# Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening
## 1 Introduction
The current testing approach is based on the earliest obtainable identification of disruptions in the internal structure of structural elements, which facilitates early diagnostics of the problem allowing relevant precautions to be taken in order to prevent a later collapse of the structure being designed. The reason this area is being focused on is because many problems with cracking of cement composites arise at early ages [6; 7; 8]. Controlling the variables that affect the volume change can minimize high stresses and cracking [7; 9; 10]. Recent measurements suggest the necessity of assessing the magnitude of shrinkage in two consecutive stages of composite aging--at an early age and at later ages [10; 11; 12; 13]. Early-age measurements reveal differences in the development of volume changes, as well as differences in  
Figure 1: Parameters reflected in an AE waveform. the initiation and propagation of cracks which cannot normally be identified by traditional methods applied after specimens have been removed from moulds (typically after 24 h) [14]. However, these methods mostly determine separate components of early-age shrinkage--e.g., methods for determining plastic or autogenous shrinkage [8; 9; 12; 15; 16; 17; 18; 19] and methods for determining shrinkage due to drying (defined mainly in the standards of various countries). Contemporary approaches to shrinkage measurement are based primarily on the determination of relative length changes. In most cases, measurement begins after specimens have been demoulded, which is typically no sooner than at 24 h of age [20]. While drying shrinkage is taking place it is very important to ascertain how microcracks develop when the material is not restrained. These microcracks develop due to aggregate limitation. Non-uniform drying leads to a moisture gradient which results in non-uniform shrinkage. The surface of the cement-based composite shrinks faster than the inside of the material, which causes surface cracking [21; 22; 23]. Furthermore, the shrinkage might be restrained by the aggregate inside the composite, generally known as a self-restriction, which is caused by the particles inside the composite that shrink more slowly. Stresses due to this type of restriction lead to severe microcracking in the vicinity of the aggregate particles [24; 25]. These microcracking greatly affects the transport properties of the cement-based material [26; 27; 28]. Nevertheless, the evaluation of microcrack formation and their dependence on the size and volume fraction of aggregate is not yet fully understood [20; 29]. The measurement of shrinkage and thermal dilation, as well as the resulting stress in the case of restraint, may be performed with so-called temperature-stress testing machines [2]. These tests provide information about potential cracking and failure of the concrete specimen. No information, however, is given concerning the cracking process. Since early-age cracking may lead to severe problems with respect to the durability of the concrete structure it is necessary to detect and quantify the cracks. The AE technique is difficult to use for early-age concrete because AE sensors are mainly designed for hardened concrete. [PERSON] et al. [30] had to use protruding bars (waveguides), which were used to introduce the signal to the AE sensors. Such waveguides can be preferably used for monitoring the structural changes during whole time of cementitious composite solidification, including the stage of setting and early hardening. This paper describes the hydration process of cementitious composites, which was studied by means of the acoustic emission method and measurement of length changes and temperature development. For this purpose, cement-based mortars were produced with a \(w/c\) ratio of 0.46 and 0.35 (with super-plasticizer). The fresh mixture was placed in a mould so that its tendency towards cracking could be monitored while it hardened. A steel waveguide was then inserted into the fresh mixture and a magnetic acoustic emission (AE) sensor was attached to its free end.