Use of Rheology to Develop and Optimize Self Compacting Concrete Olafur H. Wallevik IBRI Rheocenter

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1 Use of Rheology to Develop and Optimize Self Compacting Concrete Olafur H. Wallevik IBRI Rheocenter ABSTRACT Self-compacting concrete (SCC) is considered by many to be the greatest breakthrough in concrete technology for many decades due to the improved performance and working environment. Its origin is usually traced back to the work by professor Okamura and his students at Tokyo University in 1988, even if earlier attempts to design concrete not needing external consolidation are known. The use of SCC has increased greatly in some countries like Denmark where some 20 % of the production is characterized as SCC, while in other as Japan its market share is probably still short of 1 %. Also, the design of SCC varies considerably over countries, partly due to conservatism, partly due to differences in available materials and traditions in making conventional vibrated concrete. For instance the viscosity is generally very high in Asia, while in Iceland and New Zealand it is very low. The design of SCC is greatly enhanced by use of rheology. Two rheological parameters are needed to describe the mix in question, plastic viscosity and yield value. These parameters can easily be determined by use of either coaxial cylinder viscometers or rheometers, both available on the market. By use of these parameters and thorough knowledge of the effects of different additives and ingredients, the design can be directed towards an optimal solution with regard to available materials and intended use of the concrete. Key-words: SCC, conventional concrete, mix design, plastic viscosity, yield value. INTRODUCTION Self compacting concrete (SCC) differs from conventional vibrated concrete (CVC) as it is very easy to place it, flows like a liquid and does not need external consolidation (normally done by poke-vibrator). Figure 1. Casting of self compacting concrete. By many SCC is considered as a high performance concrete relative to CVC due to its homogeneity and thereby better performance in hardened stage. It has typically lower permeability, less interfacial zone (between aggregate and paste) and less fluctaion in production. But one has to bear in mind that SCC is concrete and follows general 1 of 11

2 rules within concrete technology, for instance if one increases paste volume by increasing water (and cement) it will shrink more. The initiation of SCC In the preface to the proceedings from the 3 rd International Symposium of SCC, Wallevik [1] wrote: It all started around 1988 at Tokyo University by professor Okamura and his students, among others Ozawa and Maekawa, as they established the basic description of SCC. Before that, several cases of application were known in which the concrete did not need external consolidation to obtain necessary compaction and could therefore be considered as SCC, but was not defined as such. Figure 2. Casting a dam in Germany in the 1920 s [2, 3]. An example of such concrete can be found as early as 1920 during casting of a dam [2,3]. In figure 2 the placement of this concrete at a dam construction site is shown [3]. Further examples, chosen at random, can be mentioned. Soletanche, France has used SCC-like concrete in their cast-in-place piles for decades. Collepardi et al. introduced so-called Rheoplastic concrete around 1975 which is a kind of SCC [4]. In Spain, a tunnel (most likely several km long) was lined about two decades ago with very fluid, non-consolidated concrete. The Norwegians used SCC-like concrete in the bottom shell of the storage tanks in the Condeep platforms before 1978 (the author worked at that time as craftsman placing such concrete). More thoughts of the origin of SCC can be found in a paper by Collepardi [5]. Rheological definition of SCC Several (more or less precise) definitions of SCC have been proposed since Okamura came up with his almost two decades ago (and three decades since Collepardi described Rheoplastic concrete). Wallevik and Nielsson employed rheological parameters [6] in 1997 to describe SCC. Note; two skilled specialists or scholars (even from same country) can often have quite different opinions regarding what is good mix-design of concrete or good workability. In addition, the meanings of labcrete and real-crete (phrases referring to concrete made in laboratory and at ready-mix plant, respectively) can also be very different. 2

3 Further, Wallevik [7] proposed a defining area for SCC using a so-called rheograph (yield value plastic viscosity diagram) as shown in figure 3. If the plastic viscosity is low or below some 40 Pa s, the SCC should have a significant yield value (depending on the viscosity). On the other hand if the SCC is viscous i.e. with plastic viscosity over 70 Pa s, the yield value has to be about zero. The outer box in figure 3 represents the values of yield value and plastic viscosity for SCC that Wallevik and Nielsson came up with about a decade ago. The inner box in the figure represents the recommended values. The figure also shows the slump-flow necessary to obtain self-compacting concrete, depending on plastic viscosity (note that slump-flow values in figure correspond to slightly higher yield values than indicated for SCC by the outer box). 160 Yield value (Pa) Min. slump-flow to obtain SCC 550 mm 600 mm 650 mm 700 mm Plastic viscosity (Pa s) Figure 3. Proposed area for SCC in a yield value-viscosity diagram [7]. Minimum slumpflow values in mm to obtain SCC depending on viscosity are also shown in the figure. The inner area/box is considered by the author to specify more robust SCC. As figure 3 shows, the properties of SCC, described through yield value and viscosity parameters can vary substantially, which can be clearly seen when typical SCC mix designs from various countries are compared. For example, Japan, Sweden and the Netherlands generally utilize very high viscosity SCC with negligible yield value. On the contrary Norway, Iceland, Switzerland and New Zealand aim for very low plastic viscosity but normally along with significant yield value. Note; this paper is largely based on hypotheses and personal opinions of the author when it comes to evaluate SCC in various countries. He has cast and tested concrete in most west-european countries, several countries in Asia, in both east and vest Canada and several states in USA. He has held courses in over fifteen countries on rheology of fresh concrete and SCC. This does not mean that he is necessarily right, just that his opinions rely on extensive experience. BASIC RHEOLOGY OF SCC Rheology is the science of flow and deformation of matter and should therefore be the appropriate tool to describe the workability and mobility of fresh cement based materials like cement paste, mortar or concrete. The theory is already used by many concrete technologists in evaluation of hardened concrete, i.e. creep. It is also used to evaluate fresh cement based materials, though most of the research has been concentrated on basic theoretical aspects. In some cases there have been difficulties 3

4 Shear stress Shear stress in applying rheological test results, as measurement techniques used were somewhat questionable. C100 Shear stress (Pa) Normal concrete SCC Rate of shear Figure 4. Flow-curves of normal-, high strength- and self-compacting concrete [6]. The rheology of self-compacting concrete differs from that of CVC ( normal concrete ) as depicted in figure 4. An SCC has almost no yield value (or about zero to 60 Pa) compared to normal concrete in which it ranges from a couple of hundreds to several thousand Pa. The viscosity applied is very variable (especially between countries). It varies from 20 Pa s to well over 100 Pa s (in extreme cases the viscosity can be more than 200 Pa s). To see how the proportions of ingredients affect the properties of SCC, one should turn to figure 5, a principal illustration of the effect on yield value and plastic viscosity with increased content of water, silica fume, air and superplasticizer in conventional concrete mix. The figure also shows the influence on the shear stress as a function of rate of shear. The influence was established in 1983 by Wallevik [8] by use of a two-point workability instrument and the illustration is still valid. τ0 Stiff Silica τ Air Wet Viscous µ Silica τ Rate of shear γ τ0 Ref. Rate of shear γ Shear stress τ Water Rate of shear γ Yield value (Pa) Air Water Plastic viscosity (Pa s) 4 SP µ Shear stress τ SP Rate of shear Figure 5. A rheograph, illustrating the effect of water, air, SP and silica fume on the flow behavior of fresh concrete [8]. γ

5 Measures to reduce the viscosity of SCC In the opinion of the author, a rheological device is essential to obtain good SCC in respect of flowability, stability and to find robust as well as economical solution. It tells (scientifically) where one is situated regarding the properties of the fresh concrete, where to go to optimize them and how to get there. Some SCCs should be very viscous while others should have very low plastic viscosity, depending on the application. If a mix is has a low plastic viscosity (say 25 Pa s) it should have at least a certain yield value (>30 Pa) to maintain stability in respect of segregation. If the SCC is very viscous (>70 Pa s) the yield value has to be approximately zero (or < 10 Pa) to be able to move. The empirical test methods like slump-flow, T50 and the V- funnel can give an indication of the plastic viscosity, but probably only when the yield value is about zero. For instance the V-funnel time is merely a function of the plastic viscosity and the thixotropy when the SCC is very viscous, and of the yield value and the plastic viscosity when the SCC is low viscous. Also the adhesion (stickiness) properties play a certain role regarding the V-funnel time. 160 Yield value (Pa) SF Air W ater Changing from crushed to rounded aggregates will reduce plastic viscosity Plastic viscosity (Pa s) Figure 6. Some measures to reduce the plastic viscosity [15]. There are several measures to alter the plastic viscosity of an SCC. The three simplest measures to reduce viscosity are to add water, air and silica fume, see figures 6 (and 5). The two first (water and air) will reduce strength whereas the third one will increase strength. Water will also reduce yield value whereas air will have little influence and silica fume will increase it. The yield value can easily be regulated by a suitable dosage of a dispersing admixture. In some cases different types of coarse aggregate are available. Use of crushed (flaky) stone will lead to high viscosity and rounded stone to low viscosity. The wide range of viscosity of SCC The applied plastic viscosity can differ considerably among SCCs and the author has experienced viscosity in the range of 7 to 160 Pa s. The pioneers like Japan, Sweden and the Netherlands generally utilize very high viscosity due to the high water powder ratio. The yield value in these countries is normally negligible or about zero. Norway and Switzerland (where very good aggregates are often available) usually aim for very low plastic viscosity; on the other hand they normally utilize significant yield value (typically 20 to 40 Pa). 5

6 Table 1. A very rough estimation of typical SCC properties in different countries [15]. Powder Water Yield value Pl. viscosity (kg/m3) (kg/m3) (Pa) (Pa s) Sweden > The Nederlands > Japan > France?? 0-10 >60 Switzerland < Norway < Iceland < Denmark < <40 UK > Germany > US > Table 1 lists a very rough estimation of the powder and water content applied in different countries and, more important, a rough estimation of the plastic viscosity. A yield value below 10 Pa can be considered negligible, but significant if it exceeds some 10 Pa. Figure 7. A rheograph showing rough estimation of rheological behavior of typical SCC in some countries. Figure 7 shows a very rough estimation of rheological behavior of typical SCC in some countries, displayed in a rheograph. WHY SCC DIFFERS BETWEEN COUNTRIES Why is SCC so different, or for that reason one can ask; why is conventional vibrated concrete so different in various countries? Likely there are three main reasons: Traditions Locally available materials How open minded people are 6

7 Tradition is probably the most important factor and may in some cases be very hard to circumvent. Our industry is in general very conservative and there is not much space for improvement except it will save money in very nearby future (preferably tomorrow ). Naturally everything has to be according to standards at least it has to be ensured that nobody is responsible. Matters regarding reliability can be big obstacle for new technology. And note; far too many use the standards to the word, but they should not remove all human creativeness. High volume share of stone is often quite sacrosanct for many in several countries and it can be quite high, or two-third of the total aggregates (often in robust design the share of coarse aggregates is about one-third). The main reason for this conservatism is fear for increased shrinkage. If stone fraction is kept about the same in SCC as in CVC it will lead to very high paste volume, even sometimes over 40% which further will lead to high cement content and water content about or over 200 kg/m 3. Often one can reduce the paste volume significantly in the SCC by reducing the stone fraction considerably. It is often disregarded that it is the cement paste that shrinks (increased stone volume reduces the total shrinkage of the concrete due to increased rigidity). SCC from a ready mix plant in Norway is not so different from their conventional vibrated concrete (CVC), containing slightly more cement and sand (same Dmax and often same admixture). In USA there is quite big difference, cementious materials are significantly larger part, proportion of stone and Dmax is reduced by half and the admixtures are very different. On the other hand the pre-cast SCCs in USA are normally quite like the pre-cast CVC as they use typically around 400 kg/m 3 of cement, Dmax is about 10 til 14 mm and often they don t have to change dispersing admixtures. The difference in mix-design of CVC and SCC is very small in Norway contrary to Sweden where (until few years ago) it used to be quite big difference. SCCs in Sweden have a high requirement to flowability as their yield value is approximately zero and the plastic viscosity relatively high or up to 80 Pa s (high slump flow), but whereas the yield value is normally high (~40 Pa) in Norwegian solutions (plastic viscosity very low ~25 Pa s) so the slump flow will be low, but the result is nevertheless a good SCC. In Europe there is a bridge connecting two countries. It differs like black and white how they make/design the SCC. The difference is 30% versus 40% matrix, gap graded grading with very high stone concentration versus continuous grading with very low stone concentration, etc. And probably they have not tried each other s solutions and never will. Finally, Denmark has apparently achieved great success in producing SCC as rumor states that 20% or more of their ready-mix concrete production is SCC. But their SCC has generally very high yield value or slump flow typically 550 ±30 mm. If one would use description/definition of SCC from a neighboring country, Germany, this would drop well under 1%. This again demonstrates that views of SCC can be quite different in various countries. 7

8 EQUIPMENT TO ASSESS RHEOLOGICAL PROPERTIES OF CONCRETE Coaxial viscometers For assessing the viscosity and yield value of a concrete mix one needs an equipment which is able to determine these parameters. The ConTec BML [11,12] Viscometer and the three types of ConTec Viscometers [15] are shown in figure 8 a and b, respectively. These are coaxial cylinder viscometers for coarse particle suspension which have proven to be efficient tools to evaluate the rheological properties of cement suspensions. The ConTec Viscometers can be used to measure dilute suspensions such as cement paste as well as relatively stiff concrete mixes. a) b) Figure 8. The viscometers: a) The BML viscometer. b) The ConTec viscometers 5,4 and 6. Ideally these viscometers are applicable for concrete with plastic to flowing consistency. The more fluid and stable (in respect of segregation) the mix is, the more reliable the test results will be. When designed, they were made for concrete with a slump value of about 120 mm and higher. However, depending on the mix design, it is possible to get sufficiently good measurements on concrete having slump-value as low as 50 mm in extreme cases. Viscometers are ideal instruments to measure the rheological behavior of special concrete such as High Performance Concrete (HPC), Underwater Concrete (UWC) and in particular Self-Compacting Concrete (SCC) as the ratio τ 0 / µ is so low (<<100). They are also suitable to evaluate the effect of different ingredients such as; various admixtures, pozzolanes, cement types, different deliveries of cement etc. The more fluid the testing material is or the higher the plastic viscosity is relative to the yield value, the more accurate are the test results. Rheometers Several other instruments exist to measure the rheology of fresh concrete. Tattersall et al. developed the two-point instrument [9,10] in the seventies, see figure 9. The ConTec BML Viscometer [11], already shown in figure 8a, was developed in the end of the eighties. In the beginning of the nineties, the BTRHEOM [13] rheometer was introduced by de-larrard France and also the IBB instrument [14] (which is modified and automated version of the Tattersall MK III instrument) by Beaupré. As a rule of thumb if one is comparing values from these instruments; the Tattersall instruments gives about the same values as the ConTec Viscometers and the BTRHEOM give roughly double values of the Bingham parameters, the yield value and the plastic viscosity. 8

9 Figure 9. The Tattersall two-point (MK II), the IBB instrument and the BTRHEOM rheometer. All the above listed rheometers/viscometers are pretty big and it is quite time consuming to make a test (relative to the simple slump test), thereby not so suitable for use at the casting site. This is an obstacle in use of rheology as one can hardly verify the prescribed rheological properties of concrete or use rheology as a tool for quality control of concrete production during casting. Figure 10. The ConTec Rheometer-4SCC at different building sites. A portable rheometer, named Rheometer-4SCC has been developed at IBRI which is intended for SCC and is at least as accurate as the rheometers mentioned above. It is quite easy to use, requires about 7 liters of concrete for testing and the time from filling the material container, testing, data processing to emptying it is about 1½ minute (even shorter than needed for the simple slump test). The instrument can also be used for conventional vibrated concrete, but then a Tattersall impeller is used instead of the so-called SCC impeller. 9

10 FINAL REMARKS There is a huge difference between the rheology of fresh SCC in various countries, which largely can be explained by their tradition in making CVC. Generally high powder, viscous SCCs are more robust (safe) in respect of production, in particular regarding segregation. On the other hand they are liable to be more costly, heavier in placing, sticky and will move very slowly (viscous). It is more difficult to make stable, low-viscous SCC and then it is likely to have relative high yield value which can easily lead to a stiff or semi-scc (S-SCC), with slump-flow 550 mm or lower. S-SCC does not necessary have to be inferior product and maybe a way to obtain success as has happened in one country in Europe. To make a robust, low viscous and low yield value (slump-flow > 650 mm) SCC can be really difficult (it s almost an art) and one should be aware that it not the most suitable type of SCC for all applications. High viscous type of SCC can be more proper for some walls, as in the same construction low viscous SCC can be more appropriate for the slabs. The only real way of evaluating the viscosity is by use of rheology (the science of flow and deformation of matter) and to quantify it one has to use viscometer or other rheometer. ACKNOWLEDGEMENT The author would like to thank the Icelandic Research Fund (Rannis) for their financial support of the research on SCC. The colleagues at IBRI are also thanked for their assistance in particular Asbjorn Johannesson. REFERENCES 1. Wallevik, O.H., Nielsson, I., Editors, Rilem Proceedings no. 33, 3rd Int. Symposium on SCC, Rilem, Reykjavik, August, 2003, p Gaye, J., Sturm, A., Der Gussbeton und seine Anwendung im Bauwesen. Verlag Ernst & Sohn, Berlin Brameshuber, W., Selbstverdichtender Beton. Aachen, 2003 In: Schriftenreihe Spezialbetone Band 5, Verlag Bau + Technik, 2004, p Collepardi, M., Rheoplastic Concrete, Il Cemento, 1975, pp Collepardi, M. Self Compacting Concrete What is new? ACI SP217-1, September, 2003, pp Wallevik. O., Nielsson, I., Self Compacting Concrete A Rheological Approach, International Workshop on SCC, Kochi, Japan {JSCE Concrete Engineering Series no. 30}, August, 1998, pp Wallevik, O.H., Practical description of rheology of SCC, SF Day at the Our World of Concrete, Singapore, August, 2002, p Wallevik, O.H., Description of fresh concrete properties by use of two-point workability test instrument, Master thesis, NTH, Trondheim, 1983, p (in Norwegian) 10

11 9. Tattersall, G.H., Relationships between the British standard test for workability and two-point test, Magazine of Concrete Research, Vol. 25, No. 84, 1973, pp Tattersall, G.H., Banfill, P.F.G., The rheology of fresh concrete, Pitman, London, 1983, p Wallevik, O.H., The Rheology of Fresh Concrete and its Application on Concrete with and without Silica Fume, Dr. Ing. thesis 1990:45, NTH, Trondheim, 1990, p (in Norwegian) 12. Wallevik, O. H., Gjørv, O. E., Development of a Coaxial Cylinder Viscometer for Fresh Concrete, Properties of Fresh Concrete, Proceedings of the Rilem Colloquium, Chapman & Hall. Hanover, October, 1990, pp De Larrard, F., Sedran, T., Hu, C., Szitkar, J.C., Joly, M., Derkx, F., Evolution of the Workability of Superplasticized Concretes: Assessment with BTRHEOM Rheometer", RILEM International Conference on Production Methods and Workability of Concrete, RILEM Proceedings 32, Glasgow, Scotland, 3-5 June, 1996, pp Beaupré, D., Rheology of high performance shotcrete, Ph.D. Thesis, University of British Columbia, 1994, p Wallevik, O.H., Rheology A Scientific Approach to Develop Self- Compacting Concrete, 3rd Int. Symp. on SCC, Rilem, Reykjavik, Iceland, August, 2003, pp