Transverse Steel Reinforcement Research Articles

Seismic Upgrade with Carbon Fiber-Reinforced Polymer of Columns Containing Lap-Spliced Reinforcing Bars

Most reinforced concrete columns designed and constructed prior to 1970 may have inadequate seismic resistance. This study evaluates the effectiveness of carbon fiber-reinforced polymer (CFRP) jackets in the strengthening and repair of such columns under simulated earthquake loading. Six circular and 6 square columns were constructed and tested. The columns were 1.47 m (58 in.) long and had a 510 x 760 x 810 mm (20 x 30 x 32 in.) stub at one end with a construction joint at the interface and spliced longitudinal bars in the columns. The variables studied in this program included effect of the presence of lap splices, the effectiveness of CFRP in pre-earthquake strengthening and post-earthquake retrofitting of deficient columns, as well as effects of level of axial load, shape of column cross section, and transverse steel reinforcement details. Findings suggest that the CFRP retrofitting technique was effective in enhancing the seismic resistance of the columns and resulted in more stable hysteresis curves with lower stiffness and strength degradations compared with the unretrofitted columns. Ductility improvements in the square columns with lap splices as a result of CFRP retrofitting were significantly lower than that of comparable circular columns.

Mechanism and modeling of transverse cracking development in continuously reinforced concrete pavement

The relationship between the initiation and spacing of transverse cracking in continuously reinforced concrete pavement (CRCP) and transverse steel reinforcement has been investigated. Field evaluations of CRCP test sections included surface condition inspection using digital video, ground penetrating radar (GPR) survey to determine the relative location of transverse bars with respect to transverse cracks, falling weight deflectometer (FWD) testing to evaluate the crack load transfer efficiency (LTE), and ground-truth coring. Field-testing results suggested that the mean crack spacing was identical to the design spacing of transverse steel bars. A three-dimensional finite element (FE) model was then developed to evaluate the mechanisms that contribute to the initiation of transverse cracking in CRCP. Results of the FE model indicated that two controlling mechanisms may contribute to the initiation of transverse cracking in CRCP: build-up of uniform compressive longitudinal stress at the pavement surface and tensile stress concentration in the vicinity of the transverse steel bars. In general, a close correlation appears to exist between the spacing of transverse cracking and the design spacing of transverse steel bars.

Seismic Behavior of High-Strength Concrete Columns Confined by Fiber-Reinforced Polymer Tubes

The use of high-strength concrete (HSC) in seismically active regions poses a major concern because of the brittle nature of material. The confinement requirements for HSC columns may be prohibitively stringent when ordinary grade transverse steel reinforcement is used. An alternative to conventional confinement reinforcement is the use of fiber-reinforced polymer (FRP) tubes in the form of stay-in-place formwork which can fulfill multiple functions of: (1) formwork; (2) confinement reinforcement; and (3) protective shell against corrosion, weathering and chemical attacks. The use of stay-in-place FRP formwork is investigated as concrete confinement reinforcement for HSC and normal strength concrete (NSC) columns with circular cross sections. Large-scale specimens with 270 mm circular cross-sections and different concrete strengths were tested under constant axial compression and incrementally increasing lateral deformation reversals. FRP tubes were manufactured from carbon fiber sheets and epoxy resin. T...

Shear and Flexure Analysis of Prestressed Concrete T-Beams containing Steel Fibers over Partial or Full Depth

Twenty-three partially prestressed steel fiber reinforced concrete T-beams have been tested to investigate the shear and flexure capacity, and the resulting mode of failure. These beams have been cast using three grades of concrete (35 MPa, 65 MPa and 85 MPa) with fibers (Vf = 1,5 percent) in different locations in the cross section of the beam (web, flange or full depth). The beams designed for flexural failure were provided with a transverse steel reinforcement ratio of 0,66 percent while those designed to fail in shear had 0,33 percent of transverse steel. These beams have been tested varying the shear span to depth (a/d) ratio between 1,59 and 5,30. Based on the test results, prediction models were developed to compute flexural and shear capacities of the partially prestressed steel fiber reinforced beams. The proposed model accounts for the contribution of fiber pullout to sustain the opening up of cracks in beam. Addition of steel fibers in partial or full depth in partially prestressed concrete beams transforming its mode of failure from brittle shear to ductile flexure has been analytically investigated. The plots of the variation of the ultimate moment carrying capacity (Mu) against the shear span to depth (a/d) ratio indicate the transition of failure mode from arch to beam action or shear to flexure occurs at lower values of ‘a/d’ when fibers are introduced into the matrix with respect to the plain concrete beam. The transformation of failure mode takes place for lower value of ‘a/d’ ratio for high strength concrete beams than for normal strength concrete beams.

Fiber-Reinforced Polymer Shear Strengthening of Reinforced Concrete Beams: Experimental Study and Analytical Modeling

Fiber reinforced polymer (FRP) is used in many forms for strengthening reinforced concrete (RC), including side bonding, U-jacketing, and complete wrapping. This article reports on a study that investigated RC rectangular beams strengthened in shear with externally bonded U-wrapped carbon fiber-reinforced polymer (CFRP). The authors report on the complex failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP sheets and show some mechanisms of interaction between the externally applied FRP sheets and the internal shear steel reinforcement with different static schemes. They note that this interaction is not considered in the actual code provisions but strongly influences the efficiency of the shear strengthening rehabilitation technique. The authors conclude by proposing an analytical model that allows the estimation of the interacting contributions to the shear capacity of the strengthened beams. This model is based on simplified hypotheses but can be used as a first step toward a comprehensive approach about the shear behavior of FRP-strengthened beams.

Effect of transverse steel and shear span on the performance of RC beams strengthened in shear with CFRP

This paper presents results of an experimental investigation involving twelve tests on 3000 mm-long reinforced concrete (RC) T-beams strengthened in shear with externally bonded carbon fiber reinforced polymer (CFRP) composites. The main objective of this study was to evaluate the effect of the following parameters on the shear performance of strengthened RC beams: (i) the CFRP ratio, (ii) the transverse steel reinforcement ratio, and (iii) the type of beams (deep versus slender). The results clearly indicated the existence of an interaction between the internal transverse steel reinforcement and the externally applied CFRP in terms of the gain in the shear capacity as well as of the strains. Results also showed the influence of the type of beam on the load capacity gain due to the CFRP. Finally, comparison of the shear resistance values as recommended by the fib TG9.3 (2001), ACI-440R (2002), and CSA S806-02 (2002) guidelines, with the test results clearly indicated that guidelines fail to capture the influence of the studied parameters.

Contribution à l'étude du renforcement en cisaillement des poutres en béton armé à l'aide de matériaux composites avancés

This paper presents results of an investigation on the shear strengthening of reinforced concrete (RC) beams with externally applied fibre reinforced polymer (FRP) composites. The first part of the study reviews and synthesizes the state of the art in the subject. Also, the requirements and recommendations specified in the Canadian CSA S806-02 standards, the American ACI-440 guidelines, as well as the European fib TG9.3 recommendations are compared with the test results reported in the literature so far. This part of the study indicates that the major parameters involved in the behaviour of RC beams strengthened in shear with FRP were not fully investigated. This can explain the observed discrepancies between the resistance values predicted by the codes and guidelines, and those obtained by tests. This has been the main impetus to carry out an experimental investigation, which is the subject of the second part of this paper. The objective of this experimental investigation was to study the influence of the following parameters on the performance of RC beams strengthened in shear with FRP composites: (i) the FRP ratio, (ii) the transverse steel reinforcement ratio, and (iii) the type of beam (deep versus slender). Results clearly showed the interaction between the FRP composite and the internal transverse steel reinforcement in the shear resistance mechanism. Results also showed the influence of the type of beam on the gain due to FRP on the carrying capacity of the beam.Key words: shear, reinforcement, concrete, composites, experimental, parameters.

The Effect of External Confinement on Flexural Hinging in Drilled Pile Shafts

Six one-third-scale CIDH pile shafts were tested to simulate the subgrade moment pattern in an in situ pile shaft. The test fixture could also simulate the presence of external confinement provided by soil. Three levels of transverse reinforcement were tested. Three piles were tested with external confinement, and three without. The presence of external confinement played a significant role in enhancing flexural ductility by supporting the confining action of the transverse steel reinforcement. The presence of external confinement also increased the plastic hinge length, significantly reducing local curvature demand by allowing the hinge rotation requirements to be spread over a greater length of the pile shaft. In the absence of external confinement, the transverse reinforcement levels specified under current column design guidelines provided adequate flexural ductility for seismic response.

Effect of Confinement on Bond Strength between Steel Bars and Concrete

This work experimentally examines the local bond aspect between steel bars and concrete confined with ordinary transverse steel. The test parameters included diameter of reinforcing bar, ratio of concrete cover to bar diameter, and area of transverse reinforcement. Results were compared with those of similar specimens for concrete confined either internally using steel fiber reinforcement or externally using fiber-reinforced polymer (FRP) sheets. Based on the comparisons, a unified expression for the local bond strength of confined concrete is derived, and a general model for the local bond stress-slip response is proposed and used to conduct an analytical evaluation of the effect of confinement on development/splice strength. Results predicted by the analysis were in good agreement with experimental results. For small development/splice lengths corresponding to local bond conditions, confining the concrete only slightly increases the local bond resistance but leads to considerable improvement in the ductility of bond failure. The corresponding ductility at the local level allows, for the practical range of development/splice lengths, more bar lugs to participate in resisting the applied bar force resulting in a more uniform bond stress distribution along the development/splice length and, consequently, a sizable increase in the average bond strength at bond failure as compared with plain unconfined concrete. The bond strength due to FRP confinement increases in proportion to the modulus of elasticity of the FRP material. For the same area of transverse reinforcement per unit length along the splice, taking into account the relative modulus of elasticity of the confining materials, external confinement of concrete using FRP sheets is more effective in increasing the development/splice strength than internal confinement with ordinary transverse steel. A general design expression is proposed to estimate the development/splice length of steel bars embedded in concrete confined with ordinary transverse steel, FRP, or steel-fiber reinforcement.

Steel–concrete bond deterioration due to corrosion: finite-element analysis for different confinement levels

The effect of corrosion on bond strength of ribbed bars in reinforced concrete (RC) is studied by axisymmetric finite-element analysis, modelling different levels of confinement given by the transverse steel reinforcement. The importance of this parameter is highlighted by a summary of the experimental results in the literature. An approach is developed to model the corrosion product expansion causing concrete-cover cracking. The numerical analyses reproduce the experimental results of the bond tests performed in two different studies. A parametric study shows the effect of the varying transverse steel percentage on the residual bond strength, and the influence of different arrangements of confinement bars. These results lead to a discussion of the different findings of several experimental researches. The conclusions regard the main parameters to be included in a model to predict the bond strength of corroded bars. Some indications for the assessment of corroding structures are given.

Behavior in compression of lightweight fiber reinforced concrete confined with transverse steel reinforcement

Abstract The compressive behavior of lightweight fiber reinforced concrete confined with transverse reinforcement consisting of steel stirrups or spirals was analyzed. Pumice stone and expanded clay aggregates were utilized to decrease the weight of the composite; hooked steel fibers were also added. The investigation was carried out by testing cylindrical and prismatic specimens of different sizes in compression using an open-loop displacement control machine, recording the full load–deformation curves. The influence of the dimensions and shape on the bearing capacity and on the ductility of the specimens confined with transverse steel reinforcements was analyzed. The results show the possibility of obtaining high confinement level through the coupled effect of fibers and steel transverse reinforcement.

Nonlinear behavior of deep reinforced concrete coupling beams

Six large scale models of conventionally reinforced concrete coupling beams with span/depth ratios ranging from 1.17 to 2.00 were tested under monotonically applied shear loads to study their nonlinear behavior using a newly developed test method that maintained equal rotations at the two ends of the coupling beam specimen and allowed for local deformations at the beam-wall joints. By conducting the tests under displacement control, the post-peak behavior and complete load-deflection curves of the coupling beams were obtained for investigation. It was found that after the appearance of flexural and shear cracks, a deep coupling beam would gradually transform itself from an ordinary beam to a truss composed of diagonal concrete struts and longitudinal and transverse steel reinforcement bars. Moreover, in a deep coupling beam, the local deformations at the beam-wall joints could contribute significantly (up to the order of 50%) to the total deflection of the coupling beam, especially at the post-peak stage. Finally, although a coupling beam failing in shear would have a relatively low ductility ratio of only 5 or even lower, a coupling beam failing in flexure could have a relatively high ductility ratio of 10 or higher.

Modelling of rectangular RC columns strengthened with FRP

Existing analytical models for predicting the stress–strain or load–displacement response of fibre-reinforced polymer (FRP)-confined concrete are mostly derived for cylindrical plain concrete columns. In practice, however, typical concrete columns come in various shapes including circular, square, or rectangular and incorporate longitudinal and transverse steel reinforcements. Furthermore, strengthening or repairing is typically done while the column is under service loading. In this paper, an analytical model is proposed to predict the load–displacement response of wall-like (i.e. high aspect ratio) reinforced concrete columns strengthened with FRP wraps with and without sustained loading. The model assumes that the general load–displacement response of the strengthened column consists of two distinct branches: a parabolic ascending branch and a linear descending branch. The ascending branch is influenced by the lateral confining pressure from the transverse reinforcement as well as the FRP wraps, while the descending branch is influenced by the buckling of the longitudinal reinforcement and the failure of the core concrete. Comparisons between model results and experimental results indicate close agreement between the two.

Fiber Reinforced Polymer Shear Strengthening of Reinforced Concrete Beams with Transverse Steel Reinforcement

The paper aims to contribute to a better understanding and modeling of the shear behavior of reinforced-concrete (RC) beams strengthened with carbon fiber reinforced polymer (FRP) sheets. The study is based on an experimental program carried out on 11 beams with and without transverse steel reinforcement, and with different amounts of FRP shear strengthening. The test results provide some new insights into the complex failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP sheets. After the discussion of the above topics, a new upper bound of the shear strength is introduced. It should be capable of taking into account how the cracking pattern in the web failing under shear is modified by the presence of FRP sheets, and how such a modified cracking pattern actually modifies the anchorage conditions of the sheets and their effective contribution to the ultimate shear strength of the beams.

Effect of Matrix Ductility on Deformation Behavior of Steel-Reinforced ECC Flexural Members under Reversed Cyclic Loading Conditions

Summarizes the results of research aimed at investigating the effect of ductile deformation behavior of engineered cementitious composites (ECC) on the response of steel reinforced flexural members to lateral load reversals. The combination of a ductile cementitious matrix and steel reinforcement is found to result in improved energy dissipation capacity, reduction of transverse steel reinforcement requirements, and damage-tolerant inelastic deformation behavior. Basic concepts and composite deformation mechanisms of steel reinforced ECC are provided, experimentally verified, and compared to conventional reinforced concrete using small-scale specimens. Results indicate advantageous synergistic effects between ECC matrix and steel reinforcement with respect to compatible deformation, structural composite integrity, and damage evolution, and suggest integrating advanced materials design into the structural design process.

Stress-strain Behavior In Compression Of Lightweight Fiber Reinforced Concrete Under Rnonotonic And Cyclic Loads

Compressive experimental behavior of lightweight fiber reinforced concrete combined with traditional transverse steel reinforcement consisting in steel spirals was analyzed. Lightweight aggregates were utilized to give lightness to the composite, and steel hooked fibers were added to the matrices besides the traditional steel lateral reinforcement to increase the ductility of members in compression. The investigation was carried out through compressive tests on cylindrical specimens, by imposing monotonic or cyclic displacements and recording the full load-deformation curves. Empirical stress-strain equations were adopted to reproduce with a good level of approximation the actual behavior of cylindrical specimens under uniaxial compression loads. considering the coupled effect of fibers and steel transverse reinforcement.

連続繊維シートによるRC部材の付着割裂強度増大効果 : 第1報付着強度式の提案

Cantilever type bond tests were carried out to evaluate the bond strength of reinforced concrete members confined with FRP sheets. Clear increase in bond strength due to sheet confinement was confirmed, and a tentative bond strength equation was formulated as the similar form for the contribution due to transverse steel reinforcement in the Fujii-Morita bond equation. The tentative equation was modified on the basis of the supplementary bond test results of which variable was the width of the specimens, and at last a bond splitting strength equation for the RC members confined with FRP sheets was proposed

Confinement of reinforced concrete columns with fibre-reinforced composite sheets - an experimental study

The wrapping of fibre-reinforced composite sheets around concrete columns is a promising method for structural strengthening and repair. This rehabilitation technique is of practical interest, as the lay-up of the sheets is rather easy; it does not require specialized tools, and the epoxy resins employed cure at ambient temperatures. Here, results of an experimental investigation are reported for 16 round reinforced concrete columns 300 mm in diameter and 1200 mm high. These columns were confined by means of carbon-epoxy sheets and loaded concentrically in axial compression. The effects of various parameters on the structural behaviour of the confined concrete columns are investigated. These parameters included the concrete strength, longitudinal steel reinforcement, steel stirrups, steel corrosion, and concrete damage. The test results show that composite confinement can considerably enhance the structural performance of concrete columns, especially with regard to ductility. The potential to restore the full strength of severely damaged columns is also demonstrated, as retrofitted columns exhibit axial load carrying capacities equal or superior to those of undamaged columns, along with significant increases in ductility. The contribution of the transverse steel reinforcement is seen to be minimal, as long as the stirrup spacing is medium to large. For such cases tests on plain concrete cylinders are sufficient for further investigations of this retrofit method, as the key parameters which really affect strength and ductility are the concrete strength, composite fibre type, and sheet thickness.Key words: fibre composite sheets, confinement, concrete, column repair, rehabilitation, strengthening.

Compressive Stress-Strain Behavior of Normal and High-Strength Carbon Fiber Concrete Reinforced with Steel Spirals

Carbon-fiber reinforced concrete was combined with traditional transverse steel reinforcement in the form of steel spirals in an attempt to increase the energy absorption and strength of the concrete under both monotonic and cyclic loading. The stress-strain relationship of concrete in compression in both unconfined and confined states was examined. The descending branch of the stress-strain curve was recorded to investigate the ductility of normal and high-strength fiber reinforced concrete confined with steel spirals. The experimental program consisted of testing 100 mm-by-200 mm concrete cylinders under compression at two different strength levels: normal (48 MPa) and high strength (75 MPa), reinforced with different percentages (volume fractions of 1.5%, 2%, and 3%) and lengths (7.5 mm, 20 mm, and 30 mm) of high-strength carbon fibers with an equivalent diameter 0.78 mm. These were then repeated with the addition of spiral reinforcement, 5 mm in diameter, with different pitches (25 mm and 50 mm). This study was carried out to evaluate the optimum combination of fibers and steel spirals to obtain the same level of fracture energy dissipation as that obtained by using only a high-volume percentage of spiral steel, which is generally used in real structures.

Plasticity computations for the design of the ductility of circular concrete columns

The theory of plasticity and the deformation compatibility equations are used to evaluate the development of lateral confinement of concrete columns, and the resulting increases in strength and ductility. Concrete is modeled as an elasto-plastic material following a simple Drucker-Prager nonassociative hardening model. The failure, hardening and dilatational modeling is complicated and features 14 material parameters. However, an extensive study that has been conducted here has shown that these parameters are uniquely related to the unconfined compression strength of concrete f' c, thus establishing a one-parameter model. The solution is based on the integration of the elasto-plastic relations for the concrete core, the transverse steel reinforcement and the concrete cover. The lateral pressure on the concrete core is calculated based on the compatibility of lateral deformations of the concrete core and the surrounding reinforcement. The ability of the method to predict the response of confined compression members is demonstrated based on numerous published experimental results.