Retrofitting Reinforced Concrete Research Articles

Effect of carbon nanotubes on strengthening of RC beams retrofitted with carbon fiber/epoxy composites

The effect of carbon nanotubes (CNTs) in improving the strengthening efficiency of carbon fiber/epoxy composites retrofitted reinforced concrete (RC) beams was investigated. A total of sixteen simply supported RC beams were prepared and tested under four-point loading. The incorporation of CNTs within the systems was done by modifying the epoxy resin using CNTs and/or coating the carbon fiber sheets with CNT enriched sizing agent. The effects of epoxy modification with CNTs, incorporation of CNT enriched sizing agent, anchorage length, and number of retrofitting layers were investigated through crack patterns, failure modes, load-deflection curves, and scanning electron microscopy (SEM) micrographs of fractured surfaces. Experimental results showed that using CNT modified epoxy resin enhanced the ultimate load and stiffness of retrofitted beams. The enhancement efficiency highly depends on the level of dispersion of CNT, anchorage length, and number of retrofitting layers. SEM characterization showed that CNTs could improve the adhesion at the concrete/epoxy interface and carbon fiber/epoxy interface leading to improvement in the load transfer and ultimate load of the strengthened beams.

Retrofitting nonseismically detailed exterior beam–column joints using concrete covers together with CFRP jacket

This paper introduces a new method for retrofitting reinforced concrete (RC) exterior beamcolumn T joints, using segmental circular concrete covers together with Carbon Fibre Reinforced Polymer (CFRP). Two RC T connections without transverse reinforcement at the joints were cast and tested. The first connection (Strengthened specimen, TS) was glued with the concrete covers around the column at the joint area to modify it from a square to a circular section and then it was wrapped with CFRP to strengthen its capacity. A load was first applied on the second connection (Repaired specimen, TR) to cause a serious failure then it was repaired. The repair scheme of the second connection was identical to the first connection. Results of testing the two connections have shown that the performances of both the strengthened and the repaired connections were improved significantly. The glued concrete covers worked well with the existing concrete to resist shear load. Moreover, the wrap on the modified circular sections helped in increasing the effectiveness of CFRP by increasing the confinement effect on the concrete and reduce the possibility of debonding of CFRP at the joints.

Acoustic emission monitoring of damage progression in CFRP retrofitted RC beams

The increased use of carbon fiber reinforced polymer (CFRP) in retrofitting reinforced concrete (RC) members has led to the need to develop non-destructive techniques that can monitor and characterize the unique damage mechanisms exhibited by such structural systems. This paper presented the damage characterization results of six CFRP retrofitted RC beam specimens tested in the laboratory and monitored using acoustic emission (AE). The focus of this study was to continuously monitor the change in AE parameters and analyze them both qualitatively and quantitatively, when brittle failure modes such as debonding occur in these beams. Although deterioration of structural integrity was traceable and can be quantified by monitoring the AE data, individual failure mode characteristics could not be identified due to the complexity of the system failure modes. In all, AE was an effective non-destructive monitoring tool that can trace the failure progression in RC beams retrofitted with CFRP. It would be advantageous to isolate signals originating from the CFRP and concrete, leading to a more clear understanding of the progression of the brittle damage mechanism involved in such a structural system. For practical applications, future studies should focus on spectral analysis of AE data from broadband sensors and automated pattern recognition tools to classify and better correlate AE parameters to failure modes observed.

Flexural Strengthening of RC Beams with an Externally Bonded Fabric-Reinforced Cementitious Matrix

AbstractConcrete structures deteriorate for various reasons and upgrading is needed to ensure their continued safe working conditions. Retrofitting reinforced concrete (RC) beams have been accomplished using various techniques, namely, steel plates, external posttensioning, externally bonded fiber-reinforced polymer (FRP), and near-surface-mounted FRP systems to increase flexural and shear capacity. The objective of this paper is to investigate the feasibility of fabric-reinforced cementitious-matrix (FRCM) materials as an alternative external strengthening technique for RC members. The FRCM is a composite material consisting of one or more layers of cement-based matrix reinforced with dry-fiber fabric. The experimental program consists of testing 18 RC beams strengthened in flexure with two different FRCM schemes (one and four reinforcement fabrics). An analysis and design are conducted following the well-established formulation to calculate the flexural capacity of the beams and compare their results wi...

Experimental and Analytical Investigations on Flexural Behaviour of Retrofitted Reinforced Concrete Beams with Geopolymer Concrete Composites

Existing multi-storey buildings in earthquake prone regions of India are vulnerable to severe damage under earthquakes. So there is an urgent need for retrofitting of deficient and damaged buildings. Portland cement (PC) production is under critical review due to high amount of carbon dioxide gas released to the atmosphere and Portland cement is also one among the most energy-intensive construction material. The current contribution of greenhouse gas emission from Portland cement production is about 1.5 billion tons annually or about 7% of the total greenhouse gas emissions to the earth’s atmosphere. So retrofitting of existing deficient building using eco-friendly material which could promise higher structural performance than the original building is essential.Geopolymer concrete (GPC) is a non-Portland cement binder based on alkaline activation of industrial wastes such as fly ash and GGBS. The application Geopolymer concrete in retrofitting can reduce the production of cement and can create an ecofriendly construction atmosphere to an extent. So the feasibility of geopolymer concrete in retrofitting of damaged structures is required to be studied. This paper presents the details of the mix designs of GPC mixes, preparation of retrofitted test specimen, testing and evaluation with respect to cracking, ultimate load, deflection and failure modes. Non-linear finite element analysis of beams by 3D modeling of concrete (solid 65 element) and discrete modeling of reinforcement (Link 8 element) was carried out using ANSYS software. The load deflection characteristics, failure modes and crack patterns are obtained from the experimental and analytical studies. The results of the study indicate that the performance of retrofitted beam with GPC and control beam shows good agreement in ultimate load, failure modes and crack patterns.

Seismic protection of lap-spliced RC columns using SMA wire jackets

This study used two forms of shape memory alloy (SMA) wires, nickel–titanium–niobium (NiTiNb) and nickle–titatanium (NiTi), to retrofit reinforced concrete (RC) columns with lap splices. NiTiNb shape memory alloys are considered to show a wider temperature window compared with NiTi shape memory alloys and thus may be beneficial for applications in the area of civil structures. The goals of this study are to assess the mechanical behaviour of the two shape memory alloy wires and investigate the seismic performance of the shape memory alloy wire jackets for reinforced concrete columns with lap splices. First, the study investigates the recovery and the residual stress of NiTiNb and NiTi shape memory alloy wires. This study also evaluates the behaviour of the NiTiNb shape memory alloy wire under residual stresses. The results indicate that the shape memory alloy wire jackets show good performance for reinforced concrete columns with lap splices. The shape memory alloy wire jackets increase the ductile behaviour of the lap-spliced columns without strength degradation. The results show that their performance is superior to that of the RC column with continuous reinforcements. The study shows that the NiTiNb shape memory alloy wire jackets are more adaptive for retrofit of RC columns than the NiTi shape memory alloy wire jackets since the NiTiNb shape memory alloy shows more appropriate temperature windows for the applications of shape memory alloys in civil structures.

Nonlinear finite element modeling of RC beams strengthened with NSM FRP rods

The introduction of Near Surface Mounted (NSM) strengthening technique to retrofit reinforced concrete (RC) members in flexure using Carbon Fiber-Reinforced Polymer (CFRP) rods is becoming more attractive to both researchers and designers in recent years. Although NSM strengthening systems has been used extensively, still further experimental, analytical, and numerical research is warranted to understand the effect of the several strengthening parameters on the flexural performance of RC members. This paper presents the development of a detailed 3D nonlinear finite element (FE) numerical model that can accurately predict the load-carrying capacity and response of RC beams strengthened with NSM FRP rods subjected to four-point bending loading. The developed FE model considers the nonlinear constitutive material properties of concrete, yielding of steel reinforcement, cracking of the filler bonding materials, bond slip of the steel and NSM reinforcements with the adjacent concrete surfaces, and bond at the interface between the filling materials and concrete. The numerical FE simulations were compared with experimental measurement tested by other investigators comprising of seven specimen strengthened with NSM CFRP rods in addition to one un-strengthened control specimen. Overall, the predicted FE mid-span deflection responses agreed very well with the corresponding measured experimental tested data at all stages of flexural loading. Furthermore, the developed models were also capable of predicting the failure mode of the strengthened tested specimen such as NSM rod debonding (peeling off) and concrete cover separation. The validated FE models are then used to study the effect of different NSM material bar types and sizes to provide further information over the limited available experimental data. It is concluded that the developed FE model is suitable as a practical and economical tool especially in design-oriented parametric studies for accurate modeling and analysis of flexural strengthening of RC members with NSM reinforcement.

Comparative behaviour of hollow columns confined with FRP composites

Confining columns with fibre reinforced polymer (FRP) composites have been investigated in the last few decades to address the problem of upgrading and retrofitting reinforced concrete (RC) columns; however, most studies have concentrated on solid columns. This paper investigates the comparative behaviour of FRP confined hollow RC columns subjected to concentric loading. A total of twelve RC columns made from high strength concrete (HSC) were cast and tested. Six of the columns had a circular cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole) and the remainder columns had a square cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole). Six columns in total, three from each configuration were left unconfined as control specimens, while the others were confined with FRP. It was found that FRP confinement increased hollow RC columns’ axial load and ductility capacities; and hollow columns having circular holes had better performance compared to hollow columns having square holes.

Modeling progressive collapse in reinforced concrete buildings using direct element removal

AbstractThis paper presents a novel analytical formulation of an element removal algorithm based on dynamic equilibrium and the resulting transient change in system kinematics, by applying imposed accelerations instead of external forces at a node where an element was once connected. The algorithm is implemented into an open‐source finite element code, numerically tested using a benchmark structural system with simplified element removal criteria, and able to capture the effect of uncertainty in member capacity. Realistic element removal criteria are introduced for mode‐dependent gravity load collapse of seismically deficient and retrofitted reinforced concrete (RC) columns and unreinforced masonry (URM) infill walls. Two applications are conducted using structural systems of RC frames with URM infill walls. The first is a probabilistic study of a one‐story model subjected to an ensemble of 14 ground motion recordings from similar neighboring sites during an earthquake event. The study produces empirical probability curves for partial and complete collapse conditioned on different hazard levels, and concludes that the intra‐event variability is a major source of uncertainty affecting the outcome of progressive collapse simulations. The second application is a deterministic sensitivity study of progressive collapse response in a five‐story structural model to uncertainty in live load, stiffness, damping, and seismic hazard level, subjected to one ground motion record. The analysis identifies the time at incipient collapse as an adequate sensitivity measure, and the uncertainty in ground motion intensity as the most important, followed by the stiffness of the URM infill wall. Copyright © 2009 John Wiley & Sons, Ltd.

Interfacial stresses in FRP-plated RC beams: Effect of adherend shear deformations

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre reinforced polymer (FRP) plates to their tensile faces. An important failure mode of such plated beams is the debonding of the FRP plates from the concrete due to high level of stress concentration in the adhesive at the ends of the FRP plate. This paper presents an improved solution for interfacial stresses in a concrete beam bonded with the FRP plate by including the effect of the adherend shear deformations. The analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions.

A numerical procedure simulating RC structures reinforced with FRP using the serial/parallel mixing theory

The use of fiber reinforced polymers (FRP) to reinforce and retrofit reinforced concrete (RC) structures has become one of the main applications of composites in civil engineering. This paper describes a procedure, based on a finite element formulation, that can be used to perform numerical simulations of RC structures reinforced with FRP. Composites are treated using the serial/parallel mixing theory, which deduces the composite behavior from the constitutive equations of its components. A new construction-stages algorithm is developed for simulating retrofitted structures. The performance of the proposed formulation is proved comparing numerical and experimental results. Finally, the developed code is used to obtain the optimum FRP reinforcement configuration for a RC frame structure.

Interfacial shear stress in FRP-plated RC beams under symmetric loads

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre-reinforced polymer (FRP) plates to their soffits. An important failure mode of such plated beams is debonding of the FRP plates from the concrete due to high level interfacial stresses near the plate ends. A closed-form rigorous solution for the interfacial stresses in simply supported beams bonded with thin plates and subjected to arbitrary loads has been found, in which a non-uniform stress distribution in the adhesive layer was taken into account. This paper uses the rigorous solution to investigate the impact of symmetric loading configurations on the interfacial shear stress distributions, and concludes that the bending moments on the cross sections at the plate ends play a significant role in terms of stress concentration, while the shear forces on the same cross-section contribute little to the concentration. On the basis of this observation, this paper proposes a simplified approximate solution to the shear stress along the interface between concrete and adhesive layer. Compared with the rigorous and other approximate solutions, the simplified solution exhibits sufficient accuracy in terms of stress distribution and stress concentration localized near the plate ends. Due to its compact feature, the simplified solution is more suitable for engineering applications using a portable calculator and to be adopted in the codes of practices.

Seismic performance of repaired RC columns

Steel or fibre-reinforced polymer (FRP) composite jackets have been utilised in retrofitting reinforced concrete (RC) bridge columns and have shown to be effective in enhancing seismic performance of the structures. However, to date, only a few researches have been conducted on the behavioural characteristics of the repaired RC columns using steel or FRP jackets. In the present paper, the comparative performance of repaired RC columns using steel and CFRP jackets is presented. Also, the effect of transverse reinforcement ratio on the behaviour of the steel and the CFRP repairing is investigated. Monotonic and cyclic load tests are conducted on nine RC column specimens with different repairing strategies and transverse reinforcement ratios to compare the ultimate and the hysteretic behaviours. From the tests, it is observed that both steel and CFRP jacket repairing can significantly increase the ductility and the ultimate capacity of damaged columns. Notably, the steel jacket repaired columns show better energy dissipation capacity than the CFRP jacket repaired columns for columns with lower transverse reinforcement ratio. It is also observed that the location of plastic hinge region goes up as the transverse reinforcement ratio of RC columns increases.

Moment Redistribution in FRP and Steel-Plated Reinforced Concrete Beams

Research on retrofitting reinforced concrete (RC) beams and slabs using externally bonded (EB) fiber reinforced polymer (FRP) or steel plates has reached the stage where the flexural strength can be determined with confidence. Research has also shown that EB plated structures tend to debond at relatively low strains and to such an extent that guidelines often preclude moment redistribution which can severely restrict the use of plating. However, recent research on retrofitting using FRP and steel near surface mounted plates (NSM) has shown that NSM plates tend to debond at high strains which can allow substantial amounts of moment redistribution. A moment redistribution approach has been developed for both NSM and EB plated beams that allows for the wide range of debonding strains that can occur. This allows RC beams to be retrofitted for both strength and ductility which should help expand the use of this convenient and inexpensive form of retrofitting.

Moment redistribution parametric study of CFRP, GFRP and steel surface plated RC beams and slabs

It is common practice these days to retrofit reinforced concrete (RC) beams and slabs by adhesive bonding FRP or steel plates to their surfaces. However, research has shown that these external plates can debond prematurely at relatively low strains such that the ability of the plated section to redistribute moment to other sections is severely limited and to such an extent that guidelines often preclude moment redistribution. This restriction may severely limit the use of plating in buildings where ductility is a requirement. Tests on steel and FRP plated continuous beams have shown that moment redistribution can occur and a design procedure has been developed to determine the amount of moment redistribution for any type of adhesively bonded plated section, and for any type of plate material, such as steel or FRP plates. In this paper, an analysis approach to quantify the amount of moment redistribution is described and the concepts of positive and negative moment redistribution introduced. A parametric study is then used to illustrate how the plate material and geometric properties affect moment redistribution; in particular, the study looks at the effect of using carbon FRP plates and glass FRP plates, as well as steel plates that have been designed to either debond prior to yielding or yield prior to IC debonding. In summary, the paper will show that plated sections can redistribute moment and, hence, the present restriction can be removed which should extend the use of retrofitting by plating. Furthermore, the moment redistribution analysis procedure allows the engineer the freedom to choose the properties of the plate to design for moment redistribution.

Interfacial stresses in soffit-plated reinforced concrete beams

A popular new method for retrofitting reinforced concrete (RC) beams is to bond a fibre-reinforced polymer (FRP) plate to their soffit to increase their flexural strength. An important failure mode of such strengthened members is the debonding of the FRP plate from the concrete due to high interfacial stresses near the plate ends. This paper presents an explicit solution for interfacial stresses in a simply supported beam bonded with a thin plate and subjected to arbitrary loads. A set of arbitrary loads is decomposed into a symmetric component and an antisymmetric component. Closed-form solutions are derived for both the symmetric and antisymmetric load cases. The solution for arbitrary loading is obtained by superimposing the results for both load cases. The solution considers the non-uniform stress distributions in the adhesive, and satisfies the stress-free boundary condition at the ends of the adhesive layer and FRP.

Interfacial stresses in soffit-plated reinforced concrete beams

A popular new method for retrofitting reinforced concrete (RC) beams is to bond a fibre-reinforced polymer (FRP) plate to their soffit to increase their flexural strength. An important failure mode of such strengthened members is the debonding of the FRP plate from the concrete due to high interfacial stresses near the plate ends. This paper presents an explicit solution for interfacial stresses in a simply supported beam bonded with a thin plate and subjected to arbitrary loads. A set of arbitrary loads is decomposed into a symmetric component and an antisymmetric component. Closed-form solutions are derived for both the symmetric and antisymmetric load cases. The solution for arbitrary loading is obtained by superimposing the results for both load cases. The solution considers the non-uniform stress distributions in the adhesive, and satisfies the stress-free boundary condition at the ends of the adhesive layer and FRP.

Analysis of Retrofitted Reinforced Concrete Shear Beams using Carbon Fiber Composites

This paper presents the numerical study to simulate the behavior of retrofitted reinforced concrete (RC) shear beams. The study was carried out on the unretrofitted RC beam designated as control beam and RC beams retrofitted using carbon fiber reinforced plastic (CFRP) composites with ±45o and 90o fiber orientations. The effect of retrofitting on uncracked and precracked beams was studied too. The finite elements adopted by ANSYS were used in this study. A quarter of the full beam was used for modeling by taking advantage of the symmetry of the beam and loadings. The load deflection plots obtained from numerical study show good agreement with the experimental plots reported by Tom Norris, et al (1997). There is a difference in behavior between the uncracked and precracked retrofitted beams though not significant. The crack patterns in the beams are also presented.