Issue34

J. Saliba et alii, Frattura ed Integrità Strutturale, 34 (2015) 300-308; DOI: 10.3221/IGF-ESIS.34.32 301 are strongly influenced by the dimensions of specimens/structures. Many researchers tried to characterize the FPZ and its evolution during crack extension in order to obtain size independent fracture parameters for the application of fracture mechanics of concrete. For Hillerborg [1], the length of the FPZ was related to the length of cohesive zone or the characteristic length which is a pure property of the materials, while Bazant & al. [2, 3] modeled the FPZ as a band with a fixed width related to the size of aggregates in concrete. Recently, various experimental methods were employed to detect the fracture process as the holographic interferometry, the dye penetration, the scanning electron microscopy, the X-rays, the digital image correlation, etc. However, these methods offer either the images of the material surface to observe micro-features of the concrete with qualitative analysis, or the black-white fringe patterns of deformation on the specimen surface (except for 3D tomography), from which it is difficult to observe profiles of the cracked material. In this study, the growth of the fracture zone is investigated using the AE technique. This later allows a continuous and a real time data acquisition, and thus the damage evolution during loading tests can be recorded. The AE technique is a passive method that has been proved to be very effective to locate microcracks and to study different failure modes in concrete structure [4-7]. It presents a large potential of applications and has been used in the past to study the influence of different parameters on FPZ, such as the effect of aggregates [8, 9], porosity [10], creep [11, 12], notch depth [13], specimen geometry and type of loading [14]. The damage is then evaluated based on statistical analysis and measurement of AE activity [15] or on quantitative and signal based techniques (moment tensors) [7]. The objective of this paper is to characterize the FPZ and its evolution during the fracture process in unnotched and notched concrete beams with different notch depths based on the AE technique. First, material and experimental methods are presented. Secondly, fracture measurements are analyzed and the characterization of crack evolution at different loading stages is suggested with AE technique. Finally, the effect of the notch to depth ratio on fracture growth is described based on the experimental observations. E XPERIMENTAL PROCEDURE Materials and specimens he experiments are realised by Grégoire & al. and reported in [16, 17]. In this study, only the comparison between notched and unnotched beams was realised. The tests realised on notched beams were conducted on beams with a constant depth of 200 mm and length of 700 mm with an effective span equal to 500 mm. Two notch to depth ratio of 0.2 and 0.5 were considered and labeled respectively SN200, LN200. While tests realised on unnotched beams were conducted on beams with the same dimensions as earlier labeled UN200 and on beams with a depth of 100 mm, a length of 350 mm and an effective span equal to 250 mm labeled UN100. The thickness was kept constant for all the beams at 50 mm. Tests were conducted under closed-loop crack mouth opening displacement (CMOD) control. The CMOD measurement consists in recording the distance between two alumina plates glued on the bottom surface of the beam, on each side of the initial notch. For unnotched beams, the alumina plates were glued at a distance from midspan equal to half the depth of the beam (details may be found in [16]). Acoustic emission technique The AE system comprised of an eight channel AEwin system, a general-purpose interface bus (PCI-DISP4) and a PC for data storage analysis. A 2D analysis with an AEwin algorithm is performed for the localization of AE events. For the source to be located in 2D, a wave must reach at least three sensors. In this study, 4 piezoelectric sensors with a frequency of 50-200 kHz and a resonance frequency of 150 kHz were used. The transducers were placed around the expected location of the FPZ to minimize errors in the AE event localization. They were placed on the specimen with silicon grease as the coupling agent. The sensors form a rectangular grid location on one side of (75 x120 mm²) for UN200 beams, (60 x120 mm²) for UN100 beams, (105 x 120 mm²) for SN200 beams and (110 x 120 mm²) for LN200 beams [17] (Fig. 1). The detected signals were amplified with a 40 dB gain differential amplifier. The recorded AE amplitudes ranged from 0 to 100 dB. In order to overcome the background noise, the signal detection threshold was set at a value of 35 dB slightly above the background noise. The acquisition system was calibrated before each test using a pencil lead break procedure HSU-NIELSEN [18]. Location accuracy is measured in the range of 5 mm by applying the pencil lead fracture at a known location of the specimen. The measured effective velocity is equal to 3800 m/s. Each waveform was digitized and stored and signal descriptors such as rise time, counts, energy, duration, amplitude, average frequency and counts to peak were captured and calculated by AEwin system. T