Ultrasonic pulse velocity from path length and transit time, graded per IS 516 (Part 5/Sec 1):2018 Table 1 — plus surface-crack depth by the transit-time method. Apply the instrument zero correction before entering readings.
Direct transmission preferred; note the arrangement used (direct / semi-direct / indirect).
After zero correction from the calibration bar.
Pulse velocity
km/s
Same spacing L for both readings: t₁ on sound concrete, t₂ straddling the crack, transducers equidistant from it.
Estimated crack depth
mm
Velocity = path length ÷ transit time; in mm and microseconds the quotient is directly km/s. Quality grading follows IS 516 (Part 5/Sec 1):2018 Table 1: above 4.5 Excellent, 3.5–4.5 Good, 3.0–3.5 Medium, below 3.0 Doubtful (km/s).
Crack depth uses the transit-time method of BS 1881 Part 203 practice: d = (L/2) × √((t₂/t₁)² − 1) — this formula is not an IS-code provision, and it assumes an open, perpendicular crack in homogeneous concrete.
Velocity: 500 mm direct path, corrected time 105 µs → 500/105 = 4.76 km/s → Excellent.
Crack depth: L = 300 mm, t₁ = 68.2 µs on sound concrete (v = 4.4 km/s), t₂ = 84.9 µs straddling the crack.
d = 150 × √((84.9/68.2)² − 1) = 150 × 0.742 = ≈111 mm
Per IS 516 (Part 5/Sec 1):2018 Table 1:>4.5 km/s Excellent · 3.5–4.5 Good · 3.0–3.5 Medium · <3.0 Doubtful. The grade reflects density and homogeneity — good concrete, well compacted, no honeycombing. It does not read strength directly; correlate with cubes or cores for that.
Broadly correlated, but not reliably convertible — moisture, age, aggregate type and temperature all shift velocity without the same shift in strength. That is why IS 516 (Part 5/Sec 1):2018 grades quality, not grade of concrete. Use UPV for uniformity mapping across a member and spot-check anomalies with cores.
Direct (opposite faces) gives the truest path and is preferred. Semi-direct (adjacent faces) is acceptable; indirect (same surface) under-reads velocity by roughly 5–10% and is mainly used for crack depth and surface-layer surveys. Note the arrangement with every reading.
When the crack is not open (tight or water-filled — no delay to measure, t₂ ≤ t₁), not perpendicular to the surface, when reinforcement lies between the transducers, or when the depth approaches the spacing. Treat the result as an estimate to confirm, not a measurement.
Often used together with this calculator — pick the next step in your design or QA loop.
Deep dive: read the full significance guide — the code clauses, assumptions and site meaning behind these numbers.
Often used together with this calculator — pick the next step in your design or QA loop.