Advanced Methods of Concrete Characterization
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ADVANCED METHODS OF CONCRETE CHARACTERIZATION R. J. Lee, E. A. Draper and J. Skalny RJ Lee Group, Inc. Monroeville, PA 15146 ABSTRACT The need for the rehabilitation of the infrastructure has lead to the adaptation of modem analytical methods for the characterization of concrete. These techniques are not commonly associated with evaluating concrete but promise to be very useful both as tools of quality assurance and in the determination of existing damage. This paper describes two such techniques, namely, coordinated electron-optical microscopy and gamma-ray tomography. Examples of the use and interpretation of each method are given. ANALYTICAL TECHNIQUES:
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Traditionally, many methods of concrete examination have been used with varying degrees of success. The test methods may be divided into two broad categories: destructive and nondestructive testing. Destructive testing typically yields very good sample-specific data (petrographic thin sections, compressive core testing) which are then assumed to be representative of the entire structure. Obvious drawbacks include the possibility of missing an important structural defect due to limited sampling locations; the labor intensive nature of obtaining, preparing and analyzing such samples; and the potential for increasing mechanical weakness from destructively testing large portions or numerous areas within a structure. Because of these and other problems, numerous non-destructive test methods have been developed for concrete evaluation, including acoustic, chemical, and X and gamma radiography..2,3.' X and gamma radiography are particularly attractive because of the directly accessible and quantitative information they yield about an object. Unfortunately, the physics of basic film radiography dictates that the images produced by the passage of X and gamma rays through an object result in a complete projected view of the object. This may not be a problem if the object in question is of a homogeneous nature such as a steel alloy or ceramic. However, in nonhomogeneous materials such as concrete5 with emplaced rebar, the images resulting from this method are often difficult to interpret and do not allow the investigator to isolate and examine a particular area of interest within an object. COMPUTED AXIAL TOMOGRAPHY Computed axial tomography or cat scanning (CT) was developed because of the for diagnostic need to produce internal cross sectional images of living human patients 6 purposes. By rotating the energy source and receiver around a patient it became possible, for the first time, to produce two dimensional images by scanning areas parallel to the Xray beam. Since the beam was only traveling through that portion of the patient being imaged, there was no overlapping or projection of extraneous material and the blurring produced by projection radiography was eliminated. The applications to material testing and evaluation were a natural evolution of this technique. Industrial CT systems have been used with great success as both an on-line quality as
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