Effect of the mix design on the robustness of fresh self-compacting concrete
Introduction
Self-compacting concrete (SCC) is a highly flowable type of concrete increasingly used in the precast concrete industry. Unlike ordinary vibrated concrete, SCC does not need any external compaction energy, eliminating possible problems caused by a poor external compaction [1]. Without the labor intensive, noisy and energy consuming vibration of fresh concrete, SCC proved to be very suitable for the precast industry and applications such as structures with dense reinforcements or complex formworks [1]. However, SCC is still not the first choice concrete for many applications: SCC is more sensitive to small variations in the mix proportions [2], [3], [4], material properties [5], [6], [7], [8], [9], or variations of the mixing method [10], [11], [12], [13], [14], [15], and it is nowadays mainly used for casting situations with a thorough quality control.
Because of the more fluid behavior and therefore more complicated mix design of SCC, rheology – the study of the flow of matter – is often used to interpret experimental results. Most often, the Bingham model is used to describe the fluid behavior of fresh concrete. This model describes a linear relation between the shear stress and the shear rate using two parameters: the yield stress and the plastic viscosity (Eq. (1)). However, because non-linear behavior is often observed for SCC, the Modified Bingham model is also applicable for SCC. In this model (Eq. (2)), three parameters describe the rheological behavior: the yield stress , the Modified Bingham linear term, and the Modified Bingham 2nd order coefficient .
When designing a SCC mixture, the contradictory requirements of a high flowability and sufficient stability against segregation and bleeding can be met with multiple stability mechanisms. As shown in Fig. 1 (based on the rheograph from Wallevik and Wallevik [16]), well-performing SCC mixtures are situated in between two extremes: a relatively high yield stress and low plastic viscosity on the left side of the graph, and a zero or near-zero yield stress and high plastic viscosity on the right side of the graph. This variety of acceptable rheological parameters is necessary to meet the different workability requirements corresponding to different applications. Mixtures with a high yield stress and low plastic viscosity have a relatively small slump flow and low V-funnel flow time; mixtures with a low yield stress and high plastic viscosity have a larger slump flow and a high V-funnel flow time [17], [18].
Dependent on whether yield stress or plastic viscosity provide stability, different mechanisms lead to an insufficient filling ability, passing ability, or stability of the mixture. When the yield stress should ensure the stability, the mixture can display segregation of the coarse aggregates or a lack of flowability (slump flow < 550 mm). SCC mixtures in which the plastic viscosity is providing stability could show excessive bleeding or become viscous and unworkable. SCC mixtures with intermediate characteristics can show a combination of several failure mechanisms.
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Lack of flowability versus segregation of coarse aggregates
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Lack of flowability: the mixture still meets most criteria for SCC (easy to process, self-consolidation, …), with the exception of sufficient flowability. A low slump flow (below 550 mm) and low L-box ratio (below 0.6) are inacceptable for most applications [19].
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Segregation of the coarse aggregates: due to segregation, the coarse aggregates of the mixtures are sinking. The Sieve Segregation Index (S.S.I.), an indicator of the segregation resistance of the mixture, is high.
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Highly viscous mixture, difficult to process versus excessive bleeding
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Highly viscous mixture: although a sufficiently large slump flow is obtained, the casting of the mixture is difficult or even impossible because of the highly viscous and very sticky behavior (V-funnel flow time > 25 s).
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Excessive bleeding: water migration can result in the formation of a water layer on the top surface of the fresh SCC [20], [21].
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The robustness of a concrete mixture is the capacity to retain its filling ability, passing ability, and segregation resistance despite small variations in the mix proportions, material properties, and the mixing method, which is a basic requirement for the production of concrete on a large scale. SCC in general has a lower robustness, due to the more complex mix design compared to vibrated concrete. The risk of segregation increases with decreasing yield stress and the risk of incompatibilities is increased by a higher number of constituents [22], [23]. The delicate equilibrium between sufficient filling ability and adequate stability in terms of segregation and bleeding is the result of the use of superplasticizers and a higher powder content compared to conventional vibrated concrete. In some cases, a viscosity-modifying admixture (VMA) is also used to increase the stability of SCC [24], [25].
The mechanisms governing the robustness of SCC have not been fully understood. Some approaches and guidelines have been developed to increase the robustness of SCC:
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Between the granular skeleton of the aggregates, a surplus of fines has to be available [3], [26], [27], [28] preventing the coarse aggregate particles from dominating the rheology [28], [29].
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Based on experiments in which cement was replaced by fly ash or silica fume, an increase of the paste volume is reported to increase the robustness [27], [30], [31].
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VMA's are often reported to increase the robustness of SCC [3], [4], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]. The higher the water-to-powder ratio, the greater the possible increase in robustness using a VMA [35]. However, different VMA's have a different influence on the robustness of SCC [4], [23], [33], [34], [41], [46], [47]. In some cases, the use of VMA's even reduces the robustness of SCC [4], [41].
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The influence of the water-to-powder ratio is unclear. Some researchers claim a higher water-to-powder ratio or the use of powders with a lower water demand result in a lower sensitivity to changes in the amount of free water [41]. Other studies report a higher robustness of mixtures with a lower water-to-powder ratio and higher superplasticizer dosages [26].
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Some authors proposed a possible link between the thixotropy and robustness to explain the influence of the cement content or the inclusion of a VMA [22], [48]. For clarification, more research in this area is needed.
The main goal of this investigation was to determine the combined effect of some of the listed approaches to enhance the robustness of SCC. The impact of the volumetric water-to-powder ratio and paste volume on the robustness of SCC is described in Section 3.1. Based on the experimental results, the impact of the mixture composition and use of a VMA on the robustness is discussed in Section 3.2. Rigueira et al. [2], [49] showed that most problems are caused by variations of the water content, only the effect of small variations in the water content is considered in this experimental program. The EFNARC guidelines [19] recommend to ensure that a mix design is able to withstand changes of the water content up to 10 l/m3, which corresponds to approximately 6% of the water content.
Section snippets
Materials
Table 1 summarizes the materials used in both parts of the experimental program. The grading curves of all materials are summarized in Fig. 2, the chemical composition of the powders determined by XRF analysis is given in Table 2.
Attapulgite clay is often used as a suspending agent in pumpable concrete. During mixing, the flocculated clay breaks down into small needles with negative charges along its main axis and pH-dependent charges at the ends, strongly increasing the floc strength in cement
Part 1: influence of the water-to-powder ratio and paste volume
Fig. 4, Fig. 5, Fig. 6, Fig. 7 and Table 5, Table 6 summarize the fresh properties of all mixtures of Part 1 of the study (9 reference mixtures and 18 adjusted mixtures) caused by variations induced by ± 8 l/m3 of water: the change of the test response due to the presence of more or less water is expressed per liter water (e.g. The change in slump flow: ΔSF/16 l/m3, see Equation (4)) and the ratio of the response interval (difference between the largest and the smallest test response) divided
Discussion
Plotting the test responses of mixtures of Part 1 with regard to slump flow and V-funnel time in a single diagram, as is illustrated in Fig. 12, robustness seems to be related to the mechanism providing stability in the mixture. In the following discussion, SCC mixtures having a high yield stress and low plastic viscosity will be referred to as ‘A-SCC’ and SCC mixtures with a low yield stress and high plastic viscosity will be referred to as ‘B-SCC’.
The robustness of SCC mixtures with a smaller
Recommendation for applications with SCC
Based on the experimental program described in this paper and the EFNARC guidelines on workability demands [19], recommendations are provided for mix design with optimum robustness dependent on the rheology (Table 8).
Composing SCC with a low plastic viscosity allows fast and convenient casting of large horizontal elements. A sufficiently high yield stress is necessary to provide stability to the SCC mixture. Robust SCC mixtures with such properties can be obtained by combining a high
Conclusions
Because the lack of robustness is currently limiting the use of SCC, an experimental investigation was executed to determine the effect of different mix design parameters: the paste volume, the water-to-powder volumetric ratio, and the addition of VMA's (diutan gum and attapulgite clay). Dependent on the mechanism providing stability of the mixture, these mix design parameters can have a more or less pronounced impact on the robustness of SCC. The robustness of mixtures with a high yield stress
Acknowledgements
The Science Foundation Flanders (FWO) is gratefully acknowledged for its financial support. The authors would like to express their special thanks and gratitude to Tom Stulemeijer and Nathan Lampens for their assistance and cooperation during the experimental work.
References (56)
- et al.
Rheological characterization of SCC mortars and pastes with changes induced by cement delivery
Cem. Concr. Compos.
(2011) - et al.
Correlating cement characteristics with rheology of paste
Cem. Concr. Res.
(2007) - et al.
Influence of fine aggregate characteristics on the rheological properties of mortars
Cem. Concr. Compos.
(2008) - et al.
Rheology as a tool in concrete science: the use of rheographs and workability boxes
Cem. Concr. Res.
(2011) - et al.
From ordinary rheology concrete to self-compacting concrete: a transition between frictional and hydrodynamic interactions
Cem. Concr. Res.
(2008) - et al.
Improving performance and robustness of SCC by adding supplementary cementitious materials
Constr. Build. Mater.
(2010) Self-compacting concrete: an analysis of 11 years of case studies
Cem. Concr. Compos.
(2006)- et al.
The effect of viscosity modifying agents on mortar and concrete
Cem. Concr. Compos.
(2007) - et al.
Influence of clays on the rheology of cement pastes
Cem. Concr. Res.
(2010) - et al.
Rate of thixotropic rebuilding of cement pastes modified with highly purified attapulgite clays
Cem. Concr. Res.
(2013)
Modulus of elasticity and tensile strength of self-compacting concrete: survey of experimental data and structural design codes
Cem. Concr. Compos.
Self-compacting Concrete. Dunbeath
Self-consolidating concrete robustness in continuous production regarding fresh and hardened state properties
ACI Mater. J.
Robustness of self-consolidating concrete
Robustness of fresh VMA-modified SCC to varying aggregate moisture
NCR J.
Interaction of Cement and Admixtures and its Effect on Rheological Properties Doctoral Thesis
Influence of different Danish glacial deposited sands on SCC properties
Influence of Mixing Procedure on Robustness of Self-consolidating Concrete
Influence of temperature on the dispersibility of polycarboxylate type superplasticizer for highly fluid concrete
Design Concepts for the Robustness Improvement of Self-compacting Concrete - Effects of Admixtures and Mixture Components on the Rheology and Early Hydration at Varying Temperatures Doctoral Thesis
Influence of Addition Sequence of Materials on Rheological Properties of Self-compacting Concrete
SCC and UHPC - effect of mixing technology on fresh concrete properties
Robustness of SCC
Measurement of Properties of Fresh Self-compacting Concrete - Final Report
The European Guidelines for Self-compacting Concrete - Specification, Production and Use
A theoretical frame to study stability of fresh concrete
Mater. Struct.
An experimental study on bleeding and channeling of cement paste and mortar
Adv. Cem. Res.
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