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For a rational design of concrete structures, it is necessary to know both the ascending and the descending parts of the stress-strain curve of concrete subjected to uniaxial compression. It is difficult to determine the descending part experimentally because of the interaction between the testing system and the specimen, especially for concrete of high compressive strength. The influence of the testing machine depends on the stiffness of the machine, the frequency response of the system, the length and diameter of the specimen, the strength of concrete, the rate of loading, and the inherent brittleness of the material. For a closed-loop testing machine, the specimen-machine interaction also depends on the parameter chosen to control the loading. It is shown that the descending part of the curve can be obtained by controlling the circumferential strain even when the axial strain control produces an unstable failure. A method of testing specimens at a controlled rate of circumferential strain increase is described. Stress-strain curves of concrete subjected to uniaxial compression using both the axial and the lateral strain as a control were compared for compressive strengths ranging between 70 to 90 MPa (10 000 to 13 000 psi).
Research assistant, University of Illinois at Chicago Circle, Chicago, Ill.
Professor, Northwestern University, Evanston, Ill.
Assistant professor, Istanbul Technical University, Istanbul,
Stock #: CCA10198J