# Acoustic Emission and Modal Frequency Variation in Concrete Specimens under Four-Point Bending
## 1 Introduction  
The Acoustic Emission (AE) technique is currently used to investigate the damage evolution in ductile or brittle materials before the final failure [1, 2]. The spatio-temporal evolution of ultrasonic AE signals is a direct result of cracking bonds during the fracture of materials. For this reason, this non-destructive monitoring method is useful for studying critical phenomena and to predict the durability and remaining life-time in laboratory and full-scale structures [3, 4].  
At the same time, low frequency Dynamic Identification (DI) techniques are currently used to extract modal parameters starting from time measures of displacements (or strains), velocities or accelerations. Non-destructive dynamic testing of structures allows information to be obtained about stability [5] and/or damage level and location [6]. Indeed, it is known that variations in structural stiffness are revealed by variations in modal parameters (primarily, frequencies and curvatures/shapes).  
Today, both AE and DI techniques are thus used independently for damage identification and survey. The basic idea behind the present investigation is to combine both these techniques, therefore taking the advantages of a multi-parameter monitoring based on two independent methods. This could be of great interest for the structural health monitoring of large buildings and structures: AE signals can effectively be used as damage precursors and localizers, while experimental modal data can more conveniently be adopted to obtain information on the global stiffness and damage severity (e.g., crack depth) via inverse procedures based, for example, on finite element models. However, this study focuses on a single structural element as a first application. The benchmark is a concrete beam tested in the lab. To the best of the authors' knowledge, there are few studies in the literature where AE signals,registered during the damage evolution in cracking beams, are put into relation with variations of global dynamic characteristics.  
In this paper, the AE and DI techniques are applied to examine the stress dependent damage progress during four-point bending tests on pre-notched concrete beams. The damage mechanisms were characterized considering the AE signals obtained during the tests by increasing the vertical deflection of the sample. In particular, the dominant fracture mode was identified, and the correlations between dissipated and emitted energies were investigated [7]. Moreover, the variations of the first three natural bending frequencies, produced by the crack advancement under loading, were detected and correlated with the cumulated AE energy.  
The tests were conducted using an MTS machine, operating in displacement-controlled conditions up to complete failure. The AE signals were acquired by piezoelectric transducers, sensitive in the frequency range from 80 to 400 kHz, attached near the notch. The natural frequencies were detected by piezoelectric pickups (PZT buzzers), sensitive in the range 0-20 kHz, placed along the beam surface. These sensors, typical of music technology, were recently applied to modal testing by some of the authors and found to be effective and robust in extracting natural frequencies and mode curvatures/shapes of straight and curved beams [8; 9; 10]. Their main advantages are high accuracy, small dimensions, low weight, easy usage, and low cost; they proved to have the same reliability of common laser sensors (but offering a much wider operating frequency range at a very low cost) and of piezoelectric accelerometers. They can also be conveniently coupled to audio acquisition devices without requiring calibration and filtering [8; 9]. PZT buzzers were thus adopted in this work aiming at (i) testing their possible use on concrete specimens (so far they were only tested on metallic elements) and (ii) taking advantage of an existing experimental apparatus and the relevant know-how.