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Title:Probabilistic impact-echo method for nondestructive detection of defects around steel reinforcing bars in reinforced concrete
Author(s):Pagnotta, Alexander
Director of Research:Gardoni, Paolo
Doctoral Committee Chair(s):Gardoni, Paolo
Doctoral Committee Member(s):Popovics, John S.; Lopez-Pamies, Oscar; Trejo, David
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Impact-Echo, Defect Detection, Mechanical Wave Propagation, Probability of Detection
Abstract:Accurate assessment of engineered infrastructure using nondestructive testing (NDT) is critical to the proper scheduling of the maintenance and/or replacement of infrastructure components. In reinforced concrete structures, potentially adverse changes in the bond between concrete and reinforcing steel have been reported as a result of at least three distinct damaging mechanisms: alkali-silica reaction, corrosion, and early-age vibration. However, the extent to which bond problems occur in deteriorated field structures is largely unknown due to the lack of a standardized and industry-accepted technique for detecting defects that alter bond behavior. After reviewing possible NDT techniques, this dissertation assesses and improves the Impact-Echo (IE) method for the detection of defects around steel reinforcing bars in concrete. IE involves deducing the presence of a defect from several single-point recordings of a resonant response that results from a non-damaging impact. IE has been developed with unconfirmed assumptions of the underlying elastodynamic behavior, leading to consistent disagreement between theoretical predictions and experimental results, and subsequent compensation with empirical “correction factors” that generate distrust in data interpretation. Evidenced by closed-form solutions and simulation results for simplified elastodynamic models of the IE test, this dissertation challenges some of the assumptions made in the development of IE regarding elastodynamic behavior. Reduced-scale specimens with deliberate defects are constructed and tested in an attempt to establish the minimum size of detectable defects. A probabilistic procedure for producing quantitative indications of a defect is proposed and calibrated using the IE results for the reduced-scale specimens. The probabilistic IE procedure is applied to a full-scale specimen to assess the efficacy of IE for this application in field structures. Though the clarifications to some aspects of the underlying elastodynamic behavior produce no change in IE testing procedures, it is important to communicate accurate knowledge to other researchers. Experimental results show that the probabilistic IE can accurately indicate the presence of defects in the reduced-scale specimens. However, this procedure is found to have limited efficacy in testing of the full-scale specimens due to uncertainties in the accuracy of rebar location and the selection of a relatively noisy motion sensor. Future researchers are cautioned against the use of reduced-scale specimens for laboratory testing and are encouraged to perform additional physical verification of as-built internal features that may require lightly destructive testing. In the absence of a better option, the conclusion of this research is that IE has potential in this application, despite remaining uncertainties and implementation challenges.
Issue Date:2018-12-06
Rights Information:Copyright 2018 Alexander Pagnotta
Date Available in IDEALS:2019-02-06
Date Deposited:2018-12

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