Steffen Grünewald

Parameter-study on the influence of steel fibers and coarse aggregate content on the fresh properties of self-compacting concrete

Self-compacting concrete (SCC) offers several economic and technical benefits; the use of steel fibers extends its possibilities. Steel fibers bridge cracks, retard their propagation, and improve several characteristics and properties of the concrete. Fibers are known to significantly affect the workability of concrete. Therefore, an investigation was performed to compare the properties of plain SCC and SCC reinforced with steel fibers. Two mixtures of SCC with different aggregate contents were used as reference. Each of the concretes was tested with four types of steel fibers at different contents in order to answer the question to what extent the workability of SCC is influenced. The slump flow, a fiber funnel and the J-ring test were used to evaluate the material characteristics of the fresh concrete. This paper discusses the suitability of the applied test methods and the effect of the coarse aggregate content, the content and type of steel fibers on the workability of SCC.

Improved Tensile Performance with Fiber Reinforced Self-compacting Concrete

The use of self-compacting concrete (SCC) eliminates the need for compaction, which has benefits related to economic production, the durability, the structural performance and working circumstances. SCC is able to transport fibers which can replace in some structures conventional reinforcement. By taking into account tailor-made concrete characteristics, new fields of structural application can be explored. This paper discusses the potential for an improved performance of fibers in self-compacting concrete. In flexural tests significant differences were observed between conventional and self-compacting concrete at a given fiber type and dosage concerning the variation of results and the flexural performance. Mechanical testing and image studies on concrete cross-sections indicate how the flow influences the performance, the orientation and the distribution of the orientation of fibers. Differences between traditionally compacted and flowable concrete are pointed out.

Fibre reinforcement and the rheology of concrete

Abstract: The addition of fibres can improve the performance of cementitious materials in the hardened state. With an optimized mixture composition and controlled rheology and quality, fibres can become more effective. Significant progress has been made during the past years on the field of flow simulations and the rheology of fibre suspensions. The main difficulties are related to the non-Newtonian behaviour of fibre suspensions (i.e. shear-thinning due to fibre orientation and local flow-induced structures) and difficulties in predicting and measuring the different contributions of, for example, hydrodynamic effects and mechanical interaction. Fibre rheology and flow simulation are excellent tools to optimize fibre suspensions and form the basis for predictions of structural performance.

The Impact of VMA on the Rheology, Thixotropy and Robustness of Self-compacting Mortars

Due to the higher sensitivity of fresh self-compacting concrete (SCC) to small variations in the mix proportions – also referred to as a lower robustness – applications with SCC are still limited. Because viscosity modifying admixtures (VMAs) are often reported to increase the robustness of SCC, the relationship between robustness, rheological characteristics, and thixotropic build-up of self-compacting mortars is examined whit an experimental program. The robustness with regard to small variations in the water dosage is measured together with the rheological and thixotropic properties of mortars made with various admixtures (purified attapulgite clay, diutan gum, corn starch, and propylene carbonate). Based on those results, the possible connections between the rheology, thixotropy and robustness of self-compacting mortar were evaluated.

Maximum Fiber Content and Passing Ability of Self-Consolidating Fiber-Reinforced Concrete

This paper will discuss how self-consolidating fiber-reinforced concrete (SCFRC) combines the benefits of self-consolidating concrete (SCC) in the fresh state and an enhanced performance of fiber reinforced concrete (FRC) in the hardened state. The application of SCC improves the efficiency at building sites, allows rationally producing prefabricated concrete elements and improves the working conditions, the quality and the aesthetic appearance of concrete structures. By adding fibers to SCC bar reinforcement can be replaced, crack widths reduced, the durability improved and the load bearing capacity of a structure increased. An extensive research study was carried out on the characteristics and the mix design of SCFRC that consisted of three parts: the fresh as well as the hardened state of SCFRC and the influence of the production process determined in three full-scale studies. This paper discusses two aspects of the mix design of SCFRC: the maximum fiber content and the required spacing of reinforcement at which blocking does not occur. Based on the analysis of experimental results mix design tools are proposed that allow predicting the maximum fiber content and the passing ability of SCFRC, which is essential information to obtain a homogeneous distribution of the fibers in a structure.

Design with Highly Flowable Fiber-Reinforced Concrete: Overview of the Activity of fib TG 8.8

Self-compacting fiber-reinforced concrete (SC-FRC) combines the benefits of highly flowable concrete in the fresh state with the enhanced performance in the hardened state in terms of crack control and fracture toughness provided by the wirelike fiber-reinforcement. Thanks to the suitably adapted rheology of the concrete matrix, it is possible to achieve a uniform dispersion of fibers, which is of the foremost importance for a reliable performance of structural elements. Balanced viscosity of concrete may also be helpful to drive the fibers along the concrete flow direction. An ad-hoc designed casting process may hence lead to an orientation of the fibers “tailored” to the intended application, which is along the anticipated directions of the principal tensile stressed within the structural element when in service. This converges towards a “holistic” approach to the design of structure made with highly flowable/self-consolidating FRC, which encompasses the influence of fresh state performance and casting process on fiber dispersion and orientation and the related outcomes in terms of hardened state properties. The fib task Group 8.8 “Structural design with highly flowable concrete”, sub-group fiber concrete, appointed in April 2009, aims at drafting recommendations to facilitate and spread the use of these innovative materials, merging together research findings and practical experience.

Structural Design with Flowable Concrete - A fib-Recommendation for Tailor-Made Concrete

Flowable concrete (either compacted with some vibration or selfcompacting) is becoming a widely applied building material. Due to its flowable nature, reinforcing bars can become an obstacle, mixture components may float or segregate and the casting technique determines the orientation of fibers, if any. An increasing range of components is available to optimize concrete concerning rheological and hardened state properties and for the application under consideration. Flowable concrete offers an extended range of engineering properties and the potential for product innovation. fib Task Group (TG) 8.8 “Structural Design with Flowable Concrete” started in 2009 to facilitate the use of innovative flowable materials for the design of concrete structures. Taking into account research findings and practical experience, the main objectives of fib TG 8.8 are to write a state-of-the-art report and recommendations on the structural design with flowable concrete. fib TG 8.8 considers three aspects of flowable concrete: material properties, production effects and structural boundary conditions. This paper discusses the scope of fib TG 8.8 concerning the characteristics and the potential of flowable concrete and presents related design standards. fib TG 8.8 aims at promoting the application of flowable concrete, improving and adapting the concrete design and the production technology and its implementation in guidelines and codes.

Deliberate Deformation of Concrete in the Fresh State - Crack Risk and Efficient Production of Curved Precast Elements

The production of double-curved precast concrete elements for cladding or shell structures requires expensive CNC (computer numerical control)-milled formwork. As an alternative method, the innovative flexible mould for economically efficient and sustainable production of such elements is discussed in this paper. This method comprises the use of a flexible, CNC-controlled formwork, which is filled with self-compacting concrete. After a short period of thixotropic stabilization in the fresh state, the flexible mould is then deformed into its desired geometry, typically having a strong curvature radius of only a few metres in one or two direction(s). After hardening and de-moulding, the flexible mould can be reused for elements with the same or different curved geometry. The present paper describes the outcomes of a study focussing on two aspects relevant for the abovementioned production method: effect of change of rheological properties in the first 90 min after casting and assessment of the risk of cracking and development of cracks during the deformation process. In an experimental study the following parameters were modified: radius of deformation, moment of deformation in time, panel thickness and water-cement ratio. The presence of cracks after deformation was investigated quantitatively, using a petrographic technology. The results show that for the application of the flexible mould method the plastic stage of concrete is important to be considered.

Case-Study on the Application of Precast Double-Curved Concrete Elements for the Green Planet Shell Structure

Double-curved structures in general, and monolithic concrete shell structures more specifically, can transfer forces very efficiently. As a result, the thickness-to-span ratio can be very low, which, material-wise, can lead to a very economical design. However, the construction of shell structures is very labour-intensive and comes with high formwork costs and shells in modern building practice are rarely constructed. Concrete shell structures can be cast in-situ making use of temporary formwork and falsework, but they can be (partially) prefabricated as well, like the Palazzetto dello Sport in Rome. Although precasting is an effective technology for the repetitive production of concrete elements, for double-curved structures, having a large variety of shapes, the advantages of precasting seem to diminish quickly as a result of high formwork costs. Another disadvantage of precasting shell elements obviously seems to be the complexity of the required connections. For shell structures, the loss of stiffness of the connections might even lead to a crucial reduction of the buckling stability. A combination of both building methods, the prefabrication of the supportive structure and a finish with a cast in-situ layer, solves this before-mentioned issues and the advantages of both methods are combined: reduction of the complexity of the connections with an in-situ cast concrete layer and integration of the supportive structure in the design for a more cost-efficient erection. This paper describes the study of an innovative, partially precast, alternative solution for the construction of shell structures, and specifically addresses the influence of connections between precast elements on the overall shell behaviour. The Green Planet gas station along the A32 highway in The Netherlands was selected as a design case for such a building method.