Retrofitting Reinforced Concrete Research Articles

Flexural strengthening of reinforced concrete beam using a natural fibre reinforced polymer laminate: an experimental and numerical study

Synthetic fibres like carbon and glass fibres are commonly used as reinforcing material in polymer composites used by the construction industry to retrofit reinforced concrete (RC) structures. Natural fibres can be developed as a more sustainable alternative to these synthetic fibres in polymer composites. In this study, a kenaf fibre reinforced polymer (KFRP) laminate was designed and fabricated to strengthen an RC beam in flexure. Four-point bending test was carried out on a full-scale control and KFRP-strengthened beam. The beams were also modelled numerically with the ANSYS finite element software. The kenaf fibre laminate increased the ultimate load of the RC beam by 77.9% and reduced beam deflections at similar loads with the control beam. The results from the numerical analysis in terms of the beams load–deflection response, ultimate load and crack plots compared well with the experimental investigation. This study provided further insight into the design and use of a more sustainable material for strengthening RC beams and shows how numerical analysis can enhance the development of emerging materials.

Response Modification Factor of Haunch Retrofitted Reinforced Concrete Frames

AbstractThis paper presents experimental and numerical research carried out on low-rise reinforced concrete (RC) frames that have weaker beam-column joints, which are retrofitted with steel haunche...

Composite behavior in RC buildings retrofitted using buckling-restrained braces with elastic steel frames

Seismically retrofitting reinforced concrete (RC) building with a combination of buckling-restrained braces (BRBs) and elastic steel frames offers a practical solution that provides additional lateral stiffness and energy dissipation capacity. However, the available methods to vertically distribute the BRB sizes based on equivalent linearization do not consider the additional stiffness due to the composite behavior between the RC frame and the elastic steel frame, which may lead to an overly conservative estimate of the BRB stiffness demands. This study proposes a retrofit design method incorporating the composite behavior. Numerical models considering the detailed composite behavior are developed and calibrated against quasi-static cyclic loading tests, and a simplified evaluation method is proposed. A four-story RC school building is used as a benchmark model, and the proposed retrofit design method is validated using nonlinear response history analysis. The analysis results suggest that taking the composite behavior into account by using the proposed retrofit design method more accurately estimates the lateral stiffness of the retrofitted structure and leads to a more economic retrofit.

Seismic performance of large rupture strain FRP retrofitted RC columns with corroded steel reinforcement

Retrofitting reinforced concrete (RC) column by conventional fibre reinforced polymers (FRPs) can enhance the seismic performance to a limited level due to the small straining capacity and brittle failure of FRP. Existing findings have substantiated that the polyethylene terephthalate (PET) FRP with large rupture strain (LRS) (e.g., greater than 5%) are regarded as preferable to conventional FRP concerning enhancing the ductility of retrofitted RC columns. This paper experimentally evaluates the enhancement in seismic performance of corroded RC columns contributed by LRS-FRP strengthening based on testing of ten RC columns. The main test variables are the corrosion ratio of longitudinal reinforcement, axial force ratio, thickness of LRS-FRP and boundary condition. The seismic performance is systematically evaluated, including crack pattern, failure mode, lateral load resistance, hysteretic response, energy-dissipation capacity, stiffness degradation, ductility and strain responses of wrapped LRS-FRP. The results indicate that corrosion results in significant performance degradation in member strength and ductility. Notably, the failure mode of un-retrofitted corroded columns changes from typical flexural failure to shear-compression failure when the axial load ratio increases from 0.2 to 0.4, which case is prevented by LRS-FRP strengthening so that the retrofitted corroded column eventually experiences ductile flexural failure under larger axial load ratio. Moreover, LRS-FRP retrofitting remarkably improves the ductility and energy dissipation capacity of corroded RC columns compared with the un-retrofitted counterparts.

Simple equations for predicting the rotational ductility of fiber-reinforced-polymer strengthened reinforced concrete joints

Achieving a certain limit of rotational ductility in retrofitted reinforced concrete (R.C) joints is very important in the design of earthquake-resistant structures. Strengthening of R.C joints using wraps of fiber-reinforced polymers (FRP) is a common attractive technique and has an effect on the ductility of such joints. This study focuses on developing simple conceptual equations to predict the ductility of exterior reinforced concrete (R.C) beam-column joints as a function of the applied FRP. The equations are derived based on statistical regression through parametric study using results from a high-fidelity finite element model created using ABAQUS. The validated model includes material and geometric nonlinearities, in addition to the use of realistic nonlinear contact behavior between FRP and concrete. The proposed simple equations can be used as an initial conceptual design step for checking the adequacy of R.C beam-column joints in seismic design of R.C buildings. The proposed equations consider FRP, relative column-to-beam inertia, and transverse reinforcement in the beam and joint as the main parameter. This study defines the types of failure based on the ductility, and it develops the equations for ductile and brittle failures for both CFRP-strengthened joints and non-strengthened joints. This research confirms quantitatively the effectiveness of using CFRP to increase the ductility in most cases of the R.C beam-column joints. However, contribution of the CFRP is limited for some cases.

A nonlinear semi-analytical model for predicting debonding of FRP laminates from RC beams subjected to uniform or concentrated load

A semi-analytical model is developed to determine in FRP retrofitted reinforced concrete (RC) beams the interfacial shear and peeling stresses, the FRP laminate and the RC section strain and stresses at all loading stages up to failure. The FRP is assumed to be externally bonded to the beam but can undergo slip and relative vertical displacement at its interface with the concrete. The model is developed by satisfying the requirements of equilibrium and strain compatibility while concurrently allowing for interfacial deformations. FRP is treated as a linear elastic, steel as elasto-plastic strain hardening and concrete as fully nonlinear material in compression and tension, including tension stiffening. The governing equations are formulated as two second order differential equations with their dependent variables being the strain in the FRP and the relative normal displacement of the interface. The equations are solved for discrete states (uncracked, cracked, yielded) experienced by the RC section and their associated level of interfacial slip. The model results are compared with available experimental results for several beams retrofitted with carbon FRP or steel reinforced polymer (SRP) laminates subjected to either four point bending or simulated uniform load, with satisfactory agreement between them.

Optimal FRP Jacket Placement in RC Frame Structures Towards a Resilient Seismic Design

This paper proposes an optimal plan for seismically retrofitting reinforced concrete (RC) frame structures. In this method, the columns are wrapped by fiber-reinforced polymer (FRP) layers along their plastic hinges. This technique enhances their ductility and increases the resiliency of the structure. Two meta-heuristic algorithms (i.e., genetic algorithm and particle swarm optimization) are adopted for this purpose. The number of FRP layers is assumed to be the design variable. The objective of the optimization procedure was to provide a uniform usage of plastic hinge rotation capacity for all the columns, while minimizing the consumption of the FRP materials. Toward this aim, a single objective function containing penalty terms is introduced. The seismic performance of the case study RC frame was assessed by means of nonlinear pushover analyses, and the capacity of the plastic hinge rotation for FRP-confined columns was evaluated at the life safety performance level. The proposed framework was then applied to a non-ductile low-rise RC frame structure. The optimal retrofit scheme for the frame was determined, and the capacity curve, inter-story drift ratios, and fragility functions were computed and compared with alternative retrofit schemes. The proposed algorithm offers a unique technique for the design of more resilient structures.

Strengthening of RC beams with externally bonded and anchored thick CFRP laminate

Premature debonding of externally bonded FRP laminate from retrofitted reinforced concrete (RC) members can lead to inefficient use of FRP and can limit the level of strength increase that can be achieved. In this investigation, π-CFRP anchors are used in an attempt to delay the onset of premature debonding and to achieve higher strength in beams retrofitted with a 1.2 mm thick and 50 mm wide CFRP laminate. This investigation consisted of testing six large scale T-beams with a 4500 mm span, 400 mm height and 500 mm flange width under four-point bending. Two beams were tested without retrofit as control beams, one beam with the laminate epoxy bonded to the beam and the remaining three beams with the laminate epoxy bonded and anchored using CFRP π-anchors. One of the beams with 30 anchors exhibited a 46% increase in the debonding load over the beam without anchors while the laminate attained a maximum strain equal to 80% of its ultimate strain capacity, a 94% increase compared to the maximum strain reached in the companion beam strengthened with only the epoxy bonded laminate. The displacement ductility ratio of the latter beam at debonding exceeded 4. The results demonstrate the π-anchoring system effectiveness and a feasible way for efficiently utilizing strong and thick laminates in strengthening RC members.

Numerical investigation on low-velocity impact response of CFRP wraps in presence of concrete substrate

Under the levels of low-velocity impact far below those considered by current design codes, the carbon fiber-reinforced polymer (CFRP) wraps externally bonded for retrofitting reinforced concrete (RC) columns might suffer severe damage. In this study, explicit finite element method was used to fill the knowledge gap, which was verified by experimental tests. Twenty-four transverse impact scenarios related to a CFRP-retrofitted RC column-to-impactor system were investigated, which were designed according to the different combinations of a series of parameters, namely incident kinetic energy of impactor, CFRP thickness, and shape of impactor. The energy distribution among the different parts in the system was presented. The contact force time histories obtained from the scenarios indicated that a damage threshold load associated with a prescribed shape of impactor existed in the case of the CFRP wraps with concrete substrate. Furthermore, the significant differences among the roles of the parameters were revealed, and the reasons behind the differences were analyzed.

A Mechanical Approach for Evaluating the Distribution of Confinement Pressure in FRP-Wrapped Rectangular Columns

AbstractIn recent decades, fiber reinforced polymer (FRP) wrapping has become a common technique to retrofit reinforced concrete (RC) columns. Numerous research works have sought to verify analytic...

Simplified HYMOD non-linear simulations of a full-scale multistory retrofitted RC structure that undergoes multiple cyclic excitations – An infill RC wall retrofitting study

Having the ability to assess the earthquake resistance of retrofitted reinforced concrete (RC) structures through accurate and objective nonlinear cyclic analysis is of great importance for both scientists and professional Civil Engineers. Full-scale RC structure simulations under ultimate limit state cyclic loading conditions through the use of 3D detail modeling techniques, is currently one of the most challenging modeling tasks that any research or commercial software can undertake. The excessive computational demand and the numerical instabilities that occur when dealing with this type of cyclic nonlinear numerical analysis, make this modeling approach impractical. The simplified hybrid modeling (HYMOD) approach is adopted in this work, which overcomes the above numerical limitations and it is used herein to illustrate the capabilities of the method in capturing the experimental results of a full-scale 4-storey RC building that was retrofitted with RC infill walls and carbon fiber reinforced polymer jacketing. This work has the aim to investigate the importance of numerically accounting for the damage that has developed at the concrete and steel domains during the analysis of problems that foresee consecutive cyclic loading tests. Based on the numerical findings, it was concluded that the proposed modeling approach was able to accurately capture the experimental data and predict the capacity degradation of the building specimen. Furthermore, the proposed method was used to numerically investigate different retrofitting configurations that foresaw the use of infill RC walls. The numerical experiments performed in this work demonstrate that the proposed modeling approach provides with the ability to study the cyclic mechanical behavior of full-scale RC structures under ultimate limit state conditions, thus paves the way in performing additional parametric investigations in determining the optimum retrofitting design of RC structures by using different types of interventions.

Experimental and Numerical Studies of Debonding Monitoring of FRP Shear-Strengthened Beams Using EMI Technique

AbstractFiber-reinforced polymer (FRP) is commonly used to strengthen or retrofit reinforced concrete (RC) structures. FRP debonding may initially occur around tiny cracks and then propagate to oth...

Seismic performance of CFRP-retrofitted large-scale rectangular RC columns under lateral loading in different directions

Considerable studies have been carried out on the seismic performance of fiber-reinforced polymer (FRP) retrofitted reinforced concrete (RC) columns. However, research on the seismic performance of FRP-retrofitted RC columns under lateral loading in different directions remains limited. This paper presents an experimental investigation on both un-retrofitted and carbon FRP (CFRP) retrofitted rectangular RC columns with an emphasis on the effect of the directions of lateral loading on the seismic performance of the columns. A total of ten large-scale cantilever rectangular RC columns were constructed and five of them were retrofitted with CFRP wraps at the potential plastic hinge regions. The overall performance of each specimen is examined in terms of damage evolution, lateral load-displacement hysteretic behavior, lateral strength, ductility capacity, stiffness degradation, and energy dissipation. The direction of lateral loading is found to have significant influence on the seismic performance of both un-retrofitted and CFRP-retrofitted rectangular RC columns. It is also found that the shear strength of the un-retrofitted and FRP-retrofitted rectangular RC columns in nonprincipal directions can be predicted based on the lateral strength in the principal directions by the ellipse equation.

Simplified Design of FRP-Confined Square RC Columns under Bi-Axial Bending

Available guidelines do not provide design procedures for the general case of retrofitting reinforced concrete (RC) columns using fiber reinforced polymer (FRP) sheets subjected to simultaneous bi-axial flexural and axial loads. In many practical cases, columns essentially undergo simultaneous axial force and bi-axial bending moments, especially in in-situ construction. This paper suggests a simplified design method based on the equivalent uni-axial moment concept to calculate the required number of layers FRP sheets for retrofitting RC square columns. The proposed procedure is then verified against available bi-axial moment and axial force test data found in the literature. Results demonstrate that the proposed procedure is appropriate for practical applications with acceptable accuracy. It also appears that retrofitting RC square columns by longitudinal fiber arrangement is only effective for columns with tension-controlled behavior, while transverse and combined longitudinal-transverse arrangements are more effective in enhancing the load bearing capacity of both the compression- and tension-controlled columns. A design example will also be presented.

Influence of width and thickness of composite laminates on the flexural behavior of reinforced concrete beams and slabs

Over the last few years externally bonded carbon fiber reinforced polymer (CFRP) laminates have become a popular technique for retrofitting reinforced concrete (RC) existing structures. This paper presents an experimental study on 10 RC members externally reinforced with different architectures of CFRP laminates in order to investigate the influence of the composite configurations on the structural behavior of RC members. American guidelines ACI 440.2R and Italian Code CNR-DT 200 are employed for comparison with the experimental results. Numerical models are also developed to simulate the behavior of the FRP-RC members with the aim to investigate the different failure modes observed in various strengthening configurations. One conclusion is that the Italian Code CNR-DT 200 results to be more conservative with respect to ACI 440.2R. Finite element analyses provide good predictions of the experimental evidences in terms of load-deflection, load strain diagrams, and crack distribution and lead to an accurate prediction of the debonding failure and the post-failure response.

Behavior of FRP strengthened RC brick in-filled framessubjected to cyclic loading

Fiber reinforced polymer (FRP) sheets are the most efficient structural materials in terms of strength to weight ratio and its application in strengthening and retrofitting of a structure or structural elements are inevitable. The performance enhancement of structural elements without increasing the cross sectional area and flexible nature are the major advantages of FRP in retrofitting/strengthening work. This research article presents a detailed study on the inelastic response of conventional and retrofitted Reinforced Concrete (RC) frames using Carbon Fibre Reinforced Polymers (CFRP) and Glass Fiber Reinforced Polymers (GFRP) subjected to quasi-static loading. The hysteretic behaviour, stiffness degradation, energy dissipation and damage index are the parameters employed to analyse the efficacy of FRP strengthening of brick in-filled RC frames. Repair and retrofitting of brick infilled RC frame shows an improved load carrying and damage tolerance capacity than control frame.

Use of DIC and AE for Monitoring Effective Strain and Debonding in FRP and FRCM-Retrofitted RC Beams

AbstractThe effective strain in composites, along with their potential rupture and debonding, plays a crucial role in predicting the strength of retrofitted reinforced concrete (RC) beams. However, only limited experimental data on these phenomena are available, mainly because of the inadequacy of traditional deformation and strain measurement techniques. This paper presents a comparative analysis of instrumentation for monitoring retrofitted RC elements. In particular, the paper addresses beams retrofitted with composite materials, FRPs (fiber-reinforced polymers), and FRCMs (fiber-reinforced cementitious mortars). It also considers strain gauges, fiber Bragg grating (FBG) sensors, LVDTs, digital image correlation (DIC), and acoustic emission (AE) sensors for monitoring strain, displacement, cracking, and debonding. Experiments on six beams are carried out, and the measured data from the monitoring devices are compared. The accuracy of DIC for strain and displacement monitoring is shown to match the perf...

Seismic retrofit of reinforced concrete frames using buckling‐restrained braces with bearing block load transfer mechanism

SummaryA new method of retrofitting reinforced concrete (RC) frames with buckling‐restrained braces (BRBs) to improve frame strength, stiffness and energy dissipation is proposed. Instead of typical post‐installed anchors, load is transferred between the BRB and RC frame through compression bearing between an installed steel frame connected to the BRB, and high‐strength mortar blocks constructed at the four corners of the RC frame. This avoids complex on‐site anchor installation, and does not limit the allowable brace force by the anchor strength. Cyclic displacements of increasing amplitudes were imposed on two RC frame specimens retrofitted with different BRB strength capacities. In one of the frames, the bearing blocks were reinforced with wire mesh to mitigate cracking. A third RC frame was also tested as a benchmark to evaluate the retrofit strength and stiffness enhancements. Test results indicate that the proposed method efficiently transferred loads between the BRBs and RC frames, increasing the frame lateral strength while achieving good ductility and energy‐dissipating capacity. When the bearing block was reinforced with wire mesh, the maximum frame lateral strength and stiffness were more than 2.2 and 3.5 times the RC frame without the BRB respectively. The BRB imposes additional shear demands through the bearing blocks to both ends of the RC beam and column member discontinuity regions (D‐regions). The softened strut‐and‐tie model satisfactorily estimated the shear capacities of the D‐regions. A simplified calculation and a detailed PISA3D analysis were shown to effectively predict member demands to within 13.8% difference of the measured test results. Copyright © 2016 John Wiley & Sons, Ltd.

Innovative solution to retrofit RC members: Inhibiting-Repairing-Strengthening (IRS)

The work aims to experimentally and theoretically investigate the structural performances of an innovative solution, alternative to Externally-Bonded (EB) technique, for retrofitting of Reinforced Concrete (RC) structures with deteriorated cover concrete and/or corroded bars: Inhibiting-Repairing-Strengthening (IRS) technique. It consists in the installation of a stainless steel fabric in the cover concrete, restoring the latter with a geopolymeric matrix (Steel Reinforced Geopolymeric Matrix, SRGM). This solution includes three operations in one: corrosion inhibition, cover concrete repair and flexural strengthening.Two groups of large-scale RC beams with different geometrical properties, low concrete strength, corroded smooth round/ribbed bars, were strengthened with IRS-SRGM/EB-SRGM systems and tested under four-point bending. Test results showed that the IRS-SRGM system provided greater load carrying capacity and ductility than the EB-SRGM system. Comparing the results of the two groups the scale effect on the ultimate load was not affected. Furthermore, a theoretical prediction of the tested beams and analytical/experimental comparisons were performed.The proposed IRS solution could be an effective alternative to EB technique, reducing the time and costs of intervention with an eco-friendly technology.

Numerical evaluation of FRP composite retrofitted reinforced concrete wall subjected to blast load

High performance materials such as Fiber Reinforced Plastic (FRP) are often used for retrofitting structures against blast loads due to its ductility and strength. The effectiveness of retrofit materials needs to be precisely evaluated for the retrofitting design based on the dynamic material responses under blast loads. In this study, the blast resistance of Carbon Fiber Reinforced Plastic (CFRP) and Kevlar/Glass hybrid fabric (K/G) retrofitted reinforced concrete (RC) wall is analyzed by using the explicit analysis code LS-DYNA, which accommodates the high-strain rate dependent material models. Also, the retrofit effectiveness of FRP fabrics is evaluated by comparing the analysis results for non-retrofitted and retrofitted walls. The verification of the analysis is performed through comparisons with the previous experimental results.