Detection of defects in concrete structures using vibration technique

This thesis investigates the dynamic behaviour of reinforced concrete beams as they are loaded to failure. Four beams have been investigated. Two types of crack pattern and two types or reinforcement pattern were the main variable parameters. Partially bonded reinforcement as artificially created (by greasing the bars) and positioned at the center third span in two of the four beams investigated. The remaining two beams had conventional bonded reinforcement. Flexural and diagonal splitting patterns were created by loading mechanisms individually applied on two beams of each type of reinforcement. Stage by stage application of static loadings was used. Steady state vibration tests were applied at prior to loadings the beams and at several load stages as gradually increasing defects occurred. There are four parts to this investigation and these are presented in this thesis. The first part investigates the accuracy of several techniques dealing with signal parameters from a digital response spectrum in the signal processing. A logic geometry was developed and was applied on the line spectra of the response spectrum. Numerical evaluation found that the error induced in the proposed technique decreased exponentially with increasing numbers of cycles. A maximum of 0.17% errors may exist when examining 100 cycles of the frequency of interest. A regression analysis was used to achieve further accuracy of the results. The second part investigates the jump phenomenon of mechanical exciters and the sharp drop phenomenon of magnetic exciters. Both of which may confuse the analysis of structural dynamic behaviour. By accounting for the stiffness of the magnetic field of the magnetic exciter in a mathematical model, the jump phenomena was shown to be due to the effect of the reflected force in the excited structure. Practical equations were also proposed to relate absolute to relative parameters. The third part of the thesis concerns the algorithms required in filter processing and includes the development of a computer solution. Two algorithms were developed to obtain coefficients of a polynomial equation which was set up from elementary equations and from a rational function respectively. The algorithms were simple and easy to program. The last part of the thesis discusses the detection of flexural and diagonal splitting defects and non-linear behaviour of the beams during the vibration tests. Static and dynamic comparisons are also discussed. Based on the characteristics of the polar diagrams it was found that several possible types of non-linear damping were demonstrated in the experiments. The typical viscous and non-linear higher polynomial damping existed mostly in the models although the crack pattern and intensity of cracks contributed to changes in the type of damping. In addition the beam models in almost all conditions showed non-linear soft spring behaviour. Diagonal splitting crack patterns can be idenuried from a small decrease of resonant frequency and from the sharp drop of resonant amplitude. The presence of single deep cracks greatly reduced the stiffness. The experiments show that a sharp decrease of resonant frequency indicates that a large amount of residual strain exists. It is concluded that defects of the reinforced concrete beams can be identified from the changes of the dynamic parameters using the proper digital signal analyses. The jump phenomenon is shown to be due to the effect of the reflected force on the moving exciter mass rather than due to the presence of the non-linear soft spring system.

Resource Type

Doctoral thesis

DOI

10.48730/5erf-kb86

EThOS ID

uk.bl.ethos.413173

Date Created

1992

Former identifier

310775

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Detection of defects in concrete structures using vibration technique

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