitted control specimens, revealed gain in ductility level of up to four times. In the third study, composite fabrics were epoxy bonded to the faces of unreinforced masonry walls to improve their in-plane and out-of-plane responses during seismic loading. Significant improvements in the behavior were also observed for this technique.