he goal of using GPR in concrete structure assessment is to define the internal structure prior to cutting and coring. Knowing what’s there prevents structural damage and improve safety. Conquest® users face the operational challenge of how to most cost-effectively achieve this goal which usually takes the form of accurately marking the location of embedded structures. Conquest® provides users with two modes of operation to achieve this goal – line scanning and grid scanning.
A single line scan essentially provides a cross section through the concrete scanned (line scanning); often this approach may provide a satisfactory answer when the structural conditions are simple. Complicated concrete structures are often very difficult to understand and best addressed by developing a full three dimensional understanding of embedded elements by collecting data on a regular grid of lines (grid scanning). Hence the question, what survey mode should be used?
Cost is often related directly to the time spent on a site. Simple line scans can be quick and hence less costly. Grid scans take a little more time but provide a much more comprehensive understanding of conditions.
Cost is often related directly to the time spent on a site. Simple line scans can be quick and hence less costly. Grid scans take a little more time but provide a much more comprehensive understanding of conditions. The cost-benefit must be weighed against the risk factor in being wrong about site conditions. Risk comes in various forms such as costly damage repairs, workplace injuries and business reputation.
A recent Conquest® training session in Richmond Hill, ON, entailed scanning an elevated concrete slab in an industrial warehouse. This site illustrates the trade-offs between line scanning and grid scanning. A simple line scan shown in Figure 1 indicates irregular rebar conditions on the right accompanied by a PCD (Power Cable Detector) response.
The PCD response indicates that there is an electrical current in this area likely associated with an embedded power cable. The single line indicates the potential of a problem area but how best to avoid damage when cutting and coring is unresolved.
By carrying out a grid scan, depth slice images at several depths can be generated such as those shown in Figure 2 and 3. From the plan views, it is clear that there are features running obliquely to the regular rebar structure. These features are inferred to be electrical conduits (conduits commonly appear as irregular features within a normal rebar structure).
The data suggest there are 4 conduits present (labeled 1, 2, 3, and 4); conduit 1 is at a shallower depth than the other three. Conduits labeled “1”, “2” and “3” in Figure 3 appear to run at 45 degrees to the rebar grid and then turn parallel in the upper right of the area scanned. Conduit “4” runs from top to bottom at the right but does not align with the rebar.
From more than 25 years of delivering GPR for concrete imaging, we strongly recommend taking the time to do grid scanning and to use line scanning as a quick reconnaissance approach to establish optimal grid orientation.
