This article compares the two approaches on accuracy, speed, damage and budget, with a decision sequence showing which jobs can rely on NDT alone and which need cores. For details of each NDT method, see What NDT methods are there for concrete?

Non-destructive on-site concrete testing with a Rebound Hammer to assess uniformity and surface compressive strength
Non-destructive testing (NDT) with a Rebound Hammer — fast and covers many points, used to flag suspect areas before confirming with cores

What Are NDT and Core Testing?

NDT (Non-Destructive Testing) is a group of methods that assess concrete without damaging it, such as the Rebound Hammer, which measures surface hardness; UPV, which measures pulse velocity to indicate uniformity; and GPR, which locates rebar and voids. These methods give comparative or estimated values and fall under non-destructive structural evaluation (NDE)

A Core Test drills concrete cores from the actual structure and tests their compressive strength in the laboratory to ASTM C42/C39 (in Thailand, alongside DPT 1501-51 of the Department of Public Works and Town and Country Planning). It gives the most accurate compressive strength, but because holes must be drilled it is classed as semi-destructive.

📚 Basic principles

NDT and cores do not compete; they "complement each other." NDT answers the questions of coverage and speed, while cores answer the question of compressive-strength accuracy. The choice therefore depends on what the job mainly needs.

How Do NDT and Core Testing Compare?

NDT has the advantage in speed, coverage and not damaging the structure, while the Core Test has the advantage in compressive-strength accuracy. The table below compares them item by item.

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TopicNDT (Rebound/UPV/GPR)Core Test
Compressive-strength accuracyEstimateMost accurate
Damage to structureNoneDrilled holes (semi-destructive)
VelocityFast, results on siteSlow, must wait for lab
Area coverageWide, many pointsOnly at drilled points
Calibration needed?Yes (against cores)Is itself the calibration reference
StandardASTM C805 / C597 / D6432 · DPT 1502-51 / 1504-51 / 1507-51ASTM C42 / C39 · DPT 1501-51

NDT provides coverage and flags suspect areas, while cores confirm the numbers and serve as the calibration basis for NDT.

3D model of one concrete slab panel surveyed with an NDT grid across the whole slab, then only three cores drilled for laboratory testing (conceptual illustration, assumed values) Conceptual isometric view of a concrete slab on four columns. The slab surface has 40 NDT test points in a grid; blue points are normal readings and red points in one corner are suspect low readings, enclosed by an amber dashed frame. An engineer presses a rebound hammer onto one test point, showing that NDT is fast and covers the whole slab without damaging the structure. A core drill stands in the suspect zone, and two more drilled holes with cores represent the normal part of the slab, for three cores in total. The lower-left inset shows a laboratory compression machine crushing a core to ASTM C42 and C39 to obtain the actual compressive strength. The right-hand panel compares NDT and cores on five items: compressive-strength accuracy — NDT gives an estimate, cores are most accurate; damage — NDT none, cores require drilled holes; speed — NDT gives results on site, cores must wait for the laboratory; coverage — NDT is wide with many points, cores only at drilled points; and calibration — NDT must be calibrated against cores, while cores are themselves the reference. The number of points shown is assumed. 3D model: NDT surveys whole slab · cores confirm selected points Not to scale NDT 40 pts, full slab Suspect low zone 3 cores Sample slab panel · point counts assumed Laboratory Core crushing Actual strength ASTM C42 / C39 NDT point · normal NDT point · low Core hole NDT vs Core Test Compressive-strength accuracy NDT Estimate Core Most accurate Damage to structure NDT None Core Drilled holes (semi-destructive) Velocity NDT Result on site Core Slow, must wait for lab Area coverage NDT Wide, many points Core Only at drilled points Calibration NDT Calibrate vs cores Core Is itself the calibration reference Broad NDT + spot cores Coverage, accuracy, budget 1 NDT flags suspects Non-destructive full survey 2 Cores confirm values Typical + suspect pts 3 Engineer sets no. of pts and interprets both
Conceptual 3D view: NDT surveys the whole slab to identify low-reading zones, then only a few cores are drilled for laboratory testing (point counts are assumed)
Drilling concrete cores from an actual structure for compressive strength testing in the laboratory
Drilling concrete cores to ASTM C42 — gives the most accurate compressive strength and serves as the calibration basis for NDT values
SPN technician drilling a concrete core from a building slab with a water-cooled diamond core drill
Coring a real building slab with a diamond core barrel — the cores are sent for compressive strength testing in the lab to ASTM C42

Which Test Method Should You Choose?

Start by asking whether you need a "verifiable compressive-strength figure." If you do, a Core Test is always required. If not, NDT can be the main method. Common situations are as follows:

  • Want to survey concrete at many points or over a large area quickly → start with NDT
  • Want a verifiable compressive strength for structural assessment → a Core Test is required
  • Want to locate rebar, voids or measure thickness → use specialized NDT (GPR, Impact Echo)
  • Limited budget and need to minimize destructive drilling → use NDT as the main method and drill only the cores necessary

In practice, engineers usually plan an NDT survey first, then select core locations that are representative as well as suspect, to achieve coverage and accuracy while controlling the budget.

2D chart of the decision sequence for choosing NDT or a Core Test, and a project-specific correlation graph between NDT readings and core compressive strength (conceptual illustration) The left side is a decision flowchart. Step one: survey broadly with NDT first, such as the Rebound Hammer, UPV and GPR, which are fast and non-destructive. Then ask whether a verifiable compressive-strength figure is needed. If not — for example, checking uniformity or locating rebar, voids or thickness — NDT can be the main method with no coring. If yes, go to step two: drill cores only at representative points and suspect points identified by NDT. Step three: build a correlation between NDT readings and core compressive strength for the same concrete. Step four: use that correlation to interpret NDT readings across the whole structure, reducing the number of cores needed. Beside the flowchart, a small floor plan shows many NDT points and only three cores. The right side is an illustrative example graph: the horizontal axis is the NDT reading, such as the rebound number, and the vertical axis is the core compressive strength. Orange points are data pairs at cored locations, the blue line is the correlation line, and the green dashed line shows converting the NDT reading of an uncored point into an estimated compressive strength, with a warning that uncalibrated NDT readings are only estimates and should not be used to conclude compressive strength directly. The bottom bar summarizes that NDT is fast, wide-coverage and non-destructive but gives estimates, while cores are the most accurate but local and semi-destructive; used together they give both coverage and accuracy. NDT or cores: how to decide, and where the two complement each other Not to scale 1 Survey broadly with NDT Rebound · UPV · GPR — fast, no damage Need a verifiable strength figure? No NDT as primary Check uniformity Find rebar · voids Measure thickness No coring needed Yes 2 Core selected points Typical + suspect points from NDT 3 Build NDT–core correlation NDT readings vs core strength 4 Interpret NDT for structure More reliable, fewer cores needed NDT: many pts Core: few pts Sample plan (assumed) Project-specific correlation Conceptual example · Not measured data NDT reading (e.g. R) → Core strength → NDT–core pairs (cored pts) Correlation for this concrete Convert uncored NDT pts Uncalibrated NDT values are only estimates not direct strength N NDT: fast, wide, no damage Gives estimates C Core: most accurate Local · semi-destr. · lab wait + Combined Coverage + accuracy + budget
Decision sequence: if no compressive-strength figure is needed, NDT can be the main method; if one is needed, drill cores at selected points and build an NDT–core correlation to interpret readings across the whole structure (the graph is an illustrative example)
⚠️ Points to watch

NDT readings not yet calibrated against cores are only estimates and should not be used directly to conclude compressive strength for structural assessment. An engineer should set the number of points and interpret the results together.

Why Are NDT and Cores Commonly Used Together?

Because the weaknesses of each method offset each other exactly. NDT alone gives estimates that can be off if not calibrated, while cores alone test only a few points and may not capture the variability of concrete across the whole structure.

When used together, the engineer builds a correlation between NDT readings (such as the rebound number or UPV velocity) and core compressive strength for that concrete, then uses the correlation to interpret NDT readings across the whole structure more reliably, so fewer destructive cores are needed.

SPN team using NDT equipment to scan for rebar and suspect points on a concrete slab before selecting core locations to confirm compressive strength
Surveying a structure with NDT equipment to locate rebar and suspect points first, then selecting core locations to confirm compressive strength to ASTM C42 — used together for both coverage and accuracy
✅ Getting the most out of it

The most cost-effective formula is "broad NDT survey + cores to confirm selected points," giving coverage and accuracy while controlling both the budget and damage to the structure.