Basic Creep of Young Concrete - Sensitivity in the Evaluation Method

Detta är en Master-uppsats från KTH/Betongbyggnad; KTH/Betongbyggnad

Sammanfattning: Creep is defined as deformation that takes place under constant load after an initial elastic response. This thesis focuses on a material property problem area that concerns stress analysis. Focus is on stress development considering creep deformations occurring when a concrete structure is under load, i.e. stress analysis with viscoelastic properties of the material.From laboratory tests, both elastic modulus and deformations over time are estimated in an evaluation process. Usually, deformations of moist sealed samples are denoted basic creep. At Luleå Technical University creep measurements are evaluated according to the theory and methodology in Larson and Jonasson (2003a, 2003b). The model is denoted Linear Logarithmic Model, used for moist sealed concrete samples. This thesis involves an investigation of the evaluation procedure for basic creep performed in Thysell laboratory at LTU, to examine how sensitive the evaluation process is for the outcome from stress calculations. The calculations are performed in the Finite Element Method software ConTeSt Pro.The aim of the thesis is to analyze the sensitivity of evaluation of basic creep and of the Linear Logarithmic Model (LLM) by making changes in the evaluation process to see how different parameters sets effect calculated stresses/strains during through crack analysis. The changes are solely done in the relaxation spectra.The purpose is also to analyze how sensitive the changes made in the evaluation process are when typical cases are studied. The typical cases are defined with a structure of a newly cast wall on a mature slab, where various thickness of the wall during different temperature conditions are analyzed. The temperature conditions are named standard, winter and summer. With this, concrete is tested and evaluated to yield two material parameter sets useful for temperature - and stress calculations for young concrete.The material parameter sets were analyzed and their creep values were converted into relaxation values, i.e. relaxation spectra, according to Maxwell-chain formulation for LLM. ConTeSt calculations generate temperature development for the walls and slabs. Colour maps and values of the strain ratio for each studied case are also obtained.It can be established that the evaluation process of basic creep is sensitive. A conclusion to be drawn is that small changes in the relaxation spectra, gives changes in the results of stress calculations for the typical cases. As soon as we deviate from the temperature development for the test performed in the laboratory, either by changing the thickness of the wall or by testing different temperature conditions we get a different temperature development than the tested one. With the deviation in the calculated temperature development compared to the measured one, a difference in the calculated strain ratios for the two different material parameter sets created are found.The main discovery in this work is that when a geometry of the wall that is identical to the geometry of the concrete tested at the laboratory is analyzed, a small deviation in the calculations of strain is obtained. This is expected since the temperature development in the created wall will follow the temperature development of the tested concrete. When differing from this geometry and temperature case, differences in calculated strain ratios are observed.

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