Dipl.Ing. Markus Greim
Schleibinger Geraete Teubert u. Greim GmbH
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D-84428 Buchbach
Tel. +49 8086 / 94010
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island.htm
25 May 1997
This paper was puplished in:
Annual Transactions Of The Nordic Rheology Society, Volume 5, 1997, Conference 1997, Reykjavik, August 6-9, 1997. Editor: A. Friis
ISBN 87-7834-219-8
The materials we discuss here are most water based supsensions, which are mixed some minuts up to hours before the will worked in a fluid state. They are pumped ore smoothed ore injected, so the rheological properties are evident. This materials will harden and then solid after some time. In the solid state most of them can only absorb pressure forces, so the are often combined with steel ore fibers to get parts which can forced in all directions.
The building material mostly known here is concrete. In Germany nearly 500 kg of concrete are produced every year per person, about 40 mio. tons at all. Figure 1 [1] shows the structure of this material. The solid phase is divided in cement, sand and coarse aggregate. Sand has a diameter up to 2..4 mm the coarse aggregate up to 32 mm and more. Cement and water (=cement lime) , filled with sand (the mortar) are mostly reacting chemical. The mortar is combined with the coarse aggregate more in a mechanical way. Finally the fluid fresh concrete is pumped in forms prefilled with steel structures.
For ordinal structural concrete the compaction effect is achieved by unrestrained flow within the concrete, so taht all parts thereof ca merge together. On the other hand, if the mortar consistency is to plastic, it will cause segregation of the concrete. Up to now its not clear in which range the viscosity and the yeald stress should varriy for an "optimal" fresh concrete.
The easiest way to control the flow properties of concrete is to varie the amount of water in the concrete. This is partly usual in the praxis but this may n o t be done. Because the strength of the hardended concrete depends mainly on the water - cement ratio. Everey litre of water added to the concrete will reduce the strength dramaticly.
The granulometric composition of the cement the sand, and the coarse aggregate is also a way to control the rheometric paramters of the concrete, but milling of the cement is expensive, sand and coarse aggregate must be used as availablke in the area of the working place because the transport of these over long distances is also to expensive.
The flow properties of the fresh concrete are strong time dependent, mostly influeced by the cement-water (cement-paste) phase. After activating the system durious the mixing early electricall bindings will lead to a first stiffning of the concrete (Spanka [2]). This bindings can be broken by mixing or pumping again. Then chemical bindings will grow until the the concrete is hardened. During this time part of the water is bound from the cement-water reaction, part of the water is sucked from sand and coarse aggregate. As very simply mentioned building materials must have defined rheological properties, which are influeced by a lot of factors wich itself are influecing each other. All the factors are also time dependent, and in practice strongly restricted by the costs!
Various concepts of measuring consistence are defined in several norms varieing from country to country. All of these systems have a big disadvantage : They all mesure the deformation of mortar specimen but the force ore velocity you apply is constant. So on the assumption that building materials follws the Bingaham curve (Banfill), its not possible to get an relaitive or absolute yeald stress and viscosity.
Three main problems avoid the use of cenvenient for example cylindric rheomters for the use with coarser suspensions, and building materials especially.
Gravity and and centrifugal forces seperates fluid and solid phases during the measurement
Seperation from fluid and solid phase during the mesurement, leads to gliding effects at the specimen container wall
Using a cylindric rheometer you must realise some basic geometric conditions depending on the greatest corn diamter in your suspension. For mortar you will need a specimen volume of 7 litres (about 14 kg) (Banfill [3]). For fresh concrete with coarse aggregate up to 16 mm you need 25 litres (Wallevik [4]) for fresh concrete up to 32 mm 50 litres (ca. 100 kg) (Nukem [5]). Rheomters for such volumes have a weight of hundreds og kg, and the adaequate dimensions and costs ! Nevertheless the problems mentioned above still remain !
As shown in figure 1 cement-lime is filled with sand to mordar, which is filled with coarse aggregate to fresh concrete. For quality test purposes yoe ned only measure the end product fresh concrete. For material design you should test and control the rheological parameters of lime, mordar and concrete. Wit coarser aggregates the measurement precision decreases and the specimen volume increases.
For measuring lime (cement + water) conventionell cylindric rheometers may be used, but segregation must be avoided. Another system is the Viskomat PC (Schleibinger). In the middle of a rotating bechure a special formed probe is mounted. The momentum on this probe is measurued. The probe calle lime-paddle is designed to avoid segregation.
Measuring mortar with a cylindric or plate system in nearly immpossible avoiding segregation sedimentation and de-mixing during the measurement. As with lime measurement you may use here the Viskomat PC. Another like an impeller formed paddle avoids segregation and de - mixing. Additionally a scrapper gliding over the wall of the measuring pott avoids sedimentation on the beachre wall. The volume of the beachure is 375 ccm. This system. This mesuring geometry nearly 20 years ago by Prof. Teubert (Teubert [6]), and well introduced and used in middle Europe.
The problem of measuring the rheologiic oarameters of fresh concrete is not generally solved yet. As examples 4 systems are shown which all works with an rotating impeller. This list is not complete.
Tatersaal [7] developed a machine were a ship screw like impeller is rotating at several speeds in an 25ltr. pot filled with fresh concrete. The force needed for driving the impeller is measured. One problem with this ssystem is that the impeller dispaces the concrete and runs in air or mordar.
Designed by O.Wallevik, this system is based on a cylinder- cylinder geometry. To avoid gliding effects, the inner cylinder is formed like a cylindric basket made of steel ribs, also on the outer cylinder ribs are mounted. The system needs about 25ltr fresh concrete per measurement. The weight of the system is about 300 kg.
A french (C.Hu [8]) developement. Two ribbed plates are working as stator and rotor. The whole bot is mounted on a vibrating console.
BT2 is the shortcut for Beton-Tester 2 developed by Schleibinger GmbH, Germany (Greim [9]). Figure 3 shows the working principle.
A central shaft is set vertically in a container full of fresh concrete (12ltrs). Three paddles each with a probe for measuring the torque against the direction of rotation , are mounted on the measuring arm of the BT2. The instrument contains a tachometer for measuring speed of rotation, a temperature sensor an a mathematical processor with a graphical display. The data from the paddles are converted into a flow ciurve using the shear rate calculated from the angular velocity annd the distance from the central shaft. Because the three different paddles are set at different distances from the axis, the torque is measured simultaneosly at three different shear rates. Fresh concrete behaves like a Bingham body, with a linear flow curve whose equation can be calculated using regression analysis , which is carried out autmaticly by the processor. In the system mentioned above, in order to determine flow curves for concrete it has been necessary to measure the flow resistance of blades at various velocities. To do this many revolutions of the blade are necessary and this causes problems such bleed and structural breakdown while the mesurements are beeing taken.
The great advantage of the BT2 lies in the simultanous measurement at three velocities, so rotation in a static measuring containre is sufficient. Measurements are never taken in the same place more than once in sample of fresh concrete. Finallly, because the absolute velocity of the measuring arm is not needed in the flow curve, the BT2 can be manually driven, thus making the unit self-contained.
At three actual examples the state of reserch and application of this technices are described:
Spanka et.al. (x) worked on "Operative mechanism of plasticizing concrete admixtures". In this study they tested the influence on the relative yeald value with and withoutv admixture, and the influence of fly ash. Measuring instrument was the Viskomat PC, they measured 150 min at 4 speeds. The correlation shown in figure 3 and 4 could be translated to the properties of the fresh concrete.
In cooperation between Schleibinger Geraete and the FH München , A.Untergehrer [10] developed a regime for testing the workability of plaster. At the dry mortar and plaster industry workability and so rhelogocal parameters are least so important as at the concrete industry. In the practical the dry premixed mortar is mixed in the plaster machine with water, pumped to throug a tube and then spread to the wall. After a short time the plaster is one ore two times smoothed. First a simple rheological model was developed to guess the maximal and minimal shear rates which are applied on the material. More than concrete mordar he plaster mordar changes his properties over the time. Also ech mechanical stress changes the rheological parameters of the material. So mixing, stop times and measurement must be seen as one complex. A lot of test was made at laboratory to find an "optimal mix- and measuring regime. This regime was then validated at several building sites. After mixing 45 s. at 12 grd.C in a kitchen aid mixer, 2 minuts waiting, the speed profile shown in fig. x was run at the Viskomat PC. Figure y. shows a flow curve mesured at the building site and one measured at the laboratory. This was a first study to test the transfer from the laboratory measurement of plaster to the pratice and vice versa.
The BT2 fresh concrete rheometer comes to market in the next months. In the last 2 years a lot of studies was done, which validates the mesuring system and test the practicability of the system.
Figure x shows two flow curves measured with the BT2. Only the filling grad (volume of sand / volume cement paste) is varieed.The relative yeald value g and the realitve viscosity h increase with increased filling ratio. (Kath, [11])
In the cas of the two flow curves shown in figure 4 only the mortar content of the concretewas varied. The yeald value g and the plastic viscosity h decrease with increasing proprtion of mortar.
Fig. x shows three different types of concrete with approximately the same flow table spread (440mm, 442 mm, 450 mm). It shows that these types of concrete are clearly differntiated.
Two kinds of concrets are measured at a ready mixed plant. 100 measurement B25 and 55 measurement with B55. (Nöbauer, [12])
B25 Cement water w/c aggregate addmixture B25 CEM II 32,5R 180 kg/m3 0,62 1858 kg/m3 35%/cem 290 kg/m3 B55 CEM II/A 175 kg/m3 0.44 1755 kg/m3 1.0% 32,5 NW 400 kg/m3 Spread table B25 47cm..53cm B55 45cm..51cm
Figure x shows yeald value g and plastic viscosity h for 100 measurements of B25, the area of standard deviation is marked (68%). Figure y shows 55 measurements of B55.
Fresh concrete can be modelled with the phases: cement- paste, mordar, fresh concrete. The standard measurement methods of workability of building materials are not suitable for two point measurement. Most of the conventional rheometers are also not able to measure roar suspensions like mortar or fresh concrete. Research on rheological mesurement of building materials is shown with 3 examples.
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