Elsevier

Cement and Concrete Composites

Volume 82, September 2017, Pages 190-201
Cement and Concrete Composites

Effect of the mix design on the robustness of fresh self-compacting concrete

https://doi.org/10.1016/j.cemconcomp.2017.06.005Get rights and content

Abstract

Self-compacting concrete (SCC) has many advantages compared to vibrated concrete. A disadvantage is the lower robustness of fresh SCC. SCC is more sensitive to small changes in the mix design, material properties, and the applied production methods. In an experimental program, the influence of important mix design parameters on the robustness of SCC was studied. First, the influence of the paste volume and the water-to-powder volumetric ratio was investigated. Depending on the mechanisms providing stability in the mixture, different levels of impact were observed. When the yield stress is the main factor providing stability in the mixture, a change in the water content will mainly affect the yield stress, making the stability of the yield stress the most important factor determining the robustness of the mixture and can be improved by lowering the paste volume. Analogue, the sensitivity of the plastic viscosity is determining the robustness of mixtures in which mainly the plastic viscosity is providing stability. The robustness of such a mixture can be improved by increasing the water-to-powder volumetric ratio. The influence of two types of viscosity modifying agents (VMA's) on the robustness of fresh SCC was examined in a second stage. The two used VMA's (diutan gum and attapulgite clay) were especially effective in SCC mixtures having a high yield stress and a low plastic viscosity. In mixtures having a low yield stress and a high plastic viscosity, the inclusion of a VMA in the mix design resulted in a decrease of the robustness.

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 τ and the shear rate γ˙ using two parameters: the yield stress τ0,B and the plastic viscosity μB (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 τ0,MB, the Modified Bingham linear termμMB, and the Modified Bingham 2nd order coefficient cMB.τ=τ0,B+μB·γ˙τ=τ0,MB+μMB·γ˙+cMB·γ˙2

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.

  • Lack of flowability versus segregation of coarse aggregates

    • 1)

      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].

    • 2)

      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.

  • Highly viscous mixture, difficult to process versus excessive bleeding

    • 1)

      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).

    • 2)

      Excessive bleeding: water migration can result in the formation of a water layer on the top surface of the fresh SCC [20], [21].

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:

  • 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].

  • 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].

  • 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].

  • 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].

  • 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.

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