Experimental and theoretical developments for the acoustoelastic characterization and stress-monitoring of concrete materials and structures
Condition assessment of civil infrastructure often requires knowing the current stress acting on a given structural member. However, the development of an efficient nondestructive testing (NDT) technique for estimating current stresses in structural concrete elements remains open. To this end, previ...
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| Main Author: | |
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| Format: | doctoralThesis |
| Language: | English |
| Published: |
2020
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| Subjects: | |
| Online Access: | https://hdl.handle.net/20.500.12008/26380 |
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| Summary: | Condition assessment of civil infrastructure often requires knowing the current stress acting on a given structural member. However, the development of an efficient nondestructive testing (NDT) technique for estimating current stresses in structural concrete elements remains open. To this end, previous research have studied the dependence of mechanical wave speed with applied stress, “the acoustoelastic effect”. Recent research on concrete elements under uniaxial compression has shown that the acoustoelastic effect can also be detected with techniques based on vibration phenomena, which offers several benefits. This thesis focuses on studying, documenting and improving the use of resonance vibration for acoustoelastic characterization and current stress determination of slender concrete structural elements under compression. An exhaustive theoretical development using analytical and numerical methods is provided, where the torsional vibration mode is selected over other vibration modes. The nonlinear material parameter βG is defined based on torsional vibration, which corresponds to the rate of change of the elastic shear modulus G with respect to the uniaxial strain. The expression of βG is analytically calculated with respect to the second and third-order elastic constants (l, m, and n) and numerically verified with finite element method (FEM) models. The effect of non-uniform torsion (warping), geometric nonlinearity (P-δ effect) and changing boundary conditions is studied analytically, numerically and experimentally, to assess their effect on βG. Experiments are carried out for three concrete mixture designs using prismatic specimens of dimensions 15 × 15 × 60 cm3; values of βG are calculated for these specimens submitted to several loading and unloading cycles, which proves the existence, dominance and repeatability of the acoustoelastic effect: torsional frequency of vibration increases with increasing compressive strains (and stresses) in elongated elements. A second experimental campaign is conducted using ultrasonic wave propagation and torsional vibration techniques simultaneously on the same mortar specimen. Conversely to the theoretical predictions based on acoustoelasticity, ultrasonic results yield a βG value an order of magnitude lower than the torsional vibration-based βG. To address this apparent contradiction the theory is completed heuristically by accounting for the slight material viscosity. Finally, a case of study of a real size post-tensioned-concrete nuclear-containment structure is presented, where the containment is submitted to gradual internal pressure. Frequencies of vibration are identified using an output-only sensing system and the tracked frequencies are correlated with internal pressure. Both the experiment and an FEM model show that frequencies of vibration increase with increasing internal pressure suggesting that geometric nonlinearity dominates over acoustoelastic effects in this case. |
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