Stress Relaxation Loss Research Articles

Evaluation of prestress relaxation loss and harping characteristics of prestressing CFRP systems

This paper presents partial results of an investigation about the use of prestressing carbon fiber reinforced polymer (CFRP) systems in prestressed bridge girders. First, stress relaxation tests were conducted on prestressing CFRP cables and bars and continued over a period of one year from the initial prestressing process. Stress relaxation loss found to be linearly related to logarithm of the time and invariant of the length of the specimen. The effect of CFRP anchor loss was also evaluated and used to develop equations for predicting prestress relaxation in both pre- and post-tensioning applications. Then, harping applications of CFRP systems under various harping angles and deviator sizes were experimentally investigated. Higher harping capacity for CFRP cable compared to bar was observed. Premature failure of the CFRP due to localized shear/contact stresses at deviaor location redulted in a significat harping capacity reduction. To maximize the contact surface and reduce the localized shear stress, new deviators were developed, which improved the results. Finally, a finite element (FE) model was developed and calibrated to better understand the harping behavior of CFRP. By performing parametric study on different harping profiles, jacking stress limitations were obtained for the use of CFRP in practical range of harping angles.

Prediction model of long-term prestress loss interaction for prestressed concrete containment vessels

Conventional methods disregard the interaction effects between concrete creep and tendon relaxation to evaluate the long-term prestressed losses. The main contribution of this paper is to present a new method to compute this interaction for prestressed concrete containment vessels. A time-dependent numerical study is conducted to compare long-term prestress losses in different prediction models. A prestress loss test program is conducted to validate the numerical analysis method for a concrete containment vessel. To perform a more comprehensive numerical study, various variables such as radius to water height ratios (R/H), tendon material (improved or normal relaxation), and different constant temperature conditions are considered in the analysis models. The obtained analytical results are categorized into three classes: creep and shrinkage losses, stress relaxation losses and simultaneous long-term losses. The analytical results show that the (R/H) ratios have no significant effects on long-term prestress loss. Nevertheless, the other investigated variables have impressive effects on the long-term loss evaluation. The analytical results are used to calculate the interaction coefficients, which demonstrate the interaction between concrete creep losses and tendon relaxation. Eventually, the interaction coefficient data are expressed as a function of stress relaxation loss to creep and shrinkage loss ratio.

Influencia de las condiciones de almacenamiento sobre la relajación de aceros de pretensado

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 test carried out on a wire coiled and stored for a long-term period. This model explains the evolution on the cross-sectional stress profile along the coiling-storing-uncoiling process, as well as the influence of the residual stresses on it.

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.

Behavior of prestressing steels after a simulated fire: Fire-induced damages

Fire safety should consider not only the performance of the structure during the fire but also the behavior after cooling. Even if a fire does not give rise to apparent damage in the prestressed structure, mechanical properties of materials as well as load distribution can be affected. A verification of residual load-bearing capacity is necessary to determine if the structure can be maintained in use. In this work, the behavior of prestressing steels during and after fire has been studied. We have attempted to clarify two aspects that should be considered when assessing the security of the structures: the reduction of the prestressing loads due to the increment in the stress relaxation losses during exposure to high temperatures and the permanent damage of the residual mechanical properties of steel wires produced by high temperatures. This paper seeks to give a comprehensive and extensive explanation, in addition to a simplified estimation of the non-visible fire-induced damages produced in steel wires, with the hope that this information can be useful for engineers in the structural evaluation after a fire.

Role of Residual Stresses in Stress Relaxation of Prestressed Concrete Wires

In prestressed concrete elements, the stress relaxation losses in steel tendons are of paramount importance to structural design. In this work, experimental and numerical results show a clear relationship between stress relaxation and the presence of residual stresses in steel wires for prestressing concrete. Stress relaxation tests were performed at different initial loads, from 50 to 98% of the rupture load, with three types of wire differing only in their distribution of residual stresses. Cold-drawn wires having the same mechanical properties required by standards (ASTM E421, ISO15630/3) but with different residual stresses will exhibit different stress relaxation losses; such losses increase with increasing surface tensile residual stresses (as a consequence of cold drawing) or increasing inner residual tensile stresses (as a consequence of mechanical postdrawing treatments). Actual expressions for prediction of stress relaxation losses do not take into account residual stresses. Better performance of the wires can be achieved by changing the profile of residual stresses according to the requirements of the structures. This note shows how to deal with this challenge.

Influence of residual stresses in the performance of cold-drawn pearlitic wires

The mechanical properties of cold-drawn pearlitic wires are controlled largely by the microstructure developed during processing and, to some extent, by the residual stresses during drawing. The advent of powerful computers and the availability of equipment to perform diffraction experiments, have made possible numerical predictions and accurate measurements of residual stresses. This paper—a review of work done by the author and collaborators—shows how stress-relaxation losses, environmental assisted cracking and fatigue life of cold-drawn pearlitic wires are influenced by residual stresses. The role of pre-stretching loads, or of stress relieving treatments, on stress-relaxation can be understood when the profile of residual stresses is known. Some awkward results in times to fracture during hydrogen embrittlement tests can be explained if accurate values of residual stresses near the surface are known, and the same is true of fatigue life. In this context numerical simulations and measurements performed on cold-drawn pearlitic wires, with different profiles of residual stresses, have shown very good quantitative agreement.

Influence of residual stresses in the stress relaxation of cold drawn wires

This study demonstrates the influence of residual stresses in steel wires on the stress relaxation losses. Standard stress relaxation tests were performed on four types of wire, all with the same mechanical properties but with different residual stresses. Surface residual stresses were measured by X-ray diffraction. The experimental results show that stress relaxation losses decrease as the value of surface tensile residual stresses decrease. The role, sometimes controversial, of initial pre-stretching and heat treatments on stress relaxation losses can also be understood in the light of the residual stresses induced during cold-drawing.

Some Studies on the Stress Relaxation Behavior of Prestressed Wire

This paper presents the experimental results of the relaxation behavior of three types of prestressed wires under various testing conditions including variations of the initial stress, temperature, stressing time prestretching. Also presented are brief discussions on the mechanism of stress relaxation of the steel and the application of the metallurgical theory to the relaxation behavior of the prestressed wire under actual conditions.The mechanism responsible for the decrease of stress relaxation loss in the prestressed wire the reduction of the number of mobile dislocations by either the realignment of dislocation due to cell formation or the strain aging due to free carbon and nitrogen atoms.In practice, the low stressing rate and long prestretching time give smaller relaxation loss than the other conditions because the number of mobile dislocations decreases. When the strain fluctuates, the apparent relaxation loss is smaller than the pure relaxation loss under a constant strain. It is, therefore, necessary to take into consideration the decrease of the mobile dislocations during the stress relaxation under a constant strain before the strain start to fluctuate.