ELSEVIER Cement and Concrete Composites 20 (1998) 171-188 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 095%9465/98/$19.00 PlI:SO958-9465(97)00064-4 Viscosity-Enhancing Admixtures for Cement-Based Materials - An Overview Kamal H. Khayat UniversitC de Sherbrooke, Department of Civil Engineering, Sherbrooke, Que. JlK 2R1, Canada Abstract INTRODUCTION zyxwvutsrqponmlkjihgfedcbaZYXWVU Kscosity - enhancing admixtures, also known as unti-washout admixtures, are water-soluble poly- mers that increase the viscosity and cohesion of c.ement-based materials. Such enhancement of the liquid- phase viscosity is essential in jlowable sys- tems in order to reduce the rate of separation of material constituents and improve the homo- geneity and performance of the hardened product. Viscosity - enhancing admixtures are mostly used along with a high-range water reducer to obtain a highly fluid, y et cohesive cement- based material that can flow readily into place with minimal separation of the various constituents of different densities and minimal intermixing with the sur- rounding w ater w henever cast under w ater Viscosity-enhancing admixtures (VEAs), also known as anti-washout admixtures, are rela- tively new admixtures used to enhance the cohesion and stability of cement-based systems. Such VEAs are water-soluble polysaccharides that enhance the water retention capacity of the paste. They are used in concrete intended for underwater repair of marine and hydraulic structures, and tremie concrete for the con- struction of curtain walls and deep foundation walls, etc. Such admixtures can also reduce the risk of separation of the heterogeneous constit- uents of concrete during transport, placement, and consolidation and provide added stability to the cast concrete while in a plastic state. This paper reviews the types and modes of uction of commonly used viscosity - enhancing admixtures and highlights their influence on the rheological properties of water and cement paste. An overview of the influence of various types of \.iscosity-enhancing admixture on high- range rt,ater reducer demand, resistance to water dilu- tion, static and forced bleeding, segregation, settlement, setting time, and air entrainment is presented. The influence of such admixtures on bond to anchored reinforcing bars, frost durability, mechanical properties, and rapid-chloride perme- ability is also highlighted. Special applications 11 here such relatively new admixtures can signi@ cantly enhance pe$ormance are highlighted, including their incorporation in concrete intended for underwater placement and repail; self-consol- idating and segregation- free concrete for above- water construction, and structural grout for filling post-tensioning ducts. 0 1998 Elsevier Science Ltd. All rights reserved. Keywords: rheology enhancement, high perform- ance concrete, stability of cement suspensions. 171 Flowable concrete is commonly used for cast- ing congested structural members that are often encountered in heavily reinforced mat founda- tions and in reinforced concrete structures in seismic regions. It is also used in restricted areas where the access for placement and con- solidation is limited, as in the case of tunnel lining. The use of flowable concrete to facilitate the casting of congested or restricted areas can result in an unstable dispersion of cement paste and aggregate particles since the tendency of the heterogeneous materials to separate increases with the reduction in viscosity. This can be obtained when the consistency of the flowable concrete increases or when the con- crete is subjected to high shear rate, such as that encountered in pumping and consolidation. The incorporation of a VEA in flowable con- crete can enable the production of a stable and yet highly flowable concrete to facilitate filling congested reinforced members with minimal vibration and segregation. The improved homo- geneity of the concrete can enhance bond