Masonry Magazine February 2006 Page. 34
Technology
When the wave passes through different materials, the propagation of the wave is altered, and some or all of the wave energy reflects from the interface of the two materials. Air, steel and grout within a masonry wall reflect the radar signal at interfaces between them.
Surface Penetrating Radar
SPR UTILIZES THE principles of electromagnetic wave propagation and reflection to distinguish between different material interfaces. The SPR unit emits a series of discrete electromagnetic wave pulses that travel through the surveyed material. When the wave passes through different materials, the propagation of the wave is altered, and some or all of the wave energy reflects from the interface of the two materials. Air, steel and grout within a masonry wall reflect the radar signal at interfaces between them. Steel is a perfect reflector of electromagnetic waves, while masonry, grout and mortar reflect only a portion of the electromagnetic energy. Air causes rapid attenuation of the electromagnetic wave and is readily distinguishable in the reflected wave signal.
The output from the SPR system can be used to identify conditions in the masonry. Radar signals are processed by a mobile computer and displayed in real time on the screen as a series of bands. The shade of the band is a function of the phase and amplitude of the reflected signal. Based on the shades and shapes of the reflected signals, three basic conditions within a grout space can be detected:
* Void in Grout Space: Air inside a grout space reflects the signal as the wave passes through the masonry at the air interface. The resulting reflection (as illustrated in this publication) from masonry to air is displayed as a black-white-black band.
* Reinforcing Bar Embedded in Grout Space: Since steel objects are perfect reflectors, the reflection from a reinforcing bar is always inverted and displayed as a white-black-white band. The shape of the band is that of a hyperbola, due to the discrete diameter of the bar and reflection of the signal as the antenna passes over the bar.
* Grout Filled Grout Space, No Reinforcement: The dielectric constant of grout and masonry are very similar. Therefore, the reflection of the wave at the grout/masonry interface will have a low amplitude and appear as a gray shade.
The characteristics of the wave reflection also permit calculation of the depth of embedded steel reinforcement relative to the surface of the antenna. The SPR method, however, cannot be used to directly determine reinforcing bar diameters from the reflected wave. At left, Figure 1 illustrates representative SPR output from the survey of a reinforced concrete masonry column.
Under favorable conditions, the SPR signals can be used to determine approximate splice lengths of bars when the bars are placed side by side, relative to the line of the antenna scanning direction. The perfect reflection of the wave and the characteristic hyperbola shape of the steel reflection hides materials directly behind the steel object so that they are not detectable by this method.
Infrared Thermography
IRT USES THE principles of thermal energy absorption, transmission and emission to distinguish between hollow and grout-filled cells in masonry. Since this method can only differentiate between hollow and solid masonry cells (steel cannot be detected), it is often coupled with a