The Schmidt Hammer — also known as a rebound hammer or concrete test hammer — is one of the most widely used tools for non-destructive concrete strength assessment on site. It's fast, portable, and gives you immediate results without cutting cores or sending samples to a laboratory.
But getting reliable results requires more than simply pressing it against a concrete surface. Used incorrectly, a Schmidt Hammer will give you readings that are inconsistent, unreliable, and potentially dangerous if they inform a structural decision. This guide covers the correct test method from start to finish.
What is a Schmidt Hammer and how does it work?
A Schmidt Hammer works by firing a spring-loaded plunger against a concrete surface. The plunger rebounds, and the distance of that rebound — expressed as a Rebound Number (R value) — is displayed on the instrument's scale. Higher rebound values generally indicate harder, stronger concrete.
The principle is simple: strong, dense concrete resists the impact and returns more energy. Weak, porous, or deteriorated concrete absorbs it. By comparing your R values against calibration curves (typically supplied with the hammer or available in BS EN 12504-2), you can estimate compressive strength.
It's worth noting that the Schmidt Hammer does not directly measure compressive strength — it measures surface hardness and infers strength from it. That distinction matters when interpreting results.
When should you use a Schmidt Hammer?
The Schmidt Hammer is suitable for a range of applications, including:
- Preliminary assessment of in-situ concrete strength before coring
- Checking uniformity of concrete across a structure
- Identifying zones of potential weakness or deterioration
- Monitoring concrete maturity during construction
- Post-fire or post-impact damage assessment
It is not a substitute for core testing where structural decisions depend on accurate strength data. Use it to guide where cores should be taken, or to screen large areas quickly.
Preparing for the test
1. Select the test surface carefully
The surface must be:
- Smooth, dry, and free from surface coatings, laitance, or loose material
- Representative of the concrete being assessed — avoid honeycombing, cracks, or edges
- At least 100 mm from any edge or corner
- Structurally connected to the main body of concrete (not an isolated patch repair)
If the surface is rough or has a weak skin, grind it smooth with an abrasive stone before testing. Do not test wet concrete — moisture significantly lowers rebound values and will underestimate strength.
2. Calibrate the instrument before use
Before starting, verify the hammer is performing correctly using a calibration anvil (also called a test anvil). Strike the anvil at least five times and check that the average rebound value falls within the manufacturer's specified range — typically around 80 ±2 for a Type N hammer.
If readings are outside tolerance, the hammer requires servicing before use. Do not adjust results to compensate for a faulty instrument.
3. Mark out your test grid
Mark a grid of test points on the surface, spaced at least 25 mm apart. This prevents one impact from influencing the next by pre-compressing the concrete locally.
Taking readings — the correct technique
Hold the Schmidt Hammer firmly and perpendicular to the test surface. The hammer can be used horizontally, vertically (upward or downward), or at an angle — but the orientation affects rebound values, so you must record it and apply the appropriate correction factor from the calibration chart.
Apply steady, gradual pressure until the plunger releases. Do not jab or strike — a controlled, smooth action is essential. Read and record the Rebound Number displayed immediately after impact.
Take a minimum of 10 readings per test area. According to EN 12504-2, any individual reading that differs from the median by more than 6 units should be discarded. If more than two readings need discarding, discard the entire set and retest.
Processing your results
Once you have your valid readings:
- Calculate the average Rebound Number from the accepted readings
- Apply any correction for hammer angle if not tested horizontally
- Plot the corrected average on the appropriate conversion curve (specific to your hammer model and concrete type)
- Read off the estimated compressive strength in N/mm²
Be aware that different conversion curves exist for different cement types and aggregate types. Using the wrong curve can introduce significant error. Where possible, establish a site-specific correlation by testing cores cut from the same concrete and comparing results.
Common mistakes that affect accuracy
Several factors consistently cause unreliable results in the field:
- Testing near edges — concrete is less constrained at edges, giving falsely low readings
- Testing over reinforcement — steel bars close to the surface increase rebound values and overestimate strength
- Wet surface — moisture depresses rebound values by up to 20%
- Not discarding outliers — a single anomalous reading skews the average significantly
- Using a worn or uncalibrated hammer — spring fatigue and plunger wear both affect results
- Ignoring carbonation — carbonated concrete surfaces give falsely high rebound values that do not reflect the strength of the bulk concrete
Recording and reporting
Good site records are essential. For each test location, record:
- Date, time, and weather conditions
- Location on structure (with sketch or photo reference)
- Hammer type and serial number
- Calibration anvil reading before testing
- Hammer orientation
- All individual rebound readings (including those discarded)
- Average Rebound Number and estimated compressive strength
- Any notable observations (surface condition, visible damage, etc.)
If your results will be used in a formal assessment report, reference the test standard (BS EN 12504-2) and note any deviations from the standard method.
Summary
The Schmidt Hammer is a powerful site tool when used correctly — but its accuracy depends entirely on preparation, technique, and careful interpretation. Take the time to calibrate before use, prepare the surface properly, follow the standard method for number of readings and outlier rejection, and apply the right conversion curve for your concrete. Done right, it gives you fast, reliable data that can inform real decisions on site.
Looking for a Schmidt Hammer?
Geo Site Supply stocks a range of Schmidt Hammers and rebound hammer accessories for site investigation and concrete testing. Whether you need a standard Type N hammer for structural assessment or accessories like calibration anvils and carrying cases, we have what you need.
Browse our concrete testing equipment or get in touch if you need advice on the right tool for your project.