Issue 50

S. Gavela et alii, Frattura ed Integrità Strutturale, 50 (2019) 383-394; DOI: 10.3221/IGF-ESIS.50.32 384 shall be provided according to a predetermined decision rule. Most of the attempts to determine the uncertainty budget for a concrete compressive strength test according to European Standard EN 12390-3 are based mainly on Type B estimations, i.e., on external laboratory sources or on reasonable assumptions according to scientific knowledge. Type B estimation of uncertainty factors is simple and fast and provides a solution in cases where it is not feasible to perform an experimental (statistical) uncertainty investigation (a type A estimation). This type of assessment leads to a number of disadvantages, such as the inability to assess the degree of positive or negative correlation between the various parameters that contribute to the result of the measurement. In this study, firstly the parameters of uncertainty during an EN 12390-3:2009 testing for specimens prepared according to EN 12390-1: 2012 are analyzed using an Ishikawa (cause and effect) diagram. Two experiments were performed for further statistical investigation of the interacting behavior of the most essential parameters presented in the Ishikawa diagram. The first experiment is also proposed as a procedure for a laboratory performing the EN 12390 test method in order to reveal its type A estimation for the most essential corresponding uncertainty parameters when a specified nominal curing age is targeted. Within this framework it is essential to perform a sensitivity analysis on the effect of the specimen’s curing age. To do so, a semi-empirical model, was estimated within the second experiment for the correlation of concrete compressive strength as a function of the parameters of specimen’s curing age and W/C ratio. The proposed procedure is described in a way to be exploitable by any laboratory, especially in the case of seeking accreditation according to ISO/IEC 17025. The semi-empirical model is expected to be useful, also, for accredited testing laboratories in order to perform their internal quality control program. U NCERTAINTY PARAMETERS IDENTIFICATION AND ANALYSIS he characteristics of the specimen that is subjected to a compressive strength measurement according to European Standard EN 12390-3 are subject to a definition detailed in EN 12390-1. Any deviation from the technical specifica- tions of the specimen, i.e. in terms of its geometrical and other characteristics, also leads to a measurement error. This does not affect directly the numerical quantity resulting from the application of the compressive strength test method if a proper testing apparatus is used. It occurs indirectly, as any measurement result is attributed to a theoretically perfect cube with edges that are all of equal length, e.g. exactly 15cm. This is something that cannot be achieved perfectly, in practice. By testing as many specimens of the same characteristics as possible, is expected to yield measurement results dispersion that is affected by these definitional deviations. Such errors contribute to the overall uncertainty of the test result inherently. They are impossible to be eliminated, but it is possible to be minimized [1] by improving the preparation conditions of the specimens (e.g. by improving the manufacturing quality of the moulds being used by the laboratory). The category of factors contributing, due to definitional errors, to the increase of the combined measurement uncertainty include, also, those associated with the definition of the aimed concrete composition for the specimen prepared, and of course, of the concrete used in the corresponding construction. The values for the mix proportions of the concrete con- stituents, usually expressed in kg/m 3 , and the particle size of the aggregates used are two of these factors. In particular, however, the ratio of the amount of water to the amount of cement used in the concrete mix has already emerged from the late 19th century as essential for concrete compression strength test results. Mathematical models that value this relation- ship for a given specimen curing age have been early proposed [2,3]. Recent work has suggested mathematical models describing the apparent association of the specimen compressive strength with curing age [4-11]. Another factor that affects the result of concrete compressive strength for a specimen of a given curing age is the tempera- ture of the environment in which the curing process takes place [5]. It is characteristic that a higher value of this temperature leads to faster curing. At the nominal age (e.g. on 28 days) the specimen may exhibit different value of compressive strength as compared to the case where the same curing procedure would be performed in a lower temperature environment. Of course, the combined uncertainty of the result of a test according to EN 12390-3 depends on the calibration quality of the uniaxial compression apparatus and the degree of familiarity of its user. An apparatus that has been successfully calibrated according to the requirements of International Standard ISO 7500-1 may be considered as causing negligible random errors to the test results. However, it should be noted that any systematic errors due to the accuracy of the reference standard (e.g. a force gauge) which is used during the calibration procedure cannot be overlooked. For this reason, the measurement uncertainty for the reference standard should always be incorporated into the uncertainty budget of a test according to EN 12390-3. All of the above are summarized in the cause-and-effect diagram (Ishikawa diagram, as already been introduced by authors of this paper in a previous study [8]) which has been used according to guidance from EURACHEM [11], in a way [8,12] that aims at mapping uncertainty factors while simultaneously visualizing synergies between them (Fig.1).