What Is the Schmidt Hammer?
The Schmidt Hammer (also called a rebound hammer) is a simple, portable, battery-free device that estimates concrete compressive strength by measuring how far a spring-loaded mass rebounds off the concrete surface. Fire it at a concrete wall, column, or slab, and in seconds you know whether the concrete is likely 20 MPa, 30 MPa, or 40 MPa strong.
It's the workhorse of non-destructive testing in Kenya. Contractors use it on active construction sites. Engineers use it to assess old buildings. Building regulators use it to verify concrete quality. It's fast, cheap, and gives answers without stopping work or damaging structures.
Oville Associates operates calibrated Schmidt Hammer equipment and combines on-site testing with professional interpretation of results—not just raw numbers, but what they mean for your building's safety and compliance.
How the Schmidt Hammer Works
The mechanism is beautifully simple:
- Press the Schmidt Hammer against a concrete surface
- A spring-loaded plunger fires a hardened mass at the concrete
- The mass rebounds
- A sliding scale captures the rebound distance (the "rebound number," typically 10–100)
- You look up the rebound number on a calibration curve to get an estimated strength in MPa
The principle: Higher-strength concrete is harder and denser, so the mass rebounds farther. Lower-strength or porous concrete absorbs more energy and rebounds less.
Time per test: 10 seconds. You can test 30–50 locations in an hour.
What the Schmidt Hammer Measures
The Schmidt Hammer measures surface hardness, which correlates to compressive strength. The correlation is captured in BS EN 12504-2 (the British/European standard for concrete testing) and similar international standards.
What you get:
- A rebound number (0–100 scale)
- Estimated compressive strength in MPa (e.g., "28 MPa ± 5 MPa")
- Strength category (below spec, meets spec, exceeds spec)
Accuracy: Typically ±10–20% of true strength (compared to lab testing). Good enough for screening and quality control, but not as definitive as crushing a cube.
When to Use Schmidt Hammer Testing
1. Quality Control During Construction
A contractor pours a 1000 m³ concrete slab. By 7 days, they want to know if it's developing strength correctly. Schmidt Hammer testing at 10–20 locations takes 2 hours, costs KES 20,000–50,000, and gives a go/no-go decision. If strength is marginal, they can send 3 cores to the lab before the next pour.
2. Post-Fire Assessment
Fire weakens concrete. After a fire, regulators need to know: Is the structure still safe? Schmidt Hammer testing quickly shows areas of severe damage (low rebound numbers) versus minimal damage. It informs repair decisions and insurance assessments.
3. Age Assessment of Existing Buildings
Older concrete often has higher strength than assumed. Testing tells you the actual strength, not the design strength (which may be outdated or unknown). Useful when renovating, extending, or certifying the safety of heritage buildings.
4. Rapid Screening of Multiple Locations
You have a 15-storey building with durability concerns. Testing every floor with cores would take weeks and cost millions. Schmidt Hammer screening (100 locations, ±100 hours) identifies problem zones. Then you core just those zones for confirmation.
5. Concrete Repair Verification
After repair patching or overlay application, Schmidt Hammer confirms that the repair material has achieved expected strength and bond quality.
Limitations of Schmidt Hammer Testing
Surface Carbonation
Atmospheric CO₂ penetrates concrete over decades, making the surface harder without changing internal strength. In old Nairobi buildings exposed to weather, carbonation can inflate rebound readings by 20–30%, overestimating true strength.
Solution: Our engineers account for carbonation. If we suspect it, we take a small sample to check depth and adjust interpretation accordingly.
Moisture Content
Wet concrete rebounds less than dry concrete of the same strength. Fresh concrete (before 7 days) gives unreliable results. Test after 28 days on dry or moderately damp concrete for best accuracy.
Aggregate Type and Size
Large aggregate (20–25 mm) or weak aggregate can reduce rebound. The hammer may hit a void or soft stone, giving falsely low readings. Multiple readings per location (typically 6–10) reduce this noise.
Surface Condition
Very rough, pitted, or painted concrete gives poor results. Our engineers smooth the surface slightly if needed (light grinding) to get reliable contact.
Depth Limitation
Schmidt Hammer only measures surface hardness (top 5–10 mm). It doesn't reveal what's happening 50 mm deep. For full-depth assessment, combine with ultrasonic testing or coring.
BS EN 12504-2: The Standard
BS EN 12504-2 is the European standard for concrete strength testing, including rebound hammer methods. Key requirements:
- Hammer must be calibrated before use (test anvil provided with hammer)
- Minimum 10 readings per area (to average out local variation)
- Discard the highest and lowest reading (outliers)
- Use calibration curves specific to concrete aggregate type if possible
- Report mean and standard deviation
- Note surface condition, moisture, age of concrete
Our engineers follow BS EN 12504-2 rigorously, ensuring your results meet international standards and are defensible in disputes.
Interpreting Schmidt Hammer Results
| Rebound Number | Estimated Strength (MPa) | Interpretation |
|---|---|---|
| 15–25 | 10–15 MPa | Very low. Possible poor materials or curing. |
| 25–35 | 15–25 MPa | Low. Below typical design spec (C25 minimum). Investigate. |
| 35–45 | 25–35 MPa | Typical for C25–C30 mix designs. Meets spec. |
| 45–55 | 35–45 MPa | Good. Typical for C40 mix designs or mature C30. |
| 55+ | 45+ MPa | Excellent. Likely high-strength mix (C50+) or very mature concrete. |
Schmidt Hammer vs. Core Sampling (Destructive Testing)
When do you use Schmidt Hammer alone, and when do you add cores?
Schmidt Hammer alone: Routine quality control, rapid screening, post-fire damage assessment, when destructive testing isn't justified financially or practically.
Schmidt Hammer + cores: Critical structures, marginal results, disputes over concrete quality, when regulatory compliance requires definitive lab-tested strength.
Best practice in Kenya: Use Schmidt Hammer to screen and identify suspicious zones. If results are borderline (e.g., average 32 MPa on a C30 spec), take 2–3 cores from those zones and test in an accredited lab. This hybrid approach balances cost and certainty.
Schmidt Hammer Testing Cost and Timeline
Price per location: KES 2,000–5,000, depending on site accessibility and number of readings.
Small survey (10–20 locations): KES 30,000–80,000, including report.
Full building assessment (100+ locations): KES 200,000–350,000, typically delivered in 3–5 days on-site plus 2–3 days for analysis and reporting.
Timeline: Testing is fast. Analysis and reporting take longer—our engineers don't just report numbers, but interpret them in context of your building's design, construction method, and intended use.
Oville's Schmidt Hammer Service
We bring three things to Schmidt Hammer testing beyond the device:
1. Calibration and technique: Every hammer is calibrated before use. Our engineers follow BS EN 12504-2 rigorously. Multiple readings per location, proper positioning, documented surface conditions—the details matter.
2. Contextual interpretation: We don't just give you numbers. We compare results to design spec, construction era, aggregate type, exposure conditions, and any visible damage. We tell you what the numbers mean: "Meets spec," "Below spec—investigate further," "Consistent with age and materials."
3. Recommendations: Do you need cores? Repair? Documentation for regulatory compliance? We advise based on your situation.
Schmidt Hammer in Nairobi Construction Practice
Nairobi's building code requires concrete to meet strength specs (typically C30 minimum for columns, C25 for slabs). Schmidt Hammer has become the primary quick-check method:
- Contractors screen every floor after curing
- Engineers flag low zones for coring or re-testing
- Clients request testing before payment, ensuring quality
- Regulators trust Schmidt Hammer results for compliance sign-offs
The tool has democratized concrete quality assessment. Ten years ago, you'd wait weeks for lab results. Now you get answers on-site.
Combining Schmidt Hammer with Other NDT
Schmidt Hammer + Ferroscan: Know concrete strength AND rebar location. Ideal for comprehensive structural assessment.
Schmidt Hammer + ultrasonic: Strength estimate + internal cracking detection. Useful for investigating damage.
Schmidt Hammer + half-cell potential: Strength + corrosion risk. Vital for durability assessment in older buildings.
A holistic approach to concrete diagnostics: First screen with Schmidt Hammer and visual inspection, then deploy specialized tools (Ferroscan, ultrasonic) as needed.