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As a critical public safety-related structure, the long-term integrity of post-tensioned concrete containment buildings (PCCs) is necessary for continued operation of the reactors they house. In 2009, during preparations for a steam generator replacement, extensive subsurface laminar cracking was identified in a portion of the Crystal River 3 (CR3) PCC in Florida, and the plant was permanently shut down in 2013. This study investigates potential contributing factors to the identified cracking with particular focus on the effects of high early-age temperatures on the cracking risk of the concrete, on the development of the concrete properties, and on the late-age structural behavior of the concrete. Two planar, full-scale mock-ups of a portion of the CR3 PCC were constructed and instrumented with temperature and strain gauges to monitor the thermal and mechanical behavior during representative concrete curing and post-tensioning loading. Standard- and match-cured concrete specimens were tested for determination of the time- and temperature-dependent development of thermal and mechanical concrete properties, and hydration parameters were determined for the mock-up cement paste for modeling the heat generation in the concrete. These properties and parameters were utilized in 3D finite element analysis of the mock-ups in COMSOL Multiphysics and compared with experimental results. Non-destructive evaluation via shear wave tomography was conducted on the mock-ups to identify flaws and determine the effectiveness of the methods for identifying delaminations between post-tensioning ducts approximately 10 inches beneath the concrete surface. Though early-age thermal stresses were determined not to have caused cracking in the mock-ups, the high early-age concrete temperatures resulted in decreased late-age mechanical properties that were shown to contribute to greater concrete cracking risk when the mock-up was post-tensioned. Tensile stresses exceeding the tensile strength of the concrete were identified along the post-tensioning ducts when biaxial post-tensioning loads were applied in finite element analysis, but the stresses decreased rapidly with increased distance from the ducts. Through parametric modeling, increasing the tensile strength of the concrete was identified as an effective means of reducing the cracking risk in PCCs. Additionally, relationships between the mechanical properties for the standard- and match-cured specimens were identified that could enable prediction of in-place or match-cured concrete properties based only on the results of tests on fog-cured specimens.

Bridges with external post-tensioned tendons are considered to be more durable than bridges with internal tendons (tendons within the webs and flanges), because external tendons are easier to inspect. In addition, in the event that extensive corrosion damage is detected, it is possible to replace an external tendon. However, an appropriate inspection for detecting damage needs to be determined for external tendons. This investigation focuses on the vibration technique, which uses the dynamic properties of the external tendon to infer the effective prestress force. Four large-scale external tendons, designed to simulate one section of an external tendon between two deviators in a post-tensioned bridge, were tested. In the study, damage to the tendons was induced in a quantifiable fashion at a specific location and the tensile force was measured directly. In addition, free-vibration tests were conducted periodically. This provided a direct means of measuring the sensitivity of measured natural frequencies and measured tensile force to local damage. The measured data were correlated with an approximation of the stiff string vibration model. In addition to the laboratory specimens, field testing was conducted on a bridge with external post-tensioned tendons. The findings from the study show that a loss in tensile force was not linear with a loss in the cross-sectional area of the strand, which results from stress redistribution within the tendon. Also, the natural frequencies were much less sensitive to the level of induced damage than the tensile force. While the measured data from the laboratory data compared very well with the analytical model, the field measurements exhibited a much greater deviation from the model. Due to several factors, the difference between the laboratory specimens and the bridge tendons are believed to be caused by larger levels of inherent error in the model. The findings from the investigation support the notion that vibration testing is most appropriately used in comparing relative differences between peer tendons. / text

The environmentally friendly and high performance multi-storey LVL timber system developed at the University of Canterbury (UC) consisting of post-tensioned frames and shear walls is referred to as the Pres-Lam system. It is possible that this structural system has the ability to increase productivity and reduce construction costs when compared with concrete and steel construction materials. As the Pres-Lam system is a new technology, the actual construction time and cost are still unknown. The outcome of this research will add value to the construction industry and encourage the industry to consider the Pres-Lam system for future projects. Previous research has shown that construction using this type of structural system is feasible for multi-storey buildings. In case study (1), this research revisited the research done for the actual Biological Sciences building under construction at the University of Canterbury based on the latest information available from the UC timber research team. This research compared the construction time and cost of three virtual buildings (Pres-Lam, Concrete and Steel) for Case Study (1). The research has been able to optimise the performance of the Pres-Lam system having increased open spaces with large column spacing. The proposed fully prefabricated double “T” timber concrete composite (TCC) floor system was used and found to reduce construction time. This has also shown that the LVL components in the Pres-lam system can be fully prefabricated at a factory. In case study (1), the predicted estimated construction time for the structural system was 60 working days (12 weeks) as compared to the concrete structure which required 83 working days. In the construction time analysis only the construction time of the structural building portion was compared instead of the overall construction time of the building project. The construction cost estimation for the concrete, steel and optimised Pres-Lam overall buildings including claddings and architectural fittings were produced and compared. The construction cost analysis concluded that the construction cost of the Pres-Lam building has been estimated to be only 3.3% more than the steel building and 4.6 % more than the concrete building. In case study (2), this research evaluated the deconstructability of the Pres-Lam system and found that the Pres-Lam system was potentially a very sustainable building material where 90% of the deconstructed materials can be recycled and reused to construct a new office building at the University of Canterbury. The reconstruction time of the STIC office building has been predicted to be 15 weeks and the estimated cost for the reconstruction to be $260,118. This will be used for future construction planning, monitoring and control.

In recent years the public expectation of what is acceptable in seismic resisting construction has changed significantly. Engineers today live under demands which are far more intensive than their historical counterparts and recent seismic events have shown that preserving life is no longer sufficient, and a preservation of livelihood is now the minimum. This means that after a major seismic event a building should not only be intact but be usable with no or minimal post-quake intervention. In addition to this already high expectation these demands must be met in a green and sustainable fashion with minimal (or even negative) environmental impact. This doctoral project looks to further advance the research into a new and innovative method of timber construction which satisfies (and exceeds) these demands. In response to these higher expectations recent developments in the field of seismic design have led to the development of damage control design philosophies and innovative seismic resistant systems. Jointed ductile connections for precast concrete structures have been implemented and successfully validated. One of these systems, referred to as the hybrid system, combines the use of unbonded post-tensioned tendons with grouted longitudinal mild steel bars or any other form of dissipation reinforcing device. During the controlled rocking of the system under seismic loading the post-tensioning provides desirable recentering properties, while the devices allow adequate energy dissipation from the system as well as increased moment resistance at column bases and beam-column connections. The hybrid concept is material independent and in 2004 an extensive campaign was begun to investigate the performance of the hybrid system when applied to large engineered timber members. Numerous small and large scale tests on both subassemblies and full buildings were performed showing that post-tensioned timber meets the seismic resilience demands now imposed by society. Recently this technology has also been applied in practice with over ten structures now using post-tensioned timber walls or frames, or a combination of the two, in New Zealand. In-spite of the extensive research effort and the acceptance and adoption in practice of post-tensioned timber as a structural system, significant work was still required in the review and refinement of both the system itself and the analytical and numerical methods used to predict and analyse structural performance. The objectives of this research were to review and refine comprehension of the static and dynamic response, analytical and numerical modelling, and design of post-tensioned timber frames under lateral loading. In order to do this a three phase experimental testing campaign was devised and performed including quasi-static testing of an angle dissipative reinforcing device, quasi-static testing of a full-scale beam-column joint and the mono-directional dynamic testing of a 2/3rd scale three storey frame. All testing used glue laminated timber, which had not been previously used in post-tensioned timber structures. Insight gained from the experimental testing was used to analyse and refine existing analytical modelling techniques. These techniques were split into two categories: 1) modelling of the local behaviour of a post-tensioned timber beam-column joint, with particular focus on stiffness and energy dissipation capacity, and 2) evaluation of the seismic demand (in the form of design base shear) on post-tensioned timber frames looking at current Force Based (FBD) and Displacement Based (DBD) design methods. This analysis led to the development of recommended alterations in the existing beam-column joint analytical procedure enabling the procedure to provide better prediction of initial and post-yield stiffness. Analysis of the FBD and DBD procedures showed that both methods are capable of providing accurate prediction of seismic demand provided correct assumptions are made regarding system ductility and damping characteristics. Recommendations have been made on how designers can ensure that assumptions are either sufficiently accurate in the beginning of a design or require minimal iteration to be performed. Current numerical modelling techniques have also been compared against the quasi-static and dynamic testing results providing confidence in their accuracy when applied to post-tensioned timber frames. Modelling techniques were also extended to the widely used SAP2000 modelling programme which had not been previously used in post-tensioned timber research. Although many observations and conclusions were made, a common theme continued throughout this research. This was the importance of the deep understanding of displacements within a post-tensioned timber frame and the impact of these displacements on frame performance. Displacements occur throughout a frame in dissipative reinforcing devices, in the connection of these devices, in beams, columns and joint panels as well as at the interfaces between members. When these displacements are allowed for through proper design excellent seismic performance, possible using this innovative system, is obtained.

Post-tensioned timber buildings utilise a new construction technique developed largely as part of research undertaken at the University of Canterbury. Timber buildings are constructed using an engineered timber product, such as laminated veneer lumber (LVL), and then stressed with post-tensioned unbonded high-strength steel tendons. The tendons apply a compressive stress to timber members to create a ductile moment resisting connection between adjacent timber members. The major benefit of post-tensioned timber buildings is a significantly improved structural performance. As timber is a combustible material there is a perceived high fire risk in timber buildings. While timber buildings can be designed to perform very well in fire, a design guide for the fire safety design of post-tensioned timber buildings has not been previously developed. Furthermore, previous research has found that post-tensioned timber box beams may be susceptible to shear failure in fire conditions. This research investigated the fire performance of post-tensioned timber buildings. A design strategy for the fire performance of post-tensioned timber buildings was developed in conjunction with a simplified calculation method for determining the fire resistance of post-tensioned timber structural members. The fire performance and failure behaviour of post-tensioned timber box beam was also specifically investigated, with special focus given to the shear performance of box beams. A full scale furnace test of a LVL post-tensioned LVL box beam was conducted at the Building Research Association of New Zealand (BRANZ). Four further full scale tests of LVL box beams were conducted at ambient temperature at the University of Canterbury structural laboratory. Through this research two distinct strategies for the fire design of post-tensioned timber structures were developed. The first strategy is to rely on the residual timber of the members only. The second strategy considers specific fire protection of the post-tensioning system, which can then be used to contribute to the fire resistance of the member. The results of the full scale tests showed good agreement with the proposed the simplified calculation method. It was also determined that shear failure does not need to be specifically considered other than performing strength checks as for other design actions.

Flexible fillers has recently been implemented as corrosion protection for post-tensioning tendons used in bridge structures in Florida. There are two different explanations why corrosion could take place: 1. water is able to reach the steel 2. Microbiologica l ly Influenced Corrosion. The aim of this research is to evaluate corrosion protection effectiveness of five differe nt microcrystalline waxes under different environmental conditions. Specimens tested ranged from 7-wire steel strands to single wires (12-16 cm). Another aim is the appraisal of wax degradation by fungi species. Single wires coated with each of the investigated protection materials, were sprayed with suspensions of three different fungi species and a mix of them. For single wires, independent of the environmental condition the specimen with more corrosion was Nontribos, as well as the filler coated wires contaminated with Fungi. Fungi species investigated were able to utilize the waxes as carbon source and caused differe nt extents of MIC. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

O concreto auto adensável (CAA) é um material novo cujas propriedades mecânicas precisam ser estudadas. Ele apresenta em sua composição maior quantidade de argamassa e agregados graúdos de menores dimensões, que podem torná-lo mais deformável que o concreto convencional. Em contrapartida, o melhor empacotamento das partículas no estado fresco do CAA e a sua maior resistência (para uma mesma relação a/c) pode atuar no sentido oposto. Além disso, os modelos de previsão disponíveis na norma brasileira não levam em consideração os concretos especiais. O objetivo deste trabalho é verificar se modelos de previsão disponíveis na literatura são adequados para prever a fluência e a retração do CAA. Este estudo envolve aspectos de sua caracterização por meio de ensaios em laboratório, de retração, de fluência e de outras propriedades mecânicas como resistência à compressão, resistência á tração e módulo de elasticidade. Para o ensaio de fluência foram avaliadas as influências das condições ambientais (através de corpos de prova mantidos em ambiente controlado e sem controle de umidade e temperatura) e idade de carregamento. A partir dos resultados obtidos experimentalmente, foi feita a sua comparação com os modelos de previsão do ACI, EC2, NBR, B3 e GL. O estudo da deformação do CAA também foi feito por meio da monitoração de uma viga protótipo protendida, seguida da comparação das deformações medidas com as obtidas por um programa de elementos finitos. A partir dos resultados experimentais, observa-se que dentre os modelos de previsão de fluência e retração estudados, o que mais se adéqua aos resultados obtidos experimentalmente, é o fornecido pelo ACI e GL. A utilização do modelo de previsão do ACI pelo programa de elementos finitos gerou bons resultados de previsão de deformações quando comparados com os resultados medidos. Em relação à função de fluência, nota-se que a norma brasileira é adequada para o concreto estudado. / Self-compacting concrete (SCC) is a new material and its mechanical properties have yet to be studied. It has a higher amount of mortar and smaller coarse aggregates which make it more deformable. On the other hand, the packing of the particles in fresh SCC and its larger strength (at a constant water/cement ratio) may act in the opposite way. Besides, the prediction models provided by the Brazilian Code do not take special concretes into account. The main objective of this work is to verify if the creep and shrinkage prediction models available in the literature can predict those properties of a SCC. This study involves aspects of its characterization by tests in laboratory of creep, shrinkage and other mechanical properties, such as compressive strength, splitting tensile strength and modulus of elasticity. For the creep test, the influence of environmental conditions (for specimens kept in an environment with and without temperature and humidity controlled) and age at loading were evaluated. From the obtained experimental results, a comparison was made to the values obtained from the ACI, EC2, NBR, B3 and GL prediction models. The deformability of SCC was analyzed by monitoring a post-tensioned beam prototype followed by the comparison of the measured deflection and strain long the time to those obtained from a finite element model. From the obtained experimental results of creep and shrinkage, the most adequate prediction models are the ACI and GL models. The use of the ACI model with the finite element method produced good results when compared to the experimental measurements of deflection and strain of the prototype beam. With respect to the compliance creep function, it can be noticed that the model provided by the Brazilian Code is adequate to the studied SCC.

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