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Karin Safranek, TU Vienna, Austria

In order to make it at all possible to increa-se the strength of concrete, the original 3-component system (water, cement andaggregate) had to be expanded (fig. 2).The purposeful manufacture of concretewith a compressive strength of over 100N/mm2 was first made possible with theintroduction of silica dust and superplastici-ser or liquefier as additives [3]. This expan-sion to a 5-component system has revolutio-nised concrete as a material and allows iteven to compete with steel in somerespects.

The basis for the outstanding properties ofUHPC is the reduction of the water/binderratio to under 0.2 as well as the homogeni-sation of the concrete texture. In order toachieve the latter, not only was the concre-te technology improved during the courseof research, the mixing technique was alsocontinually updated and adapted to thespecial requirements. Compared to normal concrete, UHPC ismixed for a significantly longer time, since itis important to achieve the optimum homo-geneity. It is necessary that the fine grainslie as individual grains in the concrete anddo not form agglomerates. This is important

for good processability and high strength[4]. Modern mixers are computer-control-led and can be adapted individually to anymaterial. The speed with which the mixingdrum revolves and its direction of rotationcan be controlled. Similarly, differentmixing profiles can be created in which thematerial is mixed in a particular sequenceof speed/time combinations. Over andabove that, various mixing tools are used.

Mixing processes

The theory of mixingMixing can be understood to be the distri-bution of particles of mass in a specifiedvolume, whereby the particles differ in atleast one property [5]. The objective ofmixing is the even distribution of all compo-nents. A mixture can be said to be homoge-neous if all components are balanced intheir concentration. If the properties of thematerials of a mixture to be mixed fluctuatedue to production or on account of the rawmaterials, the compensation of this fluctuati-on is described as homogenisation [6].A great many requirements must be metwhen manufacturing concrete. Most of therequirements are precisely defined. Certaindimensions, masses, forces or a certain tem-perature must be maintained. The proper-ties of the cement, the aggregates, themixing water, and the additives are also

described in standards and specified to theuser. However, there are no binding specifi-cations or standards for the process withwhich these materials are all brought toget-her: namely the mixing of the concrete [7].There are no regulations governing therequirements for mixing, the mixing actionand the uniformity of the concrete. One important goal in mixing is to achievea prespecified mixing quality, as well asquality assurance. There are many factorsthat influence a mixing process [5]. Themost important of these are machine andproduct-influencing variables.In terms of the mixing technique, qualitymeans uniformity or, in other words, homo-geneity. The goal of every mixing process isto achieve an ideal homogeneity. The ideal or absolute homogeneity is thetotal evenness of the entire material. In mix-tures of solids in a dry or wet state there isno absolute homogeneity. In these areas ofconsistency an apparent homogeneity isachieved at the macro-level, but only a par-tial inhomogeneity is to be found at themicro-level. A chessboard pattern represents an idealmixture. However, this cannot be said to bea mixed state; it is much more a distributedstate. This is an ideal state of order. Thestate of a good mixture can therefore notbe that of an ideal order, but rather only acomplete disorder [9]. If one were to arran-

The influence of different mixing processes on the strength of ultra-high performance concretes

Results of a degree dissertation from Austria

The history of mixing and the history of the manufacture of concrete go back a long way. Even the Romans had the so-called opus caemen-titium, nowadays known as Roman concrete. This was used to manufacture compressively strong structural components from water-resi-stant mortar and stone chippings. Probably the most famous building constructed with Roman concrete is the Pantheon (fig. 1), whose domespans 43 m [1]. Even today we are still striving for ever greater heights and spans in buildings. The development of continually better buil-ding materials goes hand-in-hand with the drive towards the ever larger. Hence, over the course of time the compressive strength of con-crete has been continuously increased, which has ultimately led to a completely newly-developed building material, ultra-high performanceconcrete (UHPC for short).

Figure 1: Pantheon in Rome [2]

Figure 2: Concrete as a 5-component system

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ge an equal number and size of black and white material elementslike a chessboard in a mixer and then switch on the mixer, the stateof complete disorder would always be achieved after a not definedmixing time.

Absolute homogeneity only exists in an ideal state of order, which,however, cannot be produced by the mixing process. The final stateafter a mixing process corresponds to a complete disorder, whichat best leads to a homogeneous random mixture (fig. 3).

Mixing devicesWithin the scope of the degree dissertation by K. Safranek, investi-gations were carried out into the effects of the mixing process on thestrength characteristics of ultra-high performance concrete. To thisend the differences between a conventional mixer and the tilted panmixer used (Eirich company) were determined first of all. The main difference between a conventional pan mixer (ring-pan orplanetary mixer) and a tilted pan mixer is the tilted position androtation of the mixing container. The mixture is transported to themixing tool by the rotating movement (fig. 4). The special feature ofthe tilted pan mixer is that the transport of the mixture is decoupledfrom the actual mixing process. This separation of the transport ofthe mixture and the mixing process enables the speed of the mixingtool and, hence, also the input of power to the mixture to be varied[7]. In a simple mixer the speed of the container is zero. The movementenergy of the mixing tool is converted into mixing energy, frictionand wear. A tilted pan mixer has a rotating mixing container, forwhich reason the speed of the mixture is equal to the speed of thecontainer and the movement energy therefore directly correspondsto the mixing energy.

Ultra-high performance concrete (UHPC)

Ultra-high performance concrete is a particularly high density con-crete with a largest grain size of < 1 mm. Due to the use of very fine

DI Karin Safranek, born on 10 March 1983 in Vienna. Studied CivilEngineering at the Technical University of Vienna, with the main subjects building materials, building physics and fire protection.Currently active in the field of building physics, thermography and structural element simulationand optimisation. Winner of the Technology Award 2008 from the Österrei-chische Vereinigung für Beton- und Bautechnik (Austrian Association of

Concrete and Building Technology). karin.safranek@kabsi.at

Figure 3: Various mixing states [5]

a) completely demixed b) ordered c) textures

d) Demixing e) real interim status f) homogeneous random mixture

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aggregates as well as the addition of micro-silica as a bulking agent, ultra-high perfor-mance concrete achieves a compressivestrength of over 150 N/mm2. This strengthcan be further increased by the addition ofsteel fibres. A further feature of UHPC is itslow water/binder ratio. In order to achievevalues < 0.2 and at the same time ensuregood processability, high performancesuperplasticiser must be mixed in. Onaccount of its fine constituents, UHPC is avirtually homogeneous building material,which is distinguished by its outstandingproperties with respect to its strength anddurability (fig. 5). In order to test the limits

of the mixing device, a concrete with speci-al homogeneity re quire ments, namely ultra-high performance concrete (UHPC), wasmanufactured for the laboratory experi-ments as part of the above-mentioneddegree dissertation.

Mixing processIn a mixing process, all raw materialsshould be mixed with one another in such away that they are evenly distributed in thefinal product. Three factors are of essentialimportance for achieving the best possiblehomogeneity:• Mixing period

• Mixing speed• Mixing toolWith a tilted pan mixer it is possible to spe-cify the above-mentioned influencing varia-bles precisely and, hence, to control themixing process.The mixing period must be selected inaccordance with the mixer so that sufficientmixing can take place. If too little mixingenergy is input, the properties that would

be possible on account of the compositionof the concrete cannot be achieved [14].The mixing period is thereby dependent onthe device itself and the type of concrete.On account of their composition, ultra-highperformance concretes must be mixed for asignificantly longer time than normal con-cretes. However, over-mixing would result inthe quality of the concrete being reducedagain.

Like the mixing period, the mixing speed isdependent on the type of concrete. Fromthe results of the experiments it can be seenthat the input of energy into the mixtureincreases as the speed increases and,hence, better homogenisation is possible.However, if the speed is too high, this canlead to rapid increases in the temperatureof the mixture. This must be consideredwhen selecting the speed.The mixing tool plays a central part in themixing process. On account of its geome-try, more or less energy will be input to themixture and in this way the homogeneity ofthe concrete will be influenced (fig. 6).

The properties that are possible on accountof the composition of the concrete are onlypossible if sufficient homogenisation of themixture is achieved. In order to control themixing process and to achieve a good mix-ture, it is necessary to control the input ofenergy. This in turn is possible with a tiltedpan mixer by Eirich for example, sincehardly any friction and wear on the deviceoccurs during the mixing process. For thisreason the energy which is input to thesystem can be directly equated to the ener-gy necessary for mixing.

Tab. 1: Comparison of a simple mixer to a tilted pan mixer

Conventional mixer Tilted pan mixer

Mixing container is stationary Mixing container rotates

Rotating tools transport the mixture The container transports the mixture

Friction of the mixture against the walls Friction inside the mixture only, no wall and base of the container friction

The mixing power is reduced due The mixing power is retainedto friction

Electrical energy = mixing energy Electrical energy = mixing energy+ friction energy + wear energy

Figure 4: Mixing principle of a tilted pan mixer, Eirich company [11]

Figure 5: Texture of UHPC [15]

robust, service-friendly tool

static combination toolas floor/wall scraper

Mixture flow with highspeed difference

revolving mixing container

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Summary

The mixing process for the manufacture ofconcrete is regulated in Austria by theÖNORM B 3303 standard, in which isstated: ‘…Mixing must be carried out in alaboratory mixer in such a manner that ahomo geneous mixture results and that nocon sti tuents of the mixture are lost…’ Inorder to determine the homogeneity of amixture and to check it subsequently, it isnecessary to know how much power orenergy is input to the mixture.

Reproducible mixing has become possiblefor the first time with the tilted pan mixer.On account of its tilted, rotating mixing con-tainer, the speed of the mixture is equivalentto that of the container, for which reasonthe movement energy is directly equivalentto the mixing energy.

Significant differences can be seen whendifferent mixing tools are used. The starwhirler is most similar to the mixing tool in aconventional concrete mixer and producesgood results. However, it was only with theuse of the pin whirler that it could bedemonstrated that much better results canbe achieved with other tools.

Prospects

Ultra-high performance concrete is in manyrespects a great building material and ithas the potential to be used in the widestvariety of applications – now and in thefuture. The dense structure and very lowporosity of UHPC makes it an ideal buildingmaterial for structural elements and suppor-ting structures that are exposed to aggressi-ve media [12]. In order to meet these requi-rements, adequate homogeneity of the buil-

ding material must be guaranteed.Controllable and, above all, reproduciblemixing processes are required.

As the experiments showed, unreinforcedultra-high performance concretes exhibit asudden, brittle failure and a low tensilestrength in relation to its compressivestrength. In order to achieve sufficient ducti-lity, the addition of steel fibres is necessary,possibly in combination with polypropylenefibres [13]. In order to ensure sufficienthomogenisation of the material, controlla-ble and, above all, reproducible mixingprocesses are required.

K. Safranek’s degree dissertation showsthat one and the same building materialcan exhibit different properties dependingon the way it is manufactured. It is therefo-re important to standardise the manufactu-ring process, in this case the mixing pro-cess.

In the same way that the progressive deve-lopment of concrete technology has madesignificant progress with research intoUHPC, the invention of the tilted pan mixerhas also provided for significant progress inmixing technology. The developments gohand-in-hand, and that is a good thing too,because ultra-high performance concretecan only develop its outstanding propertieswith a suitable mixing technique, and inturn a new area of application for the tiltedpan mixer has been born.

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Bibliography

[1] Heinz-Otto Lamprecht: „Opus Caementitium,Bautechnik der Römer“, Beton-Verlag, 3. Auflage, Düsseldorf 1987

[2] Roma Antiqua: „Pantheon“, http://www.roma-antiqua.de, 08. 11.2005

[3] Johannes Horvath: „Beiträge zum Brandverhalten vonHochleistungsbetonen“, Dissertation, TU Wien, 2003

[4] Johannes Horvath: „Entwicklung von hochfestemBlähglasbeton“, Diplomarbeit, TU Wien, 1998

[5] Matthias Kraume: „Mischen und Rühren“, WILEY-VCHVerlag, Weinheim 2003

[6] Ralf Weinekötter, Hermann Gericke: „Mischen vonFeststoffen“, Springer-Verlag, Berlin Heidelberg 1995

[7] BWI - Beton Werk International: „Mischen von Beton –eine ständige Herausforderung“, Heft 03/2004

[8] ÖNORM B 3303: Betonprüfung[9] Hans- Peter Wilke, Ralf Buhse, Klaus Groß: „Mischer“,

Vulkan-Verlag, Essen 1991[10] BFT Betonwerk + Fertigteil-Technik: Symposium

„Mischen – Steuern – Protokollieren“, Heft 5/2005,Seiten 68 bis 73

[11] Eirich Druckschrift-Nr. BT 1426-0-de: „EirichMischtechnik für hochwertige Betone“, Seite 4

[12] Gert König, Frank Dehn: „Neue Entwicklungstendenzenin der Betontechnologie“, Baustofflehre, Bauphysik,Brandschutz: Festschrift zum 60. Geburtstag von U. Schneider, Wien, 2002, Seiten 95, 99-101

[13] BWI - Beton Werk International: „Forschung undEntwicklung im Dienst des Bauens“, Heft 04/2004

[14] Dirk Lowke: „Untersuchungen zum Einfluss desMischens auf die Frischbetoneigenschaften selbstver-dichtender Betone“, Vortrag beim Symposium „Mischen– Steuern – Protokollieren“, Hardheim, 26. Jänner2005

[15] BAM, Bundesanstalt für Materialforschung und -prü-fung: „Arbeitsgruppe Zementgebundene Baustoffe,erweiterte Leistungsmerkmale“, http://www.bam.de,02.09.2005

Figure 6: Mixing tools: star whirler (a) and pin whirler (b)