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Method:
The Pulse Radar, also known as Georadar or Ground Penetrating Radar (GPR), is a proper method for non-destructive testing of the subsurface, pavements, bridges and other structures concerning existing patterns and inhomogeneities at high resolution and simultaneous fast measuring progress.
The technique is based on the propagation und reflection of emitted electromagnetic waves. Electromagnetic pulses triggered by a pulse generator are transmitted antenna into the subsurface at a previously configured frequency. At interfaces of materials with differing physical properties, these pulses are partially reflected. The receiving antenna records the strength and the time required for the return of any reflected signal. The character und intensity of the reflections is basically determined by the contrast of the permittivity of the materials concerned.
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Reflection of a transmission pulse |
The recorded data are collected and stored by the control unit and simultaneously displayed on the monitor as signal amplitude (grey scale values or colour code) versus travel time.
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Exemplary radargram |
The typical display of the radar data is the so-called radargram. The individual linescans are lined up to a way/time plot (as shown above). Thus the reflecting features appear well visible traceable according to the interpretation of the problems requested
Besides the permittivity, antenna frequency is an important factor in depth penetration of the signal. High frequency antennas are limited to a shallow penetration (≤ 1 m) but permit high resolution (small targets of a few cm size). lower frequency antennas perform a higher penetration at a lower resolution. Subsurface of dry gravel or sand commonly allow depth penetration in the range of 5 - 10 m whereas in solid rocks, penetration of more than 20 m in are realisable under certain conditions
Basically the presences of cohesive soil layers or moisture are limiting factors to depth penetration due to signal attenuation effects. Thus the gainable penetration depth for particular tasks often has to be definitely determined on site with a trial measurement.
Since the definite propagation velocity of the specific layer media normally is well known, precise depth conversion of the recorded travel times of the signals requires ground-truth data (core ore outcrop) for calibration purposes. Thus the measuring error can be reduced to ± 1-3 % of calculated depth or layer thickness respectively. For a rough depth estimation the use von experience values in regard to the propagation velocities is mostly sufficient.
Because of des indirect character of the pulse radar technique, the collected data need competent interpretation of experienced experts to gain substantiated conclusions concerning the specific tasks. These conclusions are of relative nature; absolute results are commonly not to be inferred. Thus areas of increased moisture can be localised along a layer boundary, for the accurate determination of the moisture content complementary investigations are essential.
Hence the best significance results in the synergy of the pulse radar technique and further investigations for example core tests or FWD, evenness and macro texture of pavement etc. These often punctually collected data can be summarised to representative areas, respectively, a previously performed pulse radar investigation is a powerful tool for target-oriented defining of further
Measuring system:
With our multichannel systems we possess high efficient units, enabling high speed measuring progress and simultaneous operation of up to 4 antennas. The frequencies of our ground coupled antennas range from 300 up to 1500 MHz, covering the prevalent range of applications very well. For particular needs additional equipment can be obtained rapidly at any time.
Performing high speed pavement survey at net scale we apply special air-launched horn antennas. The frequencies of 1000 and 2500 MHz enable the survey and differentiation of the entire road construction from the pavement down to the base.
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Our measuring vehicle |
with vehicle safety system |
The speed independent triggering of the measuring system by the high-precision Peiseler®-distance measuring instrument (error ~± 0,1%) combined with GPS to allow the exact recording of the measuring distance the allocation to existing reference systems (road net waypoints, chainage etc.). For stand-alone measurements we dispose of different antenna coupled survey wheels.
With several beacons, a flashlight bar and the automatic halogen light arrows, the safety system of our measuring vehicle guarantees optimal safeguarding during the service on the road. Thus supporting protection, as mobile warning or advance directional trailers, is required merely on motorways.
Video inspection:
Our digital video system enables the geo referenced survey of the surface condition in combination with the radar data recording of the transportation route under investigation. This is a powerful tool to improve the interpretation of the detected subsurface phenomena in terms of allocation und significance.
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Video camera |
Video reporting |
The geo referencing enables the precise allocation of the video documentation to existing reference systems (road net waypoints, chainage etc.). Besides the road surface, transverse sections, equipment as road signs etc. as well as specifics of the direct surroundings are documented with respect to their precise position.
Of course we offer separate radar and video inspection on demand.
Documentation:
According to the instruction sheet for ground penetrating radar services (Federal Highway Research Institute “BASt“-Report S 31) the results are typically documented in a inspection, containing besides the course of events, process description, specification of the measuring parameters as well as the interpretation and assessment of the results the complete data in the form of attachments.
In the case of layerthickness data or “homogenous sections” are presented both in table form and graphically, in the form of so-called “Layershows“. For that the converted layer thicknesses are displayed below the original radargram as shown above. Construction changes, separating homogenous sections, are accentuated.
In addition to the tabular documentation of inhomogeneities and/or possible defects, a graphical presentation in the form of “section tapes“ is very useful for a rough outline.
In general the detailed documentation of the location and alignment of dowel bars occurs tabularly. Due to the large amount of individual information, presentation in plot form is suited merely for outline purpose. For that, trouble spots (whole slabs or slab edges) with missing or positional diverging dowel bars are colour-coded.
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Examplary table: threedimensional position of the dowel bars
relative to the transverse joint |
Basically less comprehensive data sets are handed out in hardcopy form as attachments of the report. Large amounts of data are attached as data files on a separate data medium .
Concerning the large scale survey of construction data at network level the radar data are encoded together with the collected reference core data following the defaults of the client (ASB, or specific code etc.) and fed to the particular PMS. | 
