Application of nanoindentation testing to study of the interfacial transition zone in steel fiber reinforced mortar Xiao Hui Wang a, ⁎, Stefan Jacobsen b , Jian Ying He b , Zhi Liang Zhang b , Siaw Foon Lee b , Hilde Lea Lein c a Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, China b Department of Structural Engineering, Faculty of Engineering Science and Technology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway c Department of Materials Science and Engineering, Faculty of Engineering Science and Technology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway abstract article info Article history: Received 14 August 2008 Accepted 13 May 2009 Keywords: Nanoindentation Scanning Electron Microscopy (SEM) Interfacial transition zone (ITZ) Elastic modulus and hardness Steel fiber reinforced mortar The characteristics of the profiles of elastic modulus and hardness of the steel fiber–matrix and fiber–matrix– aggregate interfacial zones in steel fiber reinforced mortars have been investigated by using nanoindentation and Scanning Electron Microscopy (SEM), where two sets of parameters, i.e. water/binder ratio and content of silica fume were considered. Different interfacial bond conditions in the interfacial transition zones (ITZ) are discussed. For sample without silica fume, efficient interfacial bonds across the steel fiber–matrix and fiber–matrix–aggregate interfaces are shown in low water/binder ratio mortar; while in high water/binder ratio mortar, due to the discontinuous bleeding voids underneath the fiber, the fiber–matrix bond is not very good. On the other hand, for sample with silica fume, the addition of 10% silica fume leads to no distinct presence of weak ITZ in the steel fiber–matrix interface; but the effect of the silica fume on the steel fiber– matrix–aggregate interfacial zone is not obvious due to voids in the vicinity of steel fiber. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fiber reinforced mortar is a composite structure at a microscopic scale. Its properties rely on the matrix, aggregate, fiber and the inter- facial transition zone (ITZ) between the two. The transition area con- sists of a loose unit of hydrates of the neighboring cement grains. It consolidates with the progressing hydration and reaches in its “final phase” a porosity of about 50% [1]. The origin of the ITZ mainly lies in the so called “wall” effect of packing of cement grains against the relatively flat aggregate surface [2] or fiber, or steel surface, micro- bleeding effect which results in the accumulation of water under the aggregate particles and the flocculation effect of the small cement grains [3]. Due to the way it is formed the ITZ is not a definite zone, but a region of transition. Its effective thickness varies with the micro- structural feature being studied and during the course of hydration [2]. The typical width of the ITZ between the aggregate and matrix is 50 μm; however, different researchers obtained different thicknesses of ITZ from their tests. For instance, the thickness of the transition zone between the aggregate and matrix ranged from 10 μm to about 30 μm [1]; Ollivier et al. [3] argued that there was only thickness of 15 μm to 20 μm around the aggregates, just equaling to the mean diameter of the cement grains. For the fiber–mortar interface, where amorphous cast iron fibers were used, fiber–matrix debonding generally occurred at some distance – about 5 μm – from the fiber surface where the porous zone was the weakest [4]. Although the size of the ITZ varied with different fiber type and fiber size as well as matrix details, most observations suggested a relatively large porous and weak layer on the order of 40 μm to 70 μm thickness [5]. For the ITZ around steel reinforcement, it was shown that the minimum micro-mechanical properties occurred at 10 ±30 μm from the actual steel interface [6]. In the study of ITZ, a key question is to what extent the existence of ITZ has any practical influences on the engineering properties of cementitious materials, or it is just a peculiarity of academic interest [7]. There are two contrary opinions about this problem: some re- searchers argued that the ITZ is the weakest link between the cement paste and the aggregate, so it has a significant role in determining the properties of all cementitious composites [8–10]; however, Diamond and Huang [11] pointed out, that there is no reason to assume the significant negative effects of ITZ on permeance or mechanical properties of concrete, even for concrete with a water/binder ratio of 0.5. The reason for lack of conclusive evidences provided by various experimental researches carried out to the ITZ mainly lies in the limited sensitivity of the experimental technique or inappropriate methods yielding biased information [12]. In order to resolve this issue two committees were set up by RILEM and the conclusion are as follows [7]: 1) the ITZ should be viewed as a system property which is dependent on the overall composition and the method of fabrication of the cement composite; 2) the properties of the ITZ may have a moderate influence on the mechanical properties of concrete but not a drastic one; 3) the ITZ may have a drastic effect on the mechanical Cement and Concrete Research 39 (2009) 701–715 ⁎ Corresponding author. Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, China. Tel.: +86 13167078307 or +86 47 73 597155. E-mail address: w_xiaoh@163.com (X.H. Wang). 0008-8846/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2009.05.002 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp