# Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening
## 3 Results and Discussion
It can be presumed that this activity is caused by inner tension brought about by gradual temperature equalization between the specimen and the environment [36]. Later acoustic signals may be a consequence of both structure generation and micro-cracking. The situation is different in composite \"B\". The added plasticizer slowed down and delayed setting and hardening, which the records reflect as a drop in the number of the AE events (compared with composite \"A\"). No bleeding was observed in this mixture, i.e., there was no re-absorption of water from the specimens' surface. Most events in the mixture were recorded during the first two h of aging, which is related to plastic setting and early microstructure formation. Later, an increase in shrinkage was recorded, the intensity of which rose with age, along with an increase of the specimen's internal temperature. Again, during this period the specimen exhibited an increased acoustic response. The period of about 10 to 22 h was acoustically dormant. Between 14 and 22 h of aging the shrinkage development reached a steady state. This phenomenon is supposedly connected with the generation of hydration heat, which reached its maximum value during this interval. More activity occurs around the 24 th h of aging, when the test specimens started to shrink again. The observation of the signal lengths during setting and early stage of hardening presented in Figure 6 shows that there are only minor differences between mixtures \"A\" and \"B\". In the first 10 h signals in mixture \"B\" are more dampened, which is, again, due to the slower hydration of the mixture, which is softer at this stage. The mixture \"B\" signal lengths are higher in the period between 22-32 h, which is related to the already stronger structure and better bond of the matrix than at the beginning of setting. The values of the amplitudes of an acoustic emission signal shown in Figure 7 indicate that after the formation of the solid phase (i.e., at the time when both mixtures reached the end of the setting time) these amplitudes are higher for mixture \"B\" (compared to mixture A), especially at about the 24 th h of aging, when the composite \"B\" started again to shrink. A similar effect can be observed also for composite \"A\" at about the 32 nd h of aging. Based on the results of the number of AE events (see Figure 5) it can be assumed that, in spite of the fact that the appearance and number of events in mixture \"B\" is lower than in mixture \"A\", a greater damage to the material was caused (average value  
Figure 5: Number of AE events registered during the first 68 h of specimen aging. of AE amplitude is higher for mixture B). The highest AE amplitudes are detected at the time of the cracks' initiation. During their further opening many events with small amplitudes can usually be recorded [37; 38]. The Figure 8 shows the energy of acoustic emission signals released either by the formation of new hydration products (during setting) or by the formation of bonds, products, and micro-cracks in the already hardened structure. Data for mixture \"A\" shows that most energy is released after about 3 or 4 h after casting, which may be related to the plastic settling accompanied by re-absorption of water, and an increase of temperature. Another significant amount of energy is released when composite \"A\" starts to shrink due to the decrease of temperature. After the temperature has been stabilized, the micro-cracks creation is reflected in the increased value of AE energy. In mixture \"B\", a significant portion of energy is released at about 29 th h of aging as a consequence of the above-mentioned processes. Especially, the transition from swelling to shrinkage causes tension in the material, which may subsequently lead to damage of the already-established bonds. Based on the results presented in Figures 6-8 it can be assumed that the most significant structural change in already-rigid composite \"B\" occurred at about the 24 th h of aging.