EVALUATION OF CALCULATION CORRELATION EFFICIENCY AS MENTIONED IN en 13791 IN ORDER TO DETERMINATION CONCRETE COMPRESSION STRENGTH BY NONDESTRUCTIVE TESTING
The paper presents evaluation of correlations for calculation of concrete compression strength using nondestructive testing parameters for both impact and ultrasonic pulse methods, as listed in EN 13791:2007. This standard indicates procedures relating to compression strength determination in construction and building elements. These procedures relate to compression strength evaluation based either on test specimens (drill cores) or combination of NDT and drill core testing. These nondestructive methods are not covered by procedures taking into account only nondestructive testing parameter pursuant to common calibration correlation. Correlation based on Schmidt impact hammer rebound does not respect various positions of this instrument although positioning of the same is essential for rebound value. No specific type of impact hammer is given for particular calculation correlation. In addition, another challenge is preparation of test surface along with evaluation of impact hammer measurement according to EN 12504-2 as compared with EN 13791. Dispersion of significant values varies between 15 % and 50 % depending upon specific values of impact hammer rebound. Calculation correlation based on ultrasonic pulse velocity implies that – at 4 km/s – concrete compression strength is equal to zero which is absurd conclusion, indeed. We indicate confrontation with calibration correlation as stated by technical bibliography and technical standards as well.
Experimental Studies on the Homogeneity and Compressive Strength Prediction of Recycled Aggregate Concrete (RAC) Using Ultrasonic Pulse Velocity (UPV)
The reuse of material from collapsed building debris as a natural aggregate substituent has been an alternative to solve concrete waste in Indonesia. However, its use in larger structures needs further research. The objectives of this research are to study the concrete homogeneity at a certain sample height and to find the strength prediction of Recycled Aggregate Concrete (RAC). In the previous laboratory study, the obtained concrete compressive strength with 20% RAC substitution (from the concrete ruins with fc’ = 30 MPa) was 27-30 MPa. Since, RAC contains natural aggregates and mixed cement paste, it is important to research the distribution, homogeneity and strength prediction of the concrete. The measurement was done using a nondestructive instrument called Ultrasonic Pulse Velocity (UPV). This paper presents a series of experiments investigating homogeneity and predicting compressive strength using both Destructive and Non-Destructive Tests (NDT). To do so, three steps in the experimental works were done. Firstly, UPV propagation measurements at four different levels through vertical prismatic beam specimens was performed. Two transducers were used in the direct mode of transmission with a 15 cm distance of measurement. Secondly, identical concrete samples cast in cubic form were measured using UPV and tested under compression. Thirdly, the core-drilled samples were taken from the vertical prismatic beam specimens for further investigation. The wave propagation velocity through RAC and the concrete compressive strength relationship demonstrate similar pattern as concrete with GFC (Gypsum-free-cement) performed by Brozovsky. This pulse velocity-concrete strength relationship can be used to predict the strength of RAC. Both the prediction curve and the homogeneity properties can be useful when casting a larger element of structures using RAC.
THE SLOVENIAN SOCIETY FOR NON-DESTRUCTIVE TESTING AND FACULTY OF MECHANICAL ENGINEERING Aškerčeva 6, 1000 LJUBLJANA, SLOVENIA ARE ORGANISING
In this paper a new NDT system is presented. The system is composed by a planar eddy currents probe (IOnic probe), electronic devices for signal generation, conditioning and conversion, automated mechanized scanning and analysis software. This new IOnic probe allows a 3D induced current in the material, and an easy interpretation of the signal based, not on the absolute value, but on a comprehensible perturbation of the signal. This provides enhanced liftoff immunity and improved sensitivity for defects detection. The IOnic system was developed mostly to be used for the micro-defects detection on aluminum solid state processed alloys by FSW. The features of some material imperfections lead to significant difficulties in their identification when using existent NDT techniques. Results from an analytical simulation fully agree with the experiments. The experimental results in AA2024 welded by FSW clearly show that this system is able to detect imperfections around 50 μm, which contribute to increase the reliability on NDT of micro imperfections. EC conventional probes are also applied as material characterization technique. Results are compared to microstructural analysis and hardness measurements show that eddy current can map the different zones providing a deeper understanding on precipitate dispersion and grain shape and size.