The accuracy of a compression test does not depend solely on the performance of your concrete press. In fact, poor preparation of the test specimen can lead to an artificial drop in the measured strength of more than 20%.
Here are the three critical steps to ensure reliable results that comply with standard NF EN 12390-3.
1. Surface grinding: a top priority
In order for the load of the press to be distributed perfectly evenly, the upper and lower surfaces of the test piece must be flat and parallel. If the surface is curved or has depressions, stress concentrations will occur, causing premature failure.
There are two main methods used:
- Sulfur surfacing: A mixture of sulfur and molten quartz sand is applied to the faces. This is the traditional method, which is quick and effective.
- Sand surfacing (sandboxes): A clean alternative that uses calibrated sand confined in metal cups.
- Mechanical grinding: Use of a diamond grinding machine to achieve perfect flatness (often reserved for high-volume laboratories).
2. Centering in the concrete press
A centering error of just a few millimeters creates an eccentricity in the load. This generates a bending moment in the test piece, which completely distorts the pressure gauge reading.
CTRMAT tip: Use the centering circles engraved on the bottom plate of your presse 2000 kNAlso ensure that the trays are clean and free of debris from the previous test.
3. Loading speed: don't be too impatient!
The standard requires a constant pressure rise rate (generally 0.6 MPa/s).
- Too fast: The resistance displayed will be overestimated.
- Too slow: Concrete may fatigue prematurely.
On a manual press like the CTR039E1The operator must maintain a steady rhythm on the hydraulic pump lever while monitoring the pressure gauge needle.
Summary: Checklist before “crashing”
- Was the test tube kept in a wet cure until the last moment? (Dry concrete is more fragile).
- Are the faces perfectly flat (ruler test)?
- Is the test tube centered on the lower tray?
- Is the pressure gauge reset to zero (pointer)?
Need to equip your lab? Find all our surfacing accessories and our concrete presses on CTRMAT.fr.