Non-Destructive
Testing per
IS 13311.

Codes referenced

UPV — what the test does and doesn't tell you

UPV measures the velocity of an ultrasonic pulse through concrete. Velocity depends on the concrete's elastic modulus and density — both of which correlate with compressive strength but only weakly. The pulse travels fastest through dense, defect-free concrete and slows on voids, cracks, or poor compaction.

The key distinction: UPV gives uniformity and detects defects; it does not directly give strength. A 4.0 km/s reading tells you the concrete is uniform and free of large voids; it does not tell you whether the strength is 30 MPa or 40 MPa without site-specific calibration.

Pulse velocity interpretation (IS 13311 Pt 1 §6)

Reference: IS 13311 Pt 1 §6 grading (above values typical for normal-weight concrete; values shift with aggregate type and moisture content).

Transducer arrangement

Three arrangements, in order of reliability: direct (through) — transducers on opposite faces, most reliable; semi-direct — transducers on adjacent faces; indirect (surface) — both on same face, least reliable (used when access to opposite face is blocked). Path length L is measured to nearest 1 mm; transit time t to nearest 0.1 microsecond.

Rebound hammer — when and when not to use it

The rebound (Schmidt) hammer fires a spring-loaded mass against the concrete surface and measures the rebound distance. Harder concrete rebounds more. The result is a number on a 0-100 scale called the rebound index (R).

Test-grid and reading rule

• · Grid of ≥ 9 points in a 300 × 300 mm area; points ≥ 25 mm apart and ≥ 50 mm from edges
• · Each point: 3 readings within a 30 mm circle, take the median
• · Discard any reading > 6 below the median (low outlier)
• · Discard any reading > 1.4× the median (high outlier)
• · Average the remaining readings for the area's R

Why site calibration matters

The rebound-strength correlation that the manufacturer prints on the hammer is generic — a typical curve for generic concrete, generic aggregate, generic cement. Your project has specific aggregate, specific cement, specific moisture at test time. Without a site-specific calibration (rebound R on a cast cube vs cube compressive strength), the rebound reading is only useful for relative comparison: is column A stronger than column B? It is not good enough for acceptance: does this column meet fck = 30 MPa?

Field practice — what to actually do at site

Worked example — UPV uniformity survey on a slab

Scenario. Suspect low-strength readings on a 28-day slab pour (cube average 26 MPa, target 30 MPa). UPV survey in a 5×5 grid on the slab soffit (direct arrangement, 250 mm path).

Decision: the slab is uniformly "good" per UPV. The low cube strength is a strength issue, not a quality issue — likely caused by high w/c at the plant, not by voids or cracking. Core-test the suspect area to confirm, then accept with mitigation on future pours. The single 1.95 km/s reading is the outlier that triggers a localised core test on that 1 m².

Common deviations seen on Indian sites

Field-checklist blocks

A short questionnaire, an inspector's checklist, and three worked examples that show how the UPV and rebound readings resolve into a decision to investigate, to re-test, or to cut a core.

Questionnaire — asked before the NDT survey starts, once the question is written down

1. What is this survey being asked to answer? UPV gives uniformity and finds voids and cracks. It does not give strength, so the question decides which test you should be running.
2. What is the transducer arrangement, and has the reference bar been run today? Direct, semi-direct and indirect are not interchangeable, and transducers drift.
3. Is the surface dry and smooth? UPV slows 5 to 10% on saturated concrete, and honeycombed or scaled surfaces have to be ground before they are tested.
4. Have the rebars been located with a cover meter before the UPV run? Rebar disturbs the wavefront.
5. If the arrangement is indirect, where is the 5 to 15% correction being applied? Indirect underestimates direct by that much.
6. How many readings per test area? For UPV, at least 9 readings in a grid, discarding the lowest 20% and averaging the rest. For the rebound hammer, at least 9 points in a 300 × 300 mm area.
7. Has the rebound hammer been calibrated against cubes cast from the same concrete? Without that, the rebound number answers "is this pour weaker than the one next to it?" and not "does this member meet f_ck?".
8. If a low-strength suspicion is under investigation, who is cutting the cores? IS 516 Pt 4. NDT flags the suspect area; cores prove the strength.

Inspector's checklist — what to verify on site

1. Reason for the survey written down before the first reading, along with the acceptance question it has to answer.
2. Every UPV reading recorded with its inputs: path length L to the nearest 1 mm and transit time t to the nearest 0.1 microsecond.
3. Transducers zeroed on the reference bar before each shift, and replaced if the zero-point shifts.
4. Surface prepared before testing: ground where honeycombed or scaled, dried where wet, coupling paste used, transducers held firmly against the surface.
5. Grid layout recorded with its spacing and its coverage, as in the slab soffit example where a 5 × 5 grid was run on a 250 mm direct path.
6. Grading applied to every reading and every suspect zone marked: above 4.5 km/s excellent, 3.5 to 4.5 km/s good, 3.0 to 3.5 km/s suspect, below 3.0 km/s doubtful.
7. Rebound readings taken to the rule: at least 9 points in a 300 × 300 mm area, points at least 25 mm apart and at least 50 mm from the edges, 3 readings within a 30 mm circle at each point with the median taken, outliers more than 6 below the median or more than 1.4 times the median discarded, and the remainder averaged.
8. No rebound-to-strength conversion used for acceptance without a site-specific calibration against cubes from the same concrete.

What happens if… three worked examples

Frequently Asked Questions

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