Step 01
Prepare surfaces
Test areas are ground smooth to remove loose material, render and carbonated surface layer, since any of these will distort the rebound.
Schmidt Hammer Test
The fastest way to compare concrete across a structure. It is a surface hardness measurement, not a strength test, and it earns its place when you read it as a map rather than as a number.
Schmidt Hammer Test: what it actually tells you
A spring loaded mass is released against the concrete surface and the distance it rebounds is recorded. Harder, stiffer surface concrete returns a higher rebound number. Converting that number to a compressive strength relies on a correlation curve, and the generic curve supplied with the instrument can be out by a wide margin on any particular concrete.
Where the method is genuinely strong is comparison. Take readings on a grid across a slab, a set of columns or a whole floor and the variation stands out immediately: the column that was poorly compacted, the bay that was cured badly, the area affected by a cold joint. That map is reliable even when the absolute numbers are not.
To get a defensible strength, the hammer is calibrated against cores taken from the same structure. A handful of cores establishes the site specific correlation, and the hammer then extends that correlation across hundreds of locations at a fraction of the cost. Used that way it is a serious assessment tool; used alone as a strength meter it is not.
At a glance
- Readings per location10 to 12, median reported
- Surface preparationGround smooth, carbonated layer removed
- Depth assessedSurface zone only, roughly 30 mm
- CorrelationSite specific cores strongly recommended
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Lay out the grid
A grid of test points is set on each member, spaced at least 25 mm apart and clear of reinforcement located by cover meter.
Step 03
Take readings
Ten to twelve readings are taken per location, with the hammer held perpendicular to the surface and the impact direction recorded for correction.
Step 04
Process
Outliers are discarded per the standard and the median rebound number is taken, corrected for the angle of the hammer.
Step 05
Correlate
Where a strength value is required, cores are taken at selected points and a site specific correlation is established and applied.
What you receive
- Rebound number map across the member or structure
- Relative uniformity and identification of weak zones
- Estimated compressive strength where a correlation exists
- Site specific correlation curve when paired with cores
- A targeted list of locations worth coring
When this test is the right one
- Rapid screening of an existing structure
- Comparing uniformity between pours, columns or floors
- Locating weak or poorly compacted zones before coring
- Condition survey ahead of a change of use or added load
- Where coring must be kept to a minimum
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
ASTM C805 rebound number of hardened concreteBS EN 12504 2 testing concrete in structures, rebound numberASTM C42 obtaining and testing drilled cores for correlation
Common questions
What engineers most often ask us about Schmidt hammer test.
Using the generic curve, the honest answer is plus or minus 25 percent or worse. Calibrated against cores from the same concrete, it tightens considerably. Treat an uncalibrated rebound strength as an indication, never as an acceptance figure.
Because the hammer only reads about the top 30 millimetres. A carbonated, wet, rendered or rough surface changes the rebound independently of the concrete underneath, which is why proper grinding of the test area is not optional.
Yes, but with care. Old concrete carbonates at the surface and reads harder than it is, which flatters the result. On structures more than a few years old the correlation must come from cores taken on that structure.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
Concrete Testing
Concrete is specified by a strength it will not reach for 28 days, on a site where it is placed in minutes.
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Reinforcement Bar Scanning
Find the steel before you cut, drill or core, and check that the cover protecting it is what the drawing said it would...
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Pile Integrity Test
A cast in place pile is concreted into ground nobody can see, and its defects are invisible the moment it is complete.
Read moreNeed Schmidt hammer test on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.