Mixing concrete – a constant challenge Good mixer, good ...

CC O N C R E T E T EO N C R E T E T E C H N O LC H N O L O GO G YY

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By Dr. Peter Nold, Dipl.-Ing. Ralf Löbe,Maschinenfabrik Gustav Eirich,

Hardheim, Germany

Mixing concrete

Today’s concrete is simultaneously amass building material and a high per-formance material, writes ProfessorDr.-Ing. Bernd Hillemeier [1]. “Untilnow, concrete has been considered asa simple ternary system of cement,water and aggregate. But today thereis a push to create new and innovativeconcretes. High-tech concrete is trea-ted as a six phase system, consistingof cement, aggregates, water, fluidand solid additives, and air.” [1].

The characteristics of the cements, ag-gregates, mixing water, fluid and so-lid additives are described in stan-dards and specified to the user. How-ever there are no binding guidelinesor standards for the procedure, withwhich these materials are all united:mixing the concrete.

Here both the regulations and the ce-ment manufacturers themselves trustin the feel of the user. DIN EN 206-1only requires uniformity from themixer: “9.6.2.3 Mixers: The mixer mustbe able, with their available capacity,to achieve an even distribution of theraw materials and an even workabilityof the concrete within the mixingtime” [2]. The mixing process itselfcannot be tested in any way and iscompletely subjective: “9.8 Mixing theconcrete: Mixing the raw materialsmust take place in a mixer according

to 9.6.2.3 and take such a time neededfor the mixture to appear homoge-neous” [2]. The standard does not re-cognise a definition of uniformity.

The guidelines from the cement ma-nufacturers are going in the same di-rection. “Mixing must take place in amechanical mixer and be continueduntil the mixture appears uniform.This period of time is the mixing time.Experience suggests that with normalconcrete it is a minimum of 30 se-conds, and 90 seconds with light-weight concrete. When producing con-cretes with special requirements, forexample self-compacting concrete, fairfaced concrete or when using air en-training agents, longer mixing timesmay be necessary” [3]

What is a “longer” mixing time? Andwhat happens, if the mixing time istoo short or too long? When makingwhipped cream, the housewifeknows when she must stop “mixing”.Who knows this for concrete? Thehousewife also knows that she mustcarry out her mixing tasks at a parti-cular minimum speed. Who todayknows what the minimum speed isfor mixing of concrete?

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Good mixer, good concrete: the first step on the road to success

Figure 1:Gravity mixer made ofwooden boards,around 1860[13]

Figure 2: Hand operated trough-mixing machine, Tietze design,around 1870 [13]

M i x i n g c o n c r e t e – a c o n s t a n t c h a l l e n g e

Concrete is a material that for decades has been thesubject of research and development at a variety ofinstitutions and universities throughout the world,and will probably remain so for a long time to come.New improved concretes are regularly presented atcongresses and conferences. And we regularly hearthe speakers say that a particular concrete is diffi-cult to mix, and that it should be mixed for a longerperiod of time. Some speak merely of more mixing

without actually putting any numbers on it. The factis: premium concretes are still mixed according to“feel” as there are no objective rules laid down forthe mixing procedure. Mixing means introducingenergy to a system. The purpose of this contributionis to make mixing understandable as a rational pro-cess involving energy. The mixing systems availableon the market will also be presented.

Figure 3:Trough mixer from patent

specification No. 71321Imperial Patent Office,

Germany 1893 �by

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The fact that the mixing pro-cess can be formative for con-crete is becoming ever moreapparent. Chiara F. Ferraris, ofthe National Institute of Stan-dards and Technology in theUSA, writes in his article “Con-crete Mixing Methods andConcrete Mixers: The State ofthe Art” [4]: “As for all materi-als, the performance of con-crete is determined by itsmicrostructure. Its microstruc-ture is determined by its com-position, its curing conditions,and also by the mixing methodand mixer conditions used toprocess the concrete.”

What our fathers knew

What appears to the reader as a newproblem, is not by any means new.Back in 1967 the following appeared:“ the characteristics of concrete is notonly determined by well-known factorssuch as the cement content, the grainstructure, the inherent strength andparticle shape of the aggregate, thecompacting method and subsequenttreatment, the water cement ratio etc.but also the type of mixing processthat has been carried out, for examplethe intensity of the scrambling pro-cess” [5]. Investigations are quoted,which show that increasing the mix-ing time raises the strength of theconcrete [6].

One of the effects of an “intensivepreparation of concrete” made pos-sible by special mixing technologywas described as “an improvement ofthe strength development, a reduc-tion in cement usage, the use of chea-per types of cement, an improvementin workability, an increase in the dis-persion of strength measurements

and less water separation” [5]. Thecomparison was made between “nor-mally” mixed concretes with an energyconsumption of 1.0 kWhper cubic metre of harde-ned concrete compared to“intensively” mixed con-cretes with an energy re-quirement of 2.5 kWh percubic metre of hardenedconcrete; the extra costsfor electrical energy were,at the time, 10 times grea-ter than the saving in ce-ment.

In 1973 there were re-ports on investigationscarried out with Eirichmixing technology, whichat the time was new, usingan intensive mixer thathad an inclined mixingvessel [7]. The mixing,carried out with 200 kilo-grams of cement in 60 se-conds, amounted to threekilowatt-hours per cubic metre offinished consolidated concrete. Com-pared with a mixer, which required

0.75 kilowatt-hours percubic metre in the sameperiod of time for a mix-ture with 300 kilogramsof cement, the intensivemixing resulted instrength increases ofmore than 20% - in spiteof having 100 kilogramsless cement [7].

Without exception, inthe opinion forming com-mittees, the cement pro-ducers did not voteagainst setting minimum

cement quantities in the stan-dards for construction concrete.Because of the guidelines in thestandards, the entire cementusing industry was unable to pushthe use of intensive mixing tech-nology. As a result, Eirich with-drew from the mass concrete mar-ket and limited themselves to theproduction of fine grain concre-tes such as roof tile concrete, faceconcrete and similar.

The knowledge of how to mixconcretes was lost over the courseof the decades. Of course the con-cretes themselves have changed,and knowledge gained in 1973 re-

garding intensive mixing could notsimply be transferred to today’s con-cretes. Today almost every mixer ma-

nufacturer calls his product an “inten-sive mixer”. And in their brochures,many mixer manufacturers speak ofthe possibility of saving cement withtheir mixing system. However, no ex-amples or investigations are quoted.For example, a 5% saving on cementwould have resulted in more than100,000 tonnes being saved in Ger-many’s 20,473,000 tonnes of concreteproducts used in 2002 for road con-struction, gardening and landscaping[8] .

Regarding the durability of concrete

In recent years the durability of con-crete as a material is talked about more

Figure 6:Funnel shaped mixing machine, around 1908 [19]

Figure 5: Trough mixer with a twin shaft system [18]

Figure 4: Concrete mixing machine,patent Kunz, 1910 [16]


and more. Durability means that therequired performance characteristicsremain during a fixed length of time(service life, life span) under the re-gular operational conditions and tak-ing account of costs (fair manufac-turing and maintenance costs) [9].

No investigations have been publishedconcerning how the mixing processaffects the durability of concrete. Never-theless it is known to the authors thatuniversities are working on it.

Mixing technology development- mixing systems

The Verein Deutscher Zementwerke[Association of German Cement Plants],of Düsseldorf, Germany, says: “machinemixing is carried out in charge is in adrum, pan or trough mixer or in con-tinuous mixers. Drum mixers are lesssuitable for stiff mixtures and thosethat are very cement-rich” [10].

Drum mixers are gravity mixers. Theyare not considered here. Pan andtrough mixers are forced action mix-ers. An explanation on the Internetsays [11]:

“Forced action mixers:The mix is forced through turning mix-ing tools and mixed. Forced action inmixers considered to produce goodmix quality, but this is substantially af-fected by the position of the blades.

Pan mixers: Forced action mixer for the mixing of

concrete in which fixed or rotatingmixing blades are arranged on a fixedor rotating circular mixing pan.

Trough mixerForced action mixer for mixing con-crete, where one or two horizontalmixing shafts are arranged on a fixedor tiltable mixing trough.”Grouped together with pan mixers areannular trough mixers, planetarymixers, cone mixers and Eirich mixers.Grouped together with trough mixersare shaft mixers and double shaftmixers

Gravity mixersIn the early days of concrete, earth-moist concretes, which were compac-ted by tamping, were common, andwere used in hydraulic and civil en-gineering. Since the beginning of the19th century concrete products suchas sewerage pipes, roof and floor slabs,moulded figures and reliefs have beenproduced, later followed by local con-crete constructions such as bridges,water tanks, foundations and cellarsmade of waterproof concrete. [12].

These concretes could be mixed with

Figure 7: Annular trough mixer, 1903

Figure 8: Planetary mixer, around 1910

Figure 9:Mixing station in the construction of the Kiel Canal (1907 to 1914)


a hand shovel and la-ter on, with specialgravity mixers ma-de of wood or me-tal, which were fil-led with a shovel.

Single and twinshaft mixersThe break-throughin building tech-niques withconcrete camewith the pro-duction of “ironreinforced concrete”(later referred to as “steelreinforced concrete”) astar based on the J. Monier’s patentsof 1867 and 1878 [14]. Now concreteswere necessary in order to completelyin close the reinforcing bars with ce-ment. Accordingly the mixing techno-logy had to be better. In the firstforced action mixers were developed,mainly based on the principle of thesingle shaft mixer (trough mixers orinline blenders).

Figure 2 shows a trough mixing ma-chine with an emptying flap, around1870, Figure 3 a trough mixer wherethe outer shell could be tilted foremptying [15].

Around 1900, mixers of different sizesand types became available, based onthis patent by Alfred Kunz of Kemp-ten, Bavaria (Figure 4). However,these mixers were not accepted in themarket without some reservations.Single shaft mixers produced a mix oflimited quality, in particular when mix-ing fine grain concretes. In the caseof coarse aggregates there was veryhigh wear around the walls. Further-more, the shaft supports lay in themix.

Twin shaft mixers deve-loped out of the singleshaft mixer. The mix tosome extent can avoidthe shaft, reducing wear;only around 30 per centof the revolution of a mix-er tool results in wear onthe floor of the trough[17]. In 1930, the Bayer.Berg-, Hütten- und Salz-werke AG, Zweignieder-lassung Hüttenwerk Sont-hofen [Bavarian Mining,Smelting and Salt Works,Sonthofen MetallurgicalPlant] offered both singleand twin shaft mixers[18] (Figure 5). The com-pany that grew out ofthis, BHS Sonthofen, stillproduces twin shaft mix-ers.

Today, suppliers of singleshaft mixers include, forexample, Elba und Reich.Those companies offer-

ing twin shaft mixers are, for exam-ple, Amman, Arcen, BHS, Elba, Elema-tic, Liebherr, Lintec, OMG Sicoma,ORU, Reich, Simem, Stetter, Teka andWiggert.

Conical mixers

This mixing system is also around 100years old as shown in Figure 6. The sys-

tem did not become generally ac-cepted in the market for process-

ing concrete, although it pos-sesses the advantage of

simple emptying.

The designation “conicalmixing machine” was al-

ready assigned in 1910 [20].Conical mixers are very common

in the chemical, pharmaceutical andfood industries. One or two stirringtools are arranged in a conical mixingtrough; where the mixer has twotools, in these can counter rotate. Ma-nufacturers of such mixers found onthe Internet include companies suchas Amixon, Coperion Waeschle, FrymaKoruma, AVA-Huep, EMT Euro-Misch-Technik, BOLZ-SUMMIX, Glatt, andHosokawa.

The cone mixer was reinvented forconcrete. Kniele was awarded theGerman Precast Concrete ElementSuppliers Innovation Prize 2003 atthe Ulmer Beton- und Fertigteil-Tage[Ulm Concrete And Precast ElementConference]. Two contra rotating stir-ring tools run in the Kniele conicalmixer, whose speed, according to thebrochure folder, is infinitely variable.However at the same time Knielewrites “the low number of revolutionsof the external stirring tool meansless wear on the external wall andscrapers”. Thus the tool speeds hereare quite low, according to the manu-facturer.

Suppliers of cone mixers and trunca-ted cone mixers for concrete includeKniele and Pemat.

Annular trough mixer

The market for a good mixing systemwas very open in 1900, as demonstra-ted by some remarks recorded in thememories of Willi Eirich (1900 –1985). In 1968 he wrote in a chro-nicle of the company [21] “ The be-ginning of mixing machine construc-tion at Eirich in Hardheim in the year

Figure 10: Eirich mixer,at the stage of development in 1924

Figure 11: Announcement, around 1930


1903”: “As far as I wasaware, it was in the year1903 when Uncle Ludwigfirst met a manufacturerof concrete block moulds,known as artificial stonein those days, when visit-ing an exhibition in Düs-seldorf. In this exhibitorwas showing moulds forsmall hand tamped con-crete pipes, a manuallyoperated forming tablefor concrete roofing tilesand a device for the handmanufacturing of smallconcrete blocks andslabs”. And “ the exhibitorwas complaining and thatthere was no suitable ma-chine available for mixingthe concrete required forhis forming equipment”.

Whereupon the two brothers Ludwigand Josef Eirich thought about how tohelp this man out. The result of this

was the first annular trough mixer in1903 (Figure 7). In particular the de-veloping concrete product industrytook on these mixers with enthusi-asm.

When, in the middle of the last cen-tury, Eirich withdrew from the busi-ness of processing mass concrete, se-veral mixture manufacturers dis-covered the old mixing principle.Suppliers of annular trough mixers to-day include Kniele, Liebherr, Masa,OMG Sicoma, Pemat und Teka.

Planetary mixers

In 1906 Eirich considered that the an-nular trough mixer was outdated, andthe planetary mixer was invented (Fi-gure 8).

The planetary mixer gave better mix-ing results compared to the ringtrough mixer. Numerous large bridgeand lock construction projects werecarried out using this mixer with itsfixed mixing vessel in the first decadesof the 20th century. For example, va-rious concrete works were established(Figure 9) for building the KaiserWilhelm Canal in Germany (todayknown as the Kiel Canal); Concrete

Works I at the lock building siteBrunsbuettelkoog used five mixers toproduce “2000 cubic metres of con-crete 20 hours”, and Concrete WorksII had an output of 250 cubic metresof concrete per hour [22].

Planetary mixers succeeded for thefirst time in manufacturing what atthe time were considered to be veryhigh-quality mixtures; the mixers wereused as “concrete and mortar mixingmachines” and “were unsurpassed intheir use as mixers for cast stone, ready-mix mortar, moulding sand, xylolite,rendering, Terrazzo, tar macadam,aniline, powder, artificial fertilizerand so on”. [22]. Compared to othermixing units (tube mixers) there were

Figure 12:Eirich mixer withagitators, at thestage of develop-ment in 1960

Figure 14: Eirich mixer with an inclined mixing container

Figure 13:Eirich mixer with agitators,diagrammaticsketch

savings on cement when producingconcrete. The mixers were usedthroughout the world. The manufac-turer was so confident, that even backin 1913, he lent the mixers out for onemonth’s trial. No mixers were returned,all the customers bought one.

In 1912 Eirich developed the first pla-netary mixer that worked continuous-ly [23]. At that time the only conti-nuous mixers available on the marketwere trough mixers, developedfrom tube mixers; the patent appli-cation described how “there ishowever no thorough mixing ofthe individual components ofthe mix”.

When in the middle of thelast century Eirich with-drew from the productionof mass concrete, severalmixture manufacturers disco-vered the old mixing principle.Suppliers of planetary mixers to-day include: Arcen, Damman-Croes, Fejmert, Haarup, Kniele,OMG Sicoma, ORU, Pemat, Schlos-ser-Pfeiffer, Simem, Sipe, Skako, Tau-rus, Teka and Wiggert.

Counter current intensive mixer

In 1924 Eirich developed the firstmixers with driven mixing vessels(Figure 10). This mixing principle isunique. The mixers were introducedinto many industries as “Eirich coun-ter current intensive mixers” or simp-ly “Eirich mixers”.

This innovation again brought a newlevel of performance to the market:Fill, mix and empty in 40 seconds (Fi-gure 11).

The production of planetary mixersceased in 1924. Around 1970 the cha-racteristicsof thean-

nular trough mixers , planetary mix-ers and Eirich mixers were describedas follows [24]:

Annular trough and planetary mixer:- The circular tool guide only

achieves a limited mixing effect - Heavy wear on the wall and floor

of the mixing contai-ner and the tools. - With large mixers a

large number oftools is needed.

- A high percentage ofthe energy used isconverted to friction(wear).

- The filling height islimited by the peri-pheral speed of themixing blade.

Eirich mixer: - The eccentric posi-

tioning of the mix-ing stars and theirmovement againstthe material streamproduced by the ro-tating mixing vessel

creates a large number of changes inthe spatial position of each individualparticle of the mix in both a verticaland horizontal direction, conditionsfor a rapid and thorough mixing. - At one turn in every five to 10 se-

conds, the floor surface of the mix-ing container is cleared severaltimes by the mixing tools.

- The mixing blade circulated the in-coming mixing batch in the styleof a ploughshare

- Only a few tools are necessary tokeep the mix in constant motion.

- The movements of the mixing con-tainer and the mixing tools rein-force one another. Also a relativelylow number of revolutions minuteproduces a mixing effect.

- Stationary wall and corner scrapersforce all the mix into the mixingprocess.

- Mixing tools arranged on a numberof levels give the most favourablefilling height and thus a high mixoutput.

Mixers experienced a strong upswingaround 1930. At that time a largenumber of concrete roads were built[25]. In the magazine DIE BETON-STRASSE [CONCRETE ROAD] therewere reports on projects from Europeand the USA.

In TEER UND BITUMEN [TAR ANDBITUMEN] in 1931 the Eirich mixerwas described as the only “universalmachine”, “which allowed for usewith the most diverse range of build-ing materials, like tar, bitumen andconcrete” [26].

Accepting something newer and bet-ter has always been difficult - even inthose days. In 1928 de Weerdt wrotein “Experimental mixing in one ofGermany’s largest concrete works”that “unfortunately even today, in thetechnological age, one can still findspecialists who cannot come to termswith the fact that mixing systems thatwere once usable and profitablecould now have been overtaken” [27].The trials included tube mixers, annu-lar trough mixers, planetary mixersand Eirich mixers. De Weerdt also de-scribed how “with the Eirich mixeronly 5/7 of the mixing time is required,compared to other systems, to producea 40% greater increase in strength”.

The Eirich mixer brought a consider-able increase in strength. With “iron

CPI - Concrete Plant International – # 3 - June 2004


6

Figure 15:Mixing principle

of the inclined Eirich mixer

Figure 16: Flow of mix


concrete”, a rise of 178 to377 kg/cm2 was reported af-ter one minute mixing time,while with road concretethe increase was from 354to 666 kg/cm2 [28]. Todayin Italy, Croci produces acounter current intensivemixer based on the Eirichof 1924.

Counter current high-efficiencymixer with agitators

In 1960 Eirich invented a fourth gene-ration of mixers, the counter inten-sive mixer with additional agitators,which brings an ideal fine disintegra-tion (Figure 12 and Figure 13).

The agitator can run at a peripheralspeed of 25 metres per second. Its ef-fect was described as follows [24]:“the insertion of additional high-ener-gy agitators increases the intensity ofmovement of the particles, forcingthe complete homogenisation of com-ponents that are hard to associate andthe mixing of dies or binding agentswithout any streaking, all with the

goal of improving quality and achiev-ing large savings (e.g. cement, lime,coal dust, Bentonite, oxides and thelike).”

The combination of - A horizontally rotating mixing pan - One or more eccentrically posi-

tioned slow turning mixing tools- One or more eccentrically posi-

tioned fast running agitatorsmakes it possible to achieve an excep-tionally intensive mixing process; thatis why these mixers can be found inmany different industries.

This fourth generation Eirich mixerproduced concretes of a quality ofthat until then had been unknown.Between 1960 and 1975 “model 1924”was replaced by “model 1960”, includ-ing in the concrete industry. Thereare many testimonies from this period.And mixers made at that time are stillin use today, as are the previous mo-dels. Eirich still supplies spare compo-nents and wearing parts, even formixers that have already been runningfor over 60 years. Again and again cus-tomers confirm that even these old Ei-rich mixers make outstanding con-crete make, better than new con-ventional mixers.

Intensive mixers with inclinedmixing containers

The fifth generation of the Eirich mix-er started in 1972. The mixers have arotary mixing plate that stands at anangle, a fixed floor and walls scraperand a fast turning agitator. In mixers

of up to three cubicmetres there is onlyone moving tool; inlarger mixers two orthree agitators arerunning.

The mixing principle is unique in theworld. In the mixing vessel the mix istransported upwards by friction withthe wall. From there it falls down un-der the force of gravity. Supported bythe wall scraper, the mix is directedtowards the fast turning agitator (Fi-gure 15 and 16). Within one turn ofthe container, that is to say within afew seconds, 100% of the batch hasbeen circulated. Here the agitator runswith a peripheral speed of betweentwo and 40 metres per second.

In the meantime seventh generationmixers are now being built, bringinga hitherto unknown quality of mixingto many different industries. Depend-ing upon the mixing task, the mixercan work against the current or in thesame direction. With premium con-cretes, the mixing vessels and agita-tors usually run in the same direction,because this means that the maxi-mum shear stress can be applied tothe mixture.

The characteristic difference between“simple mixers” and Eirich mixers isthat in the Eirich mixer the transportof the mix is decoupled from the ac-tual mixing process. The separationbetween transport of the mix and mix-ing process makes it possible to varythe speed of mixer tools (and thus theenergy introduced into the mixture)within a wide range (Table 1). The in-troduction of mixing energy into themixture can then be systematicallycontrolled.

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Figure 17: Relationship of powerinput and consistency

Tab. 1 – Typical tool speeds (peripheral speed)

Annular trough mixer, planetary mixer up to 4 m/sSingle shaft mixer, double shaft mixer up to 6 m/sEirich mixer, D-Type, horizontal up to 25 m/sEirich mixer, R-Type, inclined 2 to 40 m/s

Tab. 2 Volume Loose aggregates Hardened Drive power Specific power[kg] [l] concrete [m3] [kW] [kW/100 kg]

Annular trough mixer 2,400 1,500 1.0 37 1.54Annular trough mixer with agitators 2,400 1,500 1.0 37 + 22 2.46Planetary mixer 2,400 1,500 1.0 44 1.83Eirich mixer 2,400 1,500 1.0 90 * 3.75

* up to 110 kW possible


In every other mixing system the mix-ing tools have the primary task ofmoving the mix. A significant speedincrease is not possible.

Mixing –what actually is it?

Mixing means moving position. Onemust differentiate between two fun-damental processes: 1. Distributive mixing – a simple

change in the position of the par-ticles; no high shear speeds are re-quired and

2. Dispersive mixing - macerating ag-glomerates; requires high shearspeeds.

There are many parameters, whichhave an effect on the mixing process,for example particle size, particle sizedistributions, particle shape, surfaceroughness, density differences, adhe-sive forces or flowability.

The mixing process can thus be divi-ded into two mechanisms: - rough mixing (exchange of larger

particles between material flows)and

- fine mixing (change in position ofneighbouring particles).

To mix concrete, work must be done.As high a relative velocity as possiblemust be forced upon the componentsby introducing kinetic energy. The in-dividual particles must constantlychange their position relative to eachother.

Why is a high mixing speed some-times important?

In order to be able to cause those pro-cesses that result in a change of posi-tion in the mixer, energy must be in-troduced to the system. In a givenmixer, a power input / current take-uparises, which depends solely on theconsistency of the mix. Experimental-ly this is calculated from the effectiveload on the motor for a particularmixing process and weight of mix-ture, the so-called specific power con-sumption in kilowatts per 100 kilo-grams.

The maximum specific power can becalculated from the drive power ofthe mixers. An example (taken from abrochure) is shown in table 2.

A dry mixture takes up less energy, aflowable mixture a lot. In our exam-ple with simple mixers and when re-lated to a certain concrete consistency,the power input is 1.5 to 2.5 kilowattsper 100 kilogram (Figure 17).

The mixing work required to manu-facture “good” concrete depends onits composition. Everyone knows thata mortar or fine grain concrete is farmore difficult to mix than a concretewith 0/16 or 0/32 granulation; splin-tered grains macerate pigments fasterthan round grains etc.

In a conventional mixer and there isno possibility to increase the input of

energy. However, with an Eirich mix-er, the energy input can be regulatedover a wide range by the mixing toolspeed (Figure 18). In the above exam-ple the mixer is set up for an energyinput of 3.75 kilowatts per 100 kilo-grams.

Why does a higher tool speed and re-sult in a higher energy input? Theinput of energy into a mixer is verycomplex, and up until now has notbeen modelled mathematically. How-ever, to a first approximation, theEirich mixer can be compared to astirrer. Here, according to the theoryof an ideal stirrer, the power input isproportional to the cube of the peri-pheral speed.

Only an Eirich mixer can mix stably atspeeds of 2 to 40 metres per second.It can be adapted to give the best re-sults with any particular application.If a material is mixed first with two,and then with eight metres per se-cond, according to the theory thismeans the energy input is 20 timeshigher. This leads to the best mix qua-lity, in particular with fine grain con-cretes.

What effect does a higher mixingspeed have on the mixing time?

A higher speed means a greater energyinput, which in turn means shortermixing times. An example: a mixingtask with a work requirement of 2 ki-lowatt hours per tonne. A conven-

Figure 19:Clay / feldspar mixture, sample testspecimen after 2, 4and 6 minutesmixing time,manufactured onmixers with an energy input of 3, 6and 9 kW/100 kg

Figure 18: Increase of the powerinput via higher tool speed


tional mixer with a specific powerinput of 10 kW per tonne requires amixing time of 12 minutes, whereasan Eirich mixer with a specific powerinput of 50 kW per tonne requires amixing time of 2.4 minutes.

With the type R Eirich mixer, theenergy input can be considerably in-creased via the mixing tool speed, ifthe mixing task requires it. Investiga-tions are currently being carried outat a university, which show that withtool speeds of around two metres persecond on an inclined Eirich mixer,the mixing performance of a plane-tary mixer can be simulated. Such lowspeeds are mostly only used in the Ei-rich mixer in order to incorporatelightweight aggregates into a mortarsuspension that has been prepared athigher speeds. This is to produce aer-ated concretes without any destruc-tion of the grains, or to mix in coarseaggregates.

Why can’t conventional mixingsystems mix faster?

Each mixing process is overlaid by aseparation process. Separating whilemixing has been described in nume-rous publications. For example, in1996, Koch and others reported ontrials carried out with a horizontalsingle shaft mixer [29]. Heavy par-ticles tend to accumulate towards theoutside, whilst lighter particles accu-mulate near the shaft. This is ex-plained in that “a higher energy istransmitted to the heavier particles bythe movement of the blades than forlighter particles. They are thereforethrown out further, right into theouter zone of the mixing drum”.

This applies in principle to every mix-ing system. The difference with theinclined Eirich mixer is that withinone revolution of the mixing contai-ner, the mix is circulated one hun-dred percent and back mixing takesplace. This is why it can work at hightool speeds without separation.

How does one determine the mi-xing work, which is to be inputduring a certain mixing task?

The only way is by carrying out trials,until the desired mix quality is reached.With concrete, at present nobodyknows any values for the mixing workrequired in order to mix a particular

concrete adequately. The authors areaware of several universities that areworking on the determination of themixing work for “modern concrete”.

The meaning of mixing quality can bevisualised by an example from theceramic industry. If one mixes orkneads a light and dark clay together,then the homogeneity is easily recog-nisable to the eye. This is shown bythe example of a clay / feldspar mix-ture. During mixing, samples weretaken from the mixer and formed intosample test specimens. These werecut in half (Figure 19). For the visible

size of the clay particles that have notyet been dispersed a score of 1 (ho-mogeneous mixture) to 5 (clay par-ticles of up to 15 millimetres visible)is given.

The results of the experiment areshown in table 3.

Raising the energy input from 3 to 9kW/100 kg resulted in- the mixing time falling from 15

minutes to 4 minutes. - the mixing work (and thus the

energy consumption) falling from7.5 to 6 kilowatt-hours per tonne.

Figure 20: Paving stone with washed out pigment, laid 1997

Figure 21: Paving stone with pigment smears, laid 1997


This is linked to the fact that not all ofthe energy that is input becomes ef-fective as mixing energy, but is con-verted into useless friction and wear.The use of powerful mixers saves notonly time but also energy - and thusmoney.

The mixing quality

For every mixing task there is an opti-mal mixing quality. In order to achieveit, the necessary amount of energymust be put into the system. Accord-ing to theory, the mixing quality is ameasurement of the variations of a

material property in the mixture inquestion. It describes the degree ofinhomogeneity in the mixture duringthe mixing process. A comparable pa-rameter in a production process is thequality consistency. Mixing qualityand quality constancy are thus qualitycriteria, which do not themselves des-cribed properties, but rather their in-homogeneity. Unfortunately at pre-sent there is no suitable method tomeasure the mixing quality duringthe mixing of concrete. Only laterdoes it becomes apparent whether ornot it has been mixed for longenough.

With precast concrete elements andconcrete products a bad mixing quali-ty is thus identifiable on the finishedproduct, such as paving stones withsand flecks or accumulations of pig-ment.

In both cases as can be provedthat no Eirich mixer was used.

In other industries one often can seenothing on the outside of the finishedproduct; the problems only comewhen in use, when for example a pig-ment line is visible in the finished tilejoint.

Independent investigation

Independent institutes also confirmthat Eirich mixing technology achie-ves a quality of mixing better than ofother mixing systems. The Institut fürFertigteiltechnik und Fertigbau Wei-mar e.V., (IFF) [Institute for Prefabri-cated Element Technology and Prefa-bricated Construction] in Weimar,Germany carried out an investigationinto planetary mixers and Eirich mix-ers using samples from a manufactu-rer of concrete products. In the re-port “Untersuchungen zur Mischgütevon Vorsatzbetonmischern” [Investi-gations into the Mixing Quality ofMixers for Face Concrete]’ on 11 Fe-bruary 2003, one can read “The Ei-rich mixer ... is characterised by veryshort mixing times... economic advan-tages...”. Moreover, the high uniformi-ty of the concrete prepared with theEirich mixer is apparent. With freshconcrete, the gross density coeffi-cient of variation of the Eirich mixeramounted to 0.19 per cent, whereaswith the planetary mixer it was 0.46per cent. Something similar shows upwith the water content, solid contentand resistance to frost and de-icingsalt. The Eirich mixer was rated as hav-ing “a better uniformity of concretequality” - reached “in a substantiallyshorter mixing time than in theX-mixer” [30].

Determining the quality of the pig-ment dispersion (Bayer AG measuredthe colour intensity at 2 points on eachof 3 paving-stones, and comparedthem with a reference block to whichthe value of 100 was allocated), thepaving-stone made of face concretefrom an Eirich mixer resulted in afluctuation of around 8 units (136-144); blocks originating in two simple

Figure 22: Mixing process after RILEM

Mixing time Energy input Energy input[minutes] 3 kW/100 kg 9 kW/100 kg

[degree of homogeneity] [degree of homogeneity]

0.5 5 31 4 22 4 23 3 2–1

.............4............. 3 .............1.............6 38 2

10 212 2–1

.............15............. .............1.............

Table 3


mixers showed fluctuations of 33 and34 units (100- 132, 126-160).

In other industries where very goodmixtures are needed, Eirich mixing iswell spoken of. For example in thepreparation of glass batches: “The fastEirich mixers provide a higher levelof batch homogeneity”. That is whyPhilips only operates Eirich mixersworldwide. [31].

On 12 May 2004, a lecture was givenconcerning extensive investigationsinto the comparative mixing qualityprovided by different intensive mix-ers used in the preparation of glassbatches [32]. Not surprisingly, the an-nular trough mixer had the worst per-formance, and the inclined Eirichmixer became out best. The mixingquality has a substantial effect on theoperation of the melting operation.

A view of the advantages and benefits

The individual suppliers of doubleshaft mixers, annular trough mixersand planetary mixers offer very simi-lar products. Their job is to convincecustomers that their particular mixeris better than that from another ma-nufacturer who works on the same sys-tem.

A good mixer is something that should

last for decades. So anyone who needsa new mixer should carefully analysethe actual performance hiding behindthe advertising claims, which may in-clude:

- We are one the largest manufac-turers,

- We invented the planetary mixeraround 1950,

- We save cement, - We give 5 years warranty, - We have a more innovative mixing

system than the others - We mix at differential speeds

Arguments like “my mixer requiresless power” should be treated scepti-cally. Work is the product of powerand time. The laws of physics apply tothe mixing of concrete too.

One should always scrutinise the dataregarding mixing time. Following RI-LEM [33] some manufacturers under-stand it to mean the short mixingtime of the finished product, after ad-dition of all the components (Figure22, thick line). The actual mixing timeis substantially longer.

Following the expiry of the relevantEirich patents, the manufacturers ofsome annular trough and planetarymixers have integrated agitating toolsinto their mixers too. However the ef-fects are very modest, since firstly thefilling height of the mix is low and se-

condly the mix is not directed to-wards the agitator. The additionalenergy input is thus limited, as de-monstrated by the example of an an-nular trough mixer fitted with agita-tors in table 2.

In the meantime, some mixer manu-facturers claim to “increase the num-ber of revolutions during the mixingprocess and so to raise the intensityof mixing” [34]. Anyone who needs amixer with a higher number of revo-lutions per minute, should enquire ofthe suppliers what the power input isand have a look at some operatingplants as references.

The term “counter current intensivemixer” has also been taken from Ei-rich. But a new name does not make aplanetary mixer into an Eirich mixer.As already discussed, today the incli-ned Eirich mixer usually runs withthe current in the same direction forconcrete applications.

Continuous mixing

In the precast concrete element, con-crete pipe and concrete products in-dustries, the mixing is usually carriedout in batches. So we have not coveredcontinuous mixers, which are sold astrough mixers, pan mixers and Eirichmixers.

Multi-stage mixing processes

For some applications, it is appropriateto mix at different speeds. For examplewhen manufacturing lightweight con-crete or glass fibre reinforced concrete,a flowable mortar is produced withhigh tool speeds; during the flowablephase the energy input is at a maxi-mum, and an ideal dispersion of thecomponents and binding agent isachieved. Here the tool speeds can of-ten be higher than 10 metres per se-cond, so that a 30 second mixing timeis sufficient. At the end of the rapidmix phase, the remainder of the wa-ter required by the recipe is added.Then the light aggregates or glass fi-bres are mixed in, at low speeds with-out destroying them. The manufactureof porous concrete is similar.

The authors are aware that work isbeing carried out in universities onmultistage “hybrid” mixing processesfor self-compacting concrete. The mix-ing time for self-compacting concrete

Figure 23: Bridge section made of Ultra High Performance Concrete, producedby Max Bögl, Neumarkt, Germany


(SCC) in Eirich mixers (starting fromwhen one begins to add water) isaround 70 seconds at present.

Mixing in a vacuum mixer

With Ultra High Performance Concrete(UHPC) it can be an advantage to in-troduce air into the mixing processand then remove it again. The thick,honey-like consistency of the concreteprevents it from venting when it isvibrated.

Firstly the inclined Eirich mixer is idealfor the preparation of such fine grainconcrete; secondly it can always beused under a vacuum. At the end ofthe mixing process, the pressure islowered in the mixing container for30 seconds by up to 50 hPa. A co-ope-rative research project between theTechnical University of Munich, Ger-many and its partners Degussa, Eirich,Ph. Holzmann, Schwenk, Woermannand Hochtief has developed a high-

performance fine grain concreteusing this mixing technology [35].

At the 3rd Kassel Building Materialand Solid Construction Conferenceon 10 September 2003, Max Bögl ofNeumarkt, Germany showed a proto-type bridge section as part of thetheme “ Ultra High Performance Con-crete - design and construction of thefirst bridge with UHPC in Europe”[36] (Figure 23). The concrete wasprepared in an Eirich vacuum mixer(Figure 24).

Some universities that are working onconcretes of the future, use the Eirichvacuum mixer, which is available insize is from three to 7000 litres.

Hot mixing, cold mixing

A multiplicity of mixer manufacturershave overcome the problems of intro-ducing hot air, hot water or hot steamin order to produce a warm concrete.

Today cold mixing can be achievednot only by adding ice water but alsoby cooling with liquid nitrogen. Thisnew method of cooling, which has al-ready been tried out in ready mixconcrete, has also been used with anEirisch mixer for the production of aspecial fire resistant concrete where,irrespective of the weather, the tem-perature of the mixer is kept at 16°C.This technique can be used for nor-mal concretes too.

Wear

It is well known that with annulartrough mixers the mixing result alsodepends on the degree of wear of themixing blades. This may be the basisof the general statement, that in thecase of forced action mixers “ the mix-ing quality is substantially affected bythe position of the blades” [11].

How much wear is acceptable in or-der to achieve a perfect mixed de-pends on the assessment and experi-ence of the operator.

A variety of materials are availablethat offer wear protection for the floor,walls and mixing tools, includingceramics and carbides. No empiricalvalues are available from the variousmanufacturers, or at least are not pub-lished. The general rule is that in themain, wear in a mixer takes place bet-ween fixed and turning machine ele-ments. This means:

Mixers with fixed mixing vessels: Boththe mixer tools and the mixing vessel,from which the tools take the material,wear heavily.

Eirich mixers with turning mixingvessels: mainly the mixer tools thatare subject to wear (with the inclinedmixer, it is the agitator blade)

One may find this statement surpris-ing, since the Eirich mixer has subs-tantially higher tool speeds. But never-theless wear is lower. Anyone who vi-sited mixer manufacturers at Bauma2004 in Munich will have seen mixersthat in many cases were lined withceramics. But not at Eirich. Ceramicsare used with these mixers, but onlyin very special cases, such as whenthey are working continuously withthroughputs of 750 tonnes per hourfor nine months without interruption,or in cases such as iron ore mixturesFigure 24: Eirich vacuum mixer, type RV 23 Vac

where up to 5 million tonnes ofthroughput can be achieved withoutany repairs being required as a resultof wear (Figure 25).

The fact that Eirich mixers work withless wear than other mixers was re-ported back in 1928. One user, whooperated several mixing systems, re-ferred at the time to the fact that thewalls of the mixer showed hardly anywear. He attributed this to the “ fric-tion work in the mixed batch occur-ring mainly inside the batch itself”[37].

Concrete as a subject for research and development

As already mentioned in the introduc-tion, universities and colleges acrossthe world are working on producingconcrete with even higher perform-ance - concrete of the future, whichto some extent will replace steel. Forconcrete technologists - and thosewho want to have such a title - thereare mixers available today that havemore than just an on-off switch and asingle speed.

In addition to the laboratory mixerswith three to five litres (R 02) and 8to 10 litres (RV 02) Eirich specially re-commends the pilot plant mixerswith 40 litres (R 05) or 75 litres (R08) capacity. For these sizes (and like-wise for R 09, 150 litre, which is run-ning in several institutes) an intelli-gent control has been designed andthat is unique in the world: Thespeeds of the mixing container driveand the agitator drive can be adjustedwithin a wide range, and the parame-ters are recorded.

During the mixing process, the powerinput of the mixing container driveand agitator drive is measured, loggedand also displayed graphically.

The total energy input can be prese-lected; after this quantity of energyhas been introduced, the mixing pro-cess is automatically stopped and themixer switched off.

Using this equipment one cannot onlymeasure the mixing work, but also in-vestigate how changed experimentalconditions (e.g. the tool speed, toolgeometry, counter current mixing, di-rect current mixing) has affected theconcrete.

Prospects

Different mixer types are availablethe manufacturers of precast concreteelements and concrete products. As arule conventional mixing systems areadequate for standard concrete. Selec-tion should take account of whetheror not the mixer supplier can givegood references.

However, in the case of fine grain andhigh-performance concretes, conven-tional mixers are often overloaded. Atthe end of the day, the concrete pro-ducer must decide whether or not hecan afford to supply the market withproducts that are possibly inadequate(see the paving stones in Figures 20and 21). Alternatively he can segre-gate his scrap. But scrap costs money.Investing in a better mixing technolo-gy can pay for itself within a shorttime.

In the Editorial of the BFT 11/2003,Holger Karutz quoted the English so-cial reformer John Ruskin, (1819 -1900) [38]: “There is hardly anythingin the world that some man can’t makea little worse and sell a little cheaper,and the people who consider priceonly are this man’s lawful prey. It’s un-wise to pay too much, but it’s unwiseto pay too little. When you pay toomuch you lose a little money that isall. When you pay too little, you some-times lose everything, because thething you bought was incapable ofdoing the thing you bought it to do.

The common law of bu-siness balance prohibitspaying a little and get-ting a lot. It can’t bedone. If you deal withthe lowest bidder, it’swell to add somethingfor the risk you run.And if you do that, youwill have enough to payfor something better”.

Today, one hundredyears later, this still ap-plies. Today, Ruskin’ssaying is called “goodmanagement” or “a ma-xim of business”.

“Whoever works withhis grandfather’s pick-axe will only achievewhat his grandfather

achieved - and only earn what hisgrandfather earned” wrote the pub-lisher of CPI, Gerhard Kloeckner inhis editorial in BWI No. 5, October2002. “And the answer to the crisiscan only be this: be better than thecompetition, …. produce at lowercost. That means make less mistakes,and have less production scrap” [39]

For a particular application, to have aconcrete that is sufficiently well mix-ed is the first step towards being bet-ter than the others. �


Further information:

Figure 25: Continuously working mixer for iron ore,6000 litres of useable volume

Peter Nold, Ralf LöbeMaschinenfabrik Gustav Eirich Walldürner Straße 5074736 Hardheim, GermanyE-Mail: [email protected],

[email protected]: www.eirich.com


L i t e r a t u r e :

[1] Hillemeier, B.: Neue Baustoffe – Innovationen aus der Mikrowelt, Akademie-Journal 1/2001, S. 24-28

[2] DIN EN 206-1 – Produktionskontrolle, Beton Kalender 2002, Ergänzungsband, Verlag Ernst & Sohn, Weinheim, S. 233 f

[3] Bereiten und Verarbeiten von Beton, Zement-Merkblatt Betontechnik, B7 8.2002, Bauberatung Zement,Bundesverband der Deutschen Zementindustrie, August 2002

[4] Ferraris, Ch. F., J. Res. Natl. Inst. Stand. Technol. 106, 391–399 (2001)

[5] Ries, H.: Die Intensiv-Aufbereitung von Beton – Grundlagen und Ergebnisse aus der Praxis, Betonstein-Zeitung Heft 4, 1967,Sonderdruck Maschinenfabrik Gustav Eirich, 1969

[6] Wischers, G: Einfluss langen Mischens oder Lagerns auf die Betoneigenschaften, Beton 13 (1963), Heft 1, S. 23-30 und Heft 2. S. 86 –90

[7] Umek, A: Die Intensivaufbereitung des Betons, Untersuchungen Universität Ljubljana, Sonderdruck Maschinenfabrik Gustav Eirich, 1973

[8] Beton + Fertigteil-Jahrbuch, 52. Ausgabe, Bauverlag, Gütersloh 2003, S. 342

[9] Zement Taschenbuch, Verein Deutscher Zementwerke, Verlag Bau + Technik, 50. Ausgabe 2002, S. 179


[11] www.baumarkt.de/lexikon


[13] Bild entnommen der BETONSTEIN-ZEITUNG, Heft 10/1964, S. 529

[14] Zement Taschenbuch 2000, Verein Deutscher Zementwerke, Verlag Bau + Technik, 49. Ausgabe, S. 187

[15] Patentschrift Nr. 71321 Kaiserliches Patentamt, Deutschland

[16] II. Beton-, Zement- u. Kalkindustrieausstellung Berlin, 2. Juni bis 18. Juli 1910, Firmenschrift K. Bayr. Hüttenamt Sonthofen

[17] BHS, Prospekt Doppelwellenmischer, 07/2003

[18] Bayer. Berg-, Hütten- und Salzwerke AG, Zweigniederlassung Hüttenwerk Sonthofen, Prospekt Baumaschinen, 1930

[19] Maschinenfabrik Rhein und Lahn, Gauhe, Gockel & Cie., Oberlahnstein, Katalog 1908

[20] Maschinenfabriken Karl Peschke, Zweibrücken, Katalog 1910

[21] Mitteilung von Paul Eirich, Maschinenfabrik Gustav Eirich, Hardheim

[22] Prospekt ‘Universal-Mischmaschinen Patent Eirich’, Klöckner-Werke AG, Abteilung Georgs-Marienhütte, Osnabrück, um 1920

[23] Kaiserliches Patentamt, Patentschrift Nr. 267965, Deutschland

[24] Aus ‘Typenübersicht der Eirich Gegenstrom-Mischer’, Hardheim, um 1970

[25] DIE BETONSTRASSE, Nummer 4, 1.4.1928, 3. Jahrgang

[26] Seiler, E.:, Die Maschinen des neuzeitlichen Straßenbaues, TEER UND BITUMEN, 1. März 1931, Heft 7, 29. Jahrgang

[27] De Weerdt, H.E. Fehlerfreie Betonmischungen, Westfälische Bauzeitung Köln, Nr. 32 vom 8.9.1928

[28] Garbots, G.: Leistungsversuche an Beton-Mischmaschinen und O. Graf, Die wichtigsten Ergebnisse der Versuche mit Beton-Mischmaschinen, VDI Zeitschrift, Zeitschrift des Vereins Deutscher Ingenieure Heft 23, Sonderdruck Maschinenfabrik Gustav Eirich

[29] Koch, T., Hauser, G., Zettl, T und Sommer, K.: Mischzeitenbestimmung an einem horizontalen Einwellenmischer, Schüttgut, 2. Jahrgang,Nummer 1, Januar/März 1996; Sonderdruck der Fa. Zettl, München

[30] Untersuchungen des IFF Weimar: Gleichmäßigerer Beton mit Eirich-Mischtechnik, Beton Fertigteiltechnik (BFT), 4/2003, S. 93

[31] Philips values performance, reliabilty of intensive batch mixers, Case history, PBE International, September/October 2003, S. 22-24

[32] Rikken, F., Industrial Service & Support, Philips Lighting Components, Powder Technology & Project Management, Eindhoven,The Netherlands, auf Symposium Mischtechnik, Hardheim / Deutschland, 12. Mai 2004

[33] Charonnat, Y. und Beitzel, H.: RILEM TC 150 ECM, Mater. Struct. (Suppl. 196) 30, 28-32 (1997)

[34] Doppelwellen-Chargenmischer im Mittelpunkt des Messeauftritts, BHS-Sonthofen GmbH,BetonWerk International (BWI) Nr. 1 Februar 2004, S. 47

[35] Schachinger, I. A.: Hochleistungs-Feinkornbeton, TU München, Schriftenreihe Baustoffe Heft 3/2003, Jahresmitteilungen 2002/2003, S. 55

[36] Schmidt, M. und Fehling, E.: Tagungsbeiträge zu den 3. Kasseler Baustoff- und Massivbautagen, Ultra-Hochfester Beton -Planung und Bau der ersten Brücke mit UHPC in Europa, Schriftenreihe Baustoffe und Massivbau 2, kassel university press, 2003

[37] Schmidt, Der Gegenstrom-Schnellmischer, Beton und Eisen, XXVII Jahrgang 1928, Heft 4, Sonderdruck Maschinenfabrik Gustav Eirich

[38] Karutz, H.: Herbstgedanken, Editorial zu Beton Fertigteiltechnik (BFT), 11/2003, S. 1

[39] Klöckner, G.: Beton – Baustoff des 3. Jahrtausends, Editorial zu BWI BetonWerk International Nr. 5 Oktober 2002, S. 3

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