GPR Limitations within concrete structures:
Note: The below limitations do not cover all possible limitations but only the most common.
Data Collection and Interpretation – The technician providing the GPR results is potentially the biggest limiting factor involved with this science. Technicians must not only be trained in operating the technology, they also must have a sound understanding of the material/structure and application in each specific situation; however, teaching an individual how to interpret the data they receive with the equipment can take long periods of time and ongoing training. The highest quality equipment operated by an inexperienced technician will offer little information to the customer as the ability to interpret the data is essential. In short, the technology/science of ground penetrating radar is only as good as the operator’s expertise and education in data collection and interpretation.
Moisture – Moist or ‘green’ concrete can be problematic for GPR as the presence of moisture will reflect/inhibit the passage of the radar pulse and thereby limit penetration and data quality.
Depth Penetration – The depth range of GPR is limited by the electrical conductivity of the medium, the transmitted centre frequency and the radiated power. As conductivity increases, the penetration depth decreases. Higher frequencies do not penetrate as far as lower frequencies but give better resolution. The best penetration is achieved in dry materials such as granite, limestone, and dry concrete.
Size of Target – There are two main ways in which GPR is limited when discussing the size of a target. GPR technology is unable to determine the diameter of the target being located. Dimensions of objects can in certain circumstances be given within tolerances which are specific to the site conditions and scanner used. As a rule of thumb, objects smaller than half the size of the wavelength cannot be detected. Larger objects may also not be detected, depending on the size and orientation. The wavelength is correlated with the centre frequency of the antenna used.
Obstructions – Targets may be obstructed by objects positioned in front of them, prohibiting the wave propagation to the target. Closely spaced neighbouring objects may also prohibit the detection of targets beneath.
Detected Objects
Rebars which comply with one of the following standards:
Environment / Working conditions
Smooth and flat surfaces.
Rebar layout Direction
Rebar layout Diameter
Rebar layout Spacing
Rebar layout Depth calculation
Linear Scan: up to 100mm
Intended use
This scanning tool is designed for locating reinforcing bars in concrete and other non-ferritic construction
materials, and for measuring the cover and estimating the diameter of the topmost layer of the reinforcement.
▶ Use only Hilti Liion batteries of the B 12 series with this product.
▶ Use only Hilti battery chargers of the C4/12 series for charging these batteries.
Limitations of use
The scanning tool complies with the limit values specified for electromagnetic compatibility and the wireless
spectrum. Nevertheless, permission must be obtained prior to operating the scanning tool at locations such
as hospitals and nuclear power plants, or in proximity to airports and cellular telephone relay stations.
Restrictions on scanning performance
Due to the scanning principle employed, certain unfavorable circumstances may affect the result:
• Surface is uneven or rough.
• Reinforcing bars are corroded.
• Reinforcing bars are welded.
• Adjacent reinforcing bars differ in diameter or are at different depths.
• Reinforcing bars are not normal to or parallel with the scanning direction or are not parallel with the
surface.
• Reinforcing bars have a relative magnetic permeability outside the 85-105 range.
• Ferromagnetic or paramagnetic objects close to reinforcing bars.
• Ferromagnetic or paramagnetic aggregates in construction materials on which scans are taken.
• External sources of electromagnetic interference (e.g. transformers, switches in strong electrical circuits).
• Electromagnetic emitters (other devices, including other scanners) less than 2 meters (6.6 ft) from the
scanning tool.
• Interference from external magnetic fields or nearby objects with magnetic properties.
• Metallic covers, insulation with aluminum foil backing, or comparable materials on the surface to be
scanned.
• Scanner’s wheels are dirty.
• Not all 4 wheels running on the surface to be scanned.
Detection range, measurement range and accuracy
The minimum spacing between two reinforcing bars is 30 mm (1.18 in).
The ratio between spacing of the reinforcing bars and cover must be at least 1.2 : 1. Example: If the
spacing is 120 mm, cover has to be at least 100 mm.
If the spacing is closer and/or the ratio of spacing to cover is smaller, the objects cannot be recognized as
separate reinforcing bars.
The displayed depth of the objects is the shortest distance from the object to the surface of the encasing
construction material. In the case of circular-section reinforcing bars, it is the middle of the object.
PIT is standardised by ASTM D5882 Standard Test Method for Low Strain Integrity Testing of Deep Foundations
Limitations
1. Pile Integrity Test does not provide any information on the load bearing capacity of piles and deep foundations.
2. Pile Integrity Testing cannot be conducted over pile caps or any other attached structure.
3. Pile Integrity testing of cross sections below a major crack (that crosses the entire cross-sectional area) is not possible.
4. This test is not effective in piles with highly variable cross sections
5. Pile integrity is generally not suitable for testing steel sheets, H-section, or unfilled pipe piles.
6. When the toe reflection is not evident, integrity evaluation may not be conclusive.
7. In some cases, it is difficult to distinguish the soil response, and the (pile) toe response.
8. Pile Integrity testing identifies changes in the impedance of a pile, which is determined by the pile’s area, elastic modulus and density. Which of these factors has changed cannot be isolated. Any change in impedance will be interpreted as change in area.
9. If it is not possible to determine the wave speed in a single pile, the average value from neighboring piles may be used however accuracy will be decreased.
10. The pile length/diameter ratio must be between 10 and 30. The engineer may decide to test piles outside this ratio. The probability of obtaining suitable results will decrease.
What may be detected with PIT:
• Pile toe
• Lage inclusion
• Crack through parts or the whole of the pile
• Joint (e.g. from different pour stages)
• Increase in cross-section
• Decrease in cross-section
• Soil layer change
• Major material changes
What probably may not be detected with PIT:
• Gradually increasing diameter
• Gradually decreasing diameter
• Curved pile
• Gradual material changes
• Small inclusions
• Local loss of cover
• Debris at toe
1. Maximum depth of inspection in concrete = 2500 mm
2. Maximum view depth in marble and granite = 2000 mm
3. Maximum view depth in reinforcement concrete = 800mm
4. Guaranteed Minimum and Maximum Measurable Thickness in concrete = 50 – 600 mm
5. Minimal size of detected reflector = sphere with a diameter of 30 mm at a depth of 400 mm in concrete grade M400
6. Limits of permissible absolute measurement accuracy of the thickness, where Х – thickness being measured = ±(0.05∙Х+10) mm
7. Measurement range of the depth of the flaw location (a sphere 20 mm in diameter at least and 200 mm in length at least) = from 50 to 400 mm
8. Limits of permissible absolute measurement accuracy of the depth of the flaw location, where Н – depth being measured = ±(0.05∙Н+10) mm
Due to the scanning and technology principle employed, certain unfavourable circumstances may affect the result:
• Multiple layers of reinforcing, obstructions or materials and very inhomogeneous material composition. Objects that lie at an angle in the wall
• Objects behind cavities or voids cannot be detected
• Proximity to appliances that emit powerful magnetic or electromagnetic fields, such as but not limited to; mobile phone relay stations or electric generators.
• Very closely spaced reinforcing or objects may cause signal interference.
