Handbook of nondestructive testing of concrete:Radioactive(Nuclear)Methods.pdf
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1、12-1 12 Radioactive/Nuclear Methods 12.1Introduction 12-1 12.2General Principles. 12-2 Source, Interaction, and Detection Radiation Safety 12.3Radiometry 12-4 Historical Background Test Equipment and Procedures Case Histories Advantages and Limitations 12.4Radiography. 12-12 Historical Background Te
2、st Equipment and Procedures Case Histories Advantages and Limitations 12.5Neutron-Gamma Techniques. 12-19 Historical Background Test Equipment, Procedures, and Case History Advantages and Limitations Radioactive and nuclear methods can be useful analytic or diagnostic tools, but, with an exception o
3、r two, are not widely used in concrete testing currently. The methods are based on directing radiation from sources such as radioisotopes and X-ray generators against or through fresh or hardened concrete samples. The radiation collected after interaction with the concrete provides information about
4、 physical charac- teristics such as composition, density, and structural integrity. Gamma radiometry is the most widely used method, primarily for density determinations on roller- compacted and bridge deck concretes. Radiography is used occasionally in concrete laboratories for studying microstruct
5、ure and in the fi eld for confi rming the integrity in structural concrete. Infrequently used, neutron-gamma techniques provide composition information on fresh or hardened concrete. The radioactive and nuclear methods are fast and accurate, but their use has been limited by the often complex techno
6、logy involved, high initial costs, and training and licensing requirements. Where possible, the accuracy of current magnetic and electrical apparatus is indicated. 12.1Introduction Radioactive and nuclear methods for testing concrete have been the subject of numerous research studies but, with an ex
7、ception or two, are not widely used. The methods are generally fast and accurate and often provide information not available to any other means. On the other hand, their limited use is likely due to often complex technology, high initial costs, and training and licensing requirements. Another factor
8、 which limits the effectiveness of these, as well as many other nondestructive methods, is the heterogeneous nature of concrete itself, whether within a small sample, across a construction site, or from one project to another. Nevertheless, many of the radioactive and nuclear methods can be very use
9、ful analytic or diagnostic tools, and the disadvantages cited should not be overstressed. Terry M. Mitchell Federal Highway Administration 12-2Handbook on Nondestructive Testing of Concrete: Second Edition The methods available use radiation produced by radioisotope sources, X-ray generators, and nu
10、clear reactors to bombard fresh or hardened concrete samples. The radiation that is transmitted through, attenuated by, or emitted by the concrete is then collected and analyzed. The collected radiation can provide information about physical characteristics such as composition, density, and structur
11、al integrity. 12.2General Principles Although “radioactive” and “nuclear” have specifi c and distinct meanings, they are often used inter- changeably in nondestructive testing contexts to refer to test methods that use the interaction of wave or particle radiation with matter to supply analytic or d
12、iagnostic information about the material. (In this chapter, the methods will be referred to, generically, as “nuclear methods.”) The nuclear methods used to test concrete can be separated into three categories: (1) radiometry; (2) radiography; and (3) neutron-gamma techniques. Radiometry describes t
13、echniques in which a radiation source and a detector are placed on the same or opposite sides of a concrete sample; a portion of the radiation from the source passes through the concrete and reaches the detector where it produces a series of electrical pulses. When these pulses are counted, the resu
14、lting count or count rate is a measure of the dimensions or physical characteristics, e.g., density or composition, of the concrete sample. Radiography describes techniques in which a radiation source and photographic fi lm (the radiation detector) are placed on opposites sides of a concrete sample.
15、 After exposing the fi lm, the result is a photographic image of the samples interior, which is primarily used to locate defects in the concrete. Neutron-gamma techniques, rarely used in the concrete industry, are those in which a concrete sample is irradiated with neutrons, one type of radiation, a
16、nd gamma rays, a second type, are emitted and detected. The result is a series of counts that are a measure of the composition of the concrete. 12.2.1Source, Interaction, and Detection Each nuclear testing method is a system composed of a radiation source, a mode of interaction with the concrete, an
17、d a radiation detector. A general description of each of these three components is needed before focussing on individual nuclear techniques. Sources generate two types of radiation, electromagnetic waves and particles. The electromagnetic waves employed in nondestructive testing of concrete are gamm
18、a rays and X-rays. Wave radiation is characterized by the energy it carries, usually expressed in units of electron volts, eV (or kilo electron volts, keV, or mega electron volts, MeV). Gamma rays are emitted from reactions inside an atomic nucleus and typically carry energies from a few keV to seve
19、ral MeV. X-rays are emitted from interactions outside the nucleus among orbital and free electrons. They typically have energies from a few eV up to 100 keV, although much higher energies can be produced in X-ray tubes. Neutrons are the only particles of interest in concrete testing. They are unchar
20、ged particles that are also characterized by the energy they carry. Neutrons with energies greater than 10 keV are described as “fast,” between 0.5 eV and 10 keV as “epithermal,” and less than 0.5 eV as “slow.” The interaction of gamma and X-rays with concrete can be characterized as penetration wit
21、h attenu- ation. That is, if a beam of gamma rays strikes a sample of concrete, some of the radiation will pass through the sample; a portion will be removed from the beam by absorption; and another portion will be removed by being scattered out of the beam (when gamma rays scatter, they lose energy
22、 and change direction). If the rays are travelling in a narrow beam, the intensity I of the beam falls off exponentially according to the relationship: I = I0ex(12.1) Radioactive/Nuclear Methods12-3 where Io= the intensity of the incident beam x = the distance from the surface where the beam strikes
23、 = the linear absorption coeffi cient For the gamma and X-ray energies common in nuclear instruments used to test concrete, the absorp- tion coeffi cient includes contributions from a scattering reaction, called Compton scattering, and an absorption reaction, called photoelectric absorption. In Comp
24、ton scattering, a gamma or X-ray loses energy and is defl ected into a new direction by a collision with a free electron. In photoelectric absorption, a gamma or X-ray is completely absorbed by an atom, which then emits a previously bound electron. The relative contributions of Compton scattering an
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