Round-robin tests of concrete testing laboratories in 2023

Round-robin tests have been organized annually for accredited concrete testing laboratories. In 2023, compressive strength and protective pore analysis with thin and polished section were tested. Totally 15 testing units participated in the compressive strength testing and 8 testing units in the pore analyses.

In the concrete strength testing, ready-mix concrete having the strength class of C40/50 having the maximum aggregate size of 18 mm and slump class S2 was used. The average compressive strength of totally 90 test specimens was 48,6 MPa (cylinder strength) and the standard deviation was 1,26 MPa. The coefficient of variation was 2,6%. The lowest individual test result was 43,9 MPa and the highest 52,3 MPa. Two laboratories were needed to test 8 days later as required and those results slightly differed from the other results. Generally, the variation was small, and it was on the same level as in the previous years. The average density of the test specimens was 2392 kg/m³ and the standard deviation was 8,1 kg/m³. The variation was low and slightly lower compared to the previous year.

The thin and polished section analyses divided into three parts. Pair of thin sections circulated in seven laboratories (totally 9 analyses were carried out). In addition, two concrete samples were sent to the laboratories. Finally 2 thin sections and 2 polished sections were tested in two another laboratories. Generally, the variations were high. Typically the standard variation was app. 0,040 mm and the high variation was caused by some different results. Recommendations were given to reduce the variation of the test results.

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