S.B. Singh

Durability evaluation of carbon fiber-reinforced polymer strengthened concrete beams: Experimental study and design

Carbon fiber-reinforced polymer (CFRP) materials are being used for the retrofitting and repair of deficient and old infrastructures such as bridges and buildings. Over the years, these structures tend to suffer severe strength and stiffness deterioration due to aggressive environmental conditions such as humidity, saltwater, freeze-thaw, thermal expansion, dry-heat, repeated load cycles, and alkali solutions. The authors of this study present the deflections, strains, failure loads, and failure modes of strengthened beams exposed to different independent environmental conditions and repeated load cycles. The authors propose strength reduction factors associated with these various independent environmental conditions. The authors conclude that the long-term exposure to humidity is the most detrimental factor to the bond strength between CFRP plates and fabrics and reinforced concrete (RC) beams. Beams strengthened with CFRP plates and exposed to 10,000 hours of 100% humidity experienced an average of 33% reduction in their strength. RC beams strengthened with CFRP plates are more susceptible to damage than the beams strengthened with CFRP fabrics. There is no significant effect, however, of repeated load cycles on the ultimate loads of beams strengthened with CFRP plates or fabrics for at least 2 million test cycles. Delamination was the primary mode of failure for all of the test beams, with and without exposure to environmental conditions and repeated load cycles. The authors conclude by presenting a durability-based design approach that demonstrates the evaluation of nominal and design moment strengths of strengthened beams along with failure modes.

Design Approach for Carbon Fiber-Reinforced Polymer Prestressed Concrete Bridge Beams

This paper presents a design approach for carbon fiber-reinforced polymer (CFRP) concrete bridge beams prestressed using bonded pretensioning and unbonded posttensioning tendons arranged in multiple vertically distributed layers along with nonprestressing CFRP rods. Design equations to determine flexural capacity and to compute the stresses and strains in concrete and tendons are provided. Based on parabolic stress-strain relation for concrete and linear stress-strain relation for tendons, a computer program was also developed to compute the overall response of the beam. The design equations and accuracy of the nonlinear computer program were validated by comparing the analytical results with experimental results from a full-scale double-T (DT) test beam. The difference in the analytical and experimental values of the ultimate moment capacity of the DT-test beam is negligible, whereas the corresponding difference in the ultimate forces in unbonded externally draped posttensioning strands is approximately 4.1%. A detailed parametric study was conducted to examine the effect of the reinforcement ratio and the level of prestressing forces on the deflections and ultimate load-carrying capacity of the full-scale DT-beam. Other findings are given.

Experimental and Parametric Investigation of Response of NSM CFRP-Strengthened RC Beams

The present investigation deals with carbon-fiber-reinforced polymer (CFRP) rebar manufacturing, experimental flexural and shear response of near-surface-mounted (NSM) CFRP-strengthened reinforced concrete (RC) beams, and detailed parametric study of NSM-strengthened RC beams using Abaqus. The CFRP bars were manufactured locally using the twisting technique. Parametric study was performed by considering various key parameters which affect the strengthening of the RC beams such as a CFRP bar’s orientation, geometry, intergroove separation, type of groove, cross-section area of bars, and material strength. The parametric study is based on the nonlinear finite-element modeling using Abaqus for examining the flexural and shear behavior. All beams were tested under a two-point loading system. The numerical results obtained using Abaqus are validated with the experimental results. The present study results show a significant increase in the strength of RC beams strengthened with NSM CFRP bars. Furthermore, based on this study, guidelines for realistic structural applications and optimization of the use of NSM CFRP bars are proposed.

Shear response and design of RC beams strengthened using CFRP laminates

The present investigation addresses the shear strengthening of deficient reinforced concrete (RC) beams using carbon fiber-reinforced polymer (CFRP) sheets. The effect of the pattern and orientation of the strengthening fabric on the shear capacity of the strengthened beams were examined. Three beams with various lay-ups of strengthening fabric, 45°, 0°/90°, and 0°/90°/45° were examined, in addition to an unstrengthened control beam. Principal and shear strains were measured at different locations at the critical sections of the strengthened beams corresponding to each applied shear force. Experimental results showing the advantage of beam strengthened using the various lay-ups of CFRP sheets are discussed. It is concluded that Beam-45°, Beam-0°/90°, and Beam-0°/90°/45° show about 25%, 19%, and 40% increases in shear-load carrying capacity in comparison to the control beam, respectively. Also, there exists a critical value of shear force up to which there is no appreciable shear strain in the CFRP sheets/beam. This shear force marks the ultimate shear resistance of the control beam. However, the strengthened beams exhibited significant strength and stiffness even beyond the critical value of the shear force. A design example for shear strengthening shows that the design equations available in the literature underestimate the actual shear strength of the beams.

An investigation of material characterization of pultruded FRP H- and I-beams

ABSTRACT This study presents the material characterization of pultruded fiber-reinforced polymer beams of different sizes using experimental and analytical methods. Various tests were performed to determine the material properties such as Youngs and shear moduli using tensile and bending tests of coupons and beams. The stiffness measured from the four-point bending test of both beams is in close agreement with analytical methods. It is shown that the numerically obtained flexural response of the beam using ABAQUS software with material properties obtained from the four-point beam bending test is close to the experimental response.

Performance of NSM-FRP RC beams in flexure and shear using locally developed CFRP rebars

In the present study, an experimental investigation was carried out to evaluate the flexural and shear capacity of near surface mounted (NSM) reinforced concrete (RC) beams using carbon fibre reinforced polymer (CFRP) rebars. A total of 16 half scale beams were tested to study the flexural and shear responses of strengthened RC beams. The CFRP rebars were manufactured at structural testing centre using locally developed twisting equipment to strengthen the RC beams. Also, new anchor system was locally developed to predict the tensile characteristics of CFRP bars using universal testing machine. To examine the flexural and shear behaviour, all beams were tested under two-point loading system. It was observed that the NSM strengthened RC beams have higher load carrying capacity than the corresponding control beam in flexure and shear by 68% and 35.6%, respectively. Typical failure modes of the beams during the experimental investigation were CFRP rupture, CFRP debonding, crushing of concrete and shear failure. Finally, a design example has been presented for better understanding in the analysis and design of NSM FRP strengthened RC beams. Further, the values obtained from the analytical study are in good agreement with the experimental values.

Investigation of fatigue crack growth after a single cycle peak overload in IS 1020 steel

Abstract Crack propagation experiments were conducted on IS 1020 steel for various overload ratios (1·2, 1·3 and 1·4). On the basis of these experiments one power law is developed to predict the crack propagation delay period. The delay period after application of a single overload was found to increase as the magnitude of the overload increased. Crack growth also decreased after the application of an overload cycle but after a certain number of cycles it tended to return to the crack propagation rate for constant amplitude loading (CAL).

Hybrid effect of functionally graded hybrid composites of glass–carbon fibers

ABSTRACT In this work, hybrid composite having linear gradation of carbon and glass fibers (functionally graded hybrid (FGH)) is made and its material properties are compared with classical sandwich hybrid (SH) laminates, i.e., carbon fibers in between glass fibers or vice versa. The performance of FGH composite is evaluated in tensile, compressive, and flexural loadings. Using analytical approach (rule of mixtures), hybrid effect w.r.t. carbon fiber or glass fiber laminates is determined in terms of strength and stiffness. It is observed that failure of FGH specimen is progressive, i.e., layer by layer failure, due to this high ductility is noticed in comparison with SH specimens except in compression. Stiffness and strength of FGH specimens are higher than SH specimens under tension, compression, and flexural. The FGH has shown 7–8% positive hybrid effect in flexural modulus w.r.t. carbon fiber reinforced polymer laminates. In terms of compressive strains, SH and FGH specimens have shown 526% and 380% positive hybrid effect w.r.t. carbon fiber laminates, respectively. Overall FGH is proved better in comparison with classical SH laminates.

Experimental and Analytical Studies of Failure Characteristics of FRP Connections

This study presents the failure characteristics of FRP lap joints in tension using experimental and analytical techniques. The lap joint is made from coupons extruded from laminated plates and pultruded beams. FRP plates are fabricated from different fabrics and stacking sequence of fibers. Extensive study is performed on changing the end distance-to-bolt diameter ratio, bolt diameter-to-thickness ratio, and stacking sequence of the laminates. Coupons are connected through bolts and/or adhesive. It is observed that the failure of the bonded connection is very brittle while those connected by bolt and adhesive have ductile failure. Adhesive joints failed by failure of external layers, while bolted joints have bearing failure of coupons as well as shear failure of coupons and bolt. It is noted that the strength of the joint increases with increase in the overlapping length, while it decreases with increase in bolt diameter. The quasi-isotropic, i.e., (0/90/±45)s have higher failure strength than cross-ply and angle-ply laminates.

Mechanical Properties of PVA and Polyester Fibers Based Engineered Cementitious Composites

Engineered cementitious composite (ECC) is a mortar based composite reinforced with polymeric fibers and exhibits strain hardening characteristic through the process of multiple micro-cracking. In this study, two types of polymeric fibers (i.e., polyvinyl alcohol (PVA) fibers and polyester (Poly) fibers) were used for making of ECC. The uniaxial compressive, uniaxial tensile, and four point bending test were carried out to characterize the mechanical behavior of PVA-ECC and Poly-ECC with same mix proportions. The compressive, tensile and flexural stress–strain responses of ECC are plotted. The ratio of compressive strengths of cylinder (150 × 300 mm), and small cube (70.7 × 70.7 × 70.7 mm) to the compressive strength of standard cube (150 × 150 × 150 mm) is determined. The experimental results show that PVA-ECC exhibits higher compressive, tensile and flexural strength in comparison to the Poly-ECC.