PSI - Issue 13

Libor Topolář et al. / Procedia Structural Integrity 13 (2018) 1177 – 1182 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

1180

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3. Results and discussions

The measured values of flexural strength (three-point bending test) are summarized in Tab. 3. The obtained flexural strength values demonstrate that increasing the degrading temperature leads to decreases in the flexural strength. The initial maximum value for the mixture B is caused by the content of coarse aggregate. This influence of the coarse aggregate grain occurs until firing up to 600 °C when the crystalline phase of quartz changes (see Table 1). When firing the samples to 800 and 1000 °C, the strength values of all the different mixtures become almost identical. The next temperature step (1200 °C) causes the transition from hydraulic to ceramic bonds in the samples. This leads to significant increase of flexural strength in case of the mixture C with fine aggregate grain size.

Table 3. The measured values of thermally degraded concrete (obtained from three independent measurements)

Flexural strength (MPa)

Duration of AE signals (µs)

Energy of AE signals 10 -5 (Vs)

ID specimens

Mixture A Mixture B Mixture C Mixture A Mixture B Mixture C Mixture A Mixture B Mixture C Mixture A Mixture B Mixture C Mixture A Mixture B Mixture C Mixture A Mixture B Mixture C

5.82 6.50 6.33 3.15 4.10 3.59 1.86 2.10 1.82 0.60 0.81 0.80 0.33 0.40 0.28 1.47 0.86 2.15

1865 1906 1860 1607 1810 1845 1597 1681 1848 1493 1659 1750 1205 1329 1421 1418 1410 1469

6293 4303 4704 2344 2679 4250

20

400

400

1493 3909

600

77

785

800

2094

48 94

1000

802 296 183 294

1200

To describe the AE signals that are generated in the samples during the three-point bending test, we focused on the duration and energy of the AE signals. The duration of the AE signals is determined by the difference in time between the first and last exceeding of the threshold. This parameter describes the degree of damage of the samples. In case of an undamaged sample (designated 20), the signal durations are the longest (see Table 3). When testing samples that were thermally degraded, the AE signal durations shorten. This is caused by the damage to the cement bonds and their subsequent decomposition, by the change of the crystalline phase of quartz and by the decomposition of Portlandite. On the other hand, the formation of the ceramic bonds (fired at 1200 °C) exhibits longer signal durations, which is caused by sintering of the samples. The AE signal energy is defined as the size of the area under the envelope of the AE event in Landis (2002). Higher AE energy means better bonding in the inner structure of the material. The obtained values show that the best bonds of the inner structure were exhibited in the mixture C, which has the highest values for all the firing temperatures. This fact may indicate the smallest internal damage caused by elevated temperatures, which is in correlation with the obtained flexural strength values. The absolute measured values in Tab. 3 present a comparison between the individual mixtures. They, however, do not much describe the effect and degree of damage to these samples because each mixture has different initial AE values for the reference samples (designated 20). For this reason, we present graphs (Fig. 2) where the measured