Thickness gauges for asphalt and building materials

Thickness gauges for asphalt and building materials are non-destructive instruments for measuring the thickness of road pavements, sub-bases, bituminous surfacings, concrete and other construction materials. They use measuring principles adapted to the particular characteristics of building materials: low density compared with metals, composite make-up (aggregates + binders), and variability of composition and moisture. The main technologies are microwave radar sensors (GPR — Ground Penetrating Radar), ultrasonic sensors dedicated to concrete, and systems based on radiation backscatter (for regulated applications).

Available technologies

GPR systems use radar antennas (frequencies 500 MHz to 2.6 GHz) that emit electromagnetic pulses into the material: the reflections from the interfaces between different layers (asphalt / sub-base / foundation) are received and analyzed to reconstruct the stratigraphy. Ultrasonic sensors for concrete use lower frequencies (50-200 kHz) than metal UT, in order to overcome the attenuation of heterogeneous materials. There are also impact-echo technologies and wave velocity analysis, applied to assessing the quality and integrity of structural concrete.

Non-destructive thickness gauges for asphalt

For checking road pavement during laying or at acceptance, there are portable trolley-mounted GPR instruments which, run across the surface of fresh or cured asphalt, continuously record the thickness of the surfacing and the stratigraphy beneath. Typical resolution is a few mm on asphalt layers down to 30-50 cm deep. Calibration is carried out on reference sections of known thickness (cores taken for verification). The most advanced systems incorporate GPS for georeferencing the measurements and generating digital maps of the pavement.

Applications

Applications include the quality control of new road pavements at acceptance (checking the thickness of the surfacing and sub-base against the specification), the periodic inspection of motorways and primary roads for maintenance planning, checks on car parks and industrial yards, the analysis of airport pavements, the verification of the stratigraphy of historic buildings, and the detection of voids and cavities beneath pavements or load-bearing walls. Diagnostics of structural concrete by UT methods is used in construction to verify quality and to locate internal defects.

Standards and alternative technologies

GPR methods follow ASTM D4748 (asphalt) and ASTM D6087 (bridges and infrastructure). Concrete diagnostics follows UNI EN 12504 (parts 1-4, for destructive and non-destructive methods) and UNI EN 13791. Destructive measurement by coring remains the legal reference in many contexts; non-destructive technologies are accepted as complementary methods for extensive inspection. Calibration of GPR systems is performed by the manufacturer or by means of reference cores taken at the specific site.

Selection criteria

The choice of instrument for asphalt and building materials depends on the typical thickness (thin asphalt or thick pavement), on the resolution needed, on the acceptable accuracy, on the scanning speed (for extensive campaigns on motorways), on portability, on the availability of integrated GPS for georeferencing, and on the need for dedicated software for analysis and reporting. For regulated applications, conformity with the sector standards must be verified (for example for surveys on infrastructure under concession).