Limitations

GPR Scan Limitations
Due to the scanning principle employed, certain unfavourable circumstances may affect the result:

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.

PS 200 Limitations
Reliable Measurements
Base Material

  • Reinforced concrete.
  • Reinforced CMU.
  • (Brick)

Detected Objects
Rebars which comply with one of the following standards:

  • DIN 488
  • ASTM A615/A 615/M-01b
  • CAN/CSA-G30. 18-M92
  • JIS G 3112
  • GB 50012-2002 (China)
  • Metal conduit if sufficient spacing from rebars.

Environment / Working conditions
Smooth and flat surfaces.
Rebar layout Direction

  • Rebars lying orthogonal or within ±5° of 90° to scanning direction.
  • Reinforcement running parallel to the surface.

Rebar layout Diameter

  • Neighbouring bars with a similar diameter.
  • Standard diameters in between 6 – 36mm up to a depth of 60mm, if s:c ≥ 2:1.
  • Calibrated bar sizes: {6,8,10,12,14,16,20,25,28,30,36}
  • Device is calibrated to be accurate to within ± 1 of the above intervals, however this is strongly affected by environmental conditions.

Rebar layout Spacing

  • Minimum bar spacing either 36mm (1.4 in) OR Minimum ratio of spacing : coverage (s:c), whichever value is greater.
  • Minimum ratio of spacing : coverage
  • Depth calculation / bar diameter estimation s:c = min 2:1
  • Bar location s:c = min 1.5:1

Rebar layout Depth calculation

  • Neighbouring bars at similar depth / size.
  • For accurate depths readings minimum depth of 20mm (0.8 in). For readings at all min. 10mm.
  • Maximum depth determination differs with rebar diameter.
  • Typically if diameter is known (DIN) Image Scan / Block Scan:
  • ≤ 10mm: up to 100mm
  • ≤ 14mm: up to 120mm
  • ≤30mm: up to 140mm
  • 36mm: up to 160mm

Linear Scan: up to 100mm

  • Accuracy for unknown bar diameter: ± 10% for depths greater than 20mm.

Pile Integrity Test Limitations

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

Ultrasonic Limitations

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.