## 4 Discussion
For the other two types of cementitious adhesives, i.e., C2 TES1 and C2T, respectively, 14% and 12% of products not meeting the criterion of adhesion of not less than 1.0 N/mm\({}^{2}\) were observed. In the case of adhesion measurements after freeze-thaw cycles, the share of products not meeting the adhesion requirement of at least 1.0 N/mm\({}^{2}\) was 36% for C2 TES1 type, 25% for C2 adhesive, 12% C2T and only 4% for C2 TE type. For C2T and C2 TES1 cementitious adhesives, all tested samples met the initial tensile adhesion strength requirements. However, for C2 and C2 TE adhesives, the share of products that fit the requirements for initial adhesion was 88% and 78%, respectively. Figure 10 shows the results of the analysis described in this paper. For each of the analyzed types of cementitious adhesive for ceramic tiles, the average value of the results qualified by notified laboratories and construction supervision authorities as meeting the threshold criterion was given, along with the mean square error of the measurements. The analogically calculated values were given for the results classified by notified laboratories and construction supervision authorities as not meeting the threshold value. Figure 9: Percentage of cementitious adhesives for ceramic tiles type C2, C2T, C2 TE, and C2 TES1, which met/did not meet the criterion of tensile adhesion strength not lower than 1.0 N/mm\({}^{2}\) in various conditioning conditions before the measurement. As shown in Figure 10, in several cases (initial tensile adhesion strength for C1, C1T, and C2 TE adhesives, tensile adhesion strength after thermal ageing for C1T and C2 TE adhesives, and tensile adhesion strength after water immersion for C2 adhesive), taking into account the measurements' mean uncertainty would result in a different classification. The same is visible in the figures presenting the test results, e.g., in Figure 2, sample no. 4 (after thermal ageing), and sample no. 28 (after freeze-thaw cycles), in Figure 4, sample no. 4 (after thermal ageing and freeze-thaw cycles), and sample no. 5 (initial tensile adhesion strength), in Figure 6 samples no. 7 and 25 (initial tensile adhesion strength), and sample 22 (after immersion in water). Some of the tested products found to be non-compliant would become compliant products. On the other hand, one can observe a vast number of measurements that are equal to the threshold level with the 20% uncertainty. This means that with a certain probability they can or cannot meet the criteria of the EN 12004 standard. Due to the complexity of the tensile adhesion strength measurements, the influence of the concrete slab, ceramic tiles, or even the water used for tests, neglecting the uncertainty of measurements may distort the real picture. The analysis of the results of adhesion tests of 129 cementitious adhesives for ceramic tiles commissioned by the construction supervision authorities in 2016-2019 showed that:  
* in the case of adhesives for which the compliance criterion is the adhesion value of at least 0.5 N/mm\({}^{2}\), most products did not meet the requirements for adhesion after thermal ageing. For C1 adhesives, 57% did not meet the criteria, slightly less than 50% for C1T adhesives, and 25% for C1 TE adhesives,
* all tested C1 TE adhesives met the initial tensile adhesion strength requirement,
* all tested C1 adhesives met the threshold requirement of 0.5 N/mm\({}^{2}\) in the case of testing the adhesion after freeze-thaw cycles,
* all C2T and C2 TES1 adhesives met the threshold requirement of 1.0 N/mm\({}^{2}\) for the initial tensile adhesion determination,  
Figure 10: The average value of the tensile adhesion strength results at different conditioning conditions obtained for cementitious adhesives qualified by notified laboratories and construction supervision authorities as meeting/not meeting the threshold criterion (marked by the dashed line), together with the mean square error of measurements.