Flexural Damage Diagnosis in Reinforced Concrete Beams Using a Wireless Admittance Monitoring System—Tests and Finite Element Analysis

The utilization and effectiveness of a custom-made, portable and low-cost structural health monitoring (SHM) system that implements the PZT-based electro-mechanical admittance (EMA) methodology for the detection and evaluation of the damage of flexural reinforced concrete (RC) beams is presented. Tests of large-scale beams under monotonic and cyclic reversal-imposed deformations have been carried out using an integrated wireless impedance/admittance monitoring system (WiAMS) that employs the voltage measurements of PZT transducers. Small-sized PZT patches that have been epoxy-bonded on the steel bars surface and on the external concrete face of the beams are utilized to diagnose damages caused by steel yielding and concrete cracking. Excitations and simultaneous measurements of the voltage signal responses of the PZT transducers have been carried out at different levels of the applied load during the tests using the developed SHM devices, which are remotely controlled by a terminal emulator. Each PZT output voltage versus frequency response is transferred wireless and in real-time. Statistical index values are calculated based on the signals of the PZT transducers to represent the differences between their baseline response at the healthy state of the beam and their response at each loading/damage level. Finite Element Modeling (FEM) simulation of the tested beams has also been performed to acquire numerical results concerning the internal cracks, the steel strains and the energy dissipation and instability parameters. FEM analyses are used to verify the experimental results and to support the visual observations for a more precise damage evaluation. Findings of this study indicate that the proposed SHM system with the implementation of two different PZT transducer settings can be effectively utilized for the assessment of structural damage caused by concrete cracking and steel yielding in flexural beams under monotonic and cyclic loading

Behaviour of rehabilitated RC beams with self-compacting concrete jacketing – Analytical model and test results

The use of thin reinforced self-compacting concrete jackets as a method to repair and strengthen under-designed flexural concrete members is investigated by means of experimental and analytical studies. The experimental component comprises 20 beam tests that were designed to fail mostly in shear either prior to, or immediately after flexural yielding with no ductility. After initial loading to near failure, specimens were repaired with three-sided jackets having the minimum thickness required in order to provide adequate bar cover and were subsequently retested to demonstrate the strength and ductility enhancement that could be attained through the intervention. The experimental results are correlated through member analysis considering the interfacial force transfer and relative slip occurring along the contact surface between new and existing concrete. The efficacy of the repair procedure and the important parameters controlling the post-repair response are highlighted through consistent evaluation of the confining action exerted by the jacket on the encased core using equilibrium of forces and a frictional model. It is shown that with proper anchorage, the repaired members approach the strength, ductility, deformation capacity and model of failure of the ideal monolithic member having identical reinforcement details

Application of a Reinforced Self-Compacting Concrete Jacket in Damaged Reinforced Concrete Beams under Monotonic and Repeated Loading

This paper presents the findings of an experimental study on the application of a reinforced self-compacting concrete jacketing technique in damaged reinforced concrete beams. Test results of 12 specimens subjected to monotonic loading up to failure or under repeated loading steps prior to total failure are included. First, 6 beams were designed to be shear dominated, constructed by commonly used concrete, were initially tested, damaged, and failed in a brittle manner. Afterwards, the shear-damaged beams were retrofitted using a self-compacting concrete U-formed jacket that consisted of small diameter steel bars and U-formed stirrups in order to increase their shear resistance and potentially to alter their initially observed shear response to a more ductile one. The jacketed beams were retested under the same loading. Test results indicated that the application of reinforced self-compacting concrete jacketing in damaged reinforced concrete beams is a promising rehabilitation technique. All the jacketed beams showed enhanced overall structural response and 35% to 50% increased load bearing capacities. The ultimate shear load of the jacketed beams varied from 39.7 to 42.0 kN, whereas the capacity of the original beams was approximately 30% lower. Further, all the retrofitted specimens exhibited typical flexural response with high values of deflection ductility

Analysis of Residual Flexural Stiffness of Steel Fiber-Reinforced Concrete Beams with Steel Reinforcement

This paper investigates the ability of steel fibers to enhance the short-term behavior and flexural performance of realistic steel fiber-reinforced concrete (SFRC) structural members with steel reinforcing bars and stirrups using nonlinear 3D finite element (FE) analysis. Test results of 17 large-scale beam specimens tested under monotonic flexural four-point loading from the literature are used as an experimental database to validate the developed nonlinear 3D FE analysis and to study the contributions of steel fibers on the initial stiffness, strength, deformation capacity, cracking behavior, and residual stress. The examined SFRC beams include various ratios of longitudinal reinforcement (0.3%, 0.6%, and 1.0%) and steel fiber volume fractions (from 0.3% to 1.5%). The proposed FE analysis employs the nonlinearities of the materials with new and established constitutive relationships for the SFRC under compression and tension based on experimental data. Especially for the tensional response of SFRC, an efficient smeared crack approach is proposed that utilizes the fracture properties of the material utilizing special stress versus crack width relations with tension softening for the post-cracking SFRC tensile response instead of stress–strain laws. The post-cracking tensile behavior of the SFRC near the reinforcing bars is modeled by a tension stiffening model that considers the SFRC fracture properties, the steel fiber interaction in cracked concrete, and the bond behavior of steel bars. The model validation is carried out comparing the computed key overall and local responses and responses measured in the tests. Extensive comparisons between numerical and experimental results reveal that a reliable and computationally-efficient model captures well the key aspects of the response, such as the SFRC tension softening, the tension stiffening effect, the bending moment–curvature envelope, and the favorable contribution of the steel fibers on the residual response. The results of this study reveal the favorable influence of steel fibers on the flexural behavior, the cracking performance, and the post-cracking residual stress

U-Jacketing Applications of Fiber-Reinforced Polymers in Reinforced Concrete T-Beams against Shear—Tests and Design

The application of externally bonded fiber-reinforced polymer (EB-FRP) as shear transverse reinforcement applied in vulnerable reinforced concrete (RC) beams has been proved to be a promising strengthening technique. However, past studies revealed that the effectiveness of this method depends on how well the reinforcement is bonded to the concrete surface. Thus, although the application of EB-FRP wrapping around the perimeter of rectangular cross-sections leads to outstanding results, U-jacketing in shear-critical T-beams seems to undergo premature debonding failures resulting in significant reductions of the predictable strength. In this work, five shear-critical RC beams with T-shaped cross-section were constructed, strengthened and tested in four-point bending. Epoxy bonded carbon FRP (C-FRP) sheets were applied on the three sides and along the entire length of the shear-strengthened T-beams as external transverse reinforcement. Furthermore, the potential enhancement of the C-FRP sheets anchorage using bolted steel laminates has been examined. Test results indicated that although the C-FRP strengthened beams exhibited increased shear capacity, the brittle failure mode was not prevented due to the debonding of the FRP from the concrete surface. Nevertheless, the applied mechanical anchor of the C-FRP sheets delayed the debonding. Moreover, the design provisions of three different code standards (Greek Code of Interventions, Eurocode 8 and ACI Committee 440) concerning the shear capacity of T-shaped RC beams retrofitted with EB-FRP jackets or strips in U-jacketing configuration are investigated. The ability of these code standards to predict safe design estimations is checked against 165 test data from the current experimental project and data available in the literature

Cyclic Response of Steel Fiber Reinforced Concrete Slender Beams: An Experimental Study

Reinforced concrete (RC) beams under cyclic loading usually suffer from reduced aggregate interlock and eventually weakened concrete compression zone due to severe cracking and the brittle nature of compressive failure. On the other hand, the addition of steel fibers can reduce and delay cracking and increase the flexural/shear capacity and the ductility of RC beams. The influence of steel fibers on the response of RC beams with conventional steel reinforcements subjected to reversal loading by a four-point bending scheme was experimentally investigated. Three slender beams, each 2.5 m long with a rectangular cross-section, were constructed and tested for the purposes of this investigation; two beams using steel fibrous reinforced concrete and one with plain reinforced concrete as the reference specimen. Hook-ended steel fibers, each with a length-to-diameter ratio equal to 44 and two different volumetric proportions (1% and 3%), were added to the steel fiber reinforced concrete (SFRC) beams. Accompanying, compression, and splitting tests were also carried out to evaluate the compressive and tensile splitting strength of the used fibrous concrete mixtures. Test results concerning the hysteretic response based on the energy dissipation capabilities (also in terms of equivalent viscous damping), the damage indices, the cracking performance, and the failure of the examined beams were presented and discussed. Test results indicated that the SFRC beam demonstrated improved overall hysteretic response, increased absorbed energy capacities, enhanced cracking patterns, and altered failure character from concrete crushing to a ductile flexural one compared to the RC beam. The non-fibrous reference specimen demonstrated shear diagonal cracking failing in a brittle manner, whereas the SFRC beam with 1% steel fibers failed after concrete spalling with satisfactory ductility. The SFRC beam with 3% steel fibers exhibited an improved cyclic response, achieving a pronounced flexural behavior with significant ductility due to the ability of the fibers to transfer the developed tensile stresses across crack surfaces, preventing inclined shear cracks or concrete spalling. A report of an experimental database consisting of 39 beam specimens tested under cyclic loading was also presented in order to establish the effectiveness of steel fibers, examine the fiber content efficiency and clarify their role on the hysteretic response and the failure mode of RC structural members

Numerical Analysis Exterior RC Beam-Column Joints with CFRP Bars as Beam’s Tensional Reinforcement under Cyclic Reversal Deformations

In this paper the cyclic lateral response of reinforced concrete (RC) beam-column joints with composite carbon fiber-reinforced polymer (CFRP) bars as a longitudinal reinforcement in the beam is simulated with finite element (FE) modeling using software Abaqus. An experimental project of two full-scale joint specimens subjected to cyclic loading with supplementary accompanying pull-out tests of CFRP bars is also included in this study. These test results are used to calibrate the developed FE model, the constitutive laws of the materials and the bond response between CFRP bars and concrete. Comparisons between test data and numerical results indicate that the calibrated model accurately predicts the cyclic response of RC beam-column joint specimens with CFRP longitudinal bars as the beam’s tensional reinforcement. A parametric analysis is also performed to provide useful concluding remarks concerning the design of concrete joints with composite bars and the ability of CFRP bars to substitute for conventional steel bars in RC structural members under seismic excitations

Investigation of a New Strengthening Technique for RC Deep Beams Using Carbon FRP Ropes as Transverse Reinforcements

The effectiveness of a new retrofitting technique to upgrade the structural behaviour of reinforced concrete (RC) deep beams without steel stirrups using carbon fibre-reinforced polymer (CFRP) ropes as the only transverse shear reinforcement is experimentally investigated. Five shear-critical beams with rectangular and T-shaped cross-section are tested under monotonic loading. The strengthening schemes include (a) one vertical and one diagonal single-link CFRP rope that are internally applied through the web of the rectangular beam using an embedded through-section (ETS) system and (b) two vertical U-shaped double-link ropes that are applied around the perimeter of the web of the flanged beam using a near-surface-mounted (NSM) system. In both cases, the free lengths of the CFRP ropes have been properly anchored using epoxy bonded lap splices of the rope as NSM at (a) the top and the bottom of the web of the rectangular beam and (b) the top of the slab of the T-beam. Promising results have been derived, since the proposed strengthening technique enhanced the strength and altered the brittle shear failure to a ductile flexural one. The experimental results of this study were also used to check the validity of an analytical approach to predict the strength of shear strengthened deep beams using FRP ropes as transverse link reinforcement

Numerical Analysis Exterior RC Beam-Column Joints with CFRP Bars as Beam’s Tensional Reinforcement under Cyclic Reversal Deformations

In this paper the cyclic lateral response of reinforced concrete (RC) beam-column joints with composite carbon fiber-reinforced polymer (CFRP) bars as a longitudinal reinforcement in the beam is simulated with finite element (FE) modeling using software Abaqus. An experimental project of two full-scale joint specimens subjected to cyclic loading with supplementary accompanying pull-out tests of CFRP bars is also included in this study. These test results are used to calibrate the developed FE model, the constitutive laws of the materials and the bond response between CFRP bars and concrete. Comparisons between test data and numerical results indicate that the calibrated model accurately predicts the cyclic response of RC beam-column joint specimens with CFRP longitudinal bars as the beam’s tensional reinforcement. A parametric analysis is also performed to provide useful concluding remarks concerning the design of concrete joints with composite bars and the ability of CFRP bars to substitute for conventional steel bars in RC structural members under seismic excitations

Efficacy and Damage Diagnosis of Reinforced Concrete Columns and Joints Strengthened with FRP Ropes Using Piezoelectric Transducers

Recent research has indicated that the implantation of a network of piezoelectric transducer patches in element regions of potential damage development, such as the beam–column joint (BCJ) area, substantially increases the efficacy and accuracy of the structural health monitoring (SHM) methods to identify damage level, providing a reliable diagnosis. The use of piezoelectric lead zirconate titanate (PZT) transducers for the examination of the efficiency of an innovative strengthening technique of reinforced concrete (RC) columns and BCJs is presented and commented on. Two real-scale RC BCJ subassemblages were constructed for this investigation. The columns and the joint panel of the second subassemblage were externally strengthened with carbon fiber-reinforced polymer (C-FRP) ropes. To examine the efficiency of this strengthening technique we used the following transducers: (a) PZT sensors on the ropes and the concrete; (b) tSring linear variable displacement transducers (SLVDTs), diagonally installed on the BCJ, to measure the shear deformations of the BCJ panel; (c) Strain gauges on the internal steel bars. From the experimental results, it became apparent that the PZT transducers successfully diagnosed the loading step at which the primary damage occurred in the first BCJ subassemblage and the damage state of the strengthened BCJ during the loading procedure. Further, data acquired from the diagonal SLVDTs and the strain gauges provided insight into the damage state of the two tested specimens at each step of the loading procedure and confirmed the diagnosis provided by the PZT transducers. Furthermore, data acquired by the PZT transducers, SLVDTs and strain gauges proved the effectiveness of the applied strengthening technique with C-FRP ropes externally mounted on the column and the conjunction area of the examined BCJ subassemblages