# Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening
## 3 Results and Discussion  
The measurement results presented below show the relations between temperature development and acoustic responses detected during setting and early hardening of investigated materials. In this measurement, more important than monitoring the absolute values of temperatures is to monitor their development trend. Figure 3 shows the time-dependent differences between temperatures measured inside the specimens and the ambient temperatures for a mixture without (A) and with plasticizer (B). The figure shows that the setting end-time (the peak value of \(\Delta\)T) is shifted by about 5 h in the mixture with plasticizer. The addition of the super-plasticizer slowed down the hydration and delayed the onset of hydration temperatures. The progress of temperatures corresponds well with the progress of relative length changes of test specimens (shrinkage measured along the longitudinal axis of test specimens). Looking at the time-dependent changes in specimens' length shown in Figure 4, the influence of a higher \(w/c\) ratio is manifested by swelling in the initial stage of setting and hardening. This initial swelling is caused by partial re-absorption of the water appearing on the specimen surface (bleeding) [12]. The literature calls this phenomenon 'autogenous swelling' and it is thought to be related to a high \(w/c\) ratio [34,35]. This swelling is further intensified by thermal expansion due to hydration heat development. The peak of the hydration heat was reflected in the development of relative length changes. In case of mixture \"A\" approx. 11 h after the start of measurement a surge of the length increment was recorded. In case of composite \"B\" a dormant period accompanied by short expansion was observed after the maximum temperature was recorded (approx. 16 h after start of measurement). Figure 2: Measurement arrangement: (**a**) overall view; (**b**) acoustic emission measurement; (**c**) detailed view of waveguide; and (**d**) specimens after demoulding. Before the results of AE measurements are discussed, it should be noted that all measurements were started approx. 1 h after pouring the fresh mixtures into the moulds. At this time, the fresh mixtures behaved as thixotropic fluid so the movable head of the moulds was not pushed out under the composites' own weight. Shortly after that, the waveguides were also fixed in the stable positions so that results of AE measurements were not influenced by their movement. To detect AE signals in concrete, a measurement system (DAKEL XEDO) consisting of AE sensor (MDK13 with inbuilt magnetic holder), amplifier (AS3K with 35 dB gain), and filter (100-800 kHz) was used. Total amplification of the pre-amplifier and the main amplifier was set from 60 dB to 72 dB. The measurement system records such parameters as count, hit, event, maximum amplitude, rise time, duration, RMS (root mean square) voltage, and arrival-time difference. The waveform parameters are illustrated in Figure 1. Elimination of the noise is ensured by setting the threshold level (400 mV), or by filtering during a post-analysis of the data. The events recorded for mixture \"A\" presented in Figure 5 show that most AE events tend to occur in the first six hours of aging. This very early stage sees the beginning of plastic setting, microstructure  
Figure 4: Relative deformations measured during the first 68 h of specimen aging. Figure 3: Time-dependent differences between temperatures measured inside the specimens and the ambient temperatures (during the first 68 h of specimen aging). formation (hydration products) and gradual increase of hydration heat. The increased acoustic activity of the composite without plasticizer is caused also by the initial re-absorption of water from the surface of the test specimens (bleeding was observed, see above). After this stage, there is a dormant period in acoustic effects, which lasts for the whole period of swelling. Once composite \"A\" starts to shrink, acoustic activity is once again present in the specimens.