Introduction
A significant, common topic in ACBM research has involved the effects of interfaces. Unlike classical composites, in which internal interfaces are typically well-controlled and of a limited and well-defined area (and in which perfect bonding can be assumed for most purposes), the interfaces in cement-based materials are extensive, geometrically complex, and usually constitute inherently weak zones that limit performance. The studies on interfacial effects, which are being performed, are aimed at understanding, quantifying, and eventually overcoming the limitations associated with interfaces.

Several types of interfaces (or more properly, interfacial zones) are important. Interfaces between hydrated cement paste and aggregates (fillers) play a major role in the fracturing processes in cement-based materials. Special interfacial effects introduced by the incorporation of fibers are of particular theoretical and practical concern, especially in connection with creating composites of high toughness. Interfaces with steel reinforcement (which is almost universally used in construction) form a special class whose characteristics are critical in controlling deterioration due to steel corrosion in concrete structures. ^Top

Paste-Aggregate Interfaces
ACBM will pursue research to characterize the mechanics of the paste-aggregate interfacial zone and develop constitutive relationships for the interfacial zone itself through controlled closed loop testing. Experimental studies involving in situ ESEM fracture and microscopic analyses of fracture surfaces will further clarify the influence of interfacial properties in fracture. ^Top

Steel Interfaces
One of the most pressing issues in concrete construction is the corrosion of embedded reinforcing steel. Depassivation and pitting corrosion may occur, especially in the presence of chloride ions; the effects of this chloride induced corrosion constitute a multi-billion dollar problem in parking garages and in highway bridge decks alone. ACBM is studying the interfacial electrochemical effects that accompany corrosion and will examine various inhibition strategies, in an attempt to better understand the nature and stability of the pressure film and the exact mechanisms of inhibition. ^Top

Nanoscale Interfaces
More general techniques for modifying interfacial zones and binder phases, by incorporation of organic components are part of a major program on synthesis and characterization of novel polymer-cement composites. The overall objective is the development of new classes of organo-ceramic composites including organo-calcium aluminate cements, organo-phosphate cements, and organo-silicate cements using calcium silicates. The organo-calcium aluminate cements contemplated are extensions of current research involving calcium aluminate shells surrounding cores that are mixed with water-soluble polymers, and processed either by techniques analogous to those used to form MDF cements, or by more conventional casting or compaction. ^Top

Coordinator: Hamlin Jennings (Northwestern University) Characterization of the Paste Aggregate

Interface Using Confocal Microscopy
PI: Surendra Shah and Hamlin Jennings (Northwestern University)

Study of Fiber-Matrix Interface in Cement Composites
PI: Katherine Faber and Surendra Shah (Northwestern University)

Electrochemical Impedance Spectroscopy Studies of the Steel/Cement Interface
PI: Thomas Mason (Northwestern University)

Passivation Mechanism of Steel in Concrete in the Presence of Corrosion Inhibitors
Influence of Polymers on the Behavior of Cement-FiberInterface
PI: Richard Robertson and Antoine Naaman (University of Michigan)