Issue 50

S. Gavela et alii, Frattura ed Integrità Strutturale, 50 (2019) 383-394; DOI: 10.3221/IGF-ESIS.50.32 385 Figure 1 : Cause-and-effect diagram on how uncertainty factors contribute to the final result of a test according to EN 12390-3. S COPE OF THE EXPERIMENTS Scope of experiment I actors like equipment accuracy, testing method selection, e.g. the EN 12390-3 method, measurement conditions and equipment handler’s skills should be considered as inherent to the operation characteristics of the laboratory. Their reproducibility is a matter under control of the interested laboratory. A simple reproducibility experiment, in which the same equipment is used for all the needed measurements, provides adequately the contribution of all these parameters to the laboratory’s uncertainty estimation, as the specific laboratory will continually use the specific equipment, under the specific measurement conditions, by the same handlers. However, a simple reproducibility experiment cannot incorporate the effect of parameters, that have to do with the specimen’s characteristics (which define the concrete com- position under consideration), especially when the specimen is not prepared by the laboratory. Such parameters are all those related to the mix composition and to the definitional uncertainty produced by deviations from the definition given for the specimen in EN 12390-1, especially its geometrical characteristics. In experiment I of this study, the experimental design incorporated different levels of cement content (CC) and water to cement ratio (W/C), which is adequate in order to produce the majority of the corresponding combinations of mix constitu- ents that the laboratory could encounter when receiving specimens of a specific combination of types of concrete constitu- ents. By fixing the type of the materials for the proposed type A uncertainty estimation makes this protocol materials’ type specific. This is not considered to reduce its effectiveness, especially when a laboratory receives specimens from a specific producer on an ongoing basis. In real circumstances of laboratory operation, small deviations of maximum three days from the nominal curing age of performing the compressive strength test may occur for any reason. A stratification of such deviations was also included in the design of the experiment in order to produce such an uncertainty factor. Also, a large number (about 30) of different molds, of three different construction designs, where used randomly for the preparation of the specimens, providing varying geometries, representative to the corresponding variation that a laboratory could encounter when receiving any specimen from any external customer. The aim is to propose a method for determining the uncertainty for an EN 12390-3 concrete compressive strength test result, the experimental part being extended, but only in the case where the test method is applied for a specific nominal value of curing age. The method is formulated in such a way that it is reproducible and exploited by any laboratory that applies this test, especially if it seeks accreditation in accordance with ISO / IEC 17025. This protocol of the proposed method for a type A uncertainty estimation is designed in such a way that it leads to: (a) the optimum economy of used specimens, (b) the aggregation of covariation among all the parameters that vary due to the experimental design, and (c) a (safe-side) maximal estimation of the uncertainty budget, something that is, also, supported by ISO in the GUM approach. F