# Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening
## 2 Materials and Methods  
The main aim of the performed experiments was to compare the acoustic responses in the early age of setting and hardening of fine-grained cementitious composites which differ considerably in the development of hydration heat and process and development of shrinkage. For this purpose, two fine-grained cement composites (A, B) were made, each with a different \(w/c\) ratio and plasticizer content. The fresh composite formula is based on the CSN EN 196-1 standard for the testing of cement strength characteristics. Composite \"A\" was made with quartz sand with the fraction of 0-2 mm, CEM I 42.5 R Portland cement (Cement Plant Mokra, Brno, Czech Republic) and water. The ingredients were mixed at the ratio of 3:1:0.46. The composition of composite \"B\" was modified by adding a Sika Viscocrete polycarboxylate-based super-plasticizer (Sika Group, Brno, Czech Republic), which, among others, reduced the water coefficient to \(w/c=0.35\). The other components were the same. To produce both composites, a vertical axle forced concrete mixer with forced circulation and controllable mixing speed was used. Table 1 shows the composition of both composites and fresh mortar properties. Three test specimens with the length of 1000 mm and cross-section of 100 mm \(\times\) 60 mm were made from each mixture. They were primarily intended for shrinkage/swelling measurements and monitoring the material acoustic activities in the setting and the early stages of hardening. The acoustic wave sensors should be placed directly on the object being measured, which, however, is not always possible. The reason may be that the object may not be easily accessible because of its design or because the object's surface may be so hot that it may damage the sensor, or, as in this particular case, the surface is too unstable. In such cases, acoustic waveguides are used. A major disadvantage of waveguides is the loss of the resulting signal, which is caused by losses generated by the transition across the interface and by the signal being dampened inside the material. If a waveguide is used for scanning, there are two interfaces present. One is between the object's surface and the waveguide and the other between the waveguide and the sensor [31]. The shrinkage behaviour was studied using a dilatometric shrinkage drain produced by [PERSON] [15], which is primarily designed for measuring relative longitudinal shrinkage of composite materials at early stages of setting and hardening. These testing drains are equipped with a sliding face during measurement supporting an inductive sensor connected to a data exchange, while the other drain face is fixed in place. This is how longitudinal shrinkage is measured along the specimen axis for about 72 h. To allow free deformation of the material during measurement, the inner drain surfaces are coated with a 2 mm thick polyethylene foil (MIRELON). Before the actual pouring, two additional holders were attached to the drain structure to hold the acoustic emission waveguides in place--one near the sliding face and the other at the opposite end of the drain. A thermal sensor to measure the temperature changes inside the specimens was installed near each fixed face. Detailed waveguide positions are shown in Figure 2. Measurements by acoustic emission were carried out using a DAKEL XEDO instrument using suitable magnetic sensors (MDK 13) and waveguides. The acoustic emission sensors were fixed to the waveguide, which was submerged in the fresh mixture. During the measurement, the drains lay on a special weighing table [32] for continuous recording of mass losses caused by the desiccation of the surface of the test specimens placed in the shrinkage drains. Using the above described device, the measuring process could be started after the tested material has set slightly. In the experiment performed, the measurement started about 1 h after the fresh composite was poured in the dilatometric shrinkage drains.