This article builds on What is a UPV test and focuses on the steps "after the results are in": when the report shows values lower than expected, how to read them, how to rule out causes and what to check next, so you can make a real decision without jumping to the conclusion that the structure is damaged.

What UPV Velocity Counts as Low?

A low pulse velocity is one that is significantly lower than other points in the same structural member, not merely lower than a textbook figure, because each concrete mix gives a different baseline velocity. The table below therefore focuses on "what to do next" rather than assigning quality labels.

Pulse velocity (km/s)What to do next
Above 4.5Record as the structure's reference value for comparing other points; no further checks needed
3.5 – 4.5Normal; investigate further only if values are unusually scattered within the same member
3.0 – 3.5Re-measure and add points around the area; if still low, verify with another method
2.0 – 3.0Always verify, e.g. with Impact Echo or by drilling cores (Core Test)
Below 2.0Suspect a serious internal defect; have an engineer assess before continuing to use that area
Read by comparing points, not single numbers

If a column reads 4.2 km/s at every point except one that reads 3.4 km/s, the difference of about 0.8 km/s means more than the figure 3.4 itself, because it indicates that the concrete at that location differs from the rest of the same member.

Is the Value Low Because the Concrete Is Poor, or Because of the Measurement?

Low values can come from the concrete itself or from the way it was measured. Several site factors can push readings off in either direction. This table helps you tell whether a reading has likely been pushed up or pulled down.

FactorDirection of errorHow to handle it
Insufficient couplant or rough surfaceFalsely lowGrind the surface smooth, reapply gel, press the transducers firmly, then re-measure
Plaster or paint over the surfaceFalsely lowChip off the plaster only at the test point to reach the concrete
Transducer spacing measured incorrectlyEither high or lowMeasure the actual distance with a tape measure and record every point; do not use drawing dimensions
Pulse path parallel to reinforcementFalsely highScan for bars with GPR, then avoid them or apply a correction
Saturated concrete or recent rainFalsely highRecord the moisture condition in the report and compare with points in the same condition
Young concrete that has not yet gained full strengthLower than final valueWait until the age specified by the engineer before concluding
Indirect transducer arrangementLower than directDo not directly compare values from different arrangements
Different mix or aggregate typeNot comparableCompare only within the same concrete batch
2D diagram of how to read UPV results: a velocity range scale with what to do next, an example of comparing points within the same column, and six site factors that make values falsely high or low Three-part diagram. The upper left is a five-band pulse velocity scale with the next action for each: above 4.5 kilometers per second, record as a reference value; 3.5 to 4.5 is considered normal; 3.0 to 3.5, re-measure and add points; 2.0 to 3.0, always verify with Impact Echo or a Core Test; and below 2.0, suspect a serious internal defect and have an engineer assess before using the area. The upper right is a column elevation measured at five levels: four levels read about 4.2 kilometers per second and one reads 3.4. The difference of about 0.8 kilometers per second means more than the figure 3.4 itself, because it indicates that the concrete at that location differs from the rest of the same member. The lower part has six small sections of factors that distort the values. The first two make values falsely high: a pulse path parallel to reinforcement, and saturated concrete. The last four make values falsely low: insufficient couplant or a rough surface, measuring through plaster or paint, an indirect transducer arrangement, and young concrete. 2D diagram: reading UPV correctly — scale · point comparison · error factors Not to scale Pulse velocity range (km/s) → next step > 4.5 Record as structure reference Compare others; no extra checks 3.5–4.5 Normal Check if scatter is unusually wide 3.0–3.5 Re-measure + add points around Still low → verify another way 2.0–3.0 Always verify Impact Echo or Core Test < 2.0 Suspect serious defect Engineer assesses before use Ranges are general guidance; always compare within the same member Read by comparing points, not single numbers 4.2 4.2 4.1 3.4 4.2 Diff. ~0.8 3.0 3.5 4.0 4.5 Same column A difference within one member means more than 3.4 itself (km/s · article example) Site factors that distort values ▲ Falsely high ▼ Falsely low Path parallel to bar Faster in steel Saturated or recent rain Low gel, rough face Poor contact Via plaster/paint Chip to concrete Indirect setup Lower than direct Young concrete Not full strength
2D diagram summarizing how to read UPV values: a velocity range scale with the next action, an example of comparing points within the same column (4.2 vs 3.4 km/s), and small sections of site factors that make values falsely high or low

Following ASTM C597 and DPT 1504-51 (Standard for Non-Destructive Inspection of Reinforced Concrete Structures: Ultrasonic Pulse Testing of Concrete, issued by the Department of Public Works and Town & Country Planning), reported values must come with the path length, transducer arrangement and surface condition; otherwise the velocity figure alone cannot be interpreted.

What Should You Check Before Concluding the Concrete Has a Problem?

When you find a low value, go through these 5 steps in order before deciding on further testing.

  1. Calibrate the instrument with a reference bar — both before and after each set of measurements. If the reading on the reference bar is off, the problem lies with the instrument, not the structure.
  2. Re-measure 2–3 times with fresh gel — if the value jumps up after reapplying gel, the cause is the contact, not the concrete.
  3. Move the test point away from the reinforcement — shift the transducers sideways 5–10 cm and re-measure to see whether the value changes significantly.
  4. Check that the path length and arrangement are fully recorded — a 10% error in distance immediately gives a 10% error in velocity.
  5. Expand to a grid around the low point — measure all around to see whether the low value is a single point or covers a wide area, which directly guides the next testing method.
3D model of the re-check steps when UPV results on a concrete wall are low: calibrating with a reference bar, re-measuring, moving test points away from reinforcement, measuring the actual thickness, and expanding to a fine grid around the low point Conceptual 3D view of a reinforced concrete wall on the ground. The front face has a coarse grid of test points in three rows and six columns; green points are normal values and one red point is low. Around that point is an orange dashed frame with a fine five-by-five grid, where clustered red points show the extent of the low-value area. Vertical purple dashed lines mark the reinforcement that test points must avoid. The transmitter is held against the front face and cabled to the UPV unit on the ground, while the receiver cable runs over the top of the wall to the opposite face. On the ground to the left, a calibration reference bar rests on a box, and at the end of the wall a tape measure gives the actual wall thickness. The panel on the right summarizes the five checks before concluding that the concrete has a problem: calibrate with the reference bar before and after measuring; re-measure two to three times with fresh gel; move test points five to ten centimeters away from the reinforcement; check the path length and transducer arrangement; and expand to a grid around the low point. It concludes that if only a spot is low, follow up with Impact Echo or GPR, and if the whole member is low, use a Rebound Hammer together with a Core Test. 3D model: 5 checks on a real wall before calling it low Not to scale Coarse grid, full wall Find the lower points Fine grid at low point One spot or an area Receiver R, far face Cable over wall top Calibration ref. bar Before & after each set UPV unit + transmitter T Fresh gel, repeat 2–3 times Actual thickness L 10% off → 10% velocity error Normal Low Rebar: shift test point 5–10 cm 5 checks before concluding 1 Calibrate with ref. bar Off = instrument problem 2 Repeat with fresh gel Jumps up = poor contact 3 Move away from rebar Shift sideways 5–10 cm 4 Check path + arrangement Direct · semi-direct · indirect 5 Grid around low point Guides the next test Grid result → next step Spot low → Impact Echo or GPR to locate Whole member low → Rebound Hammer + Core Test
3D model of the 5 checks on a real wall: a calibration reference bar, re-measuring with fresh gel, test points shifted away from reinforcement, measuring the actual thickness, and a fine grid around the low point that shows whether the low value is a single spot or an area (conceptual image)

UPV Results Are Genuinely Low — What Should You Test Next?

Once measurement causes have been ruled out and the value is still low, the next step depends on the "distribution pattern of the low values", not on the number itself, as shown in the diagram below. Combining several non-destructive methods to confirm results before concluding is a practice described in the guidance document ACI PRC-228.2 (Report on Nondestructive Test Methods for Evaluation of Concrete in Structures).

Decision flowchart for low UPV test results, classified by the distribution pattern of the low values and the follow-up verification method It starts with a UPV value lower than the reference points in the same member. The first step is to re-measure and fix the contact or couplant. If the value is no longer low, the cause was the measurement and the new reading is taken as the actual result. If it is still low, classify the distribution of low values on the test point map into three patterns. Pattern one, low at isolated points: suspect a local void or delamination; use Impact Echo or GPR to locate it, then drill a Core Test to confirm at that spot only, per ASTM C1383 and ASTM C42. Pattern two, low across the whole member: suspect that concrete strength is below the requirement; use a Rebound Hammer in combination (SonReb), then drill a Core Test to confirm strength, per ASTM C805 and ASTM C42. Pattern three, low only around a crack: the wave detours around the crack, lengthening its path; measure crack depth indirectly and monitor whether the crack widens, per ASTM C597 indirect. Every pattern ends with a licensed civil engineer summarizing the combined results of all methods and proposing repair or strengthening. Note: UPV is not a direct substitute for concrete compressive strength; if load-carrying capacity is affected, it must be confirmed with a Core Test. Low UPV: check the pattern, then choose verification Not to scale UPV lower than reference points in same member Re-measured + contact (couplant) fixed Is the value still low? No Cause was measurement Use the new reading Yes — check low pattern on point map Low at some points Suspect void / local delam. Impact Echo or GPR to locate Core Test at that spot ASTM C1383 · C42 Whole member low Suspect strength below spec Plus Rebound Hammer (SonReb) Core Test confirms strength ASTM C805 · C42 Low around crack Wave detours crack, longer path Indirect crack depth test Monitor crack growth ASTM C597 (indirect) Licensed civil engineer sums up all methods and proposes repair or strengthening Note: UPV does not directly replace concrete compressive strength — if load capacity is affected, confirm with a Core Test Normal Low (point map)
Decision flowchart for low UPV test results — the distribution pattern of the low values determines which verification method to use next

Low at isolated points usually indicates a local defect, such as a void from incomplete placement or insufficient vibration. Use Impact Echo (per ASTM C1383) or GPR to locate the defect and its depth first, then drill to confirm only where necessary, to minimize the number of holes in the structure.

Low across the whole member points to the quality of that concrete batch as a whole. In this case, take Rebound Hammer readings as well, then drill cores for compressive strength testing per ASTM C42, because estimating strength from pulse velocity alone is not sufficient for structural decisions.

Low only around a crack is normal behavior for a wave that has to detour around the crack. It can be used to estimate crack depth, but must be paired with monitoring whether the crack is widening before concluding that repair is needed.

Fire-damaged structures

Concrete exposed to high heat gives clearly lower velocities in proportion to the depth of damage. Comparing fire-exposed and unexposed areas helps assess the extent of damage, but the residual compressive strength must always be confirmed with cores.

What Must a UPV Report Contain to Support Decisions?

A report with only a table of velocity figures often cannot support the next decision. A usable report should contain at least these 7 items.

  • A test point layout with numbers, clearly locatable on the structural drawings
  • The actual transducer spacing at each point and the arrangement (direct / semi-direct / indirect)
  • The raw pulse transit time (µs), not just the calculated velocity
  • Reference bar calibration results before and after measurement
  • Surface and moisture condition during testing, including the reinforcement locations avoided
  • Comparisons within the same member, not only against a standard table
  • Conclusions and recommendations certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number

SPN reports include all of the above and can be used for submissions to the relevant authorities. See all services under Non-destructive structural evaluation (NDE)

5 Common Mistakes When Interpreting UPV Results

  1. Comparing values with a quality table straight away without looking at what other points in the same member read, so concrete whose mix naturally gives lower velocities gets judged as defective.
  2. Converting velocity to compressive strength with an off-the-shelf formula that has not been calibrated for that concrete batch. The resulting figure can be so far off that it is useless for decisions.
  3. Not recording the transducer arrangement and then comparing values from an indirect arrangement with a direct one, which naturally gives lower values.
  4. Assuming high values are always good, even though the pulse path may run parallel to reinforcement or the concrete may be saturated, masking internal defects.
  5. Concluding without verification when values are very low. UPV can tell you that something is abnormal, but not what it is; it must be confirmed with another NDT method or a Core Test.

If the test results suggest that concrete strength may be below the requirement, the decision steps that follow are covered in Concrete compressive strength fails: what to do. Testing fees depend on the number of points and site conditions; contact us for a free quote within 24 hrs. Prices exclude travel and equipment mobilization, which are charged extra according to the distance to the site.