Effect of fiber shape and morphology on interfacial bond and cracking behaviors of sisal fiber cement based composites Flávio de Andrade Silva a , Barzin Mobasher c,⇑ , Chote Soranakom b , Romildo Dias Toledo Filho a a Civil Engineering Department, COPPE, Universidade Federal do Rio de Janeiro, P.O. Box 68506, CEP 21941-972 Rio de Janeiro, RJ, Brazil b Infrastructure Monitoring and Management System (IMMS Co., Ltd.), Bangkok, Thailand c School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA article info Article history: Received 10 November 2010 Received in revised form 9 May 2011 Accepted 9 May 2011 Available online 18 May 2011 Keywords: Natural fibers Sisal Pull-out Interface Cement based composites abstract An experimental investigation was performed to understand the pull-out behavior of sisal fibers from a cement matrix. The effect of curing age and fiber embedment length on the fiber–matrix interface was studied. Sisal fiber presents irregular cross-section with different shapes that may be beneficial for the bond strength. A scanning electron microscope coupled with image analysis was used to measure the cross-section area of individual tested fibers and to determine and classify their morphology. The results were correlated to the fiber morphology. Direct tension tests were performed on composites reinforced by 10% in volume of continuous aligned sisal fiber. A finite difference model developed earlier by authors was used to determine the bond strength versus slip constitutive relation from experimental data and to predict the composite tensile behavior and crack spacing. It was found that the sisal fiber morphology plays an important role in the bond strength. Average adhesional bond strength as high as 0.92 MPa were reported for the fiber shape that promoted the best interfacial performance. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Cement based composites reinforced with continuous aligned sisal fibers demonstrate a tension-hardening with multiple crack- ing behavior [1] with high tolerance to fatigue loading [2] and high energy absorption capacity under dynamic loading [3]. This type of composite system is reinforced with up to five layers of fibers resulting in a total volume fraction of 10%. In humid environments the sisal fiber cement composites produced with ordinary Portland cement matrices undergo an aging process during which they may suffer a reduction in post-cracking strength and toughness. This process is a result of migration of hydration products (mainly Ca(OH) 2 ) to the fiber structure. To mitigate this effect a special ma- trix that has 50% of cement replaced by calcined clays has been re- cently developed and optimized for use with sisal fiber systems [4]. This matrix lowers the calcium hydroxide production resulting in enhanced durability against fiber degradation and also providing adequate rheology in the fresh state for the fiber volume fractions proposed. The multiple cracking behavior achieved is governed by interfacial bond characteristics between fiber and matrix. A significant amount of experimental and analytical investiga- tions have been dedicated to the mechanical characterization of interface in man-made-fiber cement matrix systems. Tests for bond adhesion of cementitious composites have been performed by many researchers; however, limited data has been presented for natural fi- bers. Most of the interface characterization work has been per- formed on steel, glass and polymeric fibers. Naaman and Najm [5] state that there are four main factors that influence the bond be- tween fiber and matrix: (i) physical and chemical adhesion, (ii) mechanical component of bond such as deformed, crimped and hooked end fibers, (iii) fiber-to-fiber interlock, and (iv) friction. Peled and Bentur [6] investigated the pull-out behavior of straight and crimped polyethylene yarns. They found that increasing the crimp density enhances the mechanical anchoring and the equivalent adhesion bond strength increases from 1 to 1.84 MPa (10 mm fiber embedded length). Markovich et al. [7] studied the pull-out behavior of hooked end steel fibers for different types of matrices and reported an average frictional stress between 2.76 and 4.97 MPa depending on the mixture. Kim et al. [8] investigated steel hooked ended and torroidal shaped fibers. Equivalent bond stresses calculated from experimental pull-out of hooked end and torroidal fibers were 4.7 and 14.5 MPa, respectively. Several models have been used for prediction and characteriza- tion of pull-out behavior in fiber reinforced cement composites. Naaman et al. [9] proposed an analytical model for smooth fibers or bars with an idealized bond–stress–slip relationship of the interface. The solution led to the prediction of the bond shear stress-versus slip curve assuming that one can employ a back- calculation procedure to use the pull-out load versus slip in 0958-9465/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconcomp.2011.05.003 ⇑ Corresponding author. Tel.: +1 480 965 0141; fax: +1 480 965 0557. E-mail address: barzin@asu.edu (B. Mobasher). Cement & Concrete Composites 33 (2011) 814–823 Contents lists available at ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp