Development of a Failure Theory for Concrete
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2012-07-31
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Edition:Final report; Sept. 22, 2008-Aug. 31, 2012.
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Abstract:The failure behavior of concrete materials is not completely understood because conventional test methods fail to assess the
material response independent of the sample size and shape.
To study the influence of strength affecting test conditions, four typical concrete sample types were experimentally
evaluated in uniaxial compression and analyzed for strength, crack initiation/propagation, and fracture patterns under
varying boundary conditions. Both low friction and conventional compression interfaces were assessed. High-speed video
technology was used to monitor macrocracking.
Inferential data analysis proved reliably lower strength results for reduced surface friction at the compression interfaces,
regardless of sample shape. Reciprocal comparisons revealed statistically significant strength differences between most
sample shapes. Crack initiation and propagation was found to differ for dissimilar compression interfaces.
The principal stress and strain distributions were analyzed, and the strain domain was found to resemble the experimental
results, whereas the stress analysis failed to explain failure for reduced end confinement. Neither stresses nor strains
indicated strength reductions due to reduced friction, and therefore, buckling was considered. The high-speed video analysis
revealed buckling phenomena, regardless of end confinement. Slender elements were the result of low friction, and stocky
fragments developed under conventional confinement. The critical buckling load increased accordingly.
The research showed that current test methods do not reflect the ``true'' compressive strength and that concrete failure is
strain driven. Ultimate collapse results from buckling preceded by unstable cracking.
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