FRP Stirrups Research Articles

Shear Resistance of Concrete Beams with FRP Grating as a Shear Reinforcement

This paper presents an experimental study on the shear behavior of concrete beams with fiber-reinforced (FRP) composite grating as shear reinforcement. Corrosion resistance and non-magnetic properties of FRP reinforcement allows its use in places where application of regular steel reinforcement would face difficulties. The use of FRP composites can increase the life span of constructions and reduce its maintenance costs. Shear stirrups are more susceptible to harsh conditions, due to their placement at the outer face of the reinforcement, and the use of FRP materials can lead to lower concrete cover thickness and therefore to a more effective design of an element. FRP reinforcements are highly anisotropic material with low strength in the direction perpendicular to the fibers. This causes the strength of a FRP stirrup to be limited by its strength in the bends (corners) of a stirrup. The tensile strength in the corner of the bent stirrup is around 40 to 60% of the strength of the straight bar. FRP grating doesn’t contain a bent section limiting its strength, but its behavior as a shear reinforcement is unknown. The paper contains the results of own experimental research on concrete beams with shear reinforcement made of FRP gratings done at the Faculty of Civil Engineering at the Brno University of Technology. Test specimen consisted of nine beams with different shear reinforcement ratios. Presented experimental data are then compared with the results of tests on beams with regular shear FRP stirrups found in literature.

Shear Behavior of Basalt Fiber Reinforced Concrete Beams with and without Basalt FRP Stirrups

AbstractThis paper examines the shear strength and behavior of concrete beams reinforced with basalt fiber-reinforced polymer (basalt FRP) bars with and without shear reinforcements. Six 200×300 mm (8×12 in.) and six 300×200 mm (12×8 in.) concrete beams were, respectively, made with and without basalt FRP shear reinforcements. The flexural reinforcement ratios (ρf) ranged from 2.69 to 14.8 times the balanced ratio (ρfb) for nonshear reinforced (NSR) beams and 1.69 to 6.88 for the shear reinforced (SR) concrete beams. Two different shear span-to-depth (a/d) ratios were considered for the NSR-beams (5.65 and 7.0) and three different a/d for the SR-beams (1.5, 2.5, and 3.5). The test results are presented in terms of crack patterns, failure modes, load-deflection, load-strain behavior, and shear capacity. It was observed that for both SR- and NSR-beams, the shear capacity increased when the area of basalt FRP reinforcement increased for the same span to depth ratio (a/d), whereas the shear capacity decre...

Experimental and analytical shear evaluation of concrete beams reinforced with glass fiber reinforced polymers bars

This paper presents an experimental and analytical study on the shear behavior of concrete beams reinforced with lab produced glass fiber reinforced polymers (GFRP) bars and stirrups. The bars and stirrups are manufactured by double parts die mold using local resources raw materials at lab. A total of ten beams measuring 120mm wide, 300mm deep and 1550mm long were casted and tested up to failure under four-point load. The main parameters were concrete compressive strength and the vertical GFRP web reinforcement ratio in the form of the number of GFRP stirrups (without stirrups and with 8 @ 215, 8 @ 150 and 8 @ 100 stirrups). The mid-span deflection, inclined crack load and GFRP reinforcement bars and stirrups strains of the tested beams were recorded and compared. The test results revealed that the shear capacity increasing by 41% and 82% of the shear carrying capacity of beam without stirrups by using web GFRP reinforcement of 8 @ 215 and 8 @ 100 respectively. The shear capacity increased by 49% and 104% as the concrete compressive strength increased from 25MPa to 45MPa and 70MPa respectively. The maximum value of measured strain in GFRP stirrups reached 0.0095. New approach to calculate FRP stirrups shear strength was proposed and verified throughout an assessment of experimental results of current study and previous works. The shear capacities of the tested specimens were calculated using the strut and tie models (STM) and non-linear finite element analysis (NLFEA). The average ratio of experimental shear capacity to calculated (Vexp/Vpred) using NLFEA and STM were 1.2 and 0.9 respectively.

Steel-CFRP composite and their shear response as vertical stirrup in beams

An experimental study was conducted for the effectiveness of steel-CFRP composite (CFRP laminates sandwiched between two steel strips) as stirrups in concrete beam to carry shearing force and comparison was made with conventional steel bar stirrups. A total numbers of 8 concrete beams were tested under four point loads. Each beam measured 1,600 mm long, 160 mm width and 240 mm depth. The beams were composed of same grade of concrete, with same amount of flexural steel but different shear reinforcements. The main variables include, type of stirrups (shape of stirrups and number of CFRP layers used in each stirrup) and number of stirrups used in shear spans. After getting on an excellent closeness between the values of ultimate shear resistance and ultimate tensile load of steel-CFRP stirrups, it could be concluded that the steel-CFRP stirrups represent the effective solution of premature failure of FRP stirrups at the bends.

Shear design of reinforced concrete beams with FRP longitudinal and transverse reinforcement

The shear resisting mechanisms of reinforced concrete (RC) beams with longitudinal and transverse FRP reinforcement can be affected by the mechanical properties of the FRP rebars. This paper presents a mechanical model for the prediction of the shear strength of FRP RC beams that takes into account its particularities. The model assumes that the shear force is taken by the un-cracked concrete chord, by the residual tensile stresses along the crack length and by the FRP stirrups. Failure is considered to occur when the principal tensile stress at the concrete chord reaches the concrete tensile strength, assuming that the contribution of the FRP stirrups is limited by a possible brittle failure in the bent zone. The accuracy of the proposed method has been verified by comparing the model predictions with the results of 112 tests. The application of the model provides better statistical results (mean value Vtest/Vpred equal to 1.08 and COV of 19.5%) than those obtained using the design equations of other current models or guidelines. Due to the simplicity, accuracy and mechanical derivation of the model it results suitable for design and verification in engineering practice.

Torsional Moment Capacity and Failure Mode Mechanisms of Concrete Beams Reinforced with Carbon FRP Bars and Stirrups

AbstractFiber-reinforced-polymer (FRP) bars and stirrups have emerged as internal flexural and shear reinforcement for reinforced-concrete (RC) members in different applications. Nonetheless, the torsional behavior of FRP RC members has not yet been defined. This paper presents the results of an investigation of the torsional strength and behavior of full-scale concrete beams reinforced with carbon-FRP (CFRP) bars and stirrups. The beams measured 4,000 mm long, 250 mm wide, and 600 mm deep and were tested under pure torsion loading. The test specimens included four beams reinforced with CFRP bars and stirrups and one control beam reinforced with conventional steel reinforcement. The test variables were the type of reinforcement and CFRP stirrup spacing. The test results indicated that the CFRP RC beam exhibited similar strength, cracking behavior, and post-peak torsional stiffness compared with the counterpart steel RC beam. The hollow-tube, space-truss analogy with the 45° inclination of diagonal compres...

Probabilistic design equations for the shear capacity of RC members with FRP internal shear reinforcement

FRP reinforcement for concrete has been developed to replace steel in several applications, particularly in corrosion-prone RC structures. FRP rebars, generally, have a lower modulus of elasticity compared to steel, which leads to wider cracks and larger deflections. Furthermore, the FRP rebars have a low transverse stiffness and strength which are usually neglected. Consequently, the effectiveness of concrete contribution to shear capacity in terms of aggregate interlock and dowel action of the longitudinal rebars is lower for FRP-RC members respect to steel. Eurocode-like design equations for predicting the shear strength of FRP-RC members are herein suggested. Based on a database containing 129 flexural tests on RC beams strengthened with FRP stirrups performed in different research institutions across the world, a statistical calibration of Eurocode-like design equations for the shear capacity of RC beams with FRP shear reinforcement has been performed according to two alternative regression-based proposals and the EN 1990-procedure that is used as a benchmark.

Punching Shear Behavior of Two-Way Slabs Reinforced with FRP Shear Reinforcement

AbstractThis study investigated the punching shear behavior of two-way concrete slabs with glass-fiber-reinforced polymer (GFRP) bars as flexural reinforcement and FRP stirrups (glass or carbon) as shear reinforcement. A total of 10 full-scale interior slab-column specimens measuring 2,500×2,500 mm, with thicknesses of either 200 or 350 mm, and 300×300-mm square column stubs were fabricated and tested under monotonic concentric loading until failure. These tests aimed at investigating the behavior of GFRP-reinforced two-way concrete slabs reinforced with FRP stirrups as shear reinforcement and evaluating their contribution to the punching shear capacity. The investigated parameters were the flexural reinforcement ratio and the shear reinforcement type (glass FRP and carbon FRP stirrups) and ratio. The test results revealed that using FRP stirrups as shear reinforcement increased the punching shear strength and deformation capacity of the test slabs. The increased punching shear strength and deformation c...

Prediction of shear strength of FRP reinforced concrete beams using fuzzy inference system

The objective of this paper is to develop a more accurate and reliable alternative method using fuzzy inference system (FIS) to predict the shear strength of FRP-reinforced concrete beams. Such an accurate model, which can lead to an economical use of FRP reinforcement, is in high demand since existing design provisions for shear capacity of FRP-RC beams are either very conservative or even inadequate mainly due to two factors. Firstly, the current design codes follow the conventional assumption of superposition of concrete plus stirrup contribution to the shear strength, hence ignoring any interaction between shear resisting mechanisms. Secondly, the current design guidelines simply assume that some modified versions of the shear design equations which are empirically derived for steel-reinforced concrete beams can be easily extended to cover FRP-RC beams although the guidelines vary greatly in the manner they modify the equations. Given very different properties of FRP as compared to those of steel, such an assumption, however, needs to be examined and validated. To relax both of these assumptions, the FIS approach offers an attractive solution because it does not require a priori information. As a result, the proposed FIS model compares favorably with a large data base containing the test results of 197 FRP-RC beams assembled from literature. Moreover, the proposed FIS model outperforms the latest design provisions for shear strength of FRP-RC beams, namely ACI 440-06 and CSA S806-02. Also, a special attention is paid to differentiate between shear-compression mode and shear-tension mode of failure, which are the two common types of shear failure for FRP-RC beams with FRP stirrups. In light of the proposed FIS model, modifications to the shear-compression resistance provided by the considered design guidelines are recommended.

Shear Capacity of Concrete Beams Reinforced with Continuous FRP Rectangular Spirals

The failure modes and the shear capacity of concrete beams reinforced with FRP reinforcement were discussed through an experimental investigation, in which continuous FRP rectangular spirals were used for shear reinforcement, while ordinary deformed steel bars are used for longitudinal reinforcement. Six concrete beams reinforced with FRP spirals were tested, the main variables considered were the shear reinforcement ratios, the shear span to depth ratios and the longitudinal reinforcement ratios. Two concrete beams of equal shear capacity which reinforced with continuous steel rectangular spirals were also tested to compare the behavior of concrete beams reinforced with different materials of spirals. All beams were tested as simply supported members subjected to a three-point load, the span of the beams varied in terms of different shear span to depth ratios. The test results show that the shear capacity and shear failure modes are greatly influenced by the shear reinforcement ratios and the shear span to depth ratios, the shear resistance provided by steel spirals is higher than that provided by FRP spirals in the case of equal shear capacity of beams, which is attributed to the differences in material properties and may result in different shear failure types. Based on the experimental program, four mechanical models are derived to give more accurate predictions of the shear capacity of test beams, the calculation results of these models are compared with that of the existing shear formulas or equations for concrete beams reinforced with FRP stirrups or spirals. The rotating-angle softened truss model, the strut-and-tie model, the shear formulas derived from the truss-arch model and Zsutty equations are suggested through comparison.

Effect of confining of boundary elements of slender RC shear wall by FRP composites and stirrups

Existing concrete shear walls may be considered vulnerable for continuing service life due to changes in the seismic codes requirements, subjecting to intensive dynamic loads such as earthquake or explosion, and damages due to some local destructive loads. A review on the previous studies shows that despite the squat walls, very limited analytical and/or experimental studies have been already conducted on the FRP strengthening of slender RC shear walls under monotonic loading. In this paper the main focus is on the confinement of the boundary elements of shear walls with FRP composites and its effect on flexural behaviour of the wall. Nonlinear finite element analysis is performed on two shear wall specimens using damage plasticity model and considering tension stiffening effect. Results of the current study showed the superior effectiveness of strengthening FRP composite layers on ductile behaviour of concrete shear walls; i.e. the confinement of the boundary elements with FRP increased the ultimate displacement under almost constant load up to 50%. Furthermore, the results showed that it is sufficient to apply the FRP layers only on the boundary elements in the plastic hinge region of the wall.

Behavior of Continuous FRP Rectangular Spirals as Shear Reinforcement for Concrete Beams

The replacement of steel stirrups with FRP stirrups in concrete structures can significantly improve the durability under severe environmental conditions, increase the service life of the structure. In some cases where the application of traditional concrete structures is restricted, such as structures that require the environment without magnetic and electric interferences, the best way is to use nonmetallic materials. Considering the unique properties compared with traditional reinforcements, FRP reinforcements is very suitable in such cases. This paper discussed the behavior of FRP stirrups used as shear reinforcement for concrete structures, continuous FRP rectangular spirals, a type of FRP stirrups, was used in the experimental investigation. Inspired by the test method suggested by ACI Committee 440, L shape specimen and U shape specimen are designed to test the tensile strength of continuous FRP rectangular spirals. Through the analysis of test results, it is indicated that the strength of the bent portion of FRP spirals is significantly lower than that of the straight portion, and the strength of FRP spirals would increase as the concrete strength or the embedment length of the bent portion increased. The loading mechanism of continuous FRP rectangular spirals embedded in concrete beams can be better represented by U shape specimen compared with L shape specimens, it is suggested by the author that the U shape specimen can be use when the dimension of FRP stirrups or spirals does not meet the requirements of ACI Committee 440 test method.

Shear Performance of RC Bridge Girders Reinforced with Carbon FRP Stirrups

One of the main components in girder-type bridges is bridge girder. This paper presents experimental data on the behavior and shear strength of concrete bridge girders reinforced with carbon fiber-reinforced polymer (CFRP) stirrups. A total of four large-scale reinforced concrete beams with a total length of 7,000 mm and a T-shaped cross section were constructed and tested up to failure. The test variables were the type and ratio of shear reinforcement (stirrups). The test beams included three beams reinforced with sand-coated CFRP stirrups of 9.5-mm-diameter spaced at d/2, d/3, and d/4 (where d is the beam depth) and a control beam reinforced with conventional steel stirrups of 9.5-mm-diameter spaced at d/2. The geometry of the test prototypes were selected to simulate the New England Bulb Tee (NEBT) beams that are being used by the Ministry of Transportation of Québec, Canada. As designed, three beams failed in shear due to CFRP stirrup rupture or steel stirrup yielding. While, the forth one, reinforced with CFRP stirrups spaced at d/4, failed in flexure due to yielding of longitudinal reinforcement. The test results were compared to predictions provided by different codes and design guidelines. The current ACI 440.1R-06 design method provides conservative predictions; however, the CAN/CSA S6-06 and JSCE 1997 underestimate the contribution of the FRP stirrups due to low strain limits.

Bend Strength of FRP Stirrups: Comparison and Evaluation of Testing Methods

This paper provides a comparison and evaluation of the current test methods used to determine the strength of fiber-reinforced polymer FRP bent bars/stirrups at the bend location bend strength. The available methods depend on applying tensile forces through the straight portion of the bent bar/stirrup and keeping the bend zone restrained to generate a stress perpendicular to the bend direction in addition to the stress in the longitudinal direction. This could be achieved through the ACI 440.3R-04 B.12 test method for U-shaped bare FRP bars. Another possible method is the ACI 440.3R-04 B.5 which evaluates the bend strength of FRP stirrups by embedding them in two concrete blocks, which are pushed apart until the rupture of the FRP bent bars. Both methods were employed in testing FRP stirrups and bent bars and the bend strength was compared. The test results showed that the ACI 440.3R-04 B.12 test method consistently underestimates the bend strength of FRP stirrups. On the other hand, B.5 test method is more reliable and representative to the actual state of stresses in real concrete structural elements.

Discussion of “Comparison of Roles of FRP Sheets, Stirrups, and Steel Fibers in Confining Bond Critical Regions” by Bilal S. Hamad and Ahmad A. Rteil

Studies on confinement of tension lap splices and comparison of confinement provided by various techniques such as FRP wrapping, the use of fiber-reinforced concrete and the use of transverse steel have been very meticulously done by the authors. The authors are to be commended for their contribution to the understanding of bond performance with various levels of confinement. The discusser would like to highlight the effect of the size of the specimen on the results reported by the authors. The following points may be taken for discussion. 1. The length of the splice is fixed as 300 mm irrespective of the diameter of the bar. However, the amount of transverse reinforcement needed to provide adequate ductility depends on the splice length Azizinamani et al. 1999 . Because the length of the splice depends on the bar diameter, testing the confinement with a fixed splice length of 300 mm may not give the required results. 2. When the diameter of the bar being tested is varied, the size of the specimen tested must be proportional to the bar diameter Azizinamini et al. 1993 . In the investigations done by the authors, the depth of the beam is kept constant for all the specimens with different bar diameters. This makes the sec-

Mechanical and Structural Characterization of New Carbon FRP Stirrups for Concrete Members

This paper describes an experimental program conducted to develop new carbon fiber reinforced polymer (CFRP) stirrups as shear reinforcement for concrete members. The structural behavior of the CFRP stirrups was examined. To simulate the performance mechanism of stirrups in concrete beams, the CFRP stirrup was embedded in two concrete blocks and tested in tension by pushing the concrete blocks away from each other. A total of 12 specimens were constructed and tested to failure. The test variables were the tail length of the stirrup beyond the bent portion, the stirrup anchorage, the bar diameter, and the embedment length. In addition, two full-scale concrete beams reinforced with CFRP stirrups as shear reinforcement were constructed and tested to failure. Test results of the concrete blocks indicated that the strength capacity at the bend of the newly developed CFRP stirrups was adequate and fulfilled the design requirements of different codes and design guides. Further, the tail length was found to be no...

Performance of RC frames with hybrid reinforcement under reversed cyclic loading

The use of FRP as reinforcement in concrete structures has been growing rapidly. A potential application of FRP reinforcement is in reinforced concrete (RC) frames. However, due to FRP's predominantly elastic behaviour, FRP-RC members exhibit low ductility and energy dissipation. Hybrid steel-FRP reinforcement can be a viable solution to the lack of ductility of FRP-RC members. Using two layers of reinforcement in a section, FRP rebars can be placed in the outer layer and steel rebars in the inner layer away from the effects of carbonation and chloride intrusion. Combined with the use of FRP stirrups, this approach can enhance the corrosion resistance of RC members. However, current design standards and detailing criteria for FRP-RC structures do not provide detailed seismic provisions. In particular, the design and detailing of beam-column joints is a key issue in seismic design. During recent earthquakes, many structural collapses were initiated or caused by beam-column joint failures. Thus, research is needed to gain a better understanding of the behaviour of FRP and hybrid FRP-steel-RC under seismic loading. In this study, three full-scale beam-column joint specimens reinforced with steel, GFRP and a hybrid GFRP-steel configuration, respectively were tested in order to investigate their performance in the event of an earthquake.