External Post-tensioned Tendons Research Articles

Performance of RC frames strengthening by external post-tensioning tendons under vertical and lateral loads

In this study, the structural performance of reinforced concrete frames strengthened with external post-tension tendons was investigated. An experimental study was performed to investigate the failure modes and generate the data required to validate the numerical model. The experimental study includes a reinforced concrete (RC) frame control specimen without strengthening and three strengthened specimens under vertical loads. The specimens were strengthened using external post-tensioning with three different tendon layouts, including a straight tendon in the beam-positive moment zone, straight tendons in the beam-positive and column-negative moment zones, and U-shape tendons along the frame beam. The numerical model was built for the four specimens, validated using the experimental results, and used to analyze the lateral load performance. Results showed that the most effective strengthening technique for the vertical load was the beam U-shape tendons, and for the lateral load, it was the beam and column strengthening. It can be concluded from this case study that strengthening frames with external post-tensioning techniques can enhance the first crack load, yielding load, and ultimate load by 80–100 %, 180–250 %, and 35–70 %, respectively.

(Digital Presentation) Bipolar Electrochemical Impedance Spectroscopy for Corrosion Monitoring of Steel Reinforced Cementitious Structural Components

Bipolar electrochemical impedance spectroscopy (BPEIS) was previously explored as a potential nondestructive method for detecting corrosion of steel in post-tensioned tendons which are used as reinforcing structural elements in segmental bridge construction. The tendons are composed of a grout-filled cylindrical plastic duct that contains high-strength steel strands. A proof of concept was demonstrated by experiments performed on tendons with passive and corroded strands and through finite-element simulations of the indirect impedance response1. Further work showed that bipolar impedance was sensitive to local corrosion in field tendons if the measurement is obtained near the corroding region2. This work further scrutinizes BPEIS as a corrosion monitoring method for reinforced cementitious structural components with direct comparison between corrosion damage estimates and actual metal loss.Bi-polar electrochemistry is a noncontact method in which a conducting material placed within an electric field induced by two feeder electrodes is indirectly polarized while maintaining charge neutrality such that the conducting material forms two poles, a positively charged pole which acts as the anode and negatively charge one that acts as the cathode. If the electric field is applied between the feeder electrodes under alternating current, the poles fluctuate between anode and cathode extremes corresponding to the direction of the induced field. Under such polarization conditions, the opportunity to assess the electrochemical impedance between the feeder electrodes is provided. The measured impedance includes the interfacial impedance of the feeder electrodes, the conducting material interface impedance, and the impedance of the conductor itself which can be assumed to be negligible. By placing two reference electrodes between the feeder electrodes, the interfacial impedance of the feeder electrodes can be eliminated from the measurement, thus allowing one to obtain an impedance reflective of the electrolyte impedance between the reference electrodes and the impedance of the conducting material interface.A finite element model is used to relate the measured impedance response to the impedance of the steel and cement interface and therefore the corrosion rate. The ability to accurately monitor the corrosion rate will be assessed by comparing the estimated corrosion damage obtained by integration of periodic estimates of corrosion rate to the actual damage. The role of a corrosion product layer, reinforcement geometry, and electrode configuration to the sensitivity of the measurement to corrosion rate will be described.[1] Alexander, C. L., & Orazem, M. E. (2020). Indirect electrochemical impedance spectroscopy for corrosion detection in external post-tensioned tendons: 1. Proof of concept. Corrosion Science, 164, 108331.[2] Alexander, C. L., & Orazem, M. E. (2020). Indirect impedance for corrosion detection of external post-tensioned tendons: 2. Multiple steel strands. Corrosion Science, 164, 108330.

Feasibility Analysis of Post-Tensioned Beam for Industrial Building: A Review

Abstract: In recent years, due to corrosion and the constant demand for increased traffic flow, there is an eagerly need for an efficient system that can be used to repair or strengthen bridges and structures with steel-concrete composite sections. There have been so many strengthening techniques used to prevent such serious structural problems. One of these most useful techniques, which has successfully proven effective in repairing and strengthening structural members, is to use the externally post-tensioned tendons technique. The various study has also provided a recently completed experimental program on scaled bridge composite steel and concrete beams strengthened with external post-tensioning tendons to explore the fundamental cyclic loading behavior. This article summarizes the design of Post-Tensioned Beam for industrial building and a comparative study between PT Beam and RCC Beam based on a literature survey. We are analyzing such structure in our research having a large span. Manual design of PT Beam for three different span lengths without any intermediate supports and manual design of RCC Beam. We are doing a comparative study of PT Beam and RCC Beam based on the results. This report is based on a study of PT Beam that will aid future research and advances in the construction industry. Keywords: Post-Tensioning Beam, RCC Beam, Industrial Building

Numerical investigation of the monotonic behavior of strengthened Steel-Concrete composite girders

The monotonic behavior of externally post-tensioned steel–concrete composite girders was numerically studied in this paper. A three-dimensional numerical model was developed and validated using experimental test results that were conducted by the authors and using an existing analytical model. A parametric study using this validated numerical model was performed to investigate the effects of various parameters on the monotonic performance of composite girders strengthened with external post-tensioned tendons. The parameters investigated include variations in the degree of shear connection, layout and diameter of shear connectors, the initial post-tensioning force, the depth of the steel beam, the eccentricity of the tendons, the compressive strength of concrete, and the shear capacity of the studs. The numerical model provided a better understanding of the effect of these parameters on the behavior of the strengthened beams. The results of the parametric study show that as the degree of shear connection decreased, the stud strains increased and the slippage between the concrete deck and steel beam increased. As the full degree of shear connection was reduced to 80%, 60%, and 40%, the stud strains were increased by 20%, 46%, and 94%, respectively. Also, as the shear connection degree decreases, its effect on the slippage behavior increases. As the degree of shear connection was reduced to 80%, 60%, and 40%, the slippage values were increased by 23%, 48%, and 102%, respectively. The study also shows that, for the same degree of shear connection, beams with one row of shear studs had up to 10% higher flexural capacity than beams with two rows of studs. A 100% increase in the stud diameter caused a 28% reduction in the ultimate load capacity and a 160% increase in the maximum slippage. The higher the post-tensioning force, the higher the ultimate load capacity and the lower the tensile strains in the steel beam. A 118% increase in the post-tensioning force resulted in a 9% increase in the ultimate load capacity and a 13% reduction in the tensile strains. Increasing the depth of the steel beam by 40% resulted in a reduction in the tensile strains at midspan by 20%. The lowest midspan tensile strains were obtained from the combination of increasing the depth of the steel beam and tendon eccentricity.

Cyclic loading behavior on strengthened composite beams using external post-tensioning tendons (experimental study)

In recent years, due to corrosion and the constant demand for increased traffic flow, it emerged that there is a need for an efficient system that can be used to repair and / or strengthen bridges and structures with steel–concrete composite section. There have been many strengthening techniques used to prevent such serious structural problems. One of these techniques, which has proven effective in repairing and strengthening structural members, is to use externally post-tensioned tendons. This study provides a recently completed experimental program on scaled bridge composite steel–concrete beams strengthened with external post-tensioning tendons to investigate the fundamental cyclic loading behavior. In addition to studying the effect of the strengthening system, there was another major factor under study which was the degree of shear connection. The study mainly relied on testing the beams under the influence of cyclic loads in addition to changing the degree of shear contact. Reducing the degree of shear connection simulates a form of composite action loss of composite beams due to cyclic loads. The study results provides modes of failure, characteristic points, load–deflection curves, slip between steel beam top flange and the concrete flange, the shear connector strains, and the concrete and steel beam bottom flanges strains. Based on the experimental results, adding external post-tensioning enhanced the overall performance of the composite beams under cyclic loading by significantly reducing strains of the shear studs, the concrete flange, and the steel I-beam at all loading stages. The strengthened specimens experienced an increase in the ultimate capacity and residual stiffness comparable to non-strengthened specimens, attributable to significant loss of composite action between concrete flange and steel beam through cyclic load test.

The Application of Indirect Impedance to Monitor Corrosion Rates of Inhibitor-Impregnated External Post-Tensioned Tendons

An indirect electrochemical impedance spectroscopy method was previously explored as a potential nondestructive method for detecting corrosion of steel in post-tensioned tendons which are used as reinforcing structural elements in segmental bridge construction. The tendons are composed of a grout-filled cylindrical plastic duct that contains high-strength steel strands. A proof of concept was demonstrated by experiments performed on tendons with passive and corroded strands and through finite-element simulations of the indirect impedance response.1 Further work showed that the indirect impedance method was sensitive to local corrosion in field tendons if the measurement is obtained near the corroding region.2 This work expands the application of the indirect impedance methodto corrosion monitoring of inhibitor-impregnated post-tensioned tendon sections. A corrosion inhibitor that is injected into the tendons through the interstitial space of the strands may be capable of preventing or stopping corrosion. The sensitivity of indirect impedance measurements to changes in both electrical properties of the grout and the corrosion rate of the steel is assessed by comparison to traditional measurements of the impedance in which direct contact is made to the steel. Additionally, further assessment of the indirect impedance method is made by relating the analysis results to the actual corrosion damage obtained by autopsies of the tendon sections. C. L. Alexander and M. E. Orazem. "Indirect Impedance for Corrosion Detection of External Post-tensioned Tendons: 2. Multiple Steel Strands." Corrosion Science 164 (2020): 108330.C. L. Alexander and M. E. Orazem. "Indirect electrochemical impedance spectroscopy for corrosion detection in external post-tensioned tendons: 1. Proof of concept." Corrosion Science164 (2020): 108331.

Comparison of novel imaging sensor and gamma ray tomography imaging of grout deficiencies in external post-tensioned structural tendons

A relatively simple and economical novel NDT method (Tendon Imaging Unit, TIU) for imaging grout anomalies in external post-tensioned structural tendons combines magnetic and impedance measurements. Validation comparison was conducted against a well-established but more resource intensive NDT technology, gamma-ray tomography (GRT), on test tendons with steel strands and grout voids, unhydrated grout, and excess water. GRT could resolve individual strands but TIU still provided viable strand envelope data. Both TIU and GRT detected full voids, outer voids, and unhydrated grout but neither adequately sensed a small inner void. TIU provided only limited detection of a water filled void compared with better GRT identification. Results support TIU deployment for wide range screening inspections that would otherwise be prohibitively costly, reserving advanced techniques like GRT for more focused targets.

Practical Cross-Section Imaging of External Tendons to Reveal Grout Deficiencies Relative to Strand Pattern

External post-tensioned tendons in concrete segmental bridges have had strand corrosion failures due to grouting deficiencies such as voids or bleed water. Nondestructive assessment of the grout condition is thus often needed, preferably by a cross-section imaging method. Here, a magnetic sensing approach to image the position of the steel strand bundle is combined with an electric impedance method to evaluate the condition of the grout space. Both are embodied in a device that images the tendon’s cross section. The magnetic sensor travels around the circumference of the tendon and measures the force of attraction to the steel strands from which an image of the strand pattern inside the tendon is created. Simultaneously, a traveling plate rotates around the tendon, and variations of the electric impedance between the plate and strands identify grout deficiencies. The impedance and strand position data create a complete color-coded image of the tendon cross-section flagging grout deficiencies. The approach was validated by experiments with tendon specimens with known deficiencies and under field conditions. The method is rapid, economical, easy to replicate, with a small and safe device not requiring specialized operator training.

Flexural behavior of reinforced concrete one-way slabs strengthened via external post-tensioned FRP tendons

External prestressing is an efficient technology to strengthen existing structures, and using fiber-reinforced polymers instead of steel in external tendons is advantageous to avoid corrosion-related problems, though anchoring external fiber-reinforced polymer tendons is still a major issue. An innovative anchoring technology is proposed in this paper for the external fiber-reinforced polymer tendons aimed to strengthen one-way reinforced concrete slabs. Six slab specimens were designed and manufactured to check the feasibility and reliability of the proposed technology. In one slab no external prestressing was applied (reference slab), while the remaining five slabs were strengthened by means of external post-tensioned glass fiber-reinforced polymer tendons, using the proposed anchoring system. The strengthened slabs differ for the prestressing level and external-tendon ratio. All slabs were tested up to failure and their flexural behaviors and failure modes were thoroughly investigated. The tests show that the proposed anchoring technology is reliable and efficient. A method for the calculation of the stress in the tendons at ultimate is also proposed. Finally, the numerical results are shown to agree fairly well with the test results.

Indirect Impedance for Corrosion Detection of External Post-tensioned Tendons: 2. Multiple Steel Strands

The use of indirect impedance measurements to detect corrosion is extended to systems in which many steel strands are encased within a plastic duct. Indirect impedance measurements were applied to an extracted external tendon section from a segmentally-constructed bridge. The results, supported by numerical finite-element simulations, showed that indirect impedance measurements are sensitive to localized corrosion in field applications only if the measurements are made very near to the corroding strands.

Indirect electrochemical impedance spectroscopy for corrosion detection in external post-tensioned tendons: 1. Proof of concept

An indirect electrochemical impedance spectroscopy method, previously applied to corrosion detection in reinforced concrete structures, was explored as a potential nondestructive method for detecting corrosion of steel in post-tensioned tendons used as reinforcing structural elements in segmental bridge construction. A proof of concept is demonstrated by experiments performed on tendons with passive and corroded strands and through finite-element simulations of the indirect impedance response. The numerical simulations, used to explore the influence of non-uniform current distributions on the indirect impedance response, demonstrate how frequency dispersion influences the impedance measurement. Validation of the model is shown through comparison to experimental measurements.

Experimental Evaluation of Vibration Response of External Post-Tensioned Tendons with Corrosion

This study investigates the vibration responses of external post-tensioned (PT) tendons based on experimental data. The vibration signals measured from four large-scale external post-tensioned tendon specimens subjected to localized corrosion until severe level of damage were accumulated. A series of impact tests was conducted to measure lateral vibration signals and the variation in the tension was continuously monitored using a load cell. The acquired vibration signals were evaluated using an axially-tensioned beam model in which the properties were varied to reflect the loss of tension and the reduction in the bending stiffness caused by corrosion damage. A comparison between the experimental data and the analytical results indicates that the natural frequencies were shifted as the residual tension decreased but were influenced by corresponding grout damage in a limited range. As a result, the frequency response was not appreciably affected by the damage level and the tension remained as the primary structural parameter of the external PT tendon with damage.

Seismic assessment and rehabilitation of a historical masonry mosque

In order to assess the structural behaviour and to evaluate the seismic vulnerability of old masonry structures located in Lebanon, a historical masonry mosque was analysed under earthquake loading. A numerical model developed by the finite element method using Abaqus software was elaborated on the basis of previously published experimental studies. It was concluded that the numerical model can predict maximum stresses with reasonable accuracy, allowing control of a full scale wall model. This analysis shows that the stresses generated in the joints between the blocks exceed the ultimate shear stress of the mortar, resulting in cracks in the joints. The choice of an adequate structural rehabilitation method was limited because the mosque is of archaeological importance and its original appearance should not be modified. Therefore, a seismic retrofit solution using internal or external post tensioned tendons was recommended.

Experimental Study on Damaged Prestressed Concrete Beams Using External Post-Tensioned Tendons

Experimental Study on Damaged Prestressed Concrete Beams Using External Post-Tensioned Tendons

A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables: Background, Success, and New Developments

Structural cables are susceptible to the effects of high stress concentrations, corrosion, and wind-induced and other vibrations. Cables are normally the most critical elements in a cable-supported structure and their well-being is very important in the health of the structure. The laser-based vibration technique discussed in this paper is a means for health monitoring of cables and therefore the entire cable-supported structure. This technique uses a noncontact remote sensing laser vibrometer for collecting cable vibration data from distances of up to several hundreds of feet and determines its dynamic characteristics including vibration frequencies and damping ratios. A formulation specifically developed for structural cables capable of accounting for important cable parameters is then used to calculate the cable force. Estimated forces in the cables are compared to previously measured forces or designer’s prediction to detect patterns associated with damage to the cable itself and/or changes to the structure elsewhere. The estimated damping ratios are also compared against predefined criteria to infer about susceptibility against wind-induced vibrations and other vibrations. The technique provides rapid, effective, and accurate means for health monitoring of cable-supported structures. It determines the locations and elements with potential damage and the need for detailed and hands on inspection. To date, the technique has been used successfully for evaluation of twenty-five major bridges in the US and abroad. Though originally devised for condition assessment of stay cables, it has been developed further to include a variety of systems and conditions among them structural hanger ropes in suspension, truss, and arch supported bridges, ungrouted stay cables, cables with cross-ties, and external posttensioning tendons in segmental bridge construction. It has also found a valuable place in construction-phase activities for verification of forces in tension elements with minimal efforts. Future endeavors for automation and aerial delivery are being considered for this technique.

Structural Responses of External Post-Tensioned Tendons to Increasing Localized Damage

Structural Responses of External Post-Tensioned Tendons to Increasing Localized Damage

Parametric study for post-tensioned composite beams with external tendons

Strengthening of bridge superstructures composite beams with external post-tensioned tendons is a good technique for strengthening the existing structures. In this study, a numerical model is illustrated to study the nonlinear simulation of composite beams stiffened with externally post-tensioned tendons. The accuracy of the developed numerical model is validated using comparisons between the numerical and existing test data. The influence of various strengthening parameters is investigated, which include draped versus straight tendons, tendon length, the effect of post-tensioning on reinstating the flexural behavior of an overloaded beam, tendon eccentricity, and the degree of shear connection. A good agreement between the proposed model and the test data is obtained. The results demonstrate that at the same tendon eccentricity, the trapezoidal profile shows better behavior for the strengthened beams. However, more ductility is obtained when using the straight tendon profile. Applying post-tensioning through the beam of full length helps to reduce the creation of fatigue cracks, which always start at stress raisers, and subsequently increases the fatigue life of the composite beam. Also, the external post-tensioning effectively maintains the flexural behavior of the overloaded strengthened beam after unloading in comparison to the un-strengthened beam. It is observed that 80% degree of shear connection or higher is recommended to obtain the desired performance of the external post-tensioning force for strengthening composite beams.

Finite-Element Modeling of Externally Posttensioned Composite Beams

Posttensioning has been used successfully to improve the performance of existing bridge structures. High-strength tendons can be used to effectively increase the ultimate capacity of composite beams. The main purpose of this paper is to develop a reliable three-dimensional (3D) finite-element (FE) model to simulate the nonlinear flexural behavior of steel–concrete composite beams strengthened with externally posttensioned tendons. A 3D FE model was used, where the nonlinear material behavior and geometrical analysis based on incremental–iterative load methods were adopted. The effective posttensioning stress was applied as initial strain in the link element used to model the tendons. To verify the accuracy of the developed 3D FE model, comparison between the FE analysis results and previous experimental results is presented. In-depth study has been carried out on the overall behavior of the strengthened beam and the effect of external posttensioning on stiffness, induced stresses, slippage between concrete slab and steel beam, and shear connector moments. The results of the FE model compared well with the experimental results and the FE analysis. Using external posttensioning tendons in the positive moment region for steel–concrete composite beams increased their ultimate capacity by 25%, the stiffness by 33%, and significantly improved the overall response of the bridge structural system.

Corrosion failure of post-tensioning tendons in alkaline and chloride-free segregated grout: a case study

This paper describes a case study of corrosion of post-tensioning strands in segregated grout that led to failure of one of the external tendons of a bridge after less than 2 years from the construction. Unexpectedly, severe corrosion attacks took place in areas where the ducts were filled by the injection grout, but segregation of the grout led to the presence of an unhardened whitish paste. A detailed inspection of the failed tendon, as well as other tendons removed afterwards, showed that corrosion occurred mainly in the inclined parts of the external post-tensioning tendons, in the presence of whitish grout. Analysis on samples of segregated grout showed that it was highly alkaline and had a high content of sulphate ions, but was free of chloride ions, excluding thus the normal causes of corrosion of steel in cement-based materials. Microstructural analysis of failed wires also excluded the failure for hydrogen embrittlement.

Determination of the repair grout volume to fill voids in external post-tensioned tendons

Recently, investigated failures of external post-tensioned (PT) tendons have called attention to the corrosion of strands in PT bridges, and the prevention of ongoing corrosion is required to secure their structural integrity. Since voids inside ducts can be a source for the ingress of water or deleterious chemicals, the vacuum grouting (VG) method and a volumeter for estimating amount of repair grouts were employed to fill voided ducts. However, the VG method is expensive and time-consuming for infield application because it requires an air-tight condition of entire ducts. Thus, latest research assessed three different repair grouting methods, and the pressure vacuum grouting (PVG) method was recommended in the field because it showed good filling capability in voided ducts and did not require an air-tight condition. Thus, a new method is required to estimate the volume of repair grouts because the PVG method is not applied in air-tight ducts. This research assesses the relationship between voided areas on ducts identified with soundings and required grout volume for repair using experimental results. The results show that the proposed equations and assumptions for estimating repair grout volume provide a sufficient amount of repair grouts for filling voided ducts.