Non-destructive Evaluation of Damage in Concrete with Applications in Shallow Foundations

Abstract

The most widely used material for civil infrastructure is reinforced concrete. The concrete deteriorates over time because of several reasons, and therefore, inspection of concrete is necessary to ensure its compliance with the design requirements. Decision makers often have insufficient data to implement the appropriate corrective measures in the face of infrastructure failure. Better assessment methods are essential to obtain comprehensive and reliable information about the concrete elements. Although, different methods exist to inspect concrete members, there is no comprehensive technique available for condition assessment of concrete of shallow foundations. To ensure the integrity of shallow foundations during construction and during its service life, it is necessary to monitor their conditions periodically. To achieve this goal a new NDT methodology is developed to reliably evaluate the conditions of new shallow foundations without changing their future performances. Recently, there is a trend to overcome coupling issues between the transducers and the object under investigation, by installing sensor networks in concrete to assess its integrity. Although many NDT approaches are designed to evaluate the integrity of concrete structural elements, shallow foundations, which are concrete elements embedded in soil, have received less attention. The challenging aspect of characterizing shallow foundations is limited accessibility for in-service foundation inspections because of structural restrictions. Even when accessibility is possible, the NDT methods (ultrasonic pulse velocity, UPV) used may produce measurements with high uncertainties because of inconsistent coupling between the transducer and the surface of the material being tested. In the current research project, a new NDT procedure is developed based on design of new transducers embedded at the base of lab-scale concrete foundation models, and these transducers are waterproof and used as receivers. The transducers consist of radial-mode piezoceramics that can detect waves from different orientations. The developed methodology relies mainly on two methods to emit the transmission pulse; either using a direct contact method by gluing the transducer to the concrete surface or using a plastic tube partially embedded in concrete and filled with water. The first procedure is used when the accessibility to the top surface of the foundations is possible; otherwise, the second option is employed to reach the concrete surface of foundations. The new methodology can be used in different stages: during construction of foundations to monitor the uniformity and quality of the concrete, and during in-service life to periodically assess the condition of the foundations, specifically after an event that may cause severe damage in concrete such as earthquake and overloading. To verify the applicability of the methodology, unreinforced and reinforced shallow foundation lab-scale concretemodels were tested in the laboratory under uniaxial compression loads. In this work, all ultrasonic measurements are averaged 16 times to ensure the consistency of the results and to eliminate high frequency noise. The average coefficient of variance obtained is less than 3.5%; which is considered acceptable in this type of measurements (typical measurement error ~5%). Also, different tests were repeated more than three times by removing and putting back all the ultrasonic transducers to enhance the statistical significance of the results. The main contributions of the research presented in this thesis are:  Characterization of low and high frequency transducers using laser vibrometer to characterize their responses for better ultrasonic measurements.  Characterization of a single fracture growth in a homogenous material based on wave velocity and wave attenuation.  Characterization of cement-based materials using ultrasonic pulse velocity and laser vibrometer methods.  Evaluation of freeze/thaw damage and monitoring progressive damage in concrete specimens subjected to uniaxial compression load using ultrasonic pulse velocity and laser vibrometer methods.  Fabrication of thirty-six new radial ultrasonic transducers to embed in concrete models for quality control purposes and to monitor progressive damage using new transmission pulse methodology.