It is often necessary to verify construction detail of structures. This could be to ensure compliance with current legislation and standards, proposed modifications or change of use, or simply part of a maintenance regime.

The following are all common scenarios:

  • There are no as-built documents for an existing structure. Drawings may have been misplaced or not handed on with a change of ownership.
  • The accuracy of as-built construction drawings is unknown.
  • Drawings are marked as “Verify-in Field” (VIF).
  • Older, historic buildings were often built by craftsmen using traditional construction methods. They used their knowledge to overcome construction difficulties. No records for these buildings are available.

Dimensional verification of drawings is fairly straightforward. Identification of mistakes and dimensional variabilities can be conducted using a topographic survey or 3D laser scanning.

But what about the hidden construction detail? How can that be determined without the use of destructive techniques?

Using GPR to verify construction detail

Ground Penetrating Radar (GPR) can be used to determine or verify construction detail.

GPR detects changes in the sub-surface; it is therefore not limited to detecting metallic objects. It offers a rapid means of obtaining subsurface information from a variety of materials used in the construction industry including concrete, brick, masonry, wood, asphalt and soil. It is a non-destructive (NDT) survey method, it makes no noise and emits no odour. It can be used on construction sites or in occupied buildings. Because it is non-intrusive, it is ideal for historic or listed buildings.

GPR surveyor scanning platform and marking up reinforcement
Using GPR to verify platform reinforcement detail

The following are just some of the ways GPR can help determine or verify construction detail:

  • Concrete floor slab surveys
    GPR can determine general slab construction detail, including slab thickness, reinforcement distribution, adequate mesh overlap, detect variations in construction detail and determine the position of ground beams and pile caps.
GPR radargram showing detected slab construction detail
GPR radargram showing variations in slab thickness and reinforcement
  • Locating Tendon Ducts
    Due to greater penetration depth and better resolution than many other techniques, GPR can be an effective way of locating tendons and confirming that they are positioned as per drawings.
  • Rebar mapping
    GPR offers excellent rebar mapping and concrete imaging to depths of 400mm (depends on the density of reinforcing steel). It is suitable for scanning smaller areas, such as beams or columns, or large areas, such as floor slabs.
  • Locating embedded metal in masonry
    Structural steel is often embedded in buildings to strengthen them. GPR can confirm the presence of structural steel members and determine their location and orientation. It can also be used to detect metal dowels, pins and cramps.
GPR surveyor recording position of embedded steel in stone facade to verify construction detail
Location of embedded metal in a stone facade
  • Void location
    GPR can locate voids beneath slabs, basements below pavements, and identify construction elements which incorporate voids such as hollow pot slab construction and hollow-core precast units.
  • Chimney Flues
    I have never seen as-built drawings showing chimney flue locations. Yet this information is often needed. Are chimney flues present? In which walls? Where do they go?
Position of chimney flues verified by GPR and marked up on CAD drawing
GPR is very effective in detecting chimney flues and ducts in walls
  • Construction thickness
    GPR detects layer interfaces in construction. It can be used to determine the thickness of slabs, screed, asphalt, brick walls, retaining walls, etc. Variations in thickness and construction can also be detected and mapped.
  • Foundations
    GPR can determine the plan view geometry of foundations such as pad foundations and pile caps.
GPR radargram showing detected pad foundations to verify construction detail
GPR radargram showing detected pad foundations

The above list includes some of the more popular examples of how GPR can be used to verify construction detail. It is by no means exhaustive.


In the hands of an experienced surveyor, GPR is an efficient and cost-effective method for the determination and verification of construction detail.

If you would like to find out more, contact us and let’s discuss how our GPR surveys could help you.

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