Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 506 explosive cracking agents on fracturing of the surrounding media inside a borehole is based on the volume thermal expansion of the cracking agent, caused by the exothermic hydration reaction of CaO under confined conditions. In the proper mixing ratio of the cracking agent to water, the subsequent coagulation and rigidity of the mixture results in the development of high expansive pressures within the pre-drilled holes that eventually causes a complex fracture network in the concrete or the rock mass [7]. In addition to commercial use, non-explosive expansion material has been utilized in laboratory experiments to evaluate its fracturing capacity of synthetic specimens, and its potential application for fracturing coal roofs in coal mines has been proposed [8]. Guo et al. used soundless cracking agent in laboratory scale experiments to form a fracture network in shale specimens, in order to establish an evaluation method for the hydraulic fracturing in shale reservoirs [9]. More recently, Zhai et al . have proposed the application of non-explosive expansion method in the field of reservoir fracturing for the extraction of coal bed methane [10]. In the previous three cases, the acoustic emission (AE) technique was used for the dynamic monitoring and analysis of the fracture network formation. To the best of our knowledge, these are the only available studies where the effect of SCDAs in fracturing has been studied by means of the AE technique. Although the optimum operation conditions of SCDAs, related to the external temperature, hole size and spacing, and mixing ratio of agent to water have been studied to some extent [2,11], the knowledge of the governing fracture generation and propagation mechanisms in concrete or rock masses is rather limited [5]. Regarding fracture mechanisms, quasi-static conditions can be considered in SCDA-induced rock fracturing, due to the observed stable crack propagation at low crack velocities, and micro-crack based sliding models are likely to be applied [5]. In this direction, the well-established monitoring technique of AE can provide valuable information on the fracture mechanisms in rocks and concrete structures under the effect of SCDAs [12-14]. The present study explores the potential use of the AE technique to study the SCDA-induced fracture process in concrete samples. The generation and propagation of micro-cracks into concrete specimens in unconfined state due to the addition of expansion mortar to pre-drilled holes is monitored for a period of 24-h, by an array of AE sensors. The experimental data are analyzed using parameter-based and statistical methodologies of acoustic emissions. E XPERIMENTAL SETUP oncrete blocks used for pavement cover were cut in prismatic shape and a cylindrical hole, of 10 mm diameter and a depth of a few cm, was drilled vertically to one of its surfaces (refer to Fig.1a). The pre-drilled hole was filled with DEXPAN soundless chemical demolition agent [5,15], in the appropriate mixing ratio of water to cementitious powder (~3.5:10), to achieve maximum expansive strenght (124 MPa, according to the manufacturer). The monitoring of concrete fragmentation in the absence of any confinement, by the AE technique was carried out at ambient pressure and temperature. Seven AE piezoelectric sensors operated at 200-750 kHz (pico-sensors of Physical Acoustics Corporation) were mounted with silicon glue on all the surfaces of the concrete specimen. Pre-amplification of 40 dB was used in all sensors and the threshold of detection was set at 39 dB, to avoid the background noise. The AE activity of the concrete block was monitored continously under the effect of the expansive mortar, for a period of 24 hours. A PCI-2 card based AE multichannel system (Physical Acoustics Corporation) was used to record the detected signals (hits) in each sensor and the AE hit para- meters (amplitude, duration, rising time, rising angle, etc) were automatically extracted and further analysed by dedicated software (Aewin and NOESIS by Mistras Group). A characteristic recorded signal (hit) with the calculated based AE parameters is shown in Fig.2. Peak definition time (PDT), hit definition time (HDT) and hit lockout time (HLT) were set as 50, 200 and 300 μs, respectively. The sampling rate was set at 5 Msamples/s. The acoustic wave velocity of concrete specimens was determined experimentally by using two of the AE sensors in pulse-receive mode and found to be (3120±70)m/s. We have to mention that the highly inhomogeneous concrete specimen combined with the finite size of the AE sensors inevitably introduces errors in determining the location of the AE sources. R ESULTS AND DISCUSSION A parameter-based analysis of acoustic emissions he time history of the AE activity, expressed as the number of hits in time bins and their amplitudes, recorded in all channels during the effect of the cracking agent on the concrete block for a 24-h monitoring period, is illustrated in Fig.3, as a three-dimensional plot. From the recording time t i ≈43618 sec (~12 h) a large number of hits with ampli- C T