Call number: LD2668 .T4 1957 D57 / Master of Science

To achieve thinner and longer floor slabs, rapid construction, and tight control of inservice deflections, modern concrete structures increasingly use high-strength, posttensioned prestressing steel as reinforcement. The resulting structures are called posttensioned (PT) concrete. Post-tensioned concrete slabs are widely believed to benefit from ‘inherent fire endurance.’ This belief is based largely on results from a series of standard fire tests performed on simply-supported specimens some five decades ago. Such tests are of debatable credibility; they do not capture the true structural behaviour of real buildings in real fires, nor do they reflect modern PT concrete construction materials or optimization methods. This thesis seeks to develop a more complete understanding of the structural and thermal response of modern prestressing steel and PT concrete slabs, particularly those with unbonded prestressing steel conditions, to high temperature, in an effort to steer current practice and future research towards the development of defensible, performance-based, safe fire designs. An exhaustive literature review of previous experimentation and real case studies of fire exposed PT concrete structures is presented to address whether current code guidance is adequate. Both bonded and unbonded prestressing steel configurations are considered, and research needs are identified. For unbonded prestressing steel in a localised fire, the review shows that the interaction between thermal relaxation and plastic deformation could result in tendon failure and loss of tensile reinforcement to the concrete, earlier than predicted by available design guidance. Since prestressing steel runs continuously in unbonded PT slabs, local damage to prestressing steel will affect the integrity of adjacent bays in a building. In the event that no bonded steel reinforcement is provided (as permitted by some design codes) a PT slab could lose tensile reinforcement across multiple bays; even those remote from fire. Using existing literature and design guidance, preliminary simplified modelling is presented to illustrate the stress-temperature-time interactions for stressed, unbonded prestressing steel under localised heating. This exercise showed that the observed behaviour cannot be rationally described by the existing design guidance. The high temperature mechanical properties of modern prestressing steel are subsequently considered in detail, both experimentally and analytically. Tests are presented on prestressing steel specimens under constant axial stress at high temperature using a high resolution digital image correlation (DIC) technique to accurately measure deformations. A novel, accurate analytical model of the stresstemperature- time dependent deformation of prestressing steel is developed and validated for both transient and steady-state conditions. Modern prestressing steel behaviour is then compared to its historical prestressing steel counterparts, showing significant differences at high temperature. Attention then turns to other structural actions of a real PT concrete structure (e.g. thermal bowing, restraint, concrete stiffness loss, continuity, spalling, slab splitting etc.) all of which also play inter-related roles influencing a PT slab’s response in fire. A series of three non-standard structural fire experiments on heavily instrumented, continuous, restrained PT concrete slabs under representative sustained service loads were conducted in an effort to better understand the response of PT concrete structures to localised heating. To the author’s knowledge this is the first time a continuous PT slab which includes axial, vertical and rotational restraint has been studied at high temperature, particularly under localised heating. The structural response of all three tests indicates a complex deflection trend in heating and in cooling which differs considerably from the response of a simply supported slab in a standard fire test. Deflection trends in the continuous slab tests were due to a combination of thermal expansion and plastic damage. The test data will enable future efforts to validate computational models which account for the requisite complexities. Overall, the research presented herein shows that some of the design guidance for modern PT concrete slabs is inadequate and should be updated. The high temperature deformation of prestressing steel under localised heating, as would be expected in a real fire, should be considered, since uniform heating of simplysupported elements is both unrealistic and unconservative with respect to tensile rupture of prestressing steel tendons. The most obvious impact of this finding would be to increase the minimum concrete covers required for unbonded PT construction, and to require adequate amounts of bonded steel reinforcement to allow load shedding to the bonded steel at high temperature in the event that the prestressing steel fails or is severely damaged by fire.

Abstract not available

Due to the unbonded nature of tendons to the slab within Unbonded Post Tensioned (UPT) concrete structures, tendon stress relaxation under heating affects all regions of the slab spanned by the tendon; not just in the locality of the fire. The numerical modelling of bonded and unbonded post tensioned concrete structures in fire has been performed to some degree, notably by Bailey and Ellobody. The consideration of elevated temperature creep to the relaxation of tendon prestress however, has not been considered. This thesis attempts to incorporate a uniaxial creep strain rate function of stress and temperature into the commercial FE software package Abaqus, compatible for use within the in-built multiaxial metal plasticity constitutive framework. What follows is a validation study of the Harmathy’s uniaxial creep strain accumulation function via the modelling of stress relaxation in isolated, tensioned and heated prestressing steel tendons, against experimental data. From here, UPT concrete slab models are analysed whilst exposed to a standard fire temperature-time curve and subsequently allowed to cool. Tendon prestress relaxation and resulting UPT concrete slab deflection is compared, where tendon creep is explicitly modelled, as opposed to implicitly covered by Eurocode 2 determined temperature dependent stress-strain curves. Following this, a large scale continuous one-way spanning UPT concrete structural model is developed to consider global structural behaviour resulting from localised fire, where realistic boundary conditions such as beam rotation and deflection are permitted. The ignorance of explicit elevated temperature creep consideration, in prestressing steel tendons, is commonly justified through the implicit accountability stated within Eurocode 2 temperature dependent stress-strain curves. This however is not completely true; Eurocode 2 states implicit accountability only holds should the tendon be heating at a rate within the bounds of 2⁰C/min to 50⁰C/min. Where only heating of a UPT concrete slab is considered, evidence from this thesis suggests Eurocode 2 determined stress-strain curves can implicitly account for accumulated creep strain up to limited temperatures. Prestressing steel tendons are however embedded within a concrete slab through which thermal gradients build up during fire. This means heat transfer can continue to the tendon, increasing its temperature postfire at an ever decreasing rate until it reaches its peak. Should post-fire cooling behaviour not be considered, continued tendon heating and subsequent creep strain accumulation will be ignored. Further, during the transition from heating to cooling within the tendon, it will be exposed to elevated temperatures with a rate of change below 2⁰C/min, whereby Eurocode 2, as stated cannot implicitly account for creep. It is shown, a significant degree of subsequent relaxation of prestress, UPT concrete slab deflection and concrete damage in hogging can occur during this phase of postfire behaviour, where the tendon temperature peaks during its transition from heating to cooling. In order to justify non consideration of creep, it should be shown tendon temperature will remain suitably low throughout the entire heating-cooling regime to which the UPT concrete slab is exposed. This must be achieved through adequate specification of minimum concrete cover to tendons to limit tendon temperature exposure for a given parametric fire curve duration, including the potential continued rise post-fire. Evidence within this thesis identifies 350⁰C as a critical temperature whereby the explicit consideration of tendon creep does not significantly increase predicted prestress relaxation and subsequent UPT concrete slab deformation, compared to implicit creep consideration from Eurocode 2. The manufacturing standard to which prestressing steel tendon strands are produced has been shown experimentally by Gales to significantly influence their susceptibility to elevated temperature creep. This is reflected by Gales determining differing creep parameters as a function of stress for incorporation in Harmathy’s uniaxial creep strain function. Modelled prestress relaxation of isolated, tensioned and heated tendons within this thesis is therefore significantly reduced when tendons are manufactured to a yield stress of 1860MPa according to the BS 5896 standard, as opposed to the ASTM A416 standard. As a result Eurocode 2 determined stress-strain curves implicitly account for accumulated creep strain during heating, at 10⁰C per minute, up to approximately 400⁰C for grade 1860 ASTM A416 manufactured tendons and 500⁰C for grade 1860 BS 5896 standard tendons. The aforementioned critical temperature of 350⁰C does not in actuality apply to necessary explicit creep consideration for UPT concrete slabs modelled with grade 1860 BS 5896 standard tendons. This temperature however remains a design temperature limit, owing to the potential onset of microstructural recrystallization beyond 400⁰C and the associated degradation of mechanical properties that coincides. The reasons for such differing elevated temperature creep and stress relaxation behaviour between the two manufacturing standards of prestressing steel wires and strands has been postulated within this thesis to be due to differing chemical compositions. This relates specifically to large relative differences of phosphorus and sulphur found in wires manufactures to each standard as tested by Gales.

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2005 / The objective of this project was to monitor the high temperature creep behaviour of 441 stainless steel. Two different alloys of 441 were investigated; the main difference between them being the Niobium content. Particularly imporlant to the project was how the Niobium content and grain size affected the creep resistance of the material. Creep tests were performed using purpose built constant load creep test rigs. Initially the rigs were not suitable for the testing procedures pertaining to this project. This was due to persistent problems being experienced with regards the reliability and reproducibility of the rigs. After various modifications were made the results produced from the rigs were consistent. Creep test data was used in order to determine the mechanism of creep that is operative within the material (at a predetermined temperature) under a predetermined load. Particular attention was paid to the resulting stress exponents. in order to identify the operative creep mechanism. The identification of the operative creep mechanisms was also aided by microscopical analysis. This analysis was also necessary to monitor how the grain size had altered at various annealing temperatures. Heat treatment was used as a method to alter the high temperature strength and microstructure of the material. Heat treatments were performed at various temperatures in order to determine the ideal temperature to promote optimum creep resistance of 441. All heat treatments were performed in a purpose designed and built high temperature salt bath furnace. The commissioning of the salt bath formed part of the objectives for this project. Sag testing was also conducted, using purpose built sag test rigs. It was necessary to design and manufacture a sag test rig that could be comparable to the industry accepted method of sag testing known as the two-point beam method, as this method is believed to produce inconsistent results. Conclusions have been drawn from the results of the data and from previous research on the subject matter.

A thesis submitted to the Faculty of Engineering in fulfilment of the requirements for the degree of Master of Technology in Mechanical Engineering 2005 / The objective of this project was to monitor the high temperature creep behaviour of 441 stainless steel. Two different alloys of 441 were investigated; the main difference between them being the Niobium content. Particularly imporlant to the project was how the Niobium content and grain size affected the creep resistance of the material. Creep tests were performed using purpose built constant load creep test rigs. Initially the rigs were not suitable for the testing procedures pertaining to this project. This was due to persistent problems being experienced with regards the reliability and reproducibility of the rigs. After various modifications were made the results produced from the rigs were consistent. Creep test data was used in order to determine the mechanism of creep that is operative within the material (at a predetermined temperature) under a predetermined load. Particular attention was paid to the resulting stress exponents. in order to identify the operative creep mechanism. The identification of the operative creep mechanisms was also aided by microscopical analysis. This analysis was also necessary to monitor how the grain size had altered at various annealing temperatures. Heat treatment was used as a method to alter the high temperature strength and microstructure of the material. Heat treatments were performed at various temperatures in order to determine the ideal temperature to promote optimum creep resistance of 441. All heat treatments were performed in a purpose designed and built high temperature salt bath furnace. The commissioning of the salt bath formed part of the objectives for this project. Sag testing was also conducted, using purpose built sag test rigs. It was necessary to design and manufacture a sag test rig that could be comparable to the industry accepted method of sag testing known as the two-point beam method, as this method is believed to produce inconsistent results. Conclusions have been drawn from the results of the data and from previous research on the subject matter.