Seven-wire Strands Research Articles

Tensile capacity degradation of randomly corroded strands based on a refined numerical model

In this study, we develop a sophisticated numerical model to analyze the axial tension behavior of seven-wire steel strands subjected to corrosion by employing the ANSYS finite element software. The axial tensile performance of steel strands subjected to random corrosion is examined, and the model's accuracy is validated by comparing it with experimental results. The corrosion degree in the steel strands is quantified by the mass loss rate χ, which denotes the ratio of the mass lost due to corrosion to the total mass. The reduction factor θ is employed to characterize the diminished axial tensile performance of the steel strands following corrosion. Two corrosion modes under random corrosion in steel strands were proposed, with lower bound formulas for the θ-χ distribution derived for each. In Mode I, the largest corrosion depth is prespecified. Mode II is characterized by destructive cross-sectional corrosion. As the corrosion intensifies, the corrosion pits can expand indefinitely across the wire's cross-section, potentially leading to significant loss or complete corrosion of a section of the steel strand. The finite element analysis indicates that the wire diameter and the corrosion pit depth affect θ-χ. The element size, steel strand length, and lay length have minimal impact.

Integrative numerical and analytical approaches to bond strength in lightweight concrete: Innovative modeling and parametric insights for ribbed steel bars and seven-wire strands

This study explores the bond behavior between steel reinforcement and lightweight concrete (LWC), focusing on ribbed steel bars and seven-wire strands. Utilizing finite element (FE) analysis with ATENA 3D and ATENA-GiD software, along with analytical methods, the research examines the impact of bar diameter and concrete compressive strength on bond strength. Forty-two FE models were created to simulate pull-out tests, validated against experimental data and existing design codes. The investigation covers ribbed steel bars and seven-wire strands with diameters ranging from 8 to 17.8 mm, embedded in LWC with compressive strengths from 35 to 80 MPa. The study proposes a new empirical equation for predicting bond strength, highlighting that bond strength improves with higher concrete compressive strength but decreases with larger bar diameters. Notably, ribbed steel bars exhibit approximately double the bond strength compared to seven-wire strands in LWC. The findings provide valuable insights for optimizing the design of reinforced concrete elements in LWC, balancing reinforcement diameter and concrete strength for enhanced structural integrity.

Comparative performance analysis of straight and harped prestressing seven-wire strands: High-strength stainless steel vs. conventional carbon steel

This paper presents an experimental investigation into the utilization of high-strength stainless steel (HSSS) strands systems in prestressed bridges. The study delves into the effects of localized bent and harping of seven-wire HSSS strand systems under various harping angles and curvatures produced by the deviators. For comparison purpose, conventional carbon steel (CS) strands were also studied using the same experimental parameters. It was anticipated that CS strands would exhibit better performance due to its superior post-yied ductility. To closely mimic the state of practice in the precast plant, two types of deviators were designed and used in this investigation. A comparable harping capacity was observed for HSSS strands relative to CS strands, despite the CS strands exhibiting higher strain compared to the HSSS strands. Upon reaching their breaking load, necking of all seven wires was predominantly observed for HSSS, whereas the CS strands failed due to the fracture of only one or two outer wires. Due to the lack of ductility in the HSSS stand, it was expected to have the lower bent capacity, especially at higher harping angles. Despite the significant difference in post-yied ductility, the study revealed that both HSSS and CS strands exhibit similar performance. To further understand the similarity and difference in behaviour of these two materials, scanning electron microscopy (SEM) examinations were conducted on both HSSS and CS wires to visualize the bending effects on the microstructure level after bending the center wire at different angles.

Effects of enhanced confinement on cyclic behavior of concrete bridge columns with partially unbonded seven-wire steel strands

The effects of enhanced confinement on the proposed bridge columns under lateral cyclic loading were investigated in this research by testing large-scale column specimens. The proposed column used nonprestressed partially unbonded seven-wire strands as elastic elements to increase the post-yield stiffness of the column to reduce the residual displacement under seismic loads. The enhanced confinement included rectilinear transverse reinforcement with an increased amount (specimen CSCR) and reduced spacing or innovative five-spiral reinforcement (specimen CSCS). Test results showed that all the specimens exhibited a positive post-yield stiffness ratio of 3.5–6.4 %. However, the enhanced confinement reduced the post-yield stiffness by 42–45 % but increased the lateral strength of the column, which is beneficial for controlling the residual displacement. The enhanced confinement did not affect the drift ratio when the positive post-yield stiffness ended (6 %) but significantly increased the deformation capacity by 15–31 % and the energy dissipation capacity by 37–63 % of the column. Even with a smaller amount of transverse reinforcement, the specimen with the innovative five-spiral reinforcement showed deformation and energy dissipation capacities better than the other specimens with conventional rectilinear transverse reinforcement. A pushover analysis model was developed and modified to better account for the effect of the enhanced confinement and the strand slip, and to consider the end point of positive post-yield stiffness. The comparison with the test result showed that the proposed pushover model well captured the envelope responses of the specimens. Finally, a simplified method was proposed for estimating the moment strength of the proposed column.

Measuring stress of strand using magnetic barkhausen noise measured by solenoid-type sensor

ABSTRACT This paper presents a new approach on non-destructive stress estimation that involves measuring the magnetic Barkhausen noise (MBN) on prestressed strands using a solenoid-type sensor. Using a contact-type sensor on infrastructure is difficult because prestressed strands are wrapped with protective materials and cannot be directly touched. To test the feasibility of the new idea, a solenoid-type MBN sensor is developed that measures the stress of a seven-wire strand. The sensor consists of a magnetic-field inducing coil and a sensing coil, designed to generate a varying magnetic field and measure the change in magnetic flux, respectively. An infinite impulse response filter is recommended to obtain the MBN from the measured signal, and the root mean square (RMS) is introduced to determine the level of the MBN. An indoor test is performed on a seven-wire strand with a diameter of 15.2 mm. The test shows that the MBN’s RMS decreases as the tensile stress increases. The relationship between the MBN’s RMS and the strand’s stress exhibits a clear trend similar to those observed in previous studies using contact-type sensors. The test results indicate that it is possible to estimate the stress of a strand inside a duct using the solenoid-type MBN sensor.

Numerical Analysis of Bond Strength in Pretensioned Concrete: Impact of Varying Tension Ratios on Seven-Wire Strand Using Tensioned Pull-Out Test

Numerical Analysis of Bond Strength in Pretensioned Concrete: Impact of Varying Tension Ratios on Seven-Wire Strand Using Tensioned Pull-Out Test

Experimental and numerical investigations on tensile capacity of corroded strands

A refined finite element model of corroded seven-wire steel strand was established based on finite element software. It was then compared and validated against the experimental results, demonstrating the accurate prediction capability of the model for variations in actual loads. To better capture the influence of corrosion on axial load bearing capacity of steel strands, an analysis was conducted using a relationship between axial load reduction coefficient θ and mass loss rate χ. Furthermore, this study investigates the effects of residual diameter dc/d, geometric parameters, and yield strength on the axial load reduction coefficient of steel strands. The research findings indicate that while there is a correlation between axial load reduction coefficient and residual diameter, no significant relationship exists with geometric parameters or yield strength. Based on extensive random finite element analysis, a predictive formula for lower limits of θ-χ curves for corroded steel strands at different levels of corrosion is proposed. By measuring the mass loss rate of steel strands, it becomes possible to more effectively predict their minimum axial load-bearing capacity and ensure operational safety under corrosive conditions.

Crack width, deflection, and strain limits of concrete beams with unstressed seven-wire steel strands as longitudinal reinforcement

ABSTRACT This study designed and tested six prototype beams longitudinally reinforced with either SD420 deformed steel bars, unstressed 1860-MPa seven-wire steel strands, or a combination of these. The experiment revealed that the displacement, crack width, and crack spacing increased with the increasing number of strands used to replace deformed bars as longitudinal reinforcement. Crack width was estimated using Frosch’s equation, which conservatively predicted the maximum crack width for beams with deformed bars and those with both strands and deformed bars; a modified version of the equation yielded more accurate values for the beams reinforced with strands only. A bond coefficient for strands was used in the modified equation. Strain limits for the tension- and compression-controlled sections of the beams with strands were determined to be 0.0145 and 0.0115, respectively. The deflection of the beams was well estimated using the ACI 318–19 equation.

Real-Time Damage Self-Diagnosis and Self-Localization Brillouin Distributed Optical Fiber–Fiber-Reinforced Polymer Composite Smart Cables

Cables are important components of long-span space structures, cable-supported bridges, slopes, and so on. However, they often suffer from damage such as wire breakage, corrosion, and anchor head loosening, which threatens structural safety seriously. Therefore, it is very important to diagnose and localize their damage. To address this problem, a real-time damage self-diagnosis and self-localization smart cable is proposed. It is made of a polyethylene cover pipe and some parallel intelligent steel strands with the Brillouin distributed monitoring capability. Fiber-reinforced polymer (FRP) and optical fiber are put together in the thermosetting molding process to form the intelligent central wire of each intelligent steel strand. When the side wire of the steel strand reaches the ultimate load, the intelligent central wire only reaches about 50% of its ultimate load, so it can be used in the real-time monitoring of side wires. Numerical models of seven-wire steel strands with different lengths were established, and the broken wire damage was simulated by changing coupling conditions in single and multiple damage cases. To further verify the proposed smart cable, experiments were carried out based on two intelligent steel strands with the length of 34.5 m. Experimental damage cases were simulated by cutting side wires at different positions under different loads. Both numerical and experimental results show that the change in the tension strain of the intelligent central wire can indicate the damaged location of the broken side wire. For a healthy steel strand, the tension strain of the central wire is a constant value; for a damaged steel strand, damage can be localized according to the strain waveform of the intelligent central wire and its ratio of peak strain to stable strain. The proposed smart cable has functions of real-time stress self-sensing, damage self-diagnosis, damage self-localization, damage self-warning of any side wire, and its central wire has excellent corrosion resistance; it can be well used in life-cycle real-time structural health monitoring of cable structures/substructures, which dramatically reduces the probability of cable accidents, and lays a good foundation for smart civil structures.

Comprehensive Indicators for Evaluating and Seeking Elasto-Magnetic Parameters for High-Performance Cable Force Monitoring

The elasto-magnetic method is a promising pathway for cable force monitoring in cable-stayed bridges. Under the action of an externally applied pulsed magnetic field, both the variation in the main flux recorded by the induction coil and the localized surface magnetic field measured by the packaged magnetic sensor are typical signals for observing the elasto-magnetic effect in tensioned cables. However, the performances of the parameters extracted from the two types of elasto-magnetic signals are never strictly compared in the experiment. Meanwhile, comprehensive indicators for evaluating the ability of elasto-magnetic parameters on cable force characterization are seldom discussed. As a result, it is difficult to compare the performances of elasto-magnetic devices developed by different teams, and the pathway of seeking new parameters for cable force monitoring is obstructed. In this study, elasto-magnetic calibration experiments were performed on a cable of seven-wire steel strands to simultaneously measure the variation in the main flux and the localized surface magnetic field. Comprehensive indicators considering sensitivity, hysteresis error, and cable force resolution are proposed to examine the performances of classic elasto-magnetic parameters and new candidate ones. Through comparative study, two new parameters demonstrated outstanding ability for cable force measurement, and they are the minimum amplitude of the induced voltage and the area under the curve between two points of 3 dB height of the voltage measured by a Hall sensor. The latter is recommended for high-performance cable force monitoring from the perspective of simplicity in sensor configuration.

Effect of the concrete cover thickness ratio on the post-yield stiffness of bridge columns with partially unbonded unstressed steel strands

A new self-centering concrete bridge column has been developed by the authors. The proposed bridge column uses unstressed partially unbonded seven-wire steel strands as elastic elements to reduce the residual displacement of the column after a strong earthquake. This research aimed to study the effect of concrete cover thickness ratio on the cyclic behavior of the proposed column. Four large-scale column specimens were tested using lateral cyclic loading. One column was the conventional concrete bridge column. The other three columns were the proposed self-centering bridge columns with varying concrete cover thickness ratios. Test results showed that partial unbonding effectively prevented the strands from yielding. The proposed columns showed post-yield stiffness ratios higher than the conventional column. The concrete cover thickness ratio did not significantly influence the hysteretic energy dissipation and the strain responses of longitudinal reinforcement. However, it had a significant impact on the post-yield stiffness ratio. The post-yield stiffness ratio of the proposed column tended to be inversely proportional to the concrete cover thickness ratio. A relationship was proposed between the concrete cover thickness ratio and the post-yield stiffness ratio for the preliminary design of the proposed column. Based on the relationship, the cover concrete thickness ratio should not exceed 5.1% to achieve a post-yield stiffness ratio of at least 5%, as recommended in the literature to control the residual displacement of a column.

Cyclic Behavior of Reinforced Concrete Columns with Unstressed Steel Strands as Longitudinal Reinforcement

The use of unstressed Grade 1860 MPa seven-wire steel strands as longitudinal reinforcement has the potential to reduce steel congestion in reinforced concrete members. However, no studies which use unstressed steel strands as the longitudinal reinforcement in concrete columns have been reported in the literature. Thus, in this study, five large-scale column specimens: four of which used unstressed steel strands and one which used conventional Grade 420 MPa deformed bars as longitudinal reinforcement were subjected to cyclic loading under constant axial load to investigate the effectiveness in using strands. Columns with unstressed steel strands as longitudinal reinforcement exhibited drift capacities ranging between 4.96% and 5.70%, which is well over the conventional expectation of 3.50% drift requirement for columns in special moment frames. Also, the specimens exhibited lower energy dissipation but enhanced recentering capability owing to the fact that the strands have significantly higher tensile yield capacity compared to deformed bars. A reduction in effective elastic stiffness was also observed as the postcracking stiffness of specimens with strands dropped due to the low longitudinal reinforcement ratio of 0.78% provided. The test results indicated the relative ineffectiveness of strands in compression than in tension, where the strand bulged and unwound once the surrounding confinement deteriorated. Despite this, strands did straighten up and continued to sustain tensile stresses under load reversal. Based on the experimental observations, two constitutive models, one which considers strain hardening and the other which assumes an elasto-perfectly plastic response in tension was proposed for strands. The constitutive model with strain hardening was able to produce reasonably accurate estimates of flexural strength when used in a detailed moment-curvature analysis. Both constitutive models when used in a simplified analysis, which used equivalent stress block for concrete, was able to consistently produce conservative estimates of flexural strength, which is suitable for design purpose.

Use of Unstressed Seven-Wire Strands as Longitudinal Reinforcement of Concrete Beams

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Cyclic behavior of shear-critical concrete columns with unstressed steel strands as longitudinal reinforcement

The significantly higher tensile capacity of seven-wire steel strands compared to the conventional Grade 420 (MPa) deformed bar can effectively reduce reinforcement congestion in concrete columns. As no previous studies have been reported in the literature, the realization of the above objective necessitates the critical evaluation of the flexural and shear behavior of columns. Thus, this study focused on the shear behavior of concrete columns where unstressed seven-wire Grade 1860 (MPa) steel strands were used as longitudinal reinforcement in place of conventional deformed bars. As shear behavior was under consideration, two types of transverse reinforcement layouts in rectilinear ties and multi-spiral transverse reinforcement were also considered. The experimental program consisted of the cyclic testing of six large-scale column specimens subjected to constant axial load. The tests indicated that the specimens with strands as longitudinal reinforcement failed in shear as intended in the design. Furthermore, the multi-spiral transverse reinforcement layout was more effective than the rectilinear tie layout in confining the core concrete and restraining the steel strands. Finally, both the ACI-318 simplified and detailed shear strength prediction methods were able to estimate the shear strength of the test specimens conservatively. Compared with the ACI-318 simplified method, shear strength prediction by the ACI-318 detailed method was able to produce safer estimates with less variation. In addition to the ACI method to estimate steel contribution to shear strength, modified discrete computational shear strength (mDCSS) model was also used for columns with multi-spiral transverse reinforcement. The mDCSS method provided more accurate results with less variation than the ACI method.

Comparison of the design of composite prestressed concrete hollow-core floor units between Eurocode 2 and ACI 318

A typical 1200 mm (48 in.) wide × 200 mm (8 in.) deep prestressed concrete hollow-core floor unit is analyzed and designed as a composite slab with a 75 mm (3 in.) deep structural topping to make a comparison between Eurocode 2: Design of Concrete Structures (EC2-1-1) and the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary (ACI 318R- 08). This is the second of two papers. The first was published in PCI Journal in the March–April 2021 issue and designed a noncomposite hollow-core unit. The present comparison includes two-stage composite service and ultimate moments of resistance, ultimate composite shear capacities, camber and deflections, and interface shear between the hollow-core unit and cast-in-place concrete topping. Precast concrete cylin­der strength was 40 N/mm2 (5800 psi) and the cast-in-place concrete topping was 25 N/mm2 (3625 psi). The hollow-core unit was pretensioned using 10 number seven-wire helical strands. Worked examples are pre­sented in parallel formation for EC2 and ACI 318.

Efficient Finite Element Modeling of Steel Cables in Reinforced Rubber

Spiral steel cables feature complex deformation behavior due to their wound geometry. In applications where the cables are used to reinforce rubber components, modeling the cables is not trivial, because the cable’s outer surface must be connected to the surrounding rubber material. There are several options for modeling steel cables using beam and/or solid elements for the cable. So far, no study that lists and evaluates the performance of such approaches can be found in the literature. This work investigates such modeling options for a simple seven-wire strand that is regarded as a cable. The setup, parameter calibration, and implementation of the approaches are described. The accuracy of the obtained deformation behavior is assessed for a three-cable specimen using a reference model that features the full geometry of the wires in the three cables. It is shown that a beam approach with anisotropic beam material gives the most accurate stiffness results. The results of the three-cable specimen model indicate that such a complex cable model is quite relevant for the specimen’s deformation. However, there is no single approach that is well suited for all applications. The beam with anisotropic material behavior is well suited if the necessary simplifications in modeling the cable–rubber interface can be accepted. The present work thus provides a guide not only for calibrating but also for selecting the cable-modeling approach. It is shown how such modeling approaches can be used in commercial FE software for applications such as conveyor belts.

Temperature Compensation of Fiber Bragg Grating Sensors in Smart Strand.

Compared to other types of sensors, fiber optic sensors have improved accuracy and durability. Recently, the Smart Strand was developed to maximize the advantages of fiber optic sensors for measuring the cable forces in prestressed concrete structures or cable-supported bridges. The Smart Strand has fiber Bragg gratings (FBGs) embedded in a core wire of the seven-wire strand. Similar to other sensors, the strain measured at an FBG is affected by temperature; therefore, the temperature effect that is not related to the mechanical strain should be compensated for or corrected in the long-term measurement subjected to temperature variation. However, a temperature compensation procedure for the FBG has yet to be established, and relevant studies have used different formulas for the compensation. Moreover, when the FBG sensors are packaged with a certain material—such as fiber reinforced polymer—for protection, it is important to consider the interaction between the FBG, packaging material, and host material during thermal behavior. Therefore, this study proposed a reasonable procedure for temperature compensation for the FBG sensors embedded in packaging material and host material. In particular, the thermal sensitivity of the Smart Strand was intensively investigated. The proposed theoretical formulas were validated through comparison with data obtained from various specimens in a temperature-controlled chamber. Finally, the procedure was applied to correct the data measured using the Smart Strands in a 20-m-long full-scale specimen for about a year, thus resulting in a realistic trend of the long-term prestressing force.

Cyclic behavior of bridge columns with partially unbonded seven-wire steel strands to increase post-yield stiffness

A new method of increasing post-yield stiffness of bridge columns to reduce the residual displacement after large earthquakes is proposed. In the proposed method, partially unbonded unstressed seven-wire steel strands are used as elastic element to increase the post-yield stiffness of the column. Three large-scale column specimens were tested using lateral cyclic loading to investigate the seismic performance of the proposed column. Test results showed that the use of the unstressed partially unbonded steel strands effectively increased the post-yield stiffness ratio of a conventional bridge column from −0.3% to 6.4%, which exceeded the 5% post-yield stiffness ratio that has been shown in the literature to reduce the residual displacement significantly. Test results also showed that the post-yield stiffness could not be maintained when the strands started to bulge in compression. The ultimate drift was slightly reduced from 5.76% to 4.77%. The use of fully bonded strands further increased the post-yield stiffness ratio to 8.8%. However, the ultimate drift ratio was greatly reduced to 2.84%. To predict the behavior of the proposed column under lateral load, a pushover model was developed. The strain of the partially unbonded strand was calculated based on deformation compatibly with the longitudinal deformed steel bar. The effect of anchorage slip was considered. Comparison with the test results showed that the proposed model captured well the envelope response of the proposed column.

Quantification of non-uniform mechanical interlock and rotation in modelling bond-slip between strand and concrete

A quantification method of the non-uniform mechanical interlock and rotation is proposed for modelling the bond-slip behavior between steel strand and concrete. The bond stress incorporating the non-uniform mechanical interlock is derived based on the developed three-dimensional (3D) mechanical model. A theoretical model of strand slip including rotation is established using the energy conservation law. The proposed method is verified by comparing with the predicted and experimental results. The effects of the mechanical interlock and rotation on the bond-slip of strand are also discussed. Results show that the proposed method can reasonably model the bond-slip behavior between steel strand and concrete. For the typical seven-wire strand, the theoretical prediction accuracy of the bond stress and slip can be improved by considering the effects of the non-uniform mechanical interlock and rotation. The non-uniform mechanical interlock restrains the overestimated bond stress and the strand rotation contributes to the slip.

Testing bond behaviour of an innovative triangular strand: Experimental setup challenges and preliminary results

The use of the seven-wire strand in pretension structures so far has shown that such strands have a relatively long development length. An attempt to reduce the development length is reflected through the idea of a different strand geometry that has a more favorable shape. This new shape should have a greater inclination of the outer wires, increasing the resistance that occurs when moving through the concrete. The idea is primarily about the triangular distribution of wires in the cross-section of the strand. This paper reviews tests related to the use of innovative, triangular, and ten-wire strands. The tests exclusively relate to the strands formed by the use of steel wires.