Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Concrete water retaining structures (WRS) in South Africa are under scrutiny due to the numerous durability problems that they have experienced lately; despite the efforts by local and national authorities in conserving these structures. At the heart of these problems are the creep and shrinkage phenomena. While shrinkage is the reduction of concrete volume with time, creep is defined as the time-dependent increase of concrete strain under constant or controlled stress. Both phenomena are affected by conditions to which WRS are exposed hence their accurate prediction is required. Numerical models have been developed to calculate the extent to which concrete creeps or shrinks over time. The objective of this thesis is to identify which of these models is better equipped to be used in South African WRS design. This is achieved through a systematic method that involves an investigation into the contents of these models and a statistical comparison of model calculations to WRS representative data. In partnership with reputable universities, a pioneer experimental creep and shrinkage data base is created in this project from which the WRS related data is selected. While investigating the contents of the numerical models, their applicability to South African WRS is identified and the integrity of model contents is assessed. Indeed, a few irregularities are found in the process and are presented in this thesis. The model calculations are statistically compared to data in the form of individual experiments as well as in the form of groups of experiments with similar concretes to find the ideal prediction model for different types of concretes as well. Also pioneered in this project is a weighted criteria and point system in which the findings of the model content assessment and statistical evaluations are incorporated. It is based on this system that conclusions are drawn and the most suitable prediction model for WRS design in South Africa is selected.

published_or_final_version / abstract / Civil Engineering / Doctoral / Doctor of Philosophy

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Effective determination of long-term prestress losses is important in the design of prestressed concrete bridges. Over-predicting prestress losses results in an overly conservative design for service load stresses, and under-predicting prestress losses, can result in cracking at service loads. Creep and shrinkage produce the most significant time-dependent effect on prestress losses, and research has shown that high performance and high strength concretes (HPC and HSC) exhibit less creep and shrinkage than conventional concrete. For this reason, the majority of traditional creep and shrinkage models and methods for estimating prestress losses, over-predict the prestress losses of HPC and HSC girders. Nine HPC girders, with design compressive strengths ranging from 8,000 psi to 10,000 psi, and three 8,000 psi lightweight HPC (HPLWC) girders were instrumented to determine the changes in strain and prestress losses. Several creep and shrinkage models were used to model the instrumented girders. For the HPLWC, each model over-predicted the long-term strains, and the Shams and Kahn model was the best predictor of the measured strains. For the normal weight HPC, the models under-estimated the measured strains at early ages and over-estimated the measured strains at later ages, and the B3 model was the best-predictor of the measured strains. The PCI-BDM model was the most consistent model across all of the instrumented girders. Several methods for estimating prestress losses were also investigated. The methods correlated to high strength concrete, the PCI-BDM and NCHRP 496 methods, predicted the total losses more accurately than the methods provided in the AASHTO Specifications. The newer methods over-predicted the total losses of the HPLWC girders by no more than 8 ksi, and although they under-predicted the total losses of the normal weight HPC girders, they did so by less than 5 ksi. / Ph. D.

Please read the abstract in the section 00front of this document / Dissertation (M Eng (Structural Engineering))--University of Pretoria, 2006. / Civil Engineering / unrestricted

The thesis deals with design and assessment of all supporting parts of cast-in-place reinforced concrete tank. The thesis includes a technical report, static analysis, drawing documentations, construction proces and visualization. The drawing documentation consists of shape and reinforcement drawings of supporting parts.

Ultra-high performance concrete (UHPC) is relatively a new generation of concretes optimized at the nano and micro-scales to provide superior mechanical and durability properties compared to conventional and high performance concretes. Improvements in UHPC are achieved through: limiting the water-to-cementitious materials ratio (i.e., w/cm < 0.20), optimizing particle packing, eliminating coarse aggregate, using specialized materials, and implementing high temperature and high pressure curing regimes. In addition, and randomly dispersed and short fibers are typically added to enhance the material¡¦s tensile and flexural strength, ductility, and toughness. There is a specific interest in using UHPC for precast prestressed bridge girders because it has the potential to reduce maintenance costs associated with steel and conventional concrete girders, replace functionally obsolete or structurally deficient steel girders without increasing the weight or the depth of the girder, and increase bridge durability to between 75 and 100 years. UHPC girder construction differs from that of conventional reinforced concrete in that UHPC may not need transverse reinforcement due to the high tensile and shear strengths of the material. Before bridge designers specify such girders without using shear reinforcement, the long-term tensile performance of the material must be characterized. This multi-scale study provided new data and understanding of the long-term tensile performance of UHPC by assessing the effect of thermal treatment, fiber content, and stress level on the tensile creep in a large-scale study, and by characterizing the fiber-cementitious matrix interface at different curing regimes through nanoindentation and scanning electron microscopy (SEM) in a nano/micro-scale study. Tensile creep of UHPC was more sensitive to investigated parameters than tensile strength. Thermal treatment decreased tensile creep by about 60% after 1 year. Results suggested the possibility of achieving satisfactory microstructural refinement at the same temperature input despite the maximum temperature applied. For the first time, the presence of a 10 Ým (394 micro inch) wide porous fiber-cementitious matrix interface was demonstrated by nanoindentation and SEM for non-thermally treated UHPC only. Tensile creep at 90 days increased by 64% and 46% upon eliminating fibers for thermally and non-thermally treated UHPC, respectively. Increases in creep upon reducing the fiber content suggested that fibers carry part of the sustained load and thus reduce creep. Tensile creep strain was proportional to the stress applied up to 60% of the ultimate strength. No tensile creep failure occurred for a period of 1 year for pre-cracked UHPC under stress level of 40%. Also, no tensile creep failure occurred for a period of 90 days under stress level of 60%. Tensile creep failure occurred at stress levels of 70% and 80%. This study showed that fibers cannot be accounted for as shear reinforcement in lieu of stirrups unless micro-defect-free fiber-matrix interface is achieved.

Baigiamajame magistro darbe išnagrinėti STR ir EC 2 betono fizikinių ir mechaninių rodiklių nustatymo metodai bei šiose normose pateiktų įtempių nuostolių skaičiavimo modeliai. Aprašytas tikimybinis įtempių nuostolių vertinimo modelis. Atlikta atskirųjų ir suminių įtempių nuostolių analizė pagal STR ir EC 2 modelius, aprašyti šių modelių skirtumai. Išnagrinėta atskirų fizikinių faktorių lemiančių įtempių nuostolius įtaka. Atlikta iš anksto įtemptos gelžbetoninės sijos tikimybinė suminių įtempių nuostolių analizė. Darbe pateiktas kiaurymėtųjų perdangos plokščių suminių įtempių nuostolių skaičiavimo rezultatų pagal EC 2 ir STR modelius ir eksperimentinių įtempių nuostolių palyginimas. / Evaluation of different physical and mechanical factors of concrete according to STR and EC 2 models is considered in this master thesis. Probabilistic model for prestress loss determination is presented in this paper. The analysis of separate and total losses according to STR and EC 2 models was carried out. Some differencies of these models were described. The influence of several factors to the value of separate prestress loss has been studied in this thesis. The probabilistic analysis of long-term losses of post-tensioned RC beam has been carried out. The comparison of total prestress loss values of hollow core slabs according to EC 2, STR and experimental results is subjected in this master thesis.

Estimating the delayed strains in concrete, namely creep and shrinkage is very important to asses the condition of the structure. Time dependent deformations in concrete, both creep and shrinkage, play a critical role in prestressed concrete structures, such as bridge girders, nuclear containment vessels, etc. These strains result in lossess, through release of prestress, and thereby influence the safety of these structures. Recognizing the role of free and bound moisture movement is the primary ingredient responsible for the development of both creep and shrinkage stains as well as the degradation of concrete under high temperature, the present study has also examined the effects of high temperature on concrete degradation, experimentally and also analytically in the same modelling framework. Fire in concretes deteriorates mechanical properties of the material and lead to col-lapse under loads. Two types of spalling occur in concrete when exposed to high temperature, namely explosive and thermal spalling. Explosive spalling occurs once the hydrostatic stress (developed due to pore pressure) exceeds the tensile strength of the concrete. Where as thermal spalling of concrete happens due to degradation of material properties (elastic modulus, compressive and tensile strength) when exposed to high temperature due to decomposition of chemical bonds that release the bound water. The present study comprises of an experimental and analytical program to assess the levels of creep and shrinkage in different concrete under various loads and environmental conditions. Deformations due to high temperature in di erent concretes forms another component of the present study. Total six concrete mixes has been studied to investigate and asses the extent of creep and shrinkage taking place in the concretes under different environmental conditions, load level and age at loading. In total six mixes, three that are self compacted concrete mixes (35MPa, 55MPa and SCC70MPa), a high volume y ash concrete mix ( 45 MPa) and two normal concrete mixes (35 MPa and 45 MPa) have been considered in this study. To study the high temperature effects, the same mixes considered in the creep and shrinkage study and in addition a heavy density concrete mix (25 MPa) is used. A normal concrete having a 28 day uniaxial compressive strength of 45 MPa after proper curing, referred to as M45 concrete, was one of the six mixes. Likewise a heavy density concrete designated as H25, corresponding to a 28 day uniaxial compressive strength of 25 MPa was another mix that was studied and was made using iron ore aggregate and iron ore sand. A concrete having high volume y ash replacing cement designated as F45 offered a 28 day strength of 45MPa. Three self-compacting concretes with uniaxial compressive strengths of 35, 55 and 70 MPa were designated as SCC35 SCC55 and SCC70, respectively is studied for creep, shrinkage and high temperature effects. F45 concrete shows lower creep strain when compared to normal M45 concrete, under similar casting, curing and testing condtions. This is due to increase in stiffness of y ash based concretes with time. Where as in shrinkage it is observed that a little higher strain takes place in F45 at initial ages than in M45 concrete mix for the same conditions. But in the later age, F45 concrete shows a decreasing rate of shrinkage strain. This is because, water to cement ratio of y ash concrete is higher than the M45 concrete. The SCC35 concrete shows higher creep and shrinkage than M35 concrete even though both the concretes have the same water cement ratio. This difference comes from the aggregate cement ratio (a/c). The lower the aggregate cement ratio, the higher the creep and shrinkage. M35 concrete has a higher aggregate cement ratio than the SCC35. Concretes exposed to higher temperature and lower humidity shows higher creep and shrinkage due to its higher rate of drying. An analytical model has been developed to simulate the drying phenomena in concrete based on poromechanics. The hydration effects of blended cements is considered while developing the model. This models prediction of degree of hydration, temperature and relative humidity is used to model creep and shrinkage in concrete. To model creep and shrinkage, micro prestress solidi cation theory is implemented and validated with the present experimental results. The model is able to predict the drying phenomena of concrete realistically. Further, a benchmark problem reported in the literature is solved through this model and validated through a comparison with the experimental results (beam detection due to creep and shrinkage). Under high temperature tests, H25 concrete shows better resistance for all the ranges of temperatures. This may be because of the hematite aggregate having a high melting point and strong interfacial transition zone (ITZ) properties between aggregate and cement mortar. The SCC70 shows poor performance against explosive spalling at both the ages (28 and 365 days) due to its lower permeability when exposed to high temperature. The intensity of explosive spalling is higher in SCC70 concrete tested at 28 days than at 365 days of age. This is because of variation in moisture content. SCC70 concrete failed due to explosive spalling at temperature of 398oC when tested at 28 days and failed at 575oC when tested at 365 days. This indicates the amount of moisture content in the concrete plays an important role while causing explosive spalling. F45 concrete shows a poor resistance against temperature beyond 500oC in its residual properties. SCC55 contains cement and y ash and shows higher residual properties when compared to normal vibrated M45 mix under similar high temperature conditions. Two geopolymers pastes prepared with y ash and metakaolin as a complete cement replacement were studied for passive re protection capability. The study shows MF70 mix (containing 70% y ash and 30% metakaolin) gives better resistance against heating than MF50 mix (50% each of metakaolin and y ash). Hence y ash geopolmer is a choice of material for passive re protection. An analytical model has been developed based on poromechanics to simulate high temperature e ects in concrete. Two type of spalling is considered while modelling the high temperature e ects of concrete, namely explosive and thermal spalling. Explosive spalling is estimated based on the hydro static stress (Biotech efficient times the pore pressure). If the hydrostatic stress increases beyond the tensile strength of concrete then explosive spalling occurs. Where as the thermal spalling is estimated based on the stresses developed due to applied mechanical and thermal loading. To validate this model, two benchmark problems from the literature have been solved and validated with the reported results. This model is able to predict pore pressure and temperatures gradients accurately, and this in turn helps to predict explosive and thermal spalling realistically in concrete under elevated temperature conditions.

[pt] A fluência do concreto é um fenômeno físico complexo e apresenta características diferentes das observadas na maioria dos materiais. Apesar dos diversos estudos desenvolvidos, a compreensão desse fenômeno ainda não é completa e os modelos disponíveis na literatura para simulação numérica variam em complexidade, número de parâmetros e condições que garantem a sua aplicabilidade. Os estudos mais recentes desenvolvidos por Bažant se destacam e reportam boa concordância com resultados experimentais. Seus fundamentos são utilizados no desenvolvimento do modelo de fluência básica linear adotado neste trabalho. Dessa forma, realiza-se uma revisão da viscoelasticidade linear, na qual apresentam-se os modelos reológicos, as relações constitutivas e as principais hipóteses adotadas no estado multiaxial de tensão. Discute-se as propriedades da fluência do concreto e apresenta-se a teoria da solidificação, que tem por objetivo representar a evolução ao longo do tempo das propriedades aparentes do concreto. A partir desse estudo, adota-se um modelo de fluência básica com envelhecimento definido por cadeias de Kelvin e desenvolve-se um algoritmo para integração numérica da taxa de deformação. O modelo é implementado em um programa de elementos finitos e validado com resultados experimentais de fluência linear. Por fim, este trabalho propõe a modelagem da não linearidade da fluência do concreto com o emprego de elementos de interface coesiva. Apresenta-se uma metodologia para calibração dos parâmetros envolvidos, e compara-se a resposta do modelo com resultados experimentais de um ensaio de fluência na flexão. Os resultados obtidos numericamente mostram boa concordância com os resultados experimentais tanto para o caso de fluência linear pura quanto para o caso de fluência e dano. / [en] Concrete creep is a complex physical phenomenon and has different characteristics from those observed in most materials. Despite several studies developed, the understanding of this phenomenon is not yet complete and the models available in the literature for numerical simulation vary in complexity, number of parameters and conditions that ensure their applicability. The latest studies developed by Bazant stand out and report good agreement with experimental results. Their fundamentals are used in the development of the linear basic creep model adopted in this work. Thus, it is carried out a review of linear viscoelasticity, in which the rheological models, the constitutive relationships, and the main assumptions adopted in the multi-axial state of stress are presented. Concrete creep properties are discussed and the solidification theory is presented, aiming to represent the development over time of the apparent concrete properties. Based on this study, a basic creep model with aging defined by Kelvin chains is adopted and an algorithm for numerical integration of the strain rate is developed. The model is implemented in a finite element program and validated with experimental linear creep results. Finally, this work proposes modeling the nonlinear dependence of concrete creep with the use of cohesive interface elements. A methodology for calibrating the parameters involved is presented, and the model response is compared with the experimental results of a bending creep test. The numerically obtained results show good agreement with the experimental results for both the pure linear creep and the creep and damage.