Handbook of nondestructive testing of concrete:Short-Pulse Radar Methods.pdf
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1、13-1 0-8493-1485-2/04/$0.00+$1.50 2004 by CRC Press LLC 13 Short-Pulse Radar Methods 13.1Introduction 13-1 13.2Principle of Short-Pulse Radar 13-2 Behavior of Microwave at the Interface of Two Different Materials Propagation of Microwave Energy through a Material 13.3Instrumentation 13-4 13.4Applica
2、tions. 13-7 Detection of Delamination in Concrete Determination of the Degree of Hydration of Cement Determination of Water Content in Fresh Concrete Measurement of Thickness 13.5Standardization of Short-Pulse Radar Methods.13-19 13.6Conclusions . 13-19 Short-pulse radar is a powerful scientifi c to
3、ol with a wide range of applications in the testing of concrete. It is gaining acceptance as a useful and rapid technique for nondestructive detection of delaminations and other types of defects in bare or overlaid reinforced concrete decks. It also shows potential for other applications such as mon
4、itoring of cement hydration or strength development in concrete, study of the effect of various admixtures on curing of concrete, determination of water content in fresh concrete, and measurement of the thickness of concrete members. To facilitate the understanding of these applications, the physica
5、l principles behind short-pulse radar are presented. The advantages and limitations of radar in these applications are also discussed. 13.1Introduction Experiments in the early 1900s proved the feasibility of transmitting electromagnetic (EM) waves through space as a beam and receiving the refl ecte
6、d signal from an airborne object in the path of the beam. Because of the military signifi cance of this technology, it experienced a rapid advancement during World War II. Subsequent refi nement in microwave* sources and detection circuits made it possible to accurately locate planes. These applicat
7、ions were made possible by the realization that different objects have their own characteristic scattering and refl ection properties toward EM waves and that EM waves travel through free space with a constant speed, equivalent to that of light. Such detection systems were eventually called radar, w
8、hich is an acronym for radio detection and ranging. Since much of this technology was also applicable to transmission of EM signals through solids, experiments were conducted in the early 1950s using radar as a tool for probing solids, such as rock and *An EM wave which has a wavelength between abou
9、t 0.3 and 30 cm, corresponding to frequencies on the order of 1 to 100 GHz. Gerardo G. Clemea Virginia Transportation Research Council 13-2Handbook on Nondestructive Testing of Concrete: Second Edition soil. It was quickly recognized that the speed of microwave and its amplitude as a function of dis
10、tance traveled in a solid could vary signifi cantly from one material to another, and that these properties can be used to identify and profi le subsurface geological features. This led to the development in the late 1960s of several types of radar systems, which were called ground-probing radars (G
11、PR) because of their original intended applications. Since then, GPR has been put to a variety of uses, including determining the thickness and structure of glaciers, locating ice in permafrost,1 fi nding sewer lines and buried cables,2 measuring the thickness of sea ice,3 profi ling the bottom of l
12、akes and rivers,4 examining the subsurface of the moon,5 detecting buried containerized hazardous wase,6 and measuring scouring around bridge foundations.7 The earliest study on the use of GPR in areas related to civil engineering, but not to concrete itself, was that reported in 1974 by Bertram et
13、al., which dealt with the inspection of airfi eld for voids underneath pavements.8 This study was followed by other studies, which included locating undermining underneath concrete sidewalks9 and pavements.10,11 As the following discussion will show, GPR can be used to test concrete for other purpos
14、es. Because of the nature of the microwave pulses that are employed by the radar systems used and because the applications are no longer limited to the probing of subsurface geological features, GPR is more appropriately called short-pulse radar. 13.2Principle of Short-Pulse Radar Short-pulse radar
15、is the electromagnetic analog of sonic and ultrasonic pulse-echo methods. It is governed by the processes involved in the propagation of electromagnetic energy through materials of different dielectric constants. 13.2.1Behavior of Microwave at the Interface of Two Different Materials Consider the be
16、havior of a beam of EM energy (such as microwave) as it strikes an interface, or boundary, between two materials of different dielectric constants (see Figure 13.1). A portion of the energy is refl ected, and the remainder penetrates through the interface into the second material. The intensity of t
17、he refl ected energy, AR, is related to the intensity of the incident energy, AI, by the following relationship FIGURE 13.1 Propagation of EM energy through dielectric boundaries. Medium 1 1,2 2,3 Antenna AR AI AB AT 2 3 D2 Short-Pulse Radar Methods13-3 (13.1) where 1,2 = the refl ection coeffi cien
18、t at the interface, and 1, 2= the wave impedances of materials 1 and 2, respectively, in ohms. For any nonmetallic material, such as concrete or soil, the wave impedance is given by (13.2) where 0= the magnetic permeability of air, which is 4 107 henry/meter, and ? = the dielectric constant of the m
19、aterial in farad/meter. (Metals are perfect refl ectors of EM waves, because the wave impedances for metals are zero.) Since the wave impedance of air, 0 is (13.3) and if we defi ne the relative dielectric constant, ?r, of a material as (13.4) where ?0 = the dielectric constant of air, which is 8.85
20、 1012 farad/meter. Then, we may rewrite Equation 13.2 as (13.5) and Equation 13.1 as (13.6) where ?ri and ?r2 are the relative dielectric constants of materials 1 and 2, respectively. Equation 13.6 indicates that when a beam of microwave strikes the interface between two materials, the amount of ref
21、l ection (1,2) is dictated by the values of the relative dielectric constants of the two materials. If material 2 has larger relative dielectric constant than material 1, then 1,2 would have a negative value, i.e., with the absolute value indicating the relative strength of the refl ected energy and
22、 the negative sign indicating that the polarity of the refl ected energy is opposite of that arbitrarily set for the incident energy. 1 2 21 21 , = + AR AI = 0 ? 0 0 0 = ? ? ? ? r = 0 = 0 ?r 1 2 12 12 , = + ? ? rr rr 13-4Handbook on Nondestructive Testing of Concrete: Second Edition 13.2.2Propagatio
23、n of Microwave Energy through a Material After penetrating the interface and into material 2, the wave propagates through material 2 with a speed, V2, given by (13.7) where C is the propagation speed of EM waves through air, which is equivalent to the speed of light, or 1 ft/ns (0.3 m/ns). As the wa
24、ve propagates through material 2, its energy is attenuated as follows: (13.8) where A = attenuation, in decibel/meter* f = wave frequency, in Hz and the loss tangent (or dissipation factor) is related to , the electrical conductivity (in mho/meter) of the material by: (13.9) When the remaining micro
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