Rheological and Tribological Characterization of Concrete in the Context of Estimating its Pumpability

Transcription

1 Rheological and Tribological Characterization of Concrete in the Context of Estimating its Pumpability Egor Secrieru 1, Viktor Mechtcherine 1, Simone Palzer 2 1 Technische Universität Dresden, Institute of Construction Materials, Dresden, Germany 2 Weimar Institute of Applied Construction Research ggmbh, Weimar, Germany Abstract: The flow of fresh concrete in the pipeline is a complex process. In the first instance it is influenced by the composition of concrete and its ability to generate lubricating material, while reducing the friction at the pipe wall-concrete interface. The article at hand discusses the characteristic differences in rheological properties and pumpability of ordinary concrete with rounded and crushed aggregates, with and without partial cement replacement through fly ash, as well as self-compacting mortar (SCM) and self-compacting concrete (SCC). In the attempt to estimate the concrete pumpability, an experimental methodology based on rheometry and tribometry is employed. The prediction of pumpability is validated by the so-called Sliding Pipe Rheometer (SLIPER) and small-scale pumping tests. Keywords: Fresh concrete; Pumpability; Rheology; Tribology 1. Introduction Pumping of concrete represents one of the most efficient transportation methods, especially suitable for large and complex construction sites, while reducing the costs and the completion time. Such spectacular megatall buildings as Shanghai Tower [1] and Burj Khalifa [2] could hardly have been erected without the application of the recent developments in the concrete technology in conjunction with modern pumping techniques [3]. Future projects target even more ambitious structures rising over 1000 m like Kingdom Tower [4] or Azerbaijan Tower [5], and thus, continuously throwing down further challenges to the engineers. In the last few years a series of scientific papers dealing with the topic of concrete pumpability have been published [6 10]. Since the nature of concrete both in fresh K.H. Khayat, SCC th International RILEM Symposium on Self-Compacting Concrete, ISBN: RILEM

2 320 Egor Secrieru, Viktor Mechtcherine, Simone Palzer and hardened states is perpetually changing, conditioned by new engineering and environmental requirements, e. g. within the perspective of their durability and sustainability [11], the approaches for estimating the workability of fresh concrete in general and its pumpability in particular have to be updated as well. Obviously, full-scale pumping tests (cf. Fig. 1) cannot always be performed to insure the suitability of the material design and properties. Instead, well-founded concepts for adapting the concrete design to the demands with respect to pumpability must be developed. Besides, official regulations for the pumpability assessment which would provide specific requirements with respect to the rheological properties of fresh concrete are needed. The authors believe that a combination of the existing laboratory tools is very helpful both to promote understanding of complex phenomena occurring in concrete during pumping and to develop optimisation procedures for concrete with respect to its pumpability. (a) (b) Figure 1. Full-scale pumping experiments; measuring pumping pressure in (a) a 600 m pipeline, Burj Khalifa [8] and (b) an experimental 1000 m pipeline [12]. 2. Experimental Investigation Table I provides the composition of the mixtures under investigation. According to the initial experimental plan the reference mixtures from [13] were supposed to be tested in the small-scale pumping experiments. After a series of preliminary tests blockages occurred, since the volume and the maximum aggregate size of the mixtures exceeded the capacity of the pipeline geometry. The recipes were hence partially modified by replacing the coarse aggregates larger than 5 mm through paste and finer aggregates, the latter being proportionally added while keeping the initial sieving curves (up to 4 or 5 mm aggregate size) unchanged. The name of the modified mixtures contain an addition * in the end in order to distinguish from the reference mixtures in [13]. The new mixtures are referred as concretes, although according to the maximum aggregate size they rather correspond to mortars and finegrained mortars. The influence of the following parameters on the pumping characteristics were measured and discussed: type of concrete: ordinary concrete M1* and M2* as well as selfcompacting concretes SCC* and SCM; admixture: without fly ash M1* and M2*, with fly ash M1-2F* and M2-2F*;

3 Rheological and Tribological Characterization of Concrete 321 type of aggregates: rounded, in M1*, and crushed, in M2*; use of coarser aggregates: with, in SCC*, and without, in SCM. Table I. Compositions of the mixtures. Material Unit mass (kg/m Density ) (kg/m 3 ) M1- M2- M1* M2* SCC* SCM 2F* 2F* CEM II/A-LL 42.5 N Fly ash Quartz sand 0.06/ Quartz sand 0/ Quartz sand 2/ Basalt split 2/ Water PCE SP, Gl. Sky Vol. aggregates (%) Paste content (%) Slump flow (mm) W/B The rheological tests were performed with a Couette type viscometer ConTec 5 (Iceland), see Fig. 2a. The resulting torque from the sheared material was measured on the stationary cylinder (1) lowered into the outer cylinder (2), with the radii of 100 mm and 145 mm respectively. For the tribometer tests, the inner stationary cylinder of the viscometer (1) with protruding vanes was replaced by a smooth cylinder, see Fig. 2b. Concrete pumpability can be described by the relation between pumping pressure P and the resulting flow rate Q. Kaplan [6] proposed two simplified approaches to predict the concrete behaviour in a pipe on the basis of the Buckingham-Reiner equation [14], cf. Eqn. (1) and (2): 2L Q i P 0i R (1) 2 R k Q R R i 2L P R k 4 3 i i (2) 0 R R 1 i 4

4 322 Egor Secrieru, Viktor Mechtcherine, Simone Palzer with τ 0i yield stress parameter and μ i viscosity parameter at the interface between inner cylinder and concrete (measured with tribometer), τ 0 yield stress and μ plastic viscosity of the bulk concrete (measured with viscometer); k stands for the pipe filling coefficient, R the radius and L the pipe length. The first approach (Eqn. 1) simplifies the concrete movement to that of a plug. The second approach (Eqn. 2) is applicable for the case when the bulk of concrete is partially sheared during pumping. (a) (b) (c) (d) Figure 2. (a) Viscometer, b) tribometer, (c) Sliding Pipe Rheometer (SLIPER), and (d) small-scale pumping device. The direct measurement of pumpability was performed using SLIPER device, which enables to estimate the relationship between the pumping pressure and discharge rate for various pipe geometries [15], Fig. 2c. The pressure prediction capacities of both the tribometer and the SLIPER were validated by small-scale pumping tests, Fig. 2d.

5 Rheological and Tribological Characterization of Concrete Results and Discussion If handled properly, the viscometer measurements depict exclusively the intrinsic rheological performance of fresh concrete. A better estimation of the concrete pumpability is possible by measuring the pipe wall-concrete interface properties, thus by tribometer measurements [7,16]. Fig. 3a and 3b depicts the behaviour of the mixtures in terms of plastic viscosity μ and yield stress τ 0 (viscometer) and viscosity parameter μ i and yield stress parameter τ 0i (tribometer). (a) (b) (c) (d) Figure 3. (a) Yield stress τ 0 and plastic viscosity µ from viscometer tests, (b) yield stress parameter τ 0i and viscosity parameter µ i from tribometer tests; pumping pressure (d) measured by SLIPER and (c) predicted using data obtained from tribometer tests for flow rates Q between 10 and 40 m 3 /h.

6 324 Egor Secrieru, Viktor Mechtcherine, Simone Palzer The pressure values for each mixture were computed based on the Eqn. (1) and (2) with P-Q obeying a linear model [6,17]. With other words, the interface stress depends linearly on the slip rate with eventual partial shear of concrete and on the shear rate of the lubrication layer. The chosen geometry specifications corresponded to that of the SLIPER pipe with inner radius r of m, length l of 0.5 m, and pipe filling coefficient k of 1. Figure. 3a and 3b show that the behaviours of the mortar M1* (rounded aggregates) and M2* (crushed aggregates) are similar both from the rheological and tribological points of view, i.e. the corresponding parameters τ 0 and μ, τ 0i and μ i show very close results, independent of kind of aggregates. As expected, also in terms of pumpability the mixtures exhibit as well very similar courses of P-Q curves. The same is valid for the mixtures M1-2F* and M2-2F* containing fly ash, with only minor difference in the yield stress τ 0. It can be concluded that the actual amount of paste (44 %) is sufficient to eliminate the effect of aggregate shape on the rheological behaviour of the mortars. Both mixtures SCC* and SCM* have the highest paste contents, which enhance their flowability, cf. Table I. The relatively low W/B of the both mixtures results in high viscosity values, while the elevated contents of superplasticizer cause their low yield stress values. As soon as the yield stress is exceeded by high shear stresses generated at the wall-concrete interface during pumping, the deformed zone will comprise some of concrete bulk in addition to the lubrication layer [18]. Due to intense shearing at this interface both the plastic viscosity μ and the viscosity parameter μ i most considerably contribute to the flow resistance [19,20] resulting in higher pumping pressures in comparison to the ordinary concretes M1* and M2*, cf. Fig. 3c,d. Partial replacement of cement by fly ash (M1-2F* and M2-2F*) slightly reduces absolute values of the rheological and tribological parameters and, subsequently, improves the pumpability of concrete/mortar, see shallower P-Q curves. According to Kaplan [6], one of the two suggested approaches can be employed in order to predict the concrete pumpability, see Eqn. (1) and (2). The best correlation between SLIPER and tribometer results could be achieved by employing Eqn. (2) for all concretes. Therefore, it can be concluded that in the performed tests the bulk of the ordinary concretes was partially sheared, thus contributing with its deformation to the overall increase in pumping pressure. Fig. 4 displays cogent correlation between the pressures predicted on the basis of the tribometer tests, of the SLIPER results and directly measured in the small-scale pumping tests. The chosen geometry specifications correspond to that of the pipeline with inner radius r of m, length l of 1.5 m, and pipe filling coefficient k of 1. The pressure values predicted with SLIPER and tribometer from Fig. 4 were computed based on the flow rates measured in the pipeline, see Table II.

7 Rheological and Tribological Characterization of Concrete 325 (a) (b) Figure 4. Correlation of the pressure measured in the pipeline and predicted on the basis of (a) tribometer measurements and (b) SLIPER measurements (geomet-ry of the pipe and flow rates in the pipe are used for the predictions). Since the pipeline diameter (32 mm) is clearly smaller than the lateral gap in the tribometer (45 mm) and the SLIPER pipe diameter (126 mm), the shear rate occurring at the concrete-wall interface in the pipeline test must be higher than in the other two experimental methods for the same flow rate. Therefore, such processes like hydrodynamic energy dissipation and shear-induced particle migration are likely to be more pronounced in the pipeline. The correlation factors of 0.85 and 0.78 for both SLIPER and tribometer are statistically significant; they indicate that the amounts of the paste in the mixtures were sufficient to reduce the impact of the difference in shear rate and other previously mentioned phenomena on tribological and pumpability characteristics of the mixtures.

8 326 Egor Secrieru, Viktor Mechtcherine, Simone Palzer Table II. Pumping pressure as obtained from the small-scale pumping tests and corresponding predictions based on SLIPER and tribometer measurements. Mixture M1* M1-2F* M2* M2-2F* SCC* SCM Pipeline SLIPER Tribometer Directly measured Predicted Q (m 3 /h) P (kpa) P (kpa) P (kpa) Conclusions The paper at hand presents various approaches for predicting the pumpability of fresh concrete. The pumpability predictions based on tribometer and SLIPER measurements were validated by the small-scale pumping tests. In contrast to the experimental results obtained for mixtures with coarse aggregates (these results will be published elsewhere), the absence of coarse aggregates in the mixtures under investigation showed only minor differences in the pumpability as predicted by various methods. This indicates that for relatively homogeneous mixtures the choice of the testing approach plays a minor role in comparison to the concrete compositions with increased heterogeneity basically due to aggregate content, size and shape. Furthermore, the increase in the paste amount due to lack of coarse aggregates reduces the effect of the aggregate shape on rheological, tribological and pumpability properties of mixtures. The addition of fly ash as cement replacement leads to lower values of both rheological and tribological parameters, and as a consequence augments the pumpability of fresh mortar.

9 Rheological and Tribological Characterization of Concrete 327 Additionally, the advantage of the tribometer tests is that the fresh properties of the lubrication layer can be directly linked with the ones of the bulk material obtained in the viscometer tests. A solid and positive correlation between the prediction based on tribometer and SLIPER measurements on one hand and measured P-Q curves in the small-scale pumping test on the other hand could be achieved by considering using Eqn. (2) suggested by Kaplan [6]. This means that for mortars under investigation, in addition to the lubrication layer the shear deformation of the bulk of material occurred contributing to the overall increase in pumping pressure. This is valid for all tested mixtures. Acknowledgements The authors thank Dipl.-Ing. Markus Greim (Schleibinger GmbH) for providing the SLIPER device. The authors appreciate the help of M.Sc. Igor Serpukhov (FTA Albstadt mbh) and M.Sc. Klemens Laub (IAB Weimar ggmbh) in execution of the experimental program. References [1] Shanghai Tower - The Skyscraper Center, (2015). (accessed November 10, 2015). [2] J. Aldred, Burj Khalifa a new high for high-performance concrete, Proc. ICE - Civ. Eng. 163 (2010) [3] J. Plank, E. Sakai, C.W. Miao, C. Yu, J.X. Hong, Chemical admixtures - Chemistry, applications and their impact on concrete microstructure and durability, Cem. Concr. Res. 78 (2015) [4] Kingdom Tower-The Mile High Tower in Jeddah, (2015). (accessed November 10, 2015). [5] Azerbaijan To Build One Kilometer-Tall Skyscraper - Architizer, (2015). (accessed November 10, 2015). [6] Kaplan D., de Larrard F., Sedran T., Design of concrete pumping circuit, ACI Mater. J. 102 (2005) [7] Ngo T.T., Kadri E.H., Bennacer R., Cussigh F., Use of tribometer to estimate interface friction and concrete boundary layer composition during the fluid concrete pumping, Constr. Build. Mater. 24 (2010) [8] Kasten K., Gleitrohr Rheometer, Ein Verfahren zur Bestimmung der Fließeigenschaften von Dickstoffen in Rohrleitungen, PhD thesis, TU Dresden, 2010.

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