PSI - Issue 28

Dimos Triantis et al. / Procedia Structural Integrity 28 (2020) 502–510 8 D. Triantis, I. Stavrakas, A. Kyriazopoulos, E. D. Pasiou, S. K. Kourkoulis / Structural Integrity Procedia 00 (2019) 000–000

509

24

45.0

120

3000

16

30.0

80

2000

E [GPa]

Q [nC]

1000 PSC [pA]

8

15.0 UCS [MPa]

40

0

0

0

0.0

0

30

60

90

0

30

60

90

Age [days]

Age [days]

(a) (b) Fig. 7. The average values of the compressive strength, UCS, and of the elastic modulus, E (a) and the average values of the maximum value of the PSC and of the electric load, Q (b) versus the age of the specimens. 14 days) it is extremely stronger (see Fig.7b) in comparison with that of mature specimens (28 and 90 days), assuming that the initial background level is ignored (as it was mentioned the strong background of the PSC in the early aged specimens can be attributed to the increased moisture of the specimens). As with E and UCS, the differences for both PSC and Q between the classes of specimens with ages equal to 28 and 90 days are negligible. Another interesting conclusion drawn is related to the time evolution of the PSC during the very last loading stages before macroscopic fracture. It was systematically observed that a power law describes excellently the PSC - (t f - t) relation for a relatively long time interval of the ascending branch of the PSC. The deviation of the PSC evolution from this power law, which appears a few seconds before fracture is considered as an indication that the system is entering into its critical stage, or, in other words, that the specimen is in a state of impending fracture. The specific feature of the PSC’s time evolution has been suggested as a warning signal or as a “pre-failure” index. This observation supports similar ones drawn from a wide variety of experimental protocols with various types of specimens and loading schemes (Triantis et al. 2015; Kourkoulis and Dakanali 2007; Kourkoulis et al. 2018; Pasiou et al. 2019). The fact that this behaviour is in agreement with the respective ones concerning the time evolution of the acoustic activity, indicates that the PSC technique could be a valuable and reliable tool for continuous Structural Health Monitoring, taking into account that the equipment required for its application is relatively cheap and the procedure rather simple. References Anastasiadis, C., Triantis, D., Stavrakas, I., Vallianatos, F., 2004. Pressure stimulated currents (PSC) in marble samples after the application of various stress modes before fracture. Annals of Geophysics 47, 21–28. Enomoto, J., Hashimoto, H., 1990. Emission of charged particles from indentation fracture of rocks. Nature 346, 641–643. Finkelstein, D., Hill, R.D., Powell, J.R., 1973. The piezoelectric theory of earthquake lightning. Journal of Geophysical Research 78(6), 992–993. Holt, E., Leivo, M., 2004. Cracking risks associated with early age shrinkage. Cement and Concrete Composites 26(5), 521–530. Khan, A.A., Cook, W.D., Mitchell, D., 1995. Early age compressive stress–strain properties of low-, medium-, and high-strength concretes. American Concrete Institute - Materials Journal 92(6), 617–624. Kourkoulis, S.K., Dakanali, I., 2017. Pre-failure indicators detected by Acoustic Emission: Alfas stone, cement-mortar and cement-paste specimens under 3-point bending. Fracture and Structural Integrity 40, 74–84. Kourkoulis, S.K., Pasiou, E.D., Dakanali, I., Stavrakas, I., Triantis, D., 2018. Notched marble plates under tension: Detecting pre-failure indicators and predicting entrance to the “critical stage”. Fatigue and Fracture of Engineering Materials and Structures 41, 776–786, 2018. Kyriazopoulos, A., Anastasiadis, C., Triantis, D., Brown, C.J., 2011. Non-destructive evaluation of cement-based materials from pressure-stimulated electrical emission – Preliminary results. Construction and Building Materials 25(4), 1980–1990. Lew, H. S., Reichard, T.W., 1978. Mechanical properties of concrete at early ages. American Concrete Institute - Journal Proceedings 75(10), 533–542. Nitsan, U., 1977. Electromagnetic emission accompanying fracture of quartz-bearing rocks. Geophysical Research Letters 4(8), 333–336. Oluokun, F.A., Burdette, E.G., Deatherage, J.H., 1991. Elastic modulus, Poisson’s ratio and compressive strength relationship at early ages. American Concrete Institute - Materials Journal 88(1), 3–10. Pasiou, E.D., Stavrakas, I., Triantis, D, Kourkoulis, S.K., 2019. Marble epistyles under shear: An experimental study of the role of “Relieving Space”.