Round-robin tests 2025

In Finland, Round-robin tests have been organized annually for accredited concrete testing laboratories. In 2025, compressive strength testing was investigated both with laboratory specimens (150 mm cylinders) and drilled core specimens. The same concrete was used for the both test series, the strength class was C35/45. Totally 12 testing units were participated in the test with laboratory specimens and 11 units with drilled specimens.

With the laboratory test specimens, the average compressive strength was 41,1 MPa (cylinder strength) and the standard deviation 1,08 MPa. The coefficient of variation was 2,6 %. Based on the results it was estimated that the margin of the error with C35/45 with six specimens would be app. ± 0,8 MPa. The average density of the test specimens was 2372 kg/m3 and the standard deviation 9,7 kg/m3. Generally in case of laboratory specimens, both the variations of compressive strength and density can be considered low.

The drilled specimens showed clearly higher variation compared to the laboratory specimens. The average compressive strength was 45,5 MPa (cube strength) and the standard deviation 3,78 MPa. The average density was 2372 kg/m3 and the standard deviation 21,6 kg/m3. The coefficient of variation of density was 0,9 %. The average values (six specimens) of the test units varied app. ± 6 MPa. The single compressive strength results varied between 36,3 and 51,5 MPa. The variation in the test results is considered high and new roundrobin tests with drilled core specimems will be needed.

Read the entire article in Finnish and see the images >>