JOURNAL OF COMPOSITE MATERIALS Article Debonding mechanism of polymeric fibers in reinforced cementitious composites as a function of crystallinity degree of fibers Mohamad Ali Etminani and Farhad Sharif Abstract Engineered cementitious composites containing polymeric fibers such as poly(vinyl alcohol) show high tensile ductility and toughness. The aim of the current study was to evaluate the effect of crystallinity degree of poly(vinyl alcohol) fibers on fiber debonding from cementitious matrix in the bending test. In this work, two types of poly(vinyl alcohol) fibers and one grade of polypropylene fiber were separately incorporated in cementitious composites. The chemical structure, surface chemistry, roughness, and microstructure of fibers were examined by Fourier transform infrared, attenuated total reflection, atomic force microscopy, and wide-angle X-ray diffraction tests, respectively. The compression and flexural behaviors of cementitious composites were also assessed. Attenuated total reflection results were indicative of similar surface chemistry for both poly(vinyl alcohol) fibers, while the main difference was observed in the case of degree of crystallinity, which plays an important role in the Poisson’s ratio. Finally, the way in which the degree of crystallinity and Poisson’s ratio of fibers can lead to premature debonding was described and confirmed by scanning electron microscopic images. Keywords Engineered cementitious composites, debonding mechanism, polymer microstructure, poly(vinyl alcohol) fiber Introduction Concrete and traditional cement composites are brit- tle materials with high compressive and yet low ten- sile strengths. Because of their brittle nature, cracks can appear under very small tensile strains and propagate to cause a total fracture. Therefore, short and long fibers have been used to increase the tough- ness of cementitious matrixes. It has been found that the presence of fibers in the cementitious composites decreases the rate of crack propagation and increases toughness, ductility, and energy absorption capacity. 1,2 In the 1960s, short steel fibers were used in concrete to reduce its brittleness. 3 Various natural and synthetic fibers such as sisal, 4 cellulose, 5 asbestos, 6 glass, 7 carbon, 8,9 poly(vinyl alcohol) (PVA), 10–12 polypropyl- ene, 13 and polyethylene 14 were applied by numerous researchers to reinforce concrete. In addition, hybrid fiber systems were used to improve the tensile perform- ance of cementitious composites. 15 The fiber reinforced cementitious composites (FRCCs) have been well stu- died in the last two decades. 16–18 Typical FRCCs exhibit softening behavior after the first cracking. High-performance fiber reinforced cementitious com- posite (HPFRCC) is a class of cement composites, which shows strain hardening behavior accompanied by multiple cracking (leading to a high strain prior to failure) under a tensile load. 19 Multiple cracking and strain hardening behaviors can improve the ductility and the load bearing of the composites after the first crack, respectively. To put it simply, the strength of a strain hardened composite is higher than its first crack- ing strength. SIMCON (slurry infiltrated mat concrete) and SIFCON (slurry infiltrated fiber concrete) are two known examples of HPFRCCs, 20 which have been designed using a great volume fraction (up to 20%) Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran Corresponding author: Farhad Sharif, Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 424 Hafez Avenue, Tehran 15875- 4413, Iran. Email: sharif@aut.ac.ir Journal of Composite Materials 0(0) 1–12 ! The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998317707567 journals.sagepub.com/home/jcm