In English | ISSUE 1/2025
Round-robin tests 2024
In Finland, Round-robin tests have been organized annually for accredited concrete testing laboratories. In 2024, compressive strength and slab test for salt freeze-thaw resistance were tested. Totally 15 testing units participated in the compressive strength testing and 8 testing units in the slab test.
In the compressive strength testing, readymix concrete having the strength class of C30/37, the maximum aggregate size of 18 mm and slump class S3 was used. Three individual test results were rejected because of clearly erroneous results. The average compressive strength was 34,6 MPa (cylinder strength) and the standard deviation 1,28 MPa. The coefficient of variation was 3,7 %. Based on the results it was estimated that the margin of the error with C30/37 would be app. ± 1,3 MPa. The average density of the test specimens was 2355 kg/m3 and the standard deviation 7,9 kg/m3. Generally, both the variations of compressive strength and density can be considered low.
The slab test was carried out according to CEN/TS 12390-9:2016. Strength class of C35/45 was used with effective w/c-ratio of 0,468 and air content of 4,6 %. The cement type was CEM I 52,5 N. Large variation was observed between the testing units. The scaling values after 56 cycles varied between 23 and 513 g/m2 and the average value was 220 g/m2. Standard deviation of the individual results was 178 g/m2 and the respective coefficient of variation 81%. The variation within the testing units was reasonable, but there were large variations between the testing units. It was estimated that the margin of the error of slab test with the particular concrete would be app. ± 200 g/m2. Actions are needed to reduce the variation in the slab test, otherwise the slab test won’t be a useful test method for testing of salt freezethaw resistance of concrete.
