The Schmidt hammer test, also called the rebound hammer test, is one of the fastest ways to check the surface hardness and uniformity of hardened concrete. It takes seconds per reading, leaves the concrete intact, and can cover large areas of slabs, walls, columns, and beams in a single visit.
At Prime Test Engineering, we use the rebound hammer under ASTM C805 to compare concrete quality across a structure, find areas that may be weaker than the rest, and choose the best locations for core sampling. The test does not measure compressive strength directly, but when used correctly and correlated with cores, it is a valuable part of a concrete condition assessment.
What Is a Schmidt Hammer Test?
A Schmidt hammer is a handheld device with a spring-loaded mass inside. When the plunger is pressed against concrete, the spring releases and the mass strikes the plunger. Part of that energy is absorbed by the concrete, and the rest makes the mass bounce back. The distance it rebounds is recorded as the rebound number.
Harder, denser concrete absorbs less energy and gives a higher rebound number. Softer or weaker surface concrete gives a lower number. Modern digital hammers record each reading automatically, which speeds up testing and reporting.
What Does the Rebound Number Tell You?
The rebound number reflects the hardness of the concrete near the surface, generally within the outer inch or so. Because surface hardness is related to strength, rebound numbers can be used to:
- Check uniformity: compare concrete in different areas, pours, or members of the same structure.
- Find suspect areas: flag zones with lower readings that may need closer investigation.
- Target core locations: place cores where they will tell you the most, instead of guessing.
- Estimate in-place strength: only when the rebound numbers are correlated with cores or cylinders from the same concrete.
Can a Rebound Hammer Measure Compressive Strength?
Not by itself. The conversion charts supplied with many hammers are based on general laboratory concrete and may not match the concrete on your project. Aggregate type, mix design, age, and moisture all affect the relationship between rebound number and strength.
For a reliable strength estimate, engineers develop a project-specific correlation by testing a few locations with both the rebound hammer and concrete cores. That correlation can then be applied to the rest of the rebound readings.
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How Is the Schmidt Hammer Test Performed?
ASTM C805 sets out the standard procedure. A typical test follows these steps:
- Select the test areas. Each test area should be at least 6 inches in diameter, on concrete at least 4 inches thick, and away from edges, joints, and honeycombed or cracked areas.
- Prepare the surface. Rough or loose surface material is ground smooth with an abrasive stone. Coatings and plaster are removed so the hammer strikes the concrete itself.
- Take the readings. At least 10 readings are taken in each test area, spaced at least 1 inch apart, with the hammer held perpendicular to the surface.
- Screen the results. Readings that differ from the average by more than 6 units are discarded. If more than two readings are discarded, the whole set is rejected and the area is retested.
- Record the conditions. Hammer direction, surface condition, and any anomalies are noted, since they affect how the results are interpreted.
What Factors Affect Rebound Hammer Results?
The rebound hammer is quick, but its readings are sensitive to conditions near the surface. Factors that can change the result include:
- Surface texture: rough, troweled, or form-finished surfaces read differently.
- Moisture: a wet surface generally gives lower readings than a dry one.
- Carbonation: older concrete with a carbonated surface layer can give higher readings that overstate the strength of the concrete beneath.
- Hammer direction: testing upward, downward, or horizontally changes the reading because of gravity.
- Aggregate type: different aggregates change the relationship between rebound and strength.
- Nearby rebar or voids: a reading directly over a bar near the surface, or over a void, can be misleading.
This is why rebound results should be interpreted by an experienced engineer and compared within the same structure, not against a generic chart.
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Rebound Hammer vs. UPV vs. Core Testing
| Method | What it measures | Key limitation |
|---|---|---|
| Rebound hammer (ASTM C805) | Surface hardness of the outer concrete | Reflects only the near-surface layer; needs correlation for strength |
| Ultrasonic pulse velocity (ASTM C597) | Speed of sound waves through the concrete | Affected by moisture, rebar, and path length; also needs correlation for strength |
| Core testing (ASTM C42) | Actual compressive strength of extracted samples | Partially destructive and limited to the cored locations |
Why Combine Methods?
Using the rebound hammer together with ultrasonic pulse velocity testing gives a fuller picture: one looks at the surface, the other through the depth of the member. A small number of cores then ties both results to real strength values. This combined approach covers more area with less damage to the structure. Read more in our guide to concrete core testing.
When Is the Schmidt Hammer Test Used?
Common applications include:
- Evaluating existing buildings before renovation, tenant improvements, or change of use.
- Checking concrete when cylinder test results are low or missing.
- Comparing concrete quality between pours, floors, or structural members.
- Screening concrete after a fire as part of a fire damage assessment.
- Selecting core locations for a structural condition assessment.
Schmidt Hammer Testing at Prime Test Engineering
Prime Test Engineering provides non-destructive testing and concrete evaluation services throughout Southern California. Depending on the project, a rebound hammer survey may be combined with UPV testing, GPR scanning, coring, and laboratory compressive strength testing, with results reviewed by a licensed professional engineer.
Learn more about our non-destructive testing services and concrete testing services.
Frequently Asked Questions
Is the Schmidt hammer test destructive?
No. The hammer may leave a small mark on the surface, but it does not damage the concrete or affect its performance.
How accurate is a rebound hammer?
It is good for comparing concrete within the same structure. Its accuracy for estimating strength depends on correlation with cores from the same concrete.
What is the difference between a Schmidt hammer and a rebound hammer?
They are the same type of device. Schmidt is the name of the original inventor, and the terms are used interchangeably.
How many readings are needed?
ASTM C805 calls for at least 10 readings per test area, with outlying readings removed before the average is calculated.
Can the rebound hammer be used on new concrete?
It can be used on hardened concrete, but very young or low-strength concrete may give unreliable readings. Engineers consider the age of the concrete when interpreting results.
Does the rebound hammer replace core testing?
No. It reduces the number of cores needed and helps place them well, but cores remain the direct measure of in-place compressive strength.
Conclusion
The Schmidt hammer test is a fast, non-destructive way to check concrete surface hardness and uniformity across a structure. On its own it is a screening tool, not a strength test. When combined with UPV and a small number of cores, it becomes a practical way to evaluate existing concrete with minimal damage.
Contact Prime Test Engineering to discuss rebound hammer testing for your building, parking structure, or concrete repair project.
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