Stress relaxation losses of prestressing steel wires

Prestressed structures are susceptible to relaxation losses which are of significant importance in structural design. After being manufactured, prestressing wires are coiled to make their storage and transportation easier. The possible deleterious effects of this operation on the stress relaxation behavior of prestressing steel wires are usually neglected, though it has been noticed by manufacturers and contractors that when relaxation tests are carried out after a long-time storage, on occasions relaxation losses are higher than those measured a short time after manufacturing. The influence of coiling on the relaxation losses is checked by means of experimental work and confirmed with a simple analytical model. The results show that some factors like initial residual stresses, excessively long-time storage or storage at high temperatures, can trigger or accentuate this damage. However, it is also shown that if the requirements of standards are fulfilled (minimum coiling diameters) these effects can be neglected.

Influence of coiling on the stress relaxation of prestressing steel wires

The possible deleterious effects of coiling and long-term storage of coiled wires on the stress relaxation behaviour of prestressing steel wires has been checked by means of experimental work and a simple analytical model. The results show that if the requirements of standards are fulfilled (minimum coiling diameters), these effects can be neglected. However, some other factors, such as previous residual stresses, long-term storage or storage at high temperatures, can trigger or emphasize this damage to the material. In the authors' opinion, checking the final curvature of the wires after uncoiling prior to prestressing, as required in some standards, is to be recommended.

Influencia de las condiciones de almacenamiento sobre la relajación de aceros de pretensado = Influence of storage conditions on prestressing steel relaxation losses

La pérdida de tensión por relajación en las armaduras activas afecta de forma importante a las estructuras de hormigón pretensado. Por ello se realizan ensayos de relajación de los alambres y cordones de pretensado tras su fabricación. Después, el material es enrollado y almacenado durante periodos que en ocasiones pueden superar el año de duración. Generalmente se desprecia la influencia que estas operaciones posteriores a la fabricación pueden tener sobre el material. Sin embargo, diversos fabricantes y suministradores han constatado experimentalmente que, en ocasiones, el material almacenado durante un periodo prolongado presenta pérdidas de relajación mayores que inmediatamente tras su fabricación. En este trabajo se realizan ensayos de laboratorio para comprobar la influencia que el radio de enrollamiento y el periodo de almacenamiento tienen sobre las pérdidas de relajación. También se propone un modelo analítico que permite predecir de manera razonablemente aproximada el valor del ensayo de relajación sobre un alambre sometido a un enrollamiento prolongado. Este modelo explica la evolución del perfil de tensiones durante el proceso de enrollamiento-almacenamiento-desenrollamiento, así como la influencia de las tensiones residuales

Behaviour of steel prestressing wires under extreme conditions of strain rate and temperature

The purpose of this paper is to provide information on the behaviour of steel prestressing wires under likely conditions that could be expected during a fire or impact loads. Four loadings were investigated: a) the influence of strain rate – from 10–3 to 600 s–1 – at room temperature, b) the influence of temperature – from 24 to 600 °C – at low strain rate, c) the influence of the joint effect of strain rate and temperature, and d) damage after three plausible fire scenarios. At room temperature it was found that using “static” values is a safe option. At high temperatures our results are in agreement with design codes. Regarding the joint effect of temperature and strain rate, mechanical properties decrease with increasing temperature, although for a given temperature, yield stress and tensile strength increase with strain rate. The data provided can be used profitably to model the mechanical behaviour of steel wires under different scenarios.

Influence of coiling in stress relaxation of prestressing steel wires

The possible deleterious effects of coiling and long-time storage of coiled wires on the stress relaxation behaviour of prestressing steel wires has been checked by means of experimental work and a simple analytical model. The results show that if the requirements of Standards are fulfilled (minimum coiling diameters) these effects can be neglected. However, some other factors like previous residual stresses, long-time storage or storage at high temperatures, can trigger or emphasise this damage to the material. In the authors’ opinion it is recommendable to control the final curvature of the wires after uncoiling prior to prestressing, as required in some Standards.

Bond Modelling of prestressed concrete during the prestressing force release

This paper presents an analytical model for simulating the bond between steel and concrete, in precast prestressed concrete elements, during the prestressing force release. The model establishes a relationship between bond stress, steel and concrete stress and slip in such concrete structures. This relationship allows us to evaluate the bond stress in the transmission zone, where bond stress is not constant, along the whole prestressing force release process. The model is validated with the results of a series of tests and is extended to evaluate the transmission length. This capability has been checked by comparing the transmission length predicted by the model and one measured experimentally in a series of tests.

Bond modelling of prestressed concrete during the prestressing force release

This paper presents an analytical model for simulating the bond between steel and concrete, in precast prestressed concrete elements, during the prestressing force release. The model establishes a relationship between bond stress, steel and concrete stress and slip in such concrete structures. This relationship allows us to evaluate the bond stress in the transmission zone, where bond stress is not constant, along the whole prestressing force release process. The model is validated with the results of a series of tests, considering different steel indentation depths and concrete covers and is extended to evaluate the transmission length. This capability has been checked by comparing the transmission length predicted by the model and one measured experimentally in two series of tests.

The role of residual stresses in the performance and durability of prestressing steel wires

Residual stresses developed during wire drawing influence the mechanical behavior and durability of steel wires used for prestressed concrete structures, particularly the shape of the stress–strain curve, stress relaxation losses, fatigue life, and environmental cracking susceptibility. The availability of general purpose finite element analysis tools and powerful diffraction techniques (X-rays and neutrons) has made it possible to predict and measure accurately residual stress fields in cold-drawn steel wires. Work carried out in this field in the past decade, shows the prospects and limitations of residual stress measurement, how the stress relaxation losses and environmentally-assisted cracking are correlated with the profile of residual stresses and how the performance of steel wires can be improved by modifying such a stress profile

Influence of long-time storage. Stress relaxation losses of prestressing steel wires

Prestressed structures are susceptible to relaxation losses which are of significant importance in structural design. After being manufactured, prestressing wires are coiled to make their storage and transportation easier. The possible deleterious effects of this operation on the stress relaxation behavior of prestressing steel wires are usually neglected, though it has been noticed by manufacturers and contractors that when relaxation tests are carried out after a long-time storage, on occasions relaxation losses are higher than those measured a short time after manufacturing. The influence of coiling on the relaxation losses is checked by means of experimental work and confirmed with a simple analytical model. The results show that some factors like initial residual stresses, excessively long-time storage or storage at high temperatures,can trigger or accentuate this damage. However, it is also shown that if the requirements of standards are fulfilled (minimum coiling diameters) these effects can be neglected.

Influencia de las condiciones de almacenamiento sobre la relajación de aceros de pretensado = Influence of storage conditions on prestressing steel relaxation losses

La pérdida de tensión por relajación en las armaduras activas afecta de forma importante a las estructuras de hormigón pretensado. Por ello se realizan ensayos de relajación de los alambres y cordones de pretensado tras su fabricación. Después, el material es enrollado y almacenado durante periodos que en ocasiones pueden superar el año de duración. Generalmente se desprecia la influencia que estas operaciones posteriores a la fabricación pueden tener sobre el material. Sin embargo, diversos fabricantes y suministradores han constatado experimentalmente que, en ocasiones, el material almacenado durante un periodo prolongado presenta pérdidas de relajación mayores que inmediatamente tras su fabricación. En este trabajo se realizan ensayos de laboratorio para comprobar la influencia que el radio de enrollamiento y el periodo de almacenamiento tienen sobre las pérdidas de relajación. También se propone un modelo analítico que permite predecir de manera razonablemente aproximada el valor del ensayo de relajación sobre un alambre sometido a un enrollamiento prolongado. Este modelo explica la evolución del perfil de tensiones durante el proceso de enrollamiento- almacenamiento-desenrollamiento, así como la influencia de las tensiones residuales.Stress relaxation losses on active reinforcement have significant impact on prestressed concrete structures. This is why relaxation tests are carried out on prestressing steel wires and strands after being manufactured. Then, these materials are coiled and stored for a long-term period, sometimes in excess of one year. The influence of these operations, carried out after manufacturing, is usually neglected. Nevertheless, some manufacturers and contractors have noticed that, sometimes, when relaxation tests are carried out after a long-term storage, the relaxation losses found are higher than those measured immediately after manufacturing. In this work, lab tests are performed to check the influence of the coiling radius and the period of storage on the relaxation test. In addition to this, an analytical model is presented to predict the results of a relaxation

Cohesive-frictional modelling of bond and splitting action of prestressing wire

This work shows a numerical procedure for bond between indented wires and concrete, and the coupled splitting of the concrete. The bond model is an interface, non-associative, plasticity model. It is coupled with a cohesive fracture model for concrete to take into account the splitting of such concrete. The radial component of the prestressing force, increased by Poisson’s effect, may split the surrounding concrete, decreasing the wire confinement and diminishing the bonding. The combined action of the bond and the splitting is studied with the proposed model. The results of the numerical model are compared with the results of a series of tests, such as those which showed splitting induced by the bond between wire and concrete. Tests with different steel indentation depths were performed. The numerical procedure accurately reproduces the experimental records and improves knowledge of this complex process.

Effects of hydrogen assisted stress corrosion on damage tolerance of a high-strength duplex stainless steel wire for prestressing concrete

The study brings new insights on the hydrogen assisted stress corrosion on damage tolerance of a high-strength duplex stainless steel wire which concerns its potential use as active reinforcement for concrete prestressing. The adopted procedure was to experimentally state the effect of hydrogen on the damage tolerance of cylindrical smooth and precracked wire specimens exposed to stress corrosion cracking using the aggressive medium of the standard test developed by FIP (International Prestressing Federation). Stress corrosion testing, mechanical fracture tests and scanning electron microscopy analysis allowed the damage assessment, and explain the synergy between mechanical loading and environment action on the failure sequence of the wire. In presence of previous damage, hydrogen affects the wire behavior in a qualitative sense, consistently to the fracture anisotropy attributable to cold drawing, but it does not produce quantitative changes since the steel fully preserves its damage tolerance.

Circular concrete tanks without prestressing

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High stress monitoring of prestressing tendons in nuclear concrete vessels using fibre-optic sensors

Maintaining the structural health of prestressed concrete nuclear containments is a key element in ensuring nuclear reactors are capable of meeting their safety requirements. This paper discusses the attachment, fabrication and characterisation of optical fibre strain sensors suitable for the prestress monitoring of irradiated steel prestressing tendons. The all-metal fabrication and welding process allowed the instrumented strand to simultaneously monitor and apply stresses up to 1300 MPa (80% of steel's ultimate tensile strength). There were no adverse effects to the strand's mechanical properties or integrity. After sensor relaxation through cyclic stress treatment, strain transfer between the optical fibre sensors and the strand remained at 69%. The fibre strain sensors could also withstand the non-axial forces induced as the strand was deflected around a 4.5 m bend radius. Further development of this technology has the potential to augment current prestress monitoring practices, allowing distributed measurements of short- and long-term prestress losses in nuclear prestressed-concrete vessels. © 2014 Elsevier B.V.

Precast Concrete Panel Thickness for Epoxy-Coated Prestressing Strands, HR-353, Interim Report, 1994

A recommended minimum thickness for prestressed concrete (P/C) bridge deck panels containing 3/8-in. diameter, 270-ksi, low-relaxation, grit-impregnated, epoxy-coated prestressing strands is being evaluated by testing prototype panel specimens. As of January 1994, specimens from ten castings have been tested. The specimens in the first five castings were constructed to establish a preliminary minimum thickness for P/C panels. The specimens in the last five castings were constructed to 1) confirm the minimum panel thickness requirement, 2) measure the development length of epoxy-coated strands in specimens containing multiple strands, 3) measure the development length of uncoated strands in specimens containing multiple and single strands, 4) observe if concrete cracks form in thin panel specimens that have a raked top surface and are reinforced with welded wire fabric and either epoxy-coated or uncoated strands, 5) measure the transfer length for specimens containing a single uncoated strand, and 6) observe the seating characteristics of the grips used for uncoated strand and epoxy-coated strands. These tests have produced several initial findings. The preliminary recommended thickness for P/C panels containing grit-impregnated, epoxy-coated strands is 3 in. and the tentative development length for uncoated and coated multiple strands is approximately 45 in. and 24 in., respectively. Further tests will address confirmation of the recommended P/C panel thickness and establish the transfer and development lengths of single and multiple, uncoated and grit-impregnated epoxy-coated strands.