In older structures, it is sometimes necessary to construct "as-built" data to confirm existing design drawings, determine structural capacity of existing structures when no reliable information on the design of the structure or as-built construction are available, or create new replacement plans. Additionally, seismic retrofits or upgrades to existing structures, bridge deck widening, construction linking new ramps and elevated structures to existing ones, and drilling or coring through existing decks to install or improve drains, lighting, or safety equipment, all require that deck reinforcement be quickly, accurately, and economically located before any of the proposed work can proceed. Critical reinforcing elements such as post-tension cables, embedded electrical and other conduits, or even superstructure elements that support the deck itself must be located prior to cutting, coring, and drilling so that they are not damaged.

When corrosion rate studies are being performed, or if corrosion activity levels within a reinforced concrete deck are being measured, detailed knowledge about the spatial location(s), depths, variability, and spacing of individual reinforcement elements (rebar) in a deck contribute tremendously to the ability to accurately characterize the severity of corrosion actually taking place. All electrochemical methods either require the presence, pattern, and density (PPD) of rebar to be known, whether on a local, regional, or global scale on the deck, so that confidence levels in the measurements being made can be established. Without sufficient knowledge about the PPD of rebar in a deck, many of the conclusions from these studies are suspect, particularly if they are not supplemented with chemical or petrographic analysis of the concrete, the construction materials, and methods that were used when the deck was built, or other NDT methods capable of verifying their accuracy.

Geophysics is useful for locating individual bars, measuring spatial distances between them, determining rebar depth (often for upper and lower mats) from single side, and quantifying the presence of rebar.

Ground Penetrating Radar (GPR)

GPR is the best method to identify presence, pattern, and density of rebar. As discussed above, applications include seismic retrofits/upgrades, bridge widening and tying into existing structure, cutting holes into deck to improve drainage and reduce ponding, particularly if de-icing chemical can cause acceleration of corrosion as they penetrate into deck or membrane, and determine structural load capacity based on PPD of rebar. See Ground Penetrating Radar for a discussion of GPR for locating rebar. For vehicle mounted GPR systems, please refer to section Vehicle Mounted Ground Penetrating Radar..