Issue 50

S. Gavela et alii, Frattura ed Integrità Strutturale, 50 (2019) 383-394; DOI: 10.3221/IGF-ESIS.50.32 386 Scope of experiment II Experiment II aims at the experimental investigation of the correlation of compressive strength testing results with the parameters of curing age and water to cement ratio at the same time, through sensitivity analysis. A sigmoidal curve was used to fit the experimental results. Freiesleben Hansen & Pedersen [4] and Carino [5] firstly proposed a sigmoidal curve for modeling the curing procedure. All of the parameters presented in Fig.1 affect the compressive strength testing according to European Standard EN 12390 series. Some essential questions are: to what extent do all these parameters correlate to each other and to the result of the test procedure? Are the results of the testing procedure valid if these parameters fail to be accurately determined? For example, should a testing result be put aside if the curing age of the specimen deviates by a few days from the typical nominal 28-days value? The population of parameters affecting the result of a compressive strength test is big so unless performing sensitivity analysis for any subset of these parameters via a specialized experiment it is almost impossible to assess the impact of this subset of parameters. Studying the effect of all the above parameters in one single experiment for various levels of those parameters would lead to an enormous specimens’ population. For this reason in the frame of an extended study aiming at the creation of a function that correlates the testing result on the compressive strength of concrete specimens to all the significant of the above parameters, only the experimental investigation of the correlation of compressive strength testing results with the parameters of curing age and water to cement ratio was examined. The integration of various similar experi- ments of such a protocol by various laboratories and for various parameters of the test procedure could speed the achieve- ment of a standardized semi-empirical model on the relation of concrete compressive strength as a function of a great number of testing parameters and mix materials characteristics [8]. E XPERIMENTAL DESIGN n the context of this study, tests have been carried out on a number of specimens prepared according to the definition of a 15 cm cementitious concrete test specimen as set out in EN 12390-1. For the design of the experiments a com- bination of characteristics for the composition of the concrete was chosen for the preparation of the specimen. The cement used in the present study was CEM II 32.5. The aggregates used were crushed limestone. Determination of particle density and water absorption of fine and coarse aggregates was performed according to EN 1097-6. The superplasticizer used was Sika ViscoFlow 700. The concrete compositions of the tested specimens are shown in Table 1 as kg of constituent per 1m 3 of fresh concrete produced (kg/m 3 ). Those compositions were obtained after performing concrete mix design using the densities of raw materials. Concrete mix design can lead to errors when raw materials are “fluffy” or lightweight or can absorb big amounts of water [10] because for these materials the error in the determination of its density is bigger but also because of a degree of compaction as the air within the initial amount of this “fluffy” material (before mixing) is displaced by all the other constituents of the mixture (during mixing). In this study, conventional aggregates were used so the errors are expected to be small. Deviations in mix proportions have been addressed to the factors contributing to the uncertainty in Ishikawa diagram above. If a laboratory reproduces the experiments using concrete mix design (using densities of raw materials) will address the same systematic errors. However, the fresh concrete’s density can be determined according to EN 12350-6. Knowing the density of fresh concrete, the yield per batch can be determined as the mass of all the ingredients in a batch divided by the density. In this way one can verify if proportions of ingredients that came from concrete mix design (kg/m 3 ) produces indeed 1m 3 of fresh concrete. In experiment I, the uncertainty estimation process was performed for the case of applying the 28-day nominal curing age that is set by the corresponding national regulation on structural concrete technical specifications. For each of the experiment I compositions, ten specimens were prepared and tested for compressive strength by pairs, at curing ages equal to 27, 28, 29, 30 and 31 days, respectively. Experiment II was intentionally performed using concrete syntheses different than those used for experiment I. This way, it was possible to assess the extent to which the corresponding multifactorial model can be used for syntheses that lie outside the range of those been used for its estimation. In experiment II, 13 specimens were prepared for each of the concrete compositions. Water to cement ratio values were selected to be separated by equivalent intervals of 0.02. At the same time superplasticizer’s mix proportions were kept the same except from the mixture with the highest water to cement ratio. In this mixture a small decrease of superplasticizer’s mix proportion was needed so the mixture did not become segregated. The cement content expressed in kg/m 3 was selected to be kept constant for the entire experiment, and as mentioned before, at a value different than the range used in experiment I. So, inevitably, it was impossible to change the water-to-cement ratio and keep the content of aggregates unchanged as expressed in kg/m 3 . Otherwise the base for the calculation of constituents’ content would not be in all cases equal to 1 m 3 . I