There are more than 64 million kilometers of roads in the world, all of different age and condition. Around 65% of the roads are paved, covered, in most cases, with asphalt, as a mixture of stones and hydrocarbon binders or bituminized agents. This part of the road is the most exposed and in need of constant maintenance. To plan measures for the maintenance work, knowledge is needed on the thickness of the asphalt as well as other roadlayers. The thickness of asphalt is also critical during construction work: has the entrepreneur carried out the paving according to plan or have they used less asphalt to save money?
To check the thickness of the asphalt, pointwise sampling or drilling is the most common investigations technique today. To gain more data and a better overview of the whole road surface geophysical methods can instead be used, as a suitable solution to create a more comprehensive picture of the asphalt surface.
Solution
GPR (Ground Penetrating Radar) is a non-destructive investigation method offered by Guideline Geo that can collected dense data points, in highway speed and map the total thickness of the asphalt as well as internal layering or voids. As the method is quick, it is also cost-efficient.
Asphalt investigations are most often quite shallow and needs to be able to resolve thin layers. Antenna frequencies around 1.2 to 2.3 GHz are most often quite convenient. In some cases, frequencies of 800 or 750 MHz are needed to see a thicker asphalt cover.
It should be remembered that the depth is most dependent on the conductivity of the asphalt. A dry asphalt is most often suitable to investigate, whilst a newly salt sprayed road surface is not recommended. The conductive salt slush will efficiently prevent radar waves to penetrate the asphalt.
Measurements in 2D or 3D
GPR measurements for asphalt applications are most often done in 2D by pulling or pushing the antennas along the ground. This can be done by foot for smaller areas. For longer investigation lines, Guideline Geo also provides a convenient cart for holding 1-3 antennas behind a vehicle: the RoadCart. In this set up a lower frequency antenna can also be used, to simultaneously investigate deeper layers and features with in or beneath the road construction. When measuring with the RoadCart the data collection unit can be placed in the front seat for easy check of the data collection.
Some asphalt applications, as e.g., voids and delamination require 3D measurements for an easier understanding of the asphalt conditions. This can be carried out by dense 2D profiles or with a multichannel 3D array solution as MIRA or MIRA HDR.
During measurements, the position of the GPR antenna is tracked by an external or internal GPS for precise positioning of your investigation results.
Also check out the Application Area: Concrete Constructions and Rebars and Application Area: Roads to learn about solutions for associated infrastructure such as road embankments, bridges, revetments, bridge decks etc.
Investigation results
The results from a 2D GPR investigation on roads is rather straightforward. Data can be analysed in different types of post-processing software where the data is processed and interpreted according to application. Layer data is most often presented as X, Y and Z, where X and Y gives the positions and Z the depth to the interpreted layer. Below some results are shown, both as radargrams and as maps with easy color scheme representing the layer thickness interpretation.
3D data is most often presented as time slices over the investigated area giving the anomalous areas, as voids on a map.
The example shows a road investigation of asphalt. As seen in the mid parts of the profile, asphalt has been added on several occasions, probably due to a weak foundation.
Asphalt thickness interpretation can be shown with simple and clear color schemes on satellite images. The example above is created with Guideline Geo´s GPS Mapper and Google Earth.
Recommended setups
MALÅ CX
If asphalt is the main or only application, Guideline Geo´s CX system is the easiest and most convenient system to use, dedicated to high frequency antennas (in the range of 1.2 to 2.3 GHz). The system is fitted in a rugged case and with an easy interface, measurements can be carried out very efficiently. Single lines can be collected as well as aided grids for a 3D view of the investigated area. The system is also very suitable for concrete and rebar investigation or inspection of other types of construction, such as brick and wood.
Most often road investigation comprises investigation of thin layers of asphalt to deep road embankment constructions. With Guideline Geo´s ProEx GPR system several antenna frequencies (from low to high, up to 8 antennas) can be measured simultaneously, saving time and costs. The system is very robust and has a number of accessories suitable for road measurements including the convenient Road Cart upon which 3 different shielded antennas can be mounted.
If your main application is estimation of thicker asphalt layers, the total road embankment and the geology underneath the road structure, Guideline Geo´s GX system with antennas from 80 to 750 MHz can be suitable. This is the easiest to use system with its WiFi option, making all cables unnecessary.
GPR measurements for asphalt applications are most often done in 
Some asphalt applications, as e.g., voids and delamination require 
