IT8349338A1 - PROCEDURE AND PLANT FOR PRODUCING CEMENT CONCRETE. - Google Patents

Description

SIB 84549

83.109 IT

EC-/11 Description of the industrial invention entitled: "PROCEDURE AND PLANT FOR PRODUCING CEMENT CONCRETE"

of the Danish company SKAKO A/S

based in - FAABORG - (DENMARK)

SUMMARY

A process for producing cement concrete by thoroughly mixing aggregates, cement, optional fillers, and water in a concrete mixer. The fine aggregate fraction is added to the mixer only after the remaining components have been mixed. This results in the coarse aggregate fraction being coated with a mortar consisting of cement, optional filler, and water, into which the fine fraction is absorbed. The addition of the fine fraction is carried out subsequently, controlled by the fine fraction metering device, preferably using controlled vibration. The fine fraction is advantageously fed into a zone of the mixer where the mass, consisting of the coarse aggregate fraction coated with mortar and free-flowing cement mortar, is moved in a flow of material under the influence of pressure external to the mixing device. In the plant for carrying out the process and comprising a mixing container with dosing means for the concrete components, mixing and unloading means, the dosing means for the fine fraction of the aggregates is equipped with devices for controlling and regulating the flow of the fine fraction, and is preferably a vibrating feeder.

The result is a homogeneous concrete with excellent strength properties, with significant savings in cement.

DESCRIPTION

The present invention relates to a process for the production of cement concrete by intimately mixing aggregates, cement, optional filler and water in a concrete mixer, wherein the fine fraction of the aggregate material is added only after the remaining components have been mixed, with the result that the coarse fraction of the aggregate material is coated with a mortar consisting of cement, optional filler and water, into which the fine fraction is absorbed.

A process of this type is described in Swedish Patent No. 7700052-9, Publication No. 415349, by the same inventor as the present invention and assigned to the present applicant. This patent represents a fundamental breakthrough in traditional concrete mixing techniques, according to which the entire aggregate quantity is typically fed into the concrete mixer, including both the coarse fraction (typically stone materials with particle sizes of 32–4 mm) and the fine fraction (sand with particle sizes up to 4 mm), together with the cement, and mixed for a certain period of time before water is added. It is also possible that aggregate, water, and cement are added in the aforementioned sequence, or, in another way, water and cement are mixed to form a mortar, which is then added to the aggregates.

The process according to the said Swedish patent, specifically aimed at producing a cement concrete mass with a high filler content, low cement content, and a high content of coarse aggregates (stone), was based on the hypothesis that if the coarse aggregate fraction was added first to the mixer, followed by cement, filler, and water in the form of a mortar, or one after the other, part of the cement mortar containing filler thus produced would coat the particles contained in the coarse aggregate fraction, while the remainder would form a free-flowing mortar. When the fine aggregate fraction was then added by continuous mixing, the particles of the fine fraction would be absorbed and encapsulated in the mortar, coating the particles of the coarse aggregate fraction. The fine particles would increase the viscosity of the mortar, which is high as such due to the filler content, but the mortar would remain liquid. It was assumed that the compressive strength and workability would be similar. of the concrete were increased, with an increasing amount of fine fraction incorporated into the cement mortar during mixing.

By means of the process described in the said patent, it should also be possible to reduce the water/cement ratio in relation to the conventional dry mixing process, with unchanged quantities of aggregates and cement, when a significant quantity of filler is used.

It is a well-known fact that the water/cement ratio, which indicates the proportion by weight of water to cement in a given concrete mix,

It is of vital importance for the strength qualities of the concrete, and therefore a low water/cement ratio would also be desirable.

The drawings in the said patent, which show the results of laboratory-scale tests with concrete mixes, each weighing 13 kg, appear to confirm that the addition of filler and the modified mixing sequence provided the claimed improvements in the concrete's strength. However, this was largely dependent on the addition of filler.

The mixing process described in the patent has, however, never been applied in practice. Based on promising laboratory results, industrial-scale trials have been initiated in a Swedish concrete plant using existing equipment.

of well-proven mixing and concrete containers, however with an alteration of the sequence of addition in accordance with the teachings of the

The patent was based on the idea that the fine fraction of the aggregate was added after the remaining components had been mixed. However, it proved impossible to homogeneously absorb the entire quantity of sand into the cement mortar.

Local drainage of the mortar was observed

of cement and, even in the case of long mixing times, a heterogeneous concrete was produced, which did not show any improvement in quality?

of resistance. It was therefore impossible to demonstrate to experts that the suggested change in the mixing sequence, the introduction of which

in itself it required great efforts to overcome deeply rooted prejudices, had it any future.

The present invention is based on the surprising recognition that, to obtain the results desired in the said patent, it is necessary to overcome not only

one but two prejudices, and furthermore the addition of filler is not mandatory to obtain good resistance qualities.

As demonstrated by the unsuccessful tests mentioned, and as will be clear from the examples given below, it is in fact not appropriate to change only the sequence in which the individual components of the cement concrete mass are added.

For homogeneous absorption of fine fraction particles during the mixing process

within the freely flowing cement mortar

and in the part of the mortar that covers the coarse particles, it is vital that the particles are not added, as usual, at one time within the shortest possible period of time at the beginning of the mixing process, but rather they are added successively in a controlled flow of materials which can be provided by the application of a metering means for the fine fraction. The process according to the present invention is therefore characterized in that the addition of the fine fraction is carried out successively, controlled by the metering means for the fine fraction.

As will be seen from the following test results, it is possible in this way to produce particularly homogeneous concrete with a significant reduction in mixing time, with a resulting reduction in energy consumption and wear on the mixer, also due to the coating effect achieved by the cement mortar when the fine fraction is added.

The concrete produced according to the method of the invention exhibits, in addition to improved homogeneity, a reduced tendency to separation, and the concrete shows no sign of accumulation of loose washed stones, the so-called "stone nests", since all the stones are clearly surrounded by cement mortar. The free concrete

only a small amount of water before and after vibration, thus avoiding seepage onto the concrete surface and thus reducing its strength. Furthermore, the resistance measurements

in compression show a reduced deviation from the standard strength, compared to concrete produced according to the normal process. In reason

of the improved homogeneity of the concrete and the demonstrated "reduced deviation of strength from the standard", it will also be possible by the process according to the invention to reduce the cement content in a given concrete mix, while maintaining the strength values.

The fact that the disadvantages experienced by the reproduction of the Swedish patent on an industrial scale can be eliminated by controlling

the material flow of the fine fraction, may seem obvious in retrospect. However, it is important to realize that such control constitutes

a fundamental breakthrough in concrete mixing techniques for several decades, based on the belief that the mixer does the work, for

for which reason the feed components should be added as quickly as possible to initiate the mixing process.

Without intending to accuse concrete technologists of being more conservative than people expert in other fields, it appears that inventions in the concrete field have concentrated mainly in recent years on improving the effectiveness of the mechanical part of the process.

of mixing, or in varying the granulometry of the fine fraction, or in adding special mixtures to prepare improved products.

The fact that it would be possible to perfect the process, rejected by experts, according to Swedish patent 7,700,052-9 simply by modifying the dosing medium for the fine fraction, thus achieving control over the material flow, must be considered to far exceed the expectations of those skilled in the art.

The importance of this simple modification is better understood by a more precise observation of the actual mixing process. Through the prevalent dry mixing process, the water content in the cement mortar is increased during mixing from 0, by starting the water supply, to the final value corresponding to the required water/cement ratio. During mixing, the concrete thus changes from a dry to a moist state.

In contrast, from the very beginning of the addition of the fines, the cement mortar has a predetermined water content, and all the cement particles are pre-ground and possibly activated by mixing with the coarse fraction. With the subsequent addition of the fines, water is bound by the formation of water menisci between the fine grains. This increases the viscosity of the mortar and the volume of fines in the mix. Therefore, during the mixing process, the concrete passes from a moist to a less moist state.

The qualities of the concrete mass, such as homogeneity and workability, increase as the volume of fines increases, which contributes to the aforementioned improved qualities of the finished concrete.

A particularly suitable addition of the fine fraction is achieved by the controlled vibration technique, more precisely by applying a so-called shock distributor.

These shock-absorbing conveyors have a wide range of applications within the most diverse material handling lines, from the transport of granulated sugar in sugar factories to the loading of pellets in blast furnaces.

The application of a shock distributor in the process according to the invention allows for particularly effective control of the flow of fine fraction material, which can be adjusted as required.

An examination of different types of concrete mixers has shown that the optimal mixing process and the best concrete mass are achieved if the fraction is added to a zone of the mixer where the mass, including the mortar-coated coarse fraction and the free-flowing cement mortar, is moved in a material flow under the influence of a pressure external to the mixing means, such that the material flow is pressed together and the particles are thus pressed and rolled against each other. By subsequently adding the fine fraction particles to the material flow, they are drawn into the flow and, due to the pressure between the coarse fraction particles and the individual particle movements generated by the mixing means, the fine fraction particles are "ground" into the mortar coating the coarse particles.

Considering this, it will be especially advantageous to design the mixing equipment and arrange the curved path described in such a way as to establish zones of high and low pressure in the concrete mass during the mixing process. This can be achieved, for example, by using a stationary mixer in which the mixing blades are moved in pairs in a planetary motion.

The subsequent addition of the fine fraction is advantageously regulated in relation to the rotation of the mixing means, so that the added fine fraction does not encounter previously added fine fraction material that has not yet been absorbed into the mortar, which would then form a double layer of fine fraction that would be difficult to work homogeneously into the mass.

Although the application of fillers, contrary to Swedish patent 415349, is not mandatory for the process according to the invention, it may be desirable to add a filler, among other things, to be able to regulate the viscosity of the cement mortar.

Such regulation could be carried out by means of measuring devices for determining the viscosity of the mortar, connected to the dosing device for the filler.

It should be emphasized that the term "filler" is used here in the broadest sense of the word, and not only to mean fine fraction material with particle sizes smaller than 1/4 mm, but also to include mineral mixtures, such as silica, fly ash, pozzolanas, etc., as well as more specialized mixtures, colorants, and inactive fillers, such as finely ground quartz, etc. Fillers are normally added together with the cement and or used for subsequent adjustments.

In the process according to the present invention, any mixture for the production of concrete can be used, i.e.:

1) Physically active mixtures such as plasticizers, both normal and superplasticizers, and air inclusion mixtures, such as surfactants, which reduce the surface tension of water by the formation of bubbles, and

2) Chemically active mixtures as accelerators or retarders for the reaction between water and cement.

The addition of these mixtures will normally be done with the last part of the water, before the addition of the fine fraction, which will enable the cement to react with water on its surface.

It has also been found that, to produce concrete of optimum quality, the coarse aggregate fraction can be advantageously moistened with part of the water before adding the cement, optional filler, and the remaining water. It is also important to establish an appropriate pre-draining time before subsequently adding the fine fraction. This improves the quality of the cement mortar and therefore the workability of the mix.

The invention further relates to a plant for the production of cement concrete, and of a nature which comprises a mixing container with dosing means for the components of the concrete mass, mixing means and discharge means, and the plant is characterised in that the dosing means for the fine fraction of the aggregates is equipped with devices for controlling and regulating the flow of the fine fraction.

Preferably the dosing medium for the fine fraction is a shaker dispenser, which provides very precise adjustment of the dosage and which can be adjusted to provide different dosage periods in accordance with the composition and qualities of the concrete mass in question.

If desired, the dosing device can be designed to add the fine fraction to more than one zone of the mixer. For example, this can be achieved by applying two or more dosing devices or by using a rotating dosing device.

In the implementation of the process according to the invention, in which the filler is used to regulate the viscosity of the cement mortar, the system may further include means for determining the viscosity of the cement mortar, connected to the dosing means for the filler and for controlling the flow of filler.

In order to achieve particularly effective mixing of the fine fraction, it is of great importance that the flow of material in the mixer is under the influence of a pressure

>*

external to the mixing media, thus establishing zones of high and low pressure in the concrete mass, and that the fine fraction is added in a

high pressure zone. In addition to the fact that pressure can be increased or reduced in the pressure zone by increased or reduced flow

of fine material, the mixing means of the plant are primarily designed to establish alternating zones of high and low pressure which can be further sustained by adapting the mixing means to move in a curved path, for example according to a planetary motion.

The invention can be applied to the production of any type of concrete for various purposes and with different strength qualities. A person skilled in the art will be able to determine the optimal mixing process for a given type of concrete by varying the mass components, the particle size of the aggregates, the types of

of cement, the different mixes, the mixing time etc.

Laboratory-scale comparison tests To demonstrate the importance of adding the fine fraction of the aggregates (0-4 ml sand) later, tests were conducted in a laboratory mill with four blades and two lateral scrapers, in which the mill and blades rotated in opposite directions. The basic mix used had the following composition:

Rapid cement 325 kg/m<3>

Water 150 g/m<3 >W/C ratio 0.46 Aggregates:

Sand 0-4 mm 650 kg/m<3>

Stone 4-16 mm 1260 kg/m<3>

The following tests were carried out:

I Reproduction of Swedish patent 415349

The stones and half the water were mixed for 5 seconds to moisten the stone material. Then, cement and the remaining water, containing a plasticizer, were added, and the mix was premixed for 20 seconds. Then, the sand was added all at once, and mixing continued for 65 seconds. Total mixing time was 90 seconds.

II Process according to the invention (long pre-mixing time)

Stone, water containing a plasticizer, and cement were mixed as in I. Sand was then added over a period of 20 seconds, and mixing was continued for 45 seconds. Total mixing time.

90 seconds.

At the end of the mixing, the following determinations were made:

I II

Air content 1.2% 1.3% Settling time 0 0 Settling time (Vebe) 4 s. 2.5 s Mixture I was homogeneous and had a workability, expressed by a Vebe measurement, of 4 seconds.

Mixture II was also homogeneous and showed improved workability, expressed by a Vebe measurement of 2.5 seconds.

Cylinders were cast from both types of concrete and used for determinations of initial compressive strength according to DS 423, 1 and 2 days after the completed mix, respectively.

The results of the compressive strength tests are shown in the following table. For comparison, standard values are used for concrete produced with the same type of rapid-setting cement using the traditional mixing technique:

Compressive strength M Pa Period II Standard values 21 hours 11.6 13(24 hours)

22 hours 10.6

44 hours 30.1 23(48 hours)

45 hours 29.3

From the previous table, it can be seen that Test II shows better initial strength values, at least if the time differences are ignored. It is assumed that the differences would be even more significant if a mixer that provided high and low pressure zones had been used.

Industrial scale comparison tests

The trials were carried out at a very experienced Danish concrete producer with its existing 1500-litre SKAKO mixer for the production of ready-mixed concrete.

The mixer is a millstone that works on the principle of countercurrent mixing, whereby mixing is carried out in a fixed millstone by means of four blades moved in pairs by a planetary gear box.

The mixer is also equipped with two built-in lateral scrapers to direct the concrete to the most active mixing zone. The speed of the lateral scrapers is approximately half the speed of the blades. This specific countercurrent mixing principle moves the concrete mass approximately one complete revolution every 10–12 seconds, resulting in continuously developing zones of high and low pressure within the concrete mass.

The following concrete recipes were chosen for testing, according to the standard mixing procedure and the new mixing procedure.

1) Concrete 15 MN/M^

Cement (Portland) 230 kg Water 165 litres Sand (0 - 4 mm) 717 kg Gravel (4 - 8 mm) 200 kg Gravel (8 - 16 mm) 818 kg Aerating mixture 53 g

2) Concrete 30 MN/M.

Cement (Portland) 330 kg Water 165 litres Sand (0 - 4 mm) 668 kg Gravel (4 - 8 mm) 200 kg Gravel (8 - 16 mm) 789 kg Aerating mixture 60 g

3) Concrete for pipes

Cement (Portland) 346 kg Water 131 litres Sand (0 - 4 mm) 760 kg Gravel (4 - 8 mm) 285 kg Gravel (8 - 16 mm) 855 kg

The dosage cycle described below was applied.

A. Standard mixing procedure

The 4-8 ?d 8-16 mm stone fractions and sand were added at once.

After the 5-second dry mixing period, water was added for 20 seconds, and after 15 seconds of mixing time, the cement was added. The total addition took approximately 30 seconds, and then the batch was mixed for approximately 30 seconds. The total mixing time was thus 60 seconds.

B. New mixing procedure according to the invention The stone fractions 4-8 and 8-16 min were added at once together with the water. After wetting for 5 seconds, the cement dosage was started and lasted approximately

15 seconds, after which the batch was mixed for another 10 seconds (mixture mixing time). Then sand was added successively over a period of 15 seconds using a SKAKO shaker. To achieve the same total mixing time, the batch was mixed ready for another 15 seconds, but the concrete was already homogeneous after 5 seconds of ready mixing.

In addition to visual observations, three cylindrical cores, each measuring 1 dm^, were taken from each mixture.

The cylindrical cores were stored in a water bath for 14 days at a constant temperature of 21?C. The results obtained are listed in the Tables below.

All three test procedures indicate that a significant increase in mean strength was achieved by the procedure according to the invention, and that the standard deviation of strength and the coefficient of variation were reduced.

This clearly demonstrates that the process according to the invention allows for considerable savings in cement consumption compared to the standard mixing procedure, while maintaining the same strength values.

CONCRETE MIXTURE TEST S K A K 07

Resistance:15MN/M<2 >Asset. 6-10 A/C 0.8

STANDARD MIXING PROCESS NEW MIXING PROCESS

?r-

A B

Average Resistance Assett. Average Resistance Assett. Average Resistance NW of 3 samples dia convert. of 3 samples converted to for dough at assett.8 for dough assett.8

-14 17.3 8 17.3 19.4 10.5 20.3

r <15 >20.3 6 19.3 20.3 6 21.8 , 21 15.1 9 15.5 21.3 10 22.2

24 15.9 7 16.3 20.8 8 20.8 27 15.6 4.5 14.0 16.4 13.5 18.0 28 16.1 5.5 15.3 19.4 5 18.3 30 19.5 5 18.0 18.1 10 19.0 32 18.1 7 17.6 22.2 7 21.6

(MN/M<2>] Average resistance 17.1 Average resistance 20.3

AVERAGE RESISTANCE INCREASE FROM "A" TO "B": 18.6%

STANDARD DEVIATION s= \yTx i - ??? ? _ (x,_zCtl'

SA= 1, 74 MN/M s?= 1.64 MN/M<1 >

COEFFICIENT OF VARIATION ; S ? 1QQ

<v>- m

V*= 10 , 2 % V. 8, 1 % CALCULATED CEMENT SAVINGS : <5 - K ( v?i - 05

SAVINGS - 26 f<G 10.4%

DATE: 4-6-83 NAME: F. FRANDSEN

CONCRETE MIX TEST 2 S K A ?o<A>/<

Resistance 30MN/M<2 >Setting 6-10 A/C 0.5

STANDARD MIXING PROCESS NEW MIXING PROCESS

!..

A B

Average Strength Assett. Average Strength Assett. Average Strength Assett. Average Strength Nv<? >of 3 samples dia converted. of 3 samples converted to for mix to assett.8 for mix assett.8 16 <' >30.9 5 29.3 33.2 5.5 33.*2 11 -18 ; 32.3 7 31.5 36f4 6.5 36.0 4 - -. 19 31.4 8t5 3lr4 34.0 10 35.4

- 20 <1 >29.4 8 29.4 30.3 10.5 31.7

:22 27,4 9 ,

1 28.3 33.5 6 31.8

8<?>I ,4 6 36,<2>. 38<5 ? >32.6 13 36.5

1 29 <; >38.0 4 34.1 37.1 5 34.3 I _ Il_ __ 25^9_ <' >5_ <' >341.2 36.0 6 34.5 i (MN/M<2 >i Average resistance 17.1 Average resistance 20.3

AVERAGE RESISTANCE INCREASE FROM "A" to "B": 7.2%

<?>JOL

STANDARD DEVIATION; s

2.94 MN/M 1.85 MN/M*

COEFFICIENT OF VARIATION*. S * 100 io/i

- <V >ITI <1 >*

9.2% V? 5.4% CALCULATED CEMENT SAVINGS: 6 = K ( V7T - 0.5 )

SAVINGS and 20 KQ 5 , 5 %

DATE: 4-6-83 NAME: F. FRANDSEN

CONCRETE MIX TEST 3 s K A K 0 V Concrete Strength for A/C Pipes 0.38

STANDARD MIXING PROCESS NEW MIXING PROCESS

A B

_L-Resist.average Asseti. Resist. me Media Resist. Assett. Resist.medfa N?,<? >of 3 samples dia convert. of 3 samples converted to for mix ' to asseti.8 for mix assett.8 IV 42.1 0 <! >- 3773 <~>U<~~>r ?

V 43.4 0 46.8 0 -VI 44.2 0 - 0 -VII 44.6 0 - 48.9 0 -IX 46.3 0 - 47.7 0 -X 45.9 0 - 48.6 0 -XI 45.1 0 - 5<CO>0.6 0 | -XII 41.1 0 - 46.4 0 I -(MN/M Resis a ! 44 ^<2>) Average resistance Average resistance 48.1

AVERAGE RESISTANCE INCREASE FROM "A" TO "B": <9>? <% >STANDARD DEVIATION s= \/l?i-mlj+ (x -mi*- ?

SA? 1.8 MN/M* ' Se- 1?3 min<7>

COEFFICIENT OF VARIATION?;

v.S-^IQQ-(%|

VA? 4.1% V? 2.7%

CALCULATED CEMENT SAVINGS: <B * K( vir-o.s)

SAVINGS = 26 KG 6.8

DATE: &-A-S3 NAME: F.FRANOSEN

Claims (13)

1. A process for producing cement concrete by thoroughly mixing aggregates, cement, optional filler, and water in a concrete mixer, in which case the fine fraction of the aggregates is fed into the mixer only after the remaining components have been mixed, with the result that the coarse fraction of the aggregates is coated with a mortar consisting of cement, optional filler, and water, into which the fine fraction is absorbed, which process is characterized in that the addition of the fine fraction is carried out subsequently, controlled by the metering device for the fine fraction. 2. Process according to claim 1, characterised in that the addition of the fine fraction to the coarse fraction coated with mortar is carried out using a controlled shaking technique. 3. Process according to claim 1 or 2, characterised in that the fine fraction is added to a zone of the mixer in which the mass, consisting of the mortar-coated coarse fraction and the freely flowing cement mixture, is moved in a material flow under the influence of a pressure external to the mixing means. 4. Process according to claim 3, characterised in that the external pressure on the material flow is exerted in such a way that the added fine fraction particles are immediately absorbed into the material flow. 5. Process according to any of claims 1 to 4, characterised in that the addition of the fine fraction is regulated in relation to the rotation of the mixing means, so as to prevent the fine fraction from encountering previously added fine fractions which have not yet been absorbed into the mortar. 6. Process according to any of claims 1 to 5, characterised in that the viscosity of the mortar-coated coarse fraction is adjusted by the addition of filler. 7. Method according to claim 6, characterised in that the regulation is carried out by means of regulation means connected to the dosing means for the filler. 8. Process according to any of the preceding claims, characterised in that the coarse fraction of the aggregates is moistened with a part of the water, before the cement, optional filler and the remaining water are added, and this mixture is premixed before the addition of the fine fraction. 9. A plant for carrying out the process claimed in claim 1, which plant comprises a mixing vessel with means for dosing the components of the concrete mass, mixing means and discharge means, which plant is characterised in that the dosing means for the fine fraction of the aggregates is equipped with devices for controlling and regulating the flow of the fine fraction. 10. Plant according to claim 9, characterised in that the dosing means for the fine fraction is a shock distributor. 11. Plant according to claim 9 or 10, characterised in that the dosing means is designed so as to add the fine fraction in more than one zone of the mixer. 12. A plant according to claim 9, characterised in that the mixing means are designed and adapted to move along a curved path so as to establish zones of high and low pressure in the concrete mass. 13. A system according to claim 9, for carrying out the process claimed in claim 7, which system is characterised in that the dosing means for the filler is connected to means for determining the filler content and the viscosity of the cement mix. p.p. SKAKO A/S Commander Giorgio Omodeo Safl (Society lllelene Brevetti S.p?A4