Introduction
The most detrimental property of advanced cement-based materials is their brittleness as characterized by their poor tensile strength, low tensile strain to failure, and correspondingly inferior toughness or fracture resistance. Improving the toughness of these materials has been, and will continue to be, the focus of several research studies at ACBM.

Several strategies can be utilized to reach this goal: tailor the matrix composition and particle size (cement, water, mineral admixtures, aggregates, fillers, etc.); incorporate fibers; and use processing alternatives (extrusion, pultrusion, etc.). While each depends on numerous parameters, they have at least one common variable of paramount importance, namely the interface between the cement paste phase and other phases in the system, such as an aggregate, a particulate filler, or a fiber. The interfacial zone is essential for composite action in fiber and non-fiber reinforced systems, because it influences both the strength and toughness of the material. Therefore, much of the fundamental work described in the research theme on interfaces is directed towards understanding the role of the interfacial zone in fracture. The results of these studies will be incorporated into micromechanical models and subsequently into macroscopic continuum models and constitutive relationships of advanced cement-based materials. ^Top

Toughness Improvement By Fiber Reinforcement
The proper characterization of bond at the fiber-matrix interface is a key element for understanding the behavior, and for modeling the response under load of high performance fiber reinforced cement and ceramic-based composites. It allows for the rational prediction of their mechanical, fracture, and ductility properties. From a mechanical viewpoint, it has been shown that the bond at the fiber-matrix interface can be best characterized by a bond stress versus slip relationship. However, the nature of bond in these systems is very complex, involving a cohesion/adhesion component, a decaying frictional component affected by radial stresses, and a mechanical component. ^Top

Modeling Composite Behavior
The micromechanics model previously developed predicts the response in tension of fiber reinforced cement composites with continuous fibers. This model will be extended to the case of discontinuous randomly oriented fibers, using the micromechanics inclusion theory and the star shaped inclusion theory, with particular attention to optimizing composite toughness. In a parallel study, homogenization theory and composite micro-mechanical models will be linked in a finite element scheme. ^Top

Compression Failure
For design of concrete structures, one needs to know the constitutive response of concrete subjected to compression. Compression failure of quasi-brittle material, such as concrete, involves growth of axial and inclined cracks, localization, and shear bands. The macroscopic, average stress-strain response, is influenced by size and shape of the specimen, gage length, and the boundary conditions. A study is planned to develop a comparative understanding of failure mechanism of concrete in compression. ^Top

Coordinator: Antoine Naaman (University of Michigan)

Evaluation of Fracture Processes in the Cement-Based Materials Using Electronic Speckle Pattern Interferometry and Computer Vision
PI: Surendra Shah (Northwestern University)

Rate Effects in the Fracture of Concrete and Cement-Based Materials
PI: Zdenik Bazant and Katherine Faber (Northwestern University)

Microchemical Behavior of Concrete Reinforced with Arbitrarily Oriented Discontinuous Fibers
PI: Toshio Mura (Northwestern University)

Contribution of Mechanical Bond to Toughness to FRC Composites
PI: Antoine Naaman (University of Michigan)

Fracture Surface and Fracture Behavior (Using Confocal Microscopy)
PI: David Lange (University of Illinois)