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Flowable concretes can differ significantly from traditional vibrated concrete. Concrete types like self-compacting concrete (SCC), ultra high performance concrete (UHPC) and high performance fibre reinforced cementitious composites (HPFRCCs) require novel mix design approaches. This has consequences for the production and the performance in the hardened state. Mix designs for flowable concretes can incorporate a wide variety of innovative admixtures or components: e.g. superplasticisers increase the flowability and allow for significant reduction of the water content, shrinkage compensating admixtures or superabsorbent polymers support sound and damage free curing processes, viscosity modifying admixtures enhance the robustness, and new fibre types allow for sophisticated and tailored structural performance.
The new Model Code has limitations regarding the application of flowable concrete, e.g. thresholds for the minimum aggregate size and the maximum strength. Provisions are added to include fibres for structural design. fib Task Group 4.3 aims at facilitating the use of innovative flowable materials for designing concrete structures and considers three aspects of flowable concrete: material properties, production effects and structural boundary conditions and performance. This paper reports about the progress of fib TG 4.3 related to the mix design of flowable concrete and discusses the present state-of-the-art concerning admixtures and robustness.
Repair is an indispensable part of the maintenance of structures over their lifetimes. Structural grouting is a widely used remediation technique for concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. A structural grout system should be injectable in narrow spaces and hence include ingredients with finer particles. Ultrafine cements are ideal for these type of demanding grouts due to their superior properties compared to that of the less expensive, but coarser ordinary Portland cement (OPC). Supplementary cementitious materials (SCMs) are often used to replace OPC clinker based binder in order to modify certain properties and to reduce costs. The most commonly used SCMs are fly ash (FA), and ground granulated blast furnace slag (GGBS). For various special applications microsilica (MS), and metakaolin (MK) are also used. Identifying the optimum replacement contents of OPC by SCMs are a challenge during the design of such grouts. The aim of this experimental study is to investigate the effect of the selected SCMs (FA, MS and MK) on the slump flow, time of efflux, viscosity, shrinkage, and compressive and flexural strength of ultrafine cement based grouts with constant water-binder ratio and superplasticizer content. The test program was formulated using Box-Behnken design principles. Maximum percentages of replacement with ultrafine cement was 6% by volume of cement for MS and 16% for FA, and MK. The results suggest that most investigated grouts have the potential to be used for structural applications. The appropriate quadratic models are then formulated through statistical tools and presented as response surfaces. The trends indicate that fly ash improves the rheological properties, whereas microsilica and metakaolin positively affect shrinkage and mechanical properties to some extent. Based on the influence of SCMs and priorities among the properties, Decision Matrix Analysis (DMA) is carried out to select the most suitable ones among the SCMs. The analysis suggests that microsilica and fly ash are more suitable as SCMs than metakaolin without affecting the properties.