Probabilistic Engineering Mechanics 21 (2006) 201–206 www.elsevier.com/locate/probengmech Microstructure variability and macroscopic composite properties of high performance fiber reinforced cementitious composites Victor C. Li ∗ , Shuxin Wang Department of Civil and Environmental Engineering, University of Michigan, Rm 2326, GGB Building, Ann Arbor, MI 48109-2125, United States Received 22 March 2005; received in revised form 26 July 2005; accepted 18 October 2005 Available online 28 December 2005 Abstract High performance fiber reinforced cementitious composites have made major advances in recent years, to the point where they are being adopted in building and bridge constructions. The most significant advantage of HPFRCC over conventional concrete is their high tensile ductility. However, the tensile strain capacity has been observed to vary, most likely as a result of the variability of the microstructure derived from the processing of these materials. This paper describes the composite property variability, as well as the variability of the material microstructure. Scale linkage is discussed. In particular, the tensile stress–strain curves, and the crack pattern on uniaxially loaded specimens are presented. The treatment of random fibers in micromechanical models, and tailoring of matrix flaw size distribution for saturated multiple cracking are examined. It is suggested that robust composite properties can be achieved by deliberate control of microstructure variability. Some open issues concerning the randomness of microstructures and possibly related macroscopic behavior are also identified. Further gains in composite property control may be expected from improvements in characterization and modeling of the microstructure randomness. c  2005 Elsevier Ltd. All rights reserved. Keywords: Composite properties; Microstructure randomness; Fiber reinforced concrete; Tensile ductility; Micromechanics 1. Introduction Great strides have been made in the past decade in devel- oping high performance fiber reinforced cementitious compos- ites (HPFRCC) with significant strain-hardening behavior. Ad- vances in HPFRCC have been so rapid that large-scale appli- cation of HPFRCC is now emerging in the field. In particular, Engineered Cementitious Composites (ECC), a special class of HPFRCC, received broad attention due to their high perfor- mance to cost ratio and easy execution in practice. The material has been successfully applied to dam repair, bridge deck over- lay, coupling beam in high-rise building, and other structural elements and systems [2]. The most significant advantage of HPFRCC is their high tensile ductility resulted from multiple cracking, in contrast to the single-cracking and tension softening behavior of concrete and conventional fiber reinforced concrete. High ductility has been recognized, gradually, as having a close association with ∗ Corresponding author. E-mail address: vcli@umich.edu (V.C. Li). structure durability [3]. Many deterioration and premature failures of infrastructure can be traced back to the brittle nature of concrete, and hence HPFRCC are considered a promising solution to the global infrastructure deterioration problem. The majority of HPFRCC are reinforced by short discontinuous fibers. The fiber orientation is determined by processing details as well as fiber content. In most cases, fibers are randomly distributed to some extent. Once a crack is formed in a uniaxial tensile specimen, the bridging fibers across the crack determine the load carrying capacity of the composite. Cracking of the brittle matrix composites is typically initiated at a dominant flaw of the section inherited from processing. In that sense, the composite performance is largely governed by the random nature of fiber location and orientation and flaw size distribution in the cementitious matrix. Despite the apparent similarity in composition of various HPFRCC, e.g. short discontinuous fiber, cementitious binders, aggregates and of course water, the design principles embodied in these materials are rather distinct. For ECC, micromechanics links between microstructure and composite performance are emphasized and the links are used to guide the tailoring of 0266-8920/$ - see front matter c  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.probengmech.2005.10.008