Self-Consolidating High Performance Concrete – SCC – Self-Compacting & Highly Flowable Concrete

In this section, robustness is discussed vis-à-vis the sensitivity of concrete mixtures to undergo limited variations in concrete properties of successive batches due to some small changes in material characteristics and placement conditions. Under ordinary processing conditions, SCC representing high level of robustness implies that the concrete is less sensitive to changes in the characteristics of primary mixture constituents (sand fineness, aggregate gradation, sand humidity, characteristics of cementitious materials, etc.) on the filling ability, passing ability, and stability.

A lack of robustness can result in significant bleeding and segregation when the water content is greater than the intended limit (for example, due to changes in sand moisture). A more robust SCC enables the concrete supplier to provide better consistency in delivering SCC that is less prone to inaccuracies in batching. This can reduce any intervention needed at the plant or job site to adjust the mixture. Typically, SCC made with low content of VMA and relatively low water content can represent greater robustness than SCC made with low binder content and higher dosage of VMA. In such mixtures, the VMA is used to reduce the variability of the SCC that can arise from changes in material properties and placement conditions. The incorporation of VMA controls bleeding and segregation and increases the robustness of the SCC, while the low water content provides mostly the required level of viscosity. Sakata et al.[i] reported that SCC made with low w:p ratio of 0.33 (powder containing limestone filler), the incorporation of a small concentration of welan gum of 50 g/m3 can reduce the variability in slump flow of SCC due to changes in cement Blaine (318 to 342 m2/kg), fineness modulus of sand (2.08 to 3.06), and temperature of fresh concrete (10 to 30°C).

Hwang and Khayat[ii] suggested using the minimum water content (MWC) index to determine the robustness of SCC. The MWC is determined for concrete-equivalent mortar as the slope of the increase in flow diameter determined using a mini slump flow cone vs. the increase in w/cm. Concrete-equivalent mortar exhibiting greater MWC can result in lower degree of increase in flow after a given increase in water content, hence more robust. Mixtures made with naphthalene-based superplasticizer are shown to have greater robustness (greater RWD) than similar mixtures with polycarboxylate-based superplasticizer. The type of binder is also shown to affect RWD and robustness.

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[i] Sakata, N., Marruyama, K. and Minami, M., "Basic properties and effects of welan gum on self-consolidating concrete," RILEM Proceedings 32, Production Methods and Workability of Concrete, Paisely, Ed. Bartos, P.J.M., Marrs, D.L. and Cleland, D.J. (1996) 237-253.

[ii] Hwang, S.-D., Khayat, K.H., Performance of Hardened Self-Consolidating Concrete Designated for Repair Applications, ACI SP-233 "Workability of SCC: Roles of It's Constituents and Measurement Techniques," Ed. Shi, C., Khayat, K.H., 2006.