Seven-wire Prestressing Strands Research Articles

Probabilistic Models for Fatigue Resistance of Seven-Wire Prestressing Strands and Stay Cables

Parallel seven-wire steel prestressing strands are the dominant form of the main tension elements (MTE) used in stay cables. In this paper, probabilistic models for fatigue resistance of seven-wire prestressing strands that are not embedded in concrete and are subjected to axial stresses are developed. Available test data from seven-wire strand fatigue tests were collected and analyzed to develop probabilistic models and nonlinear S–N curves using survival analysis techniques. Results indicate that the fatigue resistance of classic stress-relieved strands produced before early 1980s is higher than the modern low-relaxation strands that have been manufactured since then. Neither the conventional classic nor the modern strands have a good chance of passing the fatigue qualification tests required by the latest design standards for use in stay cables. Only the cable-quality (CQ) strands can pass the latest fatigue qualification tests with a less than 2.5% probability of failure. Nonlinear S–N equations for all three strand types and stay cables made with CQ strands are proposed using a log-logistic parametric survival model.

FEM Simulations of Bond Behaviour between Concrete and Seven-Wire Prestressing Strand

The article deals with the subject related to numerical tests of the concrete bond to seven-wire strands. Nonlinear contact concrete-steel calculations with using Finite Element Method (FEM) is applicable for concrete bond to reinforcing steel analyses. At the moment, the analyzes are the subject of independent research projects. The research using several scientific and commercial computing systems allows us to look deeper into the sample and obtain information about cracks development, stresses, and internal deformations. Such analyzes are impossible when tests only experimentally. The concrete-steel behaviour of materials can be modeled with a bond-slip mechanism where the relative slip of reinforcement and concrete is phenomenologically described. The behaviour of the slip zone is then mapped with a zero thickness interface (contact) element. The constitutive laws of bond-slip adhesion are based mainly on the theory of total deformation, which expresses the pulling force as a function of total relative displacement. The correct construction of the bond model requires consideration of physical phenomena occurring not only at the interface of the materials but also inside within each of them. When designing numerical research, it is necessary to keep in mind the limits in computing memory of computers. Performing static calculations for complex numerical models containing a number of geometric, material and contact nonlinearities may prove to be too time-consuming on a PC. For this reason, it is now reasonable to construct approximate, simplified models. FEM models with a strand modeled as a solid bar (with a sidewall accurate to the real one) made it possible to observe and research, destructive mechanisms identified in experimental pull-out tests.

FEM Analysis of Bond between Concrete and Seven-Wire Prestressing Strand

Detailed FEM analysis of the pull-out test of seven-wire prestressing strand form concrete sample is the purpose of the article. A 3D numerical model of a seven-wire prestressing strand concreted in the concrete cube was made. The assumption of the most trusted modeling of the strand geometry for numerical calculations was assumed. The mechanical parameters of the materials and interface elements for analysis were adopted based on the results obtained from the experimental tests previously carried out by the author. As a result of numerical calculations, it was possible to trace the deformation state, stresses and cracking as well as their development along with the change of the strand displacement in the sample. The images of deformations and stresses in the sample confirmed theoretical considerations and suppositions from experimental tests about its accumulation of the largest values in longitudinal concrete wedges in the closest cover of the strand. During the test, the concrete inside the cube cracked quickly. The first cracks appear in the zone of the nearest cover of the strand. Numerical analysis made it possible to observe both the failure mechanisms identified in the experimental tests. The first mechanism consisted in cutting the longitudinal concrete wedges between the individual wires. The second mechanism was based on the rotation of the strand and its slip along the route of the twist.

Macrocell corrosion mechanisms of prestressing strands in various concretes

This paper investigates the mechanisms of corrosion of seven-wire strands and the reasons for failures in detecting corrosion at early stages. As per ASTM G109 and ACI 222 R-01, a 1-year study on chloride-induced corrosion of seven-wire prestressing strands embedded in various concretes was conducted. The type of corrosion current circuits in the prismatic specimens with one top and two bottom strands was investigated. Unlike solid rebars, the concrete resistivity had a limited role on the type of corrosion circuits in strand systems, with a high probability of corrosion occurring between different wires in the top strand itself. This may be due to the flower-like geometry and the complex corrosion mechanisms within the top strand. It is recommended to perform other electrochemical measurements on the top strand – prior to making conclusions. Also, until about 6% corrosion loss is reached, the rust will fill the interstitial space between the seven wires and will not exert expansive stresses on the surrounding concrete. Hence, no staining/cracking will be visible on the concrete surface – a serious structural safety issue – necessitating frequent chloride/corrosion condition assessment of prestressed-concrete structures.

Effect of temperature on the mechanical properties of low relaxation seven-wire prestressing strand

This paper presents results from experiments on the effect of temperature on the mechanical properties of low relaxation seven-wire prestressing strand. A set of steady-state tensile strength tests are carried out on prestressing strands and reinforcing bars at various temperatures. Data from these tensile strength tests is utilized to derive stress-strain response, yield strength, tensile strength, elastic modulus, and rupture strain of prestressing strand and reinforcing bars in 20–800°C temperature range. Results obtained from these property tests indicate that prestressing strand experiences higher strength degradation than that of reinforcing steel bar throughout the temperature range. In addition, presence of preload on prestressing strand slightly increases the temperature induced rate of strength degradation beyond 450°C. Results from these mechanical property tests are utilized to propose empirical relations for expressing variation of mechanical properties of low relaxation seven-wire prestressing strand as a function of temperature.

Use of Recycled Concrete Aggregates in Precast/Prestressed Concrete

This paper describes a preliminary investigation on factors related to the use of recycled concrete aggregates (RCA) in precast/prestressed concrete applications. In particular, the effects of RCA are experimentally investigated for: 1) the bond between seven-wire prestressing steel strand and concrete; 2) the compression strength gain of concrete with time; and (3) the compression stiffness gain of concrete with time. The test specimens were made by replacing the natural course aggregates in the concrete mixtures with RCA from: 1) discarded/rejected precast concrete; and 2) returned ready-mix concrete. The data to-date show that the introduction of RCA results in an effect (decrease or increase) in the strand bond strength that is consistent with the effect of RCA on the compression strength of concrete. For the materials tested, RCA from precast concrete performed better than RCA from returned ready-mix concrete. The use of RCA did not have a significant effect on the rate of concrete compression strength gain with time. The results for the stiffness gain of concrete with time were somewhat more varied than the results for the strength gain.

Corrosion Performance of Prestressing Strands in Contact with Dissimilar Grouts

To improve the corrosion protection provided to prestressing strands, anti-bleed grouts are used to fill voids in post-tensioning ducts that result from bleeding and shrinkage of older Portland Cement grouts. Environmental differences caused by exposing the strands to dissimilar grouts, however, have the potential to cause rapid corrosion. Portland Cement grout, gypsum grout, and four commercially available prepackaged grouts were analyzed to determine the chemical composition of the resulting pore solutions and tested to determine if using a second grout will provide improved corrosion protection for prestressing strands or result in accelerated corrosion. The potential consequences of leaving the voids unfilled were also evaluated. Pore solutions were analyzed for pH and sodium, potassium, fluoride, chloride, nitrite, sulfate, carbonate, nitrate, and phosphate ion content. The analyses were used to develop simulated pore solutions. Selected grouts and simulated pore solutions were paired to evaluate their potential to cause corrosion of, respectively, grout-wrapped and bare stress-relieved seven-wire prestressing strands using the rapid macrocell test. Strands were also evaluated in simulated pore solutions containing chlorides and in deionized water. Because exposure of strands to water or chlorides has the potential to cause rapid corrosion, filling voids in post-tensioning ducts with an anti-bleed grout is recommended. Gypsum grout, with its low pH and high sulfate content, will cause accelerated corrosion of strands when used in conjunction with Portland Cement grout or any of the commercially prepackaged grouts tested. When paired with Portland Cement grout, the prepackaged anti-bleed grouts evaluated in this study resulted in corrosion losses significantly below those observed for strands exposed to salt or water. The highest corrosion measured for a prepackaged grout occurred for the grout with the highest pore solution sulfate content.

Slip distribution model along the anchorage length of prestressing strands

An analytical model to predict strand slips within both transmission and anchorage lengths in pretensioned prestressed concrete members is presented. This model has been derived from an experimental research work by analysing the bond behavior and determining the transmission and anchorage lengths of seven-wire prestressing steel strands in different concrete mixes. A testing technique based on measuring the prestressing strand force in specimens with different embedment lengths has been used. The testing technique allows measurement of free end slip as well as indirect determination of the strand slip at different cross sections of a member without interfering with bond phenomena. The experimental results and the proposed model for strand slip distribution have been compared with theoretical predictions according to different equations in the literature and with experimental results obtained by other researchers.

Dilation behavior of seven-wire prestressing strand – The Hoyer effect

In prestressed concrete elements, bond between pretensioned strands and the surrounding concrete is attributed to adhesion, mechanical interlock and the friction and ‘wedge-action’ attributable to radial expansion of the strand following release. The latter effect is referred to as the Hoyer effect and is the subject of this paper. The Hoyer effect contributes primarily to the transfer of the initial prestressing force to the concrete thereby affecting the transfer length. The transfer of large prestressing forces, particularly at an early concrete age, can lead to local cracking associated with bursting stresses or splitting associated with transfer of the strand force through bond. An analysis of embedded strand behavior based on fundamental principles of mechanics clearly demonstrates the importance of strand dilation characteristics in the development of concrete stresses and therefore the transfer of prestressing forces. An experimental program demonstrates that the dilation of seven wire strand is affected by not only the Poisson’s ratio for steel but also the ‘tightening’ of the helical wires and effects of bearing between these wires. The resulting dilation of a seven wire strand is larger than Poisson’s ratio for steel and is found to be a function of strand diameter. A discussion of design code provisions for prestressing strand transfer length is included. North American practice, while conservative, does not explicitly address the mechanics of stress transfer. Improvement in this regard could result in more efficient prestressed concrete structures. European practice, on the other hand, appears to reasonably capture the Hoyer effect.

Bond of 13 mm prestressing steel strands in pretensioned concrete members

This paper presents an experimental research work to determine both the transmission and the anchorage lengths of seven-wire prestressing steel strands in different concrete mixes. A testing technique based on a bond behavior analysis by measuring the force supported by the prestressing strand on a series of specimens with different embedment lengths has been used. Relationships between the average bond stress for both the transmission length and anchorage length as a function of the concrete compressive strength have been found. Equations to compute transmission and anchorage lengths of 13mm prestressing strands have been obtained. The experimental results have been compared with the theoretical prediction from proposed equations in the literature and with experimental results from other authors.

Effects of concrete composition on transmission length of prestressing strands

The bond behavior of prestressing strands in precast pretensioned concrete members, and its transmission length, depends on several factors. However, no consensus exists on the main parameters to be considered in the expressions to predict the transmission length. Usually when the concrete properties are considered, only the concrete compressive strength is included. This study analyzes the influence of concrete composition made up of different cement contents and water/cement ratios on the bond behavior in transmission of seven-wire prestressing strands. The bond properties and the transmission lengths have been determined. The results show that the influence of the water/cement ratio is very small for concretes with lows cement contents, but the influence of the water/cement ratio on the transmission lengths is highly significant when cement content is high. The effect of cement content in the transmission lengths can reveal different tendencies based on the level of the water/cement ratio.

Determining Broken Wires in Unbonded Seven-Wire Strands Using Penetration Tests

Unbonded seven-wire prestressing strands provide the primary reinforcement in post-tensioned concrete slabs. Strand corrosion can result in broken wires, which require expensive inspections and strand replacements. During these inspections, strands that are suspected of being corroded are exposed and inspected visually and a screwdriver penetration test is performed. Penetration of the screwdriver between any two adjacent wires is an indication that one of the wires may be broken or that the strand is unstressed or de-stressed. In this study, penetration tests were conducted on corroded strands with broken wires. The purpose of the study was twofold: to determine the distance from an inspection location where a wire break may be detected using penetration tests and to determine how corrosion products between the wires of a strand affect the accuracy of the penetration tests. During testing, corroded strands were stressed to typical service-load levels and the load and boundary conditions of an unbonded, post-tensioned concrete slab were simulated. Two outer wires were cut in succession, and penetration tests were performed along the strand. It was observed that penetration can be achieved between a broken and an unbroken wire within a region where the strain in the broken wire is less than about 200*e.

Analytical model for transfer length prediction of 13 mm prestressing strand

An experimental investigation to determine the transfer length of a seven-wire prestressing strand in different concretes is presented in this paper. A testing technique based on the analysis of bond behaviour by means of measuring the force supported by the prestressing strand on a series of specimens with different embedment lengths has been used. An analytical bond model to calculate the transfer length from an inelastic bond stress distribution along the transfer length has been obtained. A relationship between the plastic bond stress for transfer length and the concrete compressive strength at the time of prestress transfer has been found. An equation to predict the average and both the lower bound and the upper bound values of transfer length is proposed. The experimental results have not only been compared with the theoretical prediction from proposed equations in the literature, but also with experimental results obtained by several researchers.

Bond Behavior of Strand Embedded in Fiber Reinforced Cementitious Composites

Due to the brittleness of concrete when subjected to tension, bond between prestressing strands and concrete is generally weak. Compared with conventional concrete, fiber reinforced cement composites (FRCCs) can potentially enhance bond because of the material's superior tensile behavior and ductility. The effects of FRCC materials on the bond strength and the bond stress-slip response of 0.5 in. (13 mm) seven-wire prestressing strands were investigated. Pullout tests comprising monotonic, cyclic, and reversed cyclic loads were carried out. Test results showed that the confinement effect provided by fibers after concrete cracking increased friction and mechanical interlocking between prestressing strands and the concrete matrix, leading to enhanced bond resistance (up to three times) compared with conventional concrete matrices. Unstressed strands embedded in FRCCs maintained high bond stresses up to large slip distances, resulting in a nearly elastic-perfectly plastic, bond stress-slip response. Under various types of cyclic loading, strands embedded in FRCC matrices showed ductile bond stress-slip responses with minor bond strength and stiffness degradation up to slip lengths in excess of one strand diameter, thereby dissipating a substantial amount of energy.

Fatigue behaviour of concrete beams post-tensioned with unbonded carbon fibre reinforced polymer tendons

Much research has been conducted in the past decade to evaluate the suitability of fibre reinforced polymer (FRP) reinforcement in concrete structures. Most of the research has concentrated on the short-term performance of FRP prestressed and reinforced concrete beams. Only a limited amount of research has considered the fatigue behaviour of FRP prestressed beams. This paper presents an experimental research program designed to examine the fatigue behaviour of unbonded carbon fibre reinforced polymer (CFRP) post-tensioned concrete beams. The fatigue test program consisted of five large-scale (4.0 m span) concrete T-beams. Three of the beams were post-tensioned with CFRP tendons, and the remaining two beams were post-tensioned with steel prestressing strands. The fatigue load limits were chosen to produce an additional stress range of about 100 MPa in the lower prestressing reinforcement. During fatigue testing, some of the prestressing strands fractured at the anchor location. In the steel post-tensioned beams, fracture of wires in the seven-wire prestressing strands did not result in total failure of the steel post-tensioned beams, as the unbroken wires continued to carry prestress force. In the CFRP post-tensioned beams, however, fracture led to splintering of the tendon between the anchors and total loss of prestress force. In general, the CFRP post-tensioned beams performed satisfactorily in fatigue, in comparison with the steel post-tensioned beams, as long as premature failure of the tendons near the anchor location was prevented.Key words: fibre reinforced polymer (FRP), anchorage, tendon, fatigue, post-tension, concrete, beam, dynamic, testing.

Transfer and Development Lengths of Concentrically Prestressed Concrete

An experimental investigation to develop a test to characterize the bond between prestressing strand and concrete has been carried out at the Polytechnic University of Valencia in Spain. The proposed test determines the transfer and development lengths by measuring the force in the strand in a series of test specimens with different embedment lengths. The test sequentially reproduces the transfer of prestress in a specimen and simulates service load through a pull-out test on the same specimen. With this technique, the Hoyer Effect is produced prior to the pull-out phase of testing. An experimental program determined the applicability of the test method and the feasibility of a standardized test for quality control of prestressing strand bonding. Transfer- and development-length tests were conducted on seven-wire prestressing strand using 12 different concretes. Results show that the test is valid and practical. Test data indicate that ACI 318 equations are conservative. Details of testing equipment and procedures are also presented in this paper.

Application of Wigner-Ville Transform to Evaluate Tensile Forces in Seven-Wire Prestressing Strands

Seven-wire steel strands are widely used in various types of prestressed concrete structures. When a strand is subjected to a tensile force, the traveling time of the stress wave will be affected due to the elongation of the strand together with the changes in wave velocities. In this paper, the Wigner-Ville Transform technique was used to analyze the measured stress waves in order to identify the arrival time of each frequency component. A numerical calculation considering the elastic waveguide theory and the acoustoelastic effect was conducted. A commonly used 12.7-mm (1/2 in.) diameter seven-wire prestressing strand (Grade 270) was tested. Tensile force up to 142 kN (32 kips) was applied to the strand. The results indicate that evaluation of the tensile force in the strand can be accomplished by measuring the traveling time of a single-frequency component of the propagating stress wave. The Wigner-Ville transform technique can efficiently identify the arrival time of each frequency component of the stress-wave signals with reasonable accuracy. The numerical and experimental results correlate well with each other. Results of this study present a technique that can provide an efficient nondestructive measurement of stress retension levels in long post-tensioned steel strands.

Measurement of Tensile Forces in a Seven-Wire Prestressing Strand Using Stress Waves

This paper presents a nondestructive, stress wave technique to evaluate prestress levels in posttensioned seven-wire strands. Seven-wire steel strands are widely used in various types of prestressed concrete structures. Both experimental measurements and theoretical analysis were conducted. A commonly used 12.7 mm diameter seven-wire prestressing strand was studied. Tensile forces up to 111.2 kN were applied to the strand. A numerical analysis was conducted by accounting for acoustoelasticity effects and the dispersion of waves to calculate the prestress levels in the strands. This analysis provides a successful description of the behavior of a longitudinal transient wave traveling through a long, prestressed, circular strand. The calculation correlates well with experimental observations. The results indicate that the time of flight of the stress waves can be related to the tension level in the strand. Change of wave velocities at different stress levels was evaluated, and the technique's accuracy is discussed. This technique can provide an efficient nondestructive measurement of stress retension levels in posttensioned steel strands.

Spacing and Concrete Cover Requirements for Epoxy-Coated Prestressing Strand in Unconfined Sections

A test program was undertaken to determine the spacing and concrete cover requirements of 0.5 in. (12.7 mm) diameter seven-wire prestressing strand coated with a grit impregnated epoxy. Spacing and concrete cover requirements were studied by varying the parameters of strand spacing, concrete cover and number of strands. These requirements were then compared with the AASHTO Specifications. The transfer length of epoxy-coated prestressing strand was also investigated. It is concluded that strand spacing and concrete cover requirements should be increased somewhat in prestressed concrete members without confinement reinforcement. However, in bridge girders and other prestressed concrete members that typically carry confinement reinforcement in the form of stirrups and end zone reinforcement, increased strand spacing and concrete cover may not be warranted.

Prestressed Polymer Impregnated Tubes as Reinforcement

The deterioration of concrete structures caused by the corrosion of reinforcement has become a major problem in the past decade 1-3 Although bridge structures are most often cited,¢-' corrosion problems are by no means unique to them. Marine and port structures, residential and office buildings constructed near waterfronts, water supply and sewage disposal systems, and industrial structures subjected to aggressive environments are examples of other structures which, likewise, may be affected by reinforcement corrosion. To cope with the problem of corrosion, the use of specifically developed tension elements termed is proposed as a substitute for ordinary deformed reinforcing bars. PIC tubes are fabricated by encasing ordinary seven-wire prestressing strands in high strength, impermeable, corrosion resistant polymer impregnated concrete (PIC) while under high tension. The subsequent release of the pretensioning force results in high compressive stresses in the PIC casing, preventing tensile cracking under service loading. Fig. la is a schematic diagram of a PIC tube and Fig. lb illustrates possible uses in reinforced concrete members.