Materials and Structures/Materiaux et Constructions, Vol. 33, March 2000, pp 82-87 RI/EM TC 159-ETC: ENGINEERING OF THE INTERFACIALTRANSITION ZONE IN CEMENTITIOUS COMPOSITES A review of the work of the RILEM TC 159-ETC: Engineering of the interfacial transition zone in cementitious composites Prepared by A. Bentur, National Building Research Institute-Faculty of Civil Engineering, Technion, Israelhlstitute of Technology, Haifa, Israel and M. G. Alexander Department of Civil Engineering, University of Cape Town, South Africa TC MEMBERSHIP: Co-chairs: A. Bentur, Israel; M. G. Alexander, South Africa. Members: D. Bentz, USA; O. Buyukozturk, USA; J. Elsen,Belgium; D. Hooton, Canada;H. Jennings,USA;A. Katz, Israel;K. O. Kjellsen, Sweden; A. Kronlof, Finland,B. Laggerblad, Sweden; S. Mindess,Canada; J.-P. OUivier,France,K. Scrivener, France; J. Skalny,USA; L:J. Struble,USA andJ. G. M. Van Mier, The Netherlands. ABSTRACT The present paper presents an overview of the work of the RILEM Technical Committee 159-ETC on the engineering of the interfacial transition zone in cementi- tious composites. The object of this committee was to resolve the practical influences of the interfacial transi- tion zone (ITZ) in cementitious composites on their engineering properties. The major issues highlighted by the committee are: (i) The ITZ should not be viewed as a well defined material property but rather as a system property which is dependent on the overall composition as well as the method of fabrication of the cement com- posite. (ii) The structure of the ITZ should be quantified experimentally and by modeling in terms of gradients of microstructure. (iii) The quantification of the structure and mechanical properties of the ITZ is based on either scaling up of the interface by making special composite specimens or by in-situ testing of the actual composite by micro-level techniques. The former can be quantified more readily, but the interface which its specimens rep- resent may be quite different than the one in the real composite. (iv) Inverse modeling can provide a strategy for determining the properties of the ITZ by testing of the whole composite and modeling its behavior. (v) In concretes the characteristics of the ITZ may have a mod- erate influence on mechanical properties but not a dras- tic one. (vi) The ITZ may have a drastic effect on the mechanical properties of fiber reinforced cement com- posites and on their long term properties. 1. INTRODUCTION Traditionally, the properties of the most important cementitious material, concrete, are considered to be a function of the characteristics of the bulk cement paste matrix. This assumption forms the basis for the working hypothesis which enables adequate modeling of the behavior of traditional concrete mixes. In the last two decades a vast body of research has developed showing that the paste structure in concrete is not necessarily uni- form, due to the formation of an interfacial transition zone (ITZ) around inclusions in cement paste (e.g. aggregates, fibers) where the microstructure of the paste is different from that of the bulk paste. The dependence of the ITZ structure on various parameters such as the composition of the binder and the inclusion has been studied intensively. Several methods have been devel- oped to characterize the composition and structure of the ITZ and its changes as a function of the distance from the inclusion surface. The state of the art of the structure and characterization of the ITZ was the topic of the previous ILILEM Technical Committee 108-ICC, Interface in Cementitious Composites. It published two reports, one the proceedings of a conference [1] and the other a State of the Art Report [2]. This committee completed its work in 1992. A key question that needed to be resolved was to what extent the existence of ITZ had any practical influ- ences on the engineering properties of cementitious materials, or was it just a peculiarity of academic interest. The question arose in view of the fact that for traditional concretes, the properties could be reasonably accounted for by the bulk properties of the cementitious matrix. To resolve this issue two committees were set up by 1LILEM, one dealing with engineering properties (TC 159-ETC: Engineering of the Interracial Transition Zone in Cementitious Composites) and the other with transport properties which are related to durability (TC 163-TPZ: Transport Properties of the Interfacial Transition Zone). In discussing the practical influence of ITZ, one has to take into consideration that in the last two decades major strides have been taken to develop advanced high quality cementitious materials such as high strength con- cretes and fiber reinforced cements. These advanced materials are essentially composites where the inclusions 1359-5997/00 9 1LILEM 82