# Using Acoustic Emission Methods to Monitor Cement Composites during Setting and Hardening
## 1 Introduction  
Acoustic emission (AE) in non-destructive testing (NDT) is physically similar to an earthquake in seismology [1]. AE is a phenomenon mostly associated with a destruction process. In structural engineering concrete cracking and reinforcement rupture can be detected by analyzing AE events that occur during loading. The AE events can be analyzed either qualitatively or quantitatively. Qualitative AE analysis methods make use of basic parameters of recorded signals and aim at identifying the load history and the stage of degradation of the material. In quantitative AE analysis methods, all characteristics of an AE source are analyzed and, therefore, the wave propagation between the source and sensors needs to be considered. Further progress depends on how wave propagation is modeled in cracking and cracked concrete, and how other elements of structural concrete, such as reinforcement, post-tensioning tendons, or inserts are taken into account [2]. To track the tension changes and/or the crack generation, the acoustic emission method is used. This method allows us to follow up active (dynamic) processes inside the structure. In consequence of local tension accumulation inside the material, there arises foci of tension and, consequently, potential sources of acoustic emission. If the tension reaches, or even exceeds, the critical value at a certain point the accumulated energy will be released resulting in an acoustic event. This event is supposed to be accompanied by the formation of a micro-crack [1]. The tension propagates through the material. The point at which the tension has arisen is called the acoustic emission source. In homogenous and isotropic propagation media a spherical wave propagates from the source, its energy decreases with the distance from the source. Once the wave reaches the specimen surface it can be recorded by means of an acoustic emission sensor. In this way the position of the crack can be determined in the case of homogeneous isotropic materials and appropriate sensor positioning [3]. For the purpose of the measurements evaluation itis necessary to analyze several parameters of an AE waveform, such as its amplitude, duration, and energy. The AE waveform parameters are illustrated in Figure 1. Amplitude is the greatest measured voltage in a waveform which determines the detectability of the signal. Signals with amplitudes below the operator-defined, minimum threshold will not be recorded. Duration is the delay between the first and last threshold crossings and the energy is directly proportional to the area under the acoustic emission waveform [4]:  
\[AE\ energy_{i}=\int\limits_{t_{0}}^{t_{1}}V_{i}(t)^{2}dt \tag{1}\]  
\(i\) = the recorded voltage transient \(V(t)\) of a channel \(t_{0}\) = the starting time of the voltage transient record \(t_{1}\) = the ending time of the voltage transient record  
The modern cementitious materials, such as high-performance concrete (HPC), show advantageous properties compared to conventional concretes, for example, good workability of the fresh concrete and high strength after hardening. Due to low \(w/c\) ratios, HPC is generally characterized by low porosity and a finer capillary pore structure [2; 5]. On the other hand, due to the restricted moisture transport, a curing by externally-supplied water is not very effective. In deeper regions shrinkage in combination with thermal dilation during cooling of the concrete leads to development of internal stresses if the deformation is restrained. When the internal stresses exceed the tensile strength of the cement paste matrix, micro- or even macro-cracking occurs. Progress in building material technology and composition require advancements in test procedures for determining the physical and mechanical parameters of new materials.