PSI - Issue 13

René Čechmánek et al. / Procedia Structural Integrity 13 (2018) 1780 – 1785 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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The passage time of ultrasonic wave motion using 54 kHz probes was measured and automatically converted using the TICO apparatus with a preset distance of the probes to the speed of passage of the ultrasonic wave. The measurement was performed twice – in A-B and B-A directions. 6.2. Determination of flexural strength A tensile test with four-point bending and loading perpendicular to the direction of compaction was performed using 250 × 40 × 40 mm test specimens. The distance between the lower supports was 210 mm, the distance between the loads was 100 mm. The measured values of the ultrasound passing speed and the tensile strength with four-point bending of the individual sets of test specimens (cut from the manufactured boards) were tabulated and plotted for mutual comparison. Already at this stage of the research it is possible to derive some relations between destructive and non destructive testing. The higher values of the flexural strength correspond to the longer time of ultrasonic waves passing through the measured specimen thanks to the higher degree of fibre reinforcement as it is depicted in the graph in Fig. 2 for the composite reinforced with steel fibres, which provide better response during ultrasonic measurement.

Fig. 2. Relation between destructive and non-destructive testing for SFRC specimens (blue line – flexural strength, individual columns – ultrasound passing speed A-B, B-A and average).

6.3. Microscopic anylysis Thin sections were prepared from the fracture area of the selected samples for polarizing microscopy in order to observe the proportion of the individual elements in the given cross section and, in particular, the distribution of the individual reinforcing fibres in correlation with the achieved flexural strength. Only one thin section of optimally reinforced and insufficiently reinforced cross section of the prepared formula of fibre reinforced concrete is assumed. The main attention will be paid to the stressed bottom side of each specimen, i.e. the side perpendicular to the direction of compaction. The orientation of fibres was monitored for each GFRC mix designs in the samples with the highest and the lowest flexural strength to find the correlation between parameters. The samples were impregnated under a vacuum with epoxy resin with fluorescent dye to fill the pores and cracks and stabilize the concrete before cutting and polishing. The samples were then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the s ample were 25 μm thick (determined on quartz). Determination of fibre orientation is based on the processing of images of thin sections of GFRC using image analysis. The images were taken in fluorescence regime using a camera attached to polarizing petrographic microscope Nikon ECLIPSE LV100ND. Six images were taken out of each thin section in magnification 50×. The angle and the length of all fibres in thin section were analyzed. The data were processed and plotted in OriginPro software.