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Tensile and Compressive Creep of Young Concrete: Testing and Modelling
PhD Awarded 2003
PhD Awarded 2003
PhD Research - Tensile and Compressive Creep of Young Concrete: Testing and Modelling
PhD Research - Tensile and Compressive Creep of Young Concrete: Testing and Modelling
Welcome! In my doctoral research, My PhD thesis, titled "Tensile and Compressive Creep of Young Concrete: Testing and Modelling," explored the fascinating complexities of early-age concrete behaviour. This research, conducted in 2003 at the University of Science and Technology (UTNU) in Norway under the supervision of Professor Terje Kanstad and co-supervisor Professor Erik Johan Sellevold, focused on understanding tensile and compressive creep, along with the associated stress relaxation, in young concrete.
Research Focus
Research Focus
The primary objective of my PhD research was to characterize the tensile and compressive creep of young concrete and its associated stress relaxation. This is vital for accurately simulating stresses in early age concrete, which affects the longevity and durability of structures. To achieve this, I embarked on a comprehensive experimental and numerical modeling journey.
Experimental Program
Experimental Program
I developed new tensile creep test equipment and established reliable test procedures to generate essential experimental data. My research involved four series of tests, each focusing on a different parameter relevant to the time-dependent behaviour of early age high-performance concrete (HPC). This extensive testing program, primarily on HPC with a water/binder ratio of 0.40, included variables such as concrete age at loading, stress/strength levels, temperature levels, and the effect of silica fume on tensile creep.
Key Findings
Key Findings
My findings revealed significant differences between tensile and compressive creep behaviours. The instantaneous deformation under tension was smaller than under compression, but the tensile creep rate eventually surpassed that of compression, leading to a greater overall tensile creep magnitude. I also discovered that temperatures above 20°C accelerate sealed tensile creep, similar to compressive creep.
Mathematical Modeling
Mathematical Modeling
The extensive experimental data enabled me to investigate and enhance mathematical models. I used the Double Power Law (DPL) model to simulate self-induced stresses, modifying it to account for irrecoverable creep. This modified model accurately described the tensile behaviour at early ages, especially under varying temperature conditions.
Implications and Contributions
Implications and Contributions
Understanding the creep behaviour at very early ages is crucial for determining creep model parameters. My research concluded that an optimal test program should include at least three loading ages, particularly the first two days. Additionally, partial replacement of cement with silica fume increased sealed tensile creep, although not uniformly.
Future Research and Collaboration
Future Research and Collaboration
My PhD work lays a solid foundation for future research in concrete technology. I am eager to collaborate with fellow researchers and students interested in advancing the understanding of concrete creep behaviour. Together, we can explore new methodologies, improve predictive models, and contribute to the development of more durable and resilient concrete structures.
I invite you to explore the full thesis for a deeper insight into this research. Additionally, if you have any questions or are interested in collaborating on further investigations into early-age concrete behaviour, please do not hesitate to contact me.
QUALIFICATION DETAILS:
QUALIFICATION DETAILS:
TITLE: Tensile and Compressive Creep of Young Concrete: Testing and Modelling
AUTHOR: Dawood Sulaiman Atrushi [M.Sc.]
ISSN: Doctoral Engineering Thesis 0809-103X; 2003:17
AWARDING BODY: Department of Civil Engineering, The University of Science and Technology (NTNU), Norway
CURRENT INSTITUTION: The University of Duhok
DATE AWARDED: 2003
Full Text Link: [PLEASE CLICK TO VIEW THE FULL TEXT OF MY PhD]
SUPERVISOR: Professor Terje Kanstad and co-supervisor Professor Erik Johan Sellevold
SPONSOR: The University of Science and Technology (UTNU), Norway
QUALIFICATION NAME: Degree of Doctor of Philosophy in Civil and Structural Engineering
QUALIFICATION LEVEL: PhD
REPOSITORY LINK: https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/231168
PUBLISHER: Faculty of Engineering Science and Technology, NTNU
Citation to this work:
Citation to this work:
Atrushi, D. S. (2003). Tensile and compressive creep of young concrete: Testing and modelling (PhD thesis). Norwegian University of Science and Technology (NTNU), Norway. [Access: https://ntnuopen.ntnu.no/ntnu-xmlui/handle/11250/231168]
Abstract:
Abstract:
The thesis deals with experimental and numerical modelling to characterize early age tensile and compressive creep and its associated stress relaxation - which are very important properties in stress simulation of early age concrete. For this purpose a comprehensive work was carried out involving construction of a new tensile creep test equipment and development of test procedures to generate basic experimental data.
The experimental program is subdivided into four series. Each of the series involves one varying parameter, which is relevant to the time-dependent behaviour of early age HPC. Most of the tests are repeated to check the reproducibility of the test results. The reproducibility of the test results for the BASE concretes confirmed that the experimental setup is reliable, and that it can be used to determine tensile creep of concrete at early ages.
An extensive test program has been performed on HPC, with w/b = 0.40. The primary parameters studied were concrete ages at loading (1, 2, 3, 4, 6 and 8 days), stress/strength levels (20-80%), and temperature levels (20, 34, 40, 57 and 60 oC) in addition to the effect of silica fume (0-15%) on tensile creep. The testing apparatus was new, and significant efforts were devoted to develop reliable procedures in terms of accuracy and reproducibility. In parallel, compressive creep tests were conducted on a separate testing apparatus, and the results are compared to tensile creep behaviour.
It was found that the instantaneous deformation under tension is smaller than under compression, and that the corresponding creep curves also are different. Creep in tension is found to be lower initially, but an almost linear rate is soon established, which is much higher than in compression. The consequence is greater creep magnitude and thus greater creep coefficient in tension than in compression. The tests on non-linearity showed that the proportionality limit between stress and sealed tensile creep strain is about 60% of the strength. Creep tests under isothermal temperatures showed that, as for compressive creep, the sealed tensile creep accelerates for temperatures higher than 20 oC. In addition, the maturity principle describes this effect reasonably well, for the tested loading ages of about 3 days.
The relatively large amount of experimental data, available in this study, has been used to investigate mathematical models. Comprehensive test results from the TSTM apparatus are analyzed with respect to creep and relaxation, where the effect of temperature on creep and relaxation is emphasized. Simulations of self-induced stresses are performed using the creep model denoted the Double Power Law (DPL). As solution method, the theory of linear viscoelasticity with ageing is used. The model (M-DPL) is modified to take into account the effect of irrecoverable creep.
For increasing temperatures during the hardening phase, the transient creep, which takes place during heating, is taken into account by an additional creep term. Its contribution to stress relaxation was found to be up to 10%. This transient creep term is considered to be irrecoverable during the subsequent temperature decrease. The modified model captures the various characteristics of sealed creep and describes the tensile behaviour at early ages more accurately than the original Double Power Law.
The effect of relaxation is found to be relatively large and significant in development of selfinduced stresses. Under isothermal temperature of 20 oC, the relaxation increases to about 40% of the fictive elastic stresses after 3 days and remains about constant after that. On the other hand, presentation of relaxation under realistic temperature histories is much more complicated, because the stresses change from compression to tension. This might also lead to increased tensile stresses because compressive creep reduces compressive stresses, but increases the subsequent tensile stresses. Underestimation of creep in this early period will lead to underestimation of the cracking risk.
Creep development at very early ages has an important effect in determination of the creep model parameters. After an evaluation of the test results using six loading ages (1, 2, 3, 4, 6 and 8 days) it was concluded that an optimal test program should include at least 3 loading ages, in which the loading ages 1 and 2 must be included.
Furthermore, the test results indicate that partial replacement of cement with silica fume (5-15%) increases the sealed tensile creep. However, the reference concrete without silica fume does not fit to this systematic pattern.
Keywords:
Keywords:
Offshore Structures, Ultimate Capacity, Finite Element Analysis, Semi-Rigid Joints, Structural Analysis
Reference List:
Reference List:
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To see the full list of References for this PhD Thesis, please click to view or download the PDF version,
To see the full list of References for this PhD Thesis, please click to view or download the PDF version,
Kurder kvalitetssikrer Bjørvika-betong (Kurds quality assure Bjørvika concrete)
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