Influence of fiber chemical coating on the acoustic emission behavior of steel fiber reinforced concrete D.G. Aggelis ⇑ , D.V. Soulioti, N.M. Barkoula, A.S. Paipetis, T.E. Matikas Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece article info Article history: Received 9 September 2010 Received in revised form 4 July 2011 Accepted 12 July 2011 Available online xxxx Keywords: Four-point bending Fracture mode Frequency Pull-out Nondestructive monitoring abstract The current study focuses on the effect of chemical coating on the acoustic emission (AE) characteristics monitored during the fracture process in steel fiber reinforced concrete (SFRC). Different shapes of chem- ically treated and un-treated steel fibers are used to create specimens which are subjected to four point bending up to failure. Sensitive AE indices demonstrate that the coating gives distinct characteristics to the interface bonding between the fiber and the concrete matrix, which are evident mainly during the pull-out stage, after the moment of macroscopic crack formation. Specifically, AE average frequency and RA value, which defines the rising angle of the waveforms indicate that coating results in extensive matrix cracking in addition to the friction between fiber and concrete which characterizes the uncoated fibers. AE analysis can be used for interpretation of the fracturing stage and characterization of the frac- ture mode. It is shown that the surface conditioning of the fibers leaves a clear fingerprint on the AE sig- nals, shedding light into the processes that occur during failure in SFRC. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Concrete is characterized by some unique properties that make it the most common construction material worldwide such as its low cost, easiness of forming and high compressive strength. How- ever, it exhibits low performance when subjected to tensile loads while its behavior is quite brittle. The traditional way to deal with the problem is the inclusion of reinforcement in the form of steel bars or more recently in the form of steel fibers. Although the com- pressive strength of fiber reinforced concrete is not reported to sig- nificantly increase, the material is characterized by higher tensile strength and significantly improved post-peak behavior and specif- ically toughness [1,2]. A fiber in the path of a propagating crack bridges the crack opening and resists further crack growth by dis- sipating energy during pull-out [3]. Different types of fibers have been used in concrete structures and pavements subjected to hea- vy dynamic loads. Fibers, contribute to improving concrete tensile and flexural strength, fatigue life, ductility, surface skin resistance to seawater wetting–drying cycles as well as to delaying concrete cracking due to alkali-aggregate reaction [4–8]. Fibers are also used in reinforcing concrete with corroding steel in order to maintain steel bond to concrete, and at the same time, reduce the resulting concrete cracking [9,10]. Straight steel fibers improve the bond be- tween concrete and reinforcement in structural joints [11], while SFRC exhibits improved resistance to jet-induced spalling during vertical take-off of aircrafts [12] and improves the torsional behav- ior of concrete beams with or without conventional reinforcement [13]. However, the bonding between the fiber and the matrix may pose a severe limitation to the effective use of the fibers in high performance cementitious materials. Enhancing interface bond properties is especially important for high strength steel fibers which are commercially available as concrete reinforcement. The stress transfer efficiency, between these fibers and the matrix is a prerequisite to improve the properties of reinforced concrete [14], as well as to prevent premature debonding due to the internal stresses caused by the shrinkage of the matrix [15]. The major mechanisms that promote interfacial bond between concrete and steel are friction, mechanical anchorage and chemical adhesion in- duced by the application of fiber coating [11]. Different kinds of modifications have been attempted to improve the interfacial strength between reinforcement and matrix. Among those are chemical coating to instigate bonding [16] and mechanical pro- cessing (surface machining or surface sanding) of the reinforcing phase in order to achieve better contact between the constituent materials [17]. The fracture process in a material as complicated as SFRC in- cludes different failure mechanisms, which concern plain concrete [18] (cracking, detachment of aggregates, aggregate crushing), the fibers, (plastic deformation and rupture), and the interface (deb- onding or friction) [3]. In addition, little is known of the chemical bond between the steel fibers and the cement matrix, as the stress transfer is mostly attributed to frictional shear stresses [15]. 0958-9465/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconcomp.2011.07.003 ⇑ Corresponding author. Tel.: +30 26510 08006; fax: +30 26510 08054. E-mail address: daggelis@cc.uoi.gr (D.G. Aggelis). Cement & Concrete Composites xxx (2011) xxx–xxx Contents lists available at ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp Please cite this article in press as: Aggelis DG et al. Influence of fiber chemical coating on the acoustic emission behavior of steel fiber reinforced concrete. Cement Concrete Comp (2011), doi:10.1016/j.cemconcomp.2011.07.003