SLUMP FLOW VALUES VS. BINGHAM PARAMETERS FOR HIGH FLOWABLE MORTARS AND CONCRETES

Transcription

1 3-5 September 7, Ghent, Belgium SLUMP FLOW VALUES VS. BINGHAM PARAMETERS FOR HIGH FLOWABLE MORTARS AND CONCRETES Oskar Esping Dept. of Civil and Environmental Engineering, Chalmers University of Technology, Sweden. Abstract In the present work, the influence of Bingham rheology parameters on the slump flow values of self-compacting concrete and mortar has been evaluated. The objective was to present experimental results, without any physical validation, addressing the complex connection between the slump flow spread, flow time (T5), yield stress and plastic viscosity. A large number of more or less self-compacting mortars (~ mixtures) and concretes (~55 mixtures) with a wide range of consistency have been used for the evaluation. The mortar rheology was measured using a Bohlin CVO rheometer with concentric setup, and the slump flow spread was measured using a mini-cone. For the concrete a ConTec Visco5 rheometer was used together with a traditional Abram s cone. The results showed a large scatter and poor correlation, but a clear tendency was found. The results indicate that the slump flow spread and T5 time are not a unique function of yield stress or viscosity, respectively, but rather a more complex function of both. The spread proved to be more closely connected with the yield stress than with the viscosity, especially at high viscosity, whereas the T5 time was more dependent on both viscosity and yield stress. 1. INTRODUCTION To quantify the workability of fresh concrete, numerous tests of different types, more or less empirical, have been developed. The oldest and most frequently used test today is the slump cone test, which, associated with the Abram s cone, has its origin in the USA around 191 [1]. For high-flowable concrete, such as self-compacting concrete (SCC), the slump cone is used to measure the spread of the concrete (slump flow) and the time for the concrete to reach a spread of 5 mm (T5). For a more scientific approach, a rheometer/viscometer can be used. The most established parameters used to define the concrete rheology are the yield stress and plastic viscosity by the Bingham equation []. These rheological parameters, unlike the measures of qualitative tests (e.g. the slump), are fundamental physical quantities, mutually independent and not dependent on operator or equipment. But rheometer/viscometer tests are often expensive, immobile, and mainly for laboratory use. In the field, e.g. at concrete plant or construction site, fast, simple and cheap tests are preferred; thus slump/slump flow and other empirical tests can hardly be replaced by a rheological test, but should rather be a complement. To describe the flow properties of concrete, especially for types with high flowability, a minimum of two independent parameters is required []-[7]. Concrete mixtures with identical measures from an empirical test (e.g. slump flow) can be 315

2 3-5 September 7, Ghent, Belgium very different in their flow behaviour. These differences can be detected by a rheological test. However, in order to determine the properties of a non-newtonian fluid, as all cementitious materials are, a multipoint flow curve has to be measured. A single point measurement does not describe these materials correctly (see Figure 1). Shear stress, σ Figure 1: Viscosity (plastic), η pl Shear rate, γ Shear stress, σ Yield stress, σ Shear rate, γ Shear stress, σ Shear rate, γ (a) (b) (c) Illustration of concrete with: (a) same viscosity but different yield stress, (b) same yield stress but different viscosity, and (c) same empirical measure but different viscosity and yield stress (from [31]). It has been shown by many that the empirical tests to some extent provide information about the concrete rheology. Slump has for a long time [3][6]-[] been considered to be a function of yield stress, without influence by plastic viscosity. When analysed, the density of the concrete is often incorporated as well as the geometry of the cone. More recently the horizontal flow spread rather than the vertical slump has been used to calculate the yield stress [][3]-[5]. Usually these correlations concern concretes in a small range of flowability (e.g. HPC, SCC, etc.), and are not valid for other or wider ranges. The increased usage of highflowable concretes has also raised the interest in more dynamic measures such as flow time and viscosity. The flow time can be measured by e.g. a V-funnel or L-box, but also by measuring the time for the concrete to spread 5 mm (T5). For SCC, the slump flow spread is generally considered to correlate with the yield stress and T5 or V-funnel with the viscosity [6]-[8]. Often the correlation is purely empirical, but there are also some physical models proposed to explain this relationship. Examples of physical models linking yield stress (σ ) to slump flow spread diameter (D) are: ρ g V σ = = a / D (Eq. 1) [14] π D 4 ρ g V σ = = b / D (Eq. ) [9] 3 π D 5 ρ g V 5 σ = = c / D (Eq. 3) [3] 5 4 π D where ρ is the material density, V the sample volume and g the gravity, while a, b and c are constants for a given sample volume and density. The objective of this paper is not to find a physical relationship between slump flow values and Bingham parameters, but merely to present experimental results addressing the interaction between the slump flow spread, flow time (T5), yield stress and plastic viscosity.. TEST METHODS.1 Slump flow test The slump flow measurement was carried out using a traditional slump cone (see Figure 316

3 3-5 September 7, Ghent, Belgium 1), for concrete the Abram s cone (EN 135- or ASTM C143) and for mortar the minislump cone (EN or ASTM C3), but without compaction. The slump flow spread diameter was measured in two perpendicular directions, presented as a mean value in millimetre. In addition to the slump flow, the time from lifting the cone to when the flow spread reaches a 5 mm circle was recorded; referred to as T5 and stated in seconds. Mini-slump cone ØD=7/1 mm H=6 mm Mortar sample Flow table Slump cone (Abram) ØD=1/ mm H=3 mm Concrete sample Flow table (non-adsorbent) Spread diameter Spread diameter T5 time Figure : Illustration of slump flow test used for mortar and for concrete (from [31]).. Rheology For mortar a Bohlin CVO rheometer was used and for concrete a ConTec Visco5, both using a rotating concentric setup at a controlled shear rate (γ& ) at ºC. The experimental geometry and measuring sequence is illustrated in Figure 3. The two rheological parameters plastic viscosity (η pl ) and yield stress (σ ) were evaluated in accordance with the Bingham model: σ = σ + & γ (Eq. 4). 6 η pl Mortar sample Shear rate [1/s] 4 No logging Logging Time [s] R i =1 mm R o =15 mm H=37.5 mm Rotating and measuring inner cylinder Fixed outer cylinder Shear rate [1/s] Figure 3: No logging Logging Time [s] R o =145 mm R i =1 mm Concrete sample H=14 mm Measuring inner cylinder Rotating outer cylinder The measuring sequence for the Bingham evaluation (followed by segregation estimation) and schematic illustration of the rheometers. For mortar the Bohlin CVO and for concrete the ConTec Visco5 (from [31]). The mixes and their constituents, as used in this study, comprise typical materials for selfcompacting concrete (SCC) in Sweden. For mortar the maximum grain size was 1. mm, and for concrete 16 mm. The consistency in some cases was very stiff, but as all mixtures incorporated both filler and superplastisizer they were considered to be self-compacting. All mixtures have their 317

4 3-5 September 7, Ghent, Belgium origin in other projects whose primary purpose was to examine the effect of different constituent properties and proportions on the rheology. Examples of this are the effect of: w/c, cement, silica, fly ash, different limestone fillers, coarse and fine aggregate content, superplasticizer, air entrainer, viscosity modifier, etc. The mortar samples were prepared in batches of.5-1. litres, using a 4.73 litre Hobart laboratory paddle mixer. The mixing sequence was based on ASTM C35 standard. The concretes were prepared in batches of 3-6 litres, mixed in a BHS6 twin-shaft paddle mixer for 4 minutes after water was added to the premixed dry materials. The admixtures were added directly after the water. 3. RESULTS AND DISCUSSION The presented results are based on a large number of more or less self-compacting mortars (~ mixtures) and concretes (~55 mixtures) with a wide range of consistency. The density is also considered to influence the slump and slump flow (see Eq. 1-3), but as the difference in density of the mixes was relatively small, it was not taken into account. Concretes and mortars with high tendency of visual segregation were excluded, as these are considered to give misleading measures and thus a poor correlation between the slump flow and rheology. For concrete slump flow spread (D) and time to 5 mm (T5) were recorded and correlated with the Bingham parameters (yield stress and plastic viscosity). For mortar only the spread was recorded and correlated with the Bingham parameters. The presented correlations are not intended as a true connection of slump flow values and Bingham parameters, but merely to show the more complex connection between the slump flow spread, flow time, yield stress and plastic viscosity. 3.1 Rheology vs. slump flow spread The experimental results of slump flow spread diameter and Bingham rheology parameters (yield stress and plastic viscosity) for the mortars and concretes are presented in Figure Figure 4: -5 mm 5-3 mm 3-35 mm >35 mm >35 mm 3-35 mm 5-3 mm -5 mm 15- mm 1-15 mm 1-15 mm 15- mm mm >7 mm 6-7 mm 5-6 mm 4-5 mm 3-4 mm -3 mm 3-4 mm 4-5 mm 5-6 mm 6-7 mm >7 mm The slump flow spread (in intervals of 5 resp. 1 mm) and Bingham rheology parameters (yield stress and plastic viscosity) for the mortar and concrete samples. 318

5 3-5 September 7, Ghent, Belgium The spread flow is grouped into intervals of 5 mm for mortar and 1 mm for concrete, starting with spread equal to the cone bottom diameter (D =1 mm respectively mm). For each spread interval, a logarithmic trend-line is plotted, as this was the tendency that was best fitted over a wide range of spreads. It can be noted that there is a large scatter and rather poor correlation (R =.1-.9) but still a clear tendency can be observed that the slump flow is affected by both yield stress and viscosity. At high viscosity or low yield stress, where the slope of the curve is lower, the effect of yield stress is more pronounced. If slump flow were to be independent of viscosity the curve would be horizontal. The slump flow can seem to be independent of viscosity for the higher flowable mortars and concretes (with slump flow >5 resp. >5 mm), as the curve looks horizontal. When excluding mixtures with lower spread flow it can be observed that also here the spread is affected by both yield stress and viscosity. Based on all the investigated mixes (> 75 mixes), the following approximate model for calculating the relative slump flow area (R p ) for both mortar and concrete is proposed: ( H / 6 ) R p ln( η pl + ) σ (Eq. 5) R ( D / D ) 1 p = (Eq. 6) where η pl is the plastic viscosity, σ the yield stress, H the cone height (6 resp 3 mm), D the measured spread diameter in millimetre and D the flow cone bottom diameter (1 resp mm). The model is purely empirical and is deficient in its units. In Figure 5 the model is plotted for mortar and concrete Figure 5: Mortar 1-15 mm 15- mm -5 mm 5-3 mm 3-35 mm >35 mm mm 3-4 mm 4-5 mm 5-6 mm 6-7 mm >7 mm Calculated slump flow spread diameter as a function of Bingham rheology parameters for mortar and concrete. Concrete In Eq. 5 the yield stress is predominant, where the effect of a change in yield stress corresponds to ~3 times larger change in viscosity on the slump flow. These differences are smaller at low viscosity, and are independent of the magnitude of yield stress. In the literature, not all studies agree that slump flow spread is a measure of concrete yield stress. For example, Nielssen & Wallevik [3] explain how a low viscosity will increase the downward speed, overcome the yield stress and thereby generate a large slump flow spread. For high viscosity the effect will be the opposite. Cauberg et al [33] state that the relation between yield stress and slump flow, without taking viscosity into account, does not show a satisfactory fit, and that these relations usually are formed for a certain range of fluidity. Furthermore, a variable Z, comprising both yield stress (σ ) and viscosity (η pl ), is proposed for a better fit to the slump flow spread (D) in millimetres. 319

6 3-5 September 7, Ghent, Belgium Z = a σ + b / η pl (Eq. 7) Z = D (Eq. 8) where a and b are constants. In the region of spread D=4-6 mm, and with a=1 and b=1, Eq. 7 and Eq. 8 generate similar results as Eq. 5 and Eq. 6. For spread D>65 mm Eq. 8 is not valid. Furthermore, the shape of the curve in a η pl -σ diagram is independent of the spread. Smeplass [34] has noted that the slump is more related to the yield stress than to the viscosity of concrete, while the flow table spread is more related to the viscosity than the yield stress. If the viscosity varies, at constant yield stress, so will the flow measures. 3. Rheology vs. slump flow time T5 The experimental results of slump flow spread time T5 and Bingham rheology parameters for the concrete samples are presented in Figure 6. Concretes with slump flow spread <5 mm have been excluded as they have no T5 measure (~38 mixtures with slump spread >5mm). The T5 times are grouped into intervals of seconds. For each T5 time interval a trend-line is plotted. It can be noted that there is a large scatter and poor correlation (R =.1-.4). Still the tendency can be observed that the T5 time is affected by both yield stress and viscosity. If the flow time were independent of yield stress the curve would be vertical >1 s 8-1 s >1 s 8-1 s 6-8 s 4-6 s -4 s < Figure 6: < s 4-6 s -4 s 6-8 s The experimental results of slump flow spread time T5 (in intervals of s) and Bingham rheology parameters (yield stress and plastic viscosity) for the concrete. Based on the results, the following empirical model for calculating the slump flow spread time T5 in seconds is proposed: T 5. σ η (Eq. 9) In the literature, there is no general agreement that T5 is a measure of concrete plastic viscosity (η pl ). For example, Tedaka et al [35] state that the slump flow spread corresponds well to the yield stress (σ ), whereas the T5 flow time does not alone represent the viscosity since slump flow also contributes. Thus, T5 can be used to evaluate the viscosity only when the slump flow is constant. Furthermore, Ferraris et al [36] show that the slump flow spread times of cement pastes with a wide range of yield stress have no correlation at all with the viscosity. Emborg [37] has also noted a weak correlation, but then for the T5 vs. viscosity of SCC. Utsi et al [38] have observed that T5 correlates not only to the viscosity but also to the yield stress, and proposes that both the yield stress and viscosity influences the T5 and slump flow respectively. By dividing the slump flow into different intervals the correlation between T5 and viscosity was shown to be acceptable, which further fortified this theory. pl 3

7 3-5 September 7, Ghent, Belgium 4. CONCLUSIONS It has been shown by many investigators that the results of empirical tests are strongly related to the concrete rheology. For SCC, the slump flow spread is generally considered to be a function of yield stress and T5 or V-funnel of viscosity. Often this correlation is purely empirical, but there are also different physical models explaining this relationship. In the presented work, the influence of Bingham rheology parameters on the slump flow values of a large number of more or less self-compacting concrete and mortar has been evaluated. Experimental results addressing the connection between the slump flow spread, flow time (T5), yield stress and plastic viscosity are presented. The results clearly indicate that the slump flow spread and T5 time are not a unique function of yield stress and viscosity respectively, but rather a more complex function of both where neither yield stress nor viscosity can be neglected. The spread proved to be more closely connected with the yield stress than the viscosity (~3 times), especially at high viscosity, whereas the T5 time was connected more equally with yield stress and viscosity. Consequently, T5 can only be used to estimate the viscosity for mixture with constant yield stress, and slump flow can only be used to estimate the yield stress with constant viscosity. ACKNOWLEDGEMENTS Financial support from Thomas Concrete Group and Färdig Betong is greatly appreciated. REFERENCES [1] Bartos P., Sonebi M., Tamimi A. K. (): Workability and rheology of fresh concrete: Compendium of tests, RILEM, France. [] Tattersall G.H. (1991): Workability and quality control of concrete, E&FN Spon, London. [3] Domone P. (1998): The slump flow test for high-workability concrete, Cement and Concrete Research, vol 8(), pp [4] Domone P.L.J., Xu Y., Banfill P.F.G. (1999): Developments of the two-point workabiliy test for high-performance concrete, Magazine of Concrete Research, Vol 51(3), pp , [5] Ferraris C.F., Measurement of the rheological properties of high performance concrete: State of art report, Journal of NIST, vol 14(5), pp [6] Tattersall G.H. (1973): The rationale of a two-point workability test, Magazine of Concrete Research, vol 5 (84), pp [7] Tattersall G.H., Banfill P.F.G. (1983): Rheology of fresh concrete, Pitman, London. [8] Chidiac S.E., Madaani O., Razaqpur A.G., Mailvaganam, N.P. (): Controlling the quality of fresh concrete - A new approach, Magazine of Concrete Research, vol 5 (5), pp [9] Clayton S., Grice T.G., and Boger D.V. (3): Analysis of the slump test for on-site yield stress measurement of mineral suspensions, Journal of Mineral Processing, Vol 7, pp 3-1. [1] de Larrard F. (1999): Concrete mixture proportioning, a scientific approach, New York. [11] de Larrard F., Hu C., Sedran T., Szitkar J.C., Joly M., Claux F., Derkx F. (1997): A new rheometer for soft-to-fluid fresh concrete, ACI Materials Journal, vol 94(3), pp [1] Ferraris C.F., de Larrard F. (1998): Testing and modelling of fresh concrete rheology, NISTIR 694, NIST, USA. [13] Hu C., de Larrard F., Sedran T., Boulay C., Bosc F., Deflorenne F. (1996): Validation of BTRHEOM, the new rheometer for soft-to-fluid concrete, Materials and Structures, vol 9(194), pp [14] Murata J. (1984): Flow and deformation of fresh concrete, Materials and Structures, vol 17(98), pp

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