# Acoustic Emission for Evaluating the Reinforcement Effectiveness in Steel Fiber Reinforced Concrete
## 1 Introduction  
The acoustic emission (AE) technique has proven very sensitive to the damage process of concrete. AE can detect the onset of failure more accurately than any other technique, something that is one of the great advantages of AE [1, 2]. It also enables localization of sources due to the arrival times of elastic waves to different stations (sensor positions) [3, 4, 5, 6]. Another important feature is the identification of the failure modes. This is feasible due to the correlation between the AE waveform parameters and the crack tip displacement [2, 7, 8]. It is widely accepted that the tensile mode of fracture leads to higher frequencies and shorter signals than the shear mode in cementitious media [2, 9, 10, 11]. When displacement occurs in an elastic medium (i.e., crack propagation event), this disturbance propagates as an elastic wave (similar to ultrasound) through the medium. There are two basic wave modes in bulk media: the fast longitudinal and slower shear wave mode. Depending on the displacement of the crack tip, the proportion of the different wave modes is defined. Shear events due to the shape change they impose on their vicinity, generate most of their energy in shear (transverse) waves, while the volumetric transient change of tensile cracks gives rise mostly to longitudinal waves, as shown in Figure 1a. Therefore, due to the components of the wave and their different propagation velocity, the shape of the waveform is also influenced. AE signals originating from shear tend to obtain longer waveforms, as the main energy arrives later while tensile signals obtain shorter durations and higher frequencies [2, 12, 13, 14]. Recently it was shown that AE is also sensitive to the preliminary displacements imposedby the stress field and can therefore, provide information on the stress field a long time before the manifestation of cracks in any measurable way [15]. The reason for this is related to the shear stress developed on the interphase between the matrix and the reinforcement (either in the form of steel bars or fibers, under the assumption of perfect bonding). Indeed, the effective contribution of the reinforcement is realized by load transfer through shear stresses between the matrix and the reinforcing element, with this either being a steel bar, patch or fiber. When load is applied, shear stresses (and strains) are automatically developed on the interphase. Even at a low level, these displacements give rise to AE that can be picked up due to the sensitivity of the sensors and allow the understanding of the stress field. This is the reason why AE behavior differs, as will be seen, between the two types of materials (plain and steel fiber reinforced concrete). While Figure 1a demonstrates a depiction of tension and shear for a general case, Figure 1b shows a case closer to the typical events expected in a matrix with reinforcement like concrete with rebar or fibers. The waveform parameters that appear mostly sensitive to the stress field and the crack tip displacement are presented in Figure 2 and are namely the rise time (RT) or delay between the onset and the highest waveform peak, and the RA value, defined as RT over maximum amplitude (A). The frequency content can be easily assessed by the average frequency (AF) which is the number of threshold crossings over the duration of the waveform, as well as the central frequency (CF) which is the frequency with the highest magnitude after Fast Fourier Transform. Due to its definition, RT receives information by both aforementioned wave modes. Concerning the \"tensile\" events that carry mostly longitudinal waves, the main energy arrives also with high speed; therefore, the RT is short. In the case of a shear event, the major part of energy comes later due to the lower speed of shear waves. Therefore, the RT (and consequently RA) tend to obtain higher values, while the total duration is also increased. This characteristic renders RT very indicative of the fracture mode [9; 11; 12; 14; 15].