GPR scanning for car park decks

Multi-storey car park decks are among the most frequently surveyed concrete structures in the UK, and for good reason. They are heavily loaded, fully exposed to the weather, and subjected to a relentless cycle of de-icing salt every winter. That combination makes them one of the harder-working concrete structures in the built environment — and one of the most likely to need investigation. Here is what GPR scanning finds in a car park deck, why these structures need regular attention, and what a survey involves.

Why car park decks deteriorate

A car park deck takes punishment that an internal slab never sees. Vehicles track in chloride-laden water from gritted roads through the winter. That chloride works its way into the concrete and, once it reaches the reinforcement, breaks down the protective layer that normally keeps the steel from corroding. Corroding steel expands, and that expansion cracks and spalls the surrounding concrete from the inside.

Decks are also exposed to thermal movement, freeze-thaw cycling, and repeated traffic loading. Many UK car parks date from the 1960s and 1970s and were not built with the durability detailing that current standards require. Cover depths are often shallow and inconsistent, which means the reinforcement has less protection than it should. The result is a structure that needs ongoing inspection rather than a one-off check.

What GPR finds in a deck

GPR scanning is well suited to car park investigation because it is non-intrusive and can cover large areas quickly. On a typical deck survey it is used to establish:

• The position and depth of the reinforcement, top and bottom.
• Concrete cover depth, and how consistent it is across the deck.
• The presence of post-tension tendons, where the deck is a PT structure.
• Embedded services, drainage, and conduit.
• Indications of voiding, delamination, or honeycombing within the slab.

Cover depth mapping is often the most valuable single output. Knowing where cover is shallow tells the engineer where chloride will reach the steel first, which is where deterioration will appear first. That turns inspection from a reactive exercise into a planned one.

GPR alongside other testing

GPR rarely works alone on a car park assessment. It locates and maps; it does not, on its own, tell you the chemical condition of the concrete or how far corrosion has progressed. A full condition assessment usually pairs GPR with other non-destructive and intrusive testing — half-cell potential surveys to map corrosion activity, chloride sampling to measure salt ingress, and core sampling to confirm concrete strength and condition at specific points.

The two work together well. GPR maps the reinforcement and cover across the whole deck and identifies the suspect zones; targeted testing then confirms what is happening in those zones. Using GPR to guide where the intrusive testing happens means fewer cores, less disruption, and a more representative result.

What a deck survey involves

A car park survey is usually run as a grid. The surveyor sets out a marked grid across the deck and scans systematically so the data can be tied back to real positions on a drawing. Open decks scan quickly; the work is straightforward where the surface is sound and clear.

The main practical constraints are access and the surface itself. A working car park means scanning around parked vehicles or arranging phased closures so bays can be cleared. Surface coatings, waterproofing membranes, and heavy contamination can all affect the data, so the deck condition should be discussed before the visit. Decks with steep ramps, tight bays, and low headroom take longer to cover than an open floor plate.

The deliverable is normally an annotated drawing showing reinforcement layout, cover depth, located services, and any anomalies, with the survey grid clearly referenced so findings can be relocated on site.

Practical advice for commissioning

Treat car park deck scanning as part of a planned inspection cycle, not a one-off. These structures deteriorate steadily, and a baseline survey is far more useful when there is an earlier survey to compare it against. Repeating the assessment on a sensible interval lets the asset owner see how the deck is changing and budget for repairs before they become urgent.

When commissioning, give the surveyor the structural history: build date, structural type, whether it is a PT deck, and any previous repairs. Plan access so the survey can be done efficiently, and expect GPR to form one part of a wider assessment alongside condition testing. A well-planned deck survey gives the owner a clear, evidence-based picture of a structure that genuinely needs watching.