154 ACI Materials Journal/March-April 2009 ACI MATERIALS JOURNAL TECHNICAL PAPER ACI Materials Journal, V. 106, No. 2, March-April 2009. MS No. M-2008-134 received April 24, 2008, and reviewed under Institute publication policies. Copyright © 2009, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including authors’ closure, if any, will be published in the January-February 2010 ACI Materials Journal if the discussion is received by October 1, 2009. The investigation reported in this paper seeks to develop a method- ology to evaluate the rheological parameters and thixotropy of self-consolidating concrete (SCC) using those of concrete-equivalent mortar (CEM). The mixture proportioning of CEMs are derived from their corresponding concrete mixtures by eliminating the coarse aggregate fraction and replacing it by a certain mass of sand of the same surface area. SCC mixtures with water-cementitious material ratios (w/cm) of 0.35, 0.38, and 0.42 and coarse aggregate-total aggregate volume ratios (CA/A) of 0.44 to 0.53 were investigated. The mixtures with a w/cm of 0.38 and 0.42 incorporated low and moderate dosages, respectively, of a viscosity-modifying admixture to enhance stability. For each SCC, the dosage of high-range water-reducer (HRWR) was varied to cover a wide range of slump flow consistencies ranging between 570 and 730 mm (22.4 and 28.7 in.). All SCC mixtures were proportioned with 450 kg/m 3 (758.5 lb/yd 3 ) of ternary silica fume- ground granulated blast-furnace slag cement. A simple method is also proposed to determine the HRWR demand of SCC from that of the corresponding CEM. Test results showed that a good correlation can be established between the yield stress, plastic viscosity, and thixotropy of SCC and their corresponding CEM mixtures. Both thixotropy and plastic viscosity of SCC mixtures and their corresponding CEM mixtures are shown to vary primarily with variations in the w/cm and relative volume of coarse aggregate. Keywords: concrete-equivalent mortar; plastic viscosity; rheology; self- consolidating concrete; thixotropy; yield stress. INTRODUCTION Self-consolidating concrete (SCC) is a high-performance concrete that can flow under its own weight to completely fill the formwork and self-consolidate without any mechanical vibration. The use of SCC facilitates the placement process of concrete in densely reinforced members and through restricted sections. Such concrete must achieve excellent deformability, low risk of blockage, and good stability to ensure high filling capacity of the formwork. SCC has a low yield stress and a moderate viscosity to ensure high filling ability and segregation resistance; proper resistance to segregation is essential to prevent blockage and ensure homogeneous deformation of the concrete through restricted sections. 1 Given the numerous factors affecting flow properties of SCC, the mixture design and production of SCC can involve balancing several factors affecting material performance, including a reduction in coarse aggregate and free water content, an increase in paste volume, sand/paste ratio, and dosage of high-range water-reducer (HRWR). 1 In general, the reproducibility of SCC and repeatability of workability test methods are lower than those for conventional concrete. These factors inherently necessitate more trial batches and greater precision when designing SCC mixtures. Therefore, attempts are made to correlate the flow properties of SCC to those of mortar to facilitate material selection and mixture optimization protocols. To understand fresh concrete behavior and predict the flow properties of concrete based on those of mortar, Banfill 2 recommended using mortar containing sand particles smaller than 2 mm (0.08 in.). Petit et al. 3 successfully investigated the yield stress and plastic viscosity of concrete by considering the micro-mortar phase of concrete mixtures where only the volume fraction of aggregate smaller or equal to 315 μm (0.012 in.) is considered. Yahia et al. 4 proposed a relationship between the slump flow of SCC and mortar sampled from the mixer before coarse aggregate addition. From the results of 27 measurements, the following relationship was proposed: SFC = 20 + 2.48 × (SFM – 10), where SFC and SFM are the slump flow values of concrete and mortar (in cm), respectively. Many studies attempted to estimate the rheological parameters of concrete by considering concrete as a suspension in which solids are dispersed into a fluid phase. For modeling the rheological properties of concrete, Noor and Uomoto 5 assumed that the yield stress of high-flowing concrete is a function of the volume fraction of aggregate and the yield stress of the mortar; in this case, the mortar was separately mixed without any wet sieving. On the other hand, Ferraris and de Larrard 6 expressed the yield stress of the concrete as a function of the contents of cement, sand, and coarse aggregate. Nielsen, 7 Geiker et al., 8 and Murata and Kikukawa 9 considered SCC as a two-phase system into which coarse aggregates are dispersed into the mortar fluid phase, allowing estimation of the rheological properties of concrete from those of the mortar phase. Wallevik 10 proposed an empirical linear relationship between the yield stress of concrete and that of the mortar for concrete mixtures prepared with fixed coarse aggregate volume. Modeling of flow properties of concrete can also be carried out using the concrete equivalent mortar (CEM) method 11 where the coarse aggregate fraction is replaced by sand of equal surface area. The total surface area of aggregate to be coated with cement paste is then unchanged. The CEM method has been employed to investigate the effect of material properties on SCC performance, including binder-admixture interaction, packing density of the binder materials, and robustness of the fresh material. 12 Limited studies are, however, available to correlate the rheological properties of CEM and SCC and to quantify the relative effect of mixture proportioning on the rheological properties of SCC and corresponding CEM. Title no. 106-M19 Correlating Rheology of Self-Consolidating Concrete to Corresponding Concrete-Equivalent Mortar by Tahir Kemal Erdem, Kamal H. Khayat, and Ammar Yahia