The formation of cracks and the development of the fracture process zone (FPZ) have typically been documented by Acoustic Emission (AE) methods and important conclusions regarding the nature of the FPZ and the propagation mechanisms of concrete have been drawn to form the basis of current fracture models for concrete. The study presented in this paper focuses on Mode I cracking of ultra-high performance concrete (UHPC) and fibre-reinforced concrete material (UHPC/steel fibres) using compact tension specimens. It compares the results of AE measurements to those obtained from documenting the cracking process by Digital Image Correlation (DIC). The findings from this comparison show that different types of AE signals from deformation mechanisms of different nature occur in distinct regions of the entire cracking process, i.e. ahead of the crack tip, at the crack tip and in the wake of the crack due to the increasing separation of the crack faces and further opening of the crack. The DIC measurements indicate that crack initiation occurs with locally corresponding AE signals and furthermore suggest a continuously connected path of the fracture process zone from initiation at the crack tip to the stress free region in the wake of the crack. Based on these comparative measurements the study shows that crack formation in UHPC is initiated by an individual, sharp micro-crack rather than by a region of diffuse micro-cracking ahead of the eventual crack tip. Later on a single macro-crack propagates. Fibrous filling of UHPC yields widespread AE sources around all crack paths and crack branches originating from the main macro-crack. AE signals detected using wideband sensors exhibit quite different characteristics in time (waveform) and frequency (bandwidth) domain. In addition to three types of AE signals from different source mechanisms in concrete already described [13] a fourth type of signals in fibre-reinforced concrete probably related to the process of fibre pull-out is detected.
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