# Acoustic Emission for Evaluating the Reinforcement Effectiveness in Steel Fiber Reinforced Concrete
## 1 Introduction
Steel fibers are used in concrete to enhance its mechanical behavior by bridging the crack surfaces and transferring tensile stresses [16]. Tensile stresses are transferred across cracks which decrease the width of the cracks, resulting in an increase in the ductility and toughness of the fiber reinforced concrete element [17; 18]. Due to their high stiffness, strength and aspect ratio, steel fibers are more commonly used than other types of fibers. The most common applications of steel fiber reinforced concrete (SFRC) are as  
Figure 1: (**a**) Correspondence between stress mode and AE waveform, (**b**) illustration of typical tensile (top) and shear (bottom) events in reinforced concrete. industrial slabs, tunnel linings, high performance concrete members and precast structural members [19]. In the field of SFRC bending, AE has been applied to monitor the different stages of the fracture process. Initially, the AE behavior is influenced by the tensile bending cracking exhibiting low RT and high frequency values, while just after the moment of main cracking, a sudden shift to lower frequencies and longer RT is noticed. This is interpreted through the addition of the fiber pull-out mechanism, that before main cracking seemed inactive. These changes can be as strong as a 50% drop in AF and even higher increase in RT, showing the sensitivity of AE to the fracture process [11]. Studies on SFRC have been conducted with regard to the orientation of the fibers. Specimens with fibers more aligned to the load direction exhibited higher mechanical properties and a more shear character of AE expressed by AF and RA [20] or a higher AE energy [21]. In general, the conclusion in similar studies is that the addition of fibers shifts the mode of fracture towards shear based on the AE parameters calculation [22]. However, this behavior is addressed for the whole duration of the loading, including the post-peak stage after the main cracking event, when the damage is even visually obvious and when the effect of fibers is stronger. Therefore, the possibility of prediction of the final mechanical performance is not exploited. The present paper studies the AE behavior in SFRC, focusing on the early load AE. Although the main contribution of fibers is expected in the post-peak regime by increasing the toughness, the sensitivity of AE can help in gathering signals before the main fracturing events, therefore supplying more information on the initial contribution and allowing projections to the final mechanical properties. Experiments were independently conducted in different laboratories with different experimental protocols (3 point- and 4 point-bending) while different concrete mixes, specimen sizes and fibers were used. A higher content of fibers renders the reinforcing character more effective. Since the reinforcement is activated through shear stresses, it is assumed that early load AE will exhibit more shear character as the fiber content increases, and this is the hypothesis that the present study wishes to test. In general, the failure process in both cases (4p- and 3p-bending) reasonably starts with tensile cracks of the matrix. Concerning 4p-bending, it is also true that nominally there is no shear between the load application points (center of specimen) for a homogeneous material. However, it is a point to be highlighted that although the external loading does not nominally cause shear in the center, the local fields responsible for AE creation are influenced by the fiber presence. Due to heterogeneity (difference between stiffness of concrete matrix and steel fibers) the externally imposed load creates local variations in the stress field. Although the purpose of the paper is not the exact correspondence between AE values and stress values, this has been recently shown in [15], where even in the center of the specimen, shear strains develop in the interface between the matrix and the  
Figure 2: Typical AE waveform with main parameters. reinforcement.