Date of Award

1-1-2012

Document Type

Campus Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Paul H Ziehl

Abstract

Assessing the condition of in-service concrete bridges has recently gained significant concern from both public and private entities. This is a direct result of the aging infrastructure in the United States and the resultant effect on the national economy. The use of current advances in non-destructive testing methods in early detection of damage or condition assessment is still limited due to insufficient research and variability of data. The series of studies presented in this dissertation utilizes the high sensitivity of acoustic emission (AE) as a non-destructive evaluation technique for reinforced and prestressed concrete structures. The study has two main objectives: a) use AE as an early detection method for deterioration or cracking mechanisms that may lead to brittle (sudden) failure, shear cracks in reinforced concrete and corrosion in prestressed concrete, and b) utilize AE parameters for condition assessment of in-service structures. Three studies were completed in an effort to fulfill the objectives, and are presented in a series of journal articles as chapters in this dissertation.

The first study included four prestressed T-girders measuring 4.98 m (16 ft. 4 in.) subjected to long term corrosion. Corrosion was applied by subjecting the specimens to wet-dry cycles using a 3% NaCl solution to accelerate corrosion. The specimens were continuously monitored using acoustic emission (AE) and standardized electrochemical tests. Half-cell potential (HCP) measurements and linear polarization resistance (LPR) were performed daily to serve as a benchmark for corrosion detection.

The results showed that AE has the ability to detect corrosion initiation with the same sensitivity as HCP. To better assess the corrosion, an AE intensity analysis chart was correlated to corrosion rate as calculated from LPR results. In the second study, AE was used to monitor eight prestressed T-girders load tested using the proposed cyclic load test (CLT) method. The purpose of the test was to calibrate AE as a substitute for conventional measurements used to assess the condition of the structure using the ACI 437 CLT acceptance criteria. The results of the study also showed that AE can be used as a means of structural evaluation. These techniques are correlated to existing test acceptance criteria and are shown to be able to assess conditions of previously damaged specimens in a manner correlated to a pristine specimen. Unlike conventional criteria AE is able to assess structural condition without knowledge of load history. Additionally these charts can be used to reduce the maximum load applied during the test to be within the service load limit.

The third study was conducted to determine the cracking mechanism developed in a specimen, which may give an indication of the anticipated failure mode of the specimen. The specimens contained Glass Fiber Reinforced Polymer (GFRP) longitudinal reinforcement and no shear reinforcement. Five scaled GFRP reinforced concrete (RC) beams with effective depth up to 290 mm (11.4 in.) and constant shear span-to-effective depth ratio of 3.1 were load tested under four-point bending. The results indicated that appropriate AE parameters can be used to discriminate between developing flexural and shear cracks irrespective of scale, and provide warning of impending failure irrespective of the failure mode (flexural and shear).

The outcomes of the studies described in this dissertation demonstrate the potential of AE as a viable technique for condition assessment and structural health monitoring as well as prognosis systems and techniques. AE condition assessment charts were also developed for different deterioration mechanisms. The demonstrated ability of AE to accurately map of crack growth and damage activity can provide guidance for rehabilitation efforts and/or further inspection.

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