EP0199884A2

EP0199884A2 - Method of producing concrete in a mobile concrete station

Info

Publication number: EP0199884A2
Application number: EP85850157A
Authority: EP
Inventors: Torsten Nikolaus Ljung
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-11-07
Filing date: 1985-05-03
Publication date: 1986-11-05
Also published as: SE443741B; SE8306098D0; EP0199884A3; SE8306098L; WO1986006677A1

Abstract

The invention refers to a method of producing concrete in a mobile computer controlled concrete station (22) for the production of concrete mixtures, comprising cement, aggregate material of gravel and sand, and water, with or without superplasticizers. In the concrete station, that is realized for computer control of the mixing processes, the aggregate material is supplied to a telescopic sorter device (2) for screening and distributing the aggregate material according to size to a plurality of collecting cells (3). A system of trap doors (4) and valves (5) in co-operation with electronic equipment for the weighing of the aggregate material in exact proportion by means of a weighing drum (6), is computer controlled. The content of the weighing drum (6) is transferred to a mixer drum (7) by pneumatic means. A preprogrammed amount of water, ahd, if desired, amounts of superplasticizers are supplied from tanks (12) to the mixer drum through pipes. The mixer drum is provided with means for electric measurement of the ZETA-potential difference or of the conductivity in the cement paste. Using these measuring quantities and nominal values for water and for the used kind of superplasticizers, the consistency of the mix can be controlled through computer controlled supply of water and superplasticizers under due observance of said nominal values.

Description

The present invention has for its object a method of producing concrete in a mobile, automatically controlled concrete station for the production of concrete mixtures, comprising cement, ballast material and water, with or without superplasti- cizing admixtures.
The production of concrete comprising admixtures of superplasticizers requires a very close supervision of the amount of aggregate of gravel and sand, water and cement. The development of such superplasticizers has mainly taken place during the last 20 years in Japan, the Federal Republic of Germany, the USA, Great Britain and Canada. Such superplasticizers are, for instance:

1. sulfonated melamine-formaldehyde condensates
2. sulfonated naphtalene-formaldehyde condensates
3. modified ligninsulfonates.

Observations made in different countries about the use of superplasticizers clearly show that they should be added directly to the mix just before placing the concrete at the building site, in order that their full effect should be obtained. Several controlled and verified experiments in several countries show that important gains can be made with respect to an increased strength, reduced consumption of cement and easier processing. An addition of superplasticizers in the amount of 1 to 2 % of the cement weight has shown, that, in spite of a reduction of the cement weight of around 27 %, an increased strength of 15 or more can be obtained. The reduction in volume obtained due to the reduced addition of cement, can easily be compensated by the addition of fly ashes or silicon powder.
The present invention refers to a method of producing concrete in a mobile concrete mixer at the building site in order to give the concrete a desired strength. The invention also refers to the use of automatic control technique in the form of programmable computer technique to perform a fully controlled production of programmed contcrete mixtures.
The characterizing features of the invention are, according to claim 1, that the aggregate of gravel and sand is added by means of a telescopic sorter device comprising a plurality of collecting cells, and that a programmable amount of aggregate material is delivered from preselected cells through automatically controlled trap doors to a weighing drum, and from that drum to a mixer drum, to which a preselected amount of water is added, and that the consistency of the concrete paste in the mixer drum is determined by directly measuring in said drum the electrostatic voltage drop and the resistance between the ends of the concrete paste, and that the consistency is controlled by adding an additional amount of water, and possibly of superplasticizers by presetting in the computer a nominal value of the amount of water and one or more nominal values of the amount of superplasticizers for said voltage drop value and said resistance value, and by first adding water until the nomimal value of water is reached and after that adding superplasticizers until the nominal values of superplasticizers are reached, said latter values being negative with respect to the nominal value of water, and thereafter discharging (emptying) the mixed concrete for use at the building site.
The control according to the invention comprises the use of fully determinable aggregate material curves, and by continuously screening sand and gravel and distributing the fractions of said aggregate material to a required number of separate collecting cells. The discharge of the aggregate material through the controllable trap doors is made by summing up the weights of the separate fractions according to a definite aggregate material curve for the concrete mixture. This weighing procedure is combined with a continuous indication of the moistness of the discharged aggregate material from the collecting cells and an automatic compensation of the preselected dry weights by adding to the moisture weights, the weight of water supplied with the aggregate. If an aggregate material of a certain fraction is missing in one or more of the collecting cells, the weighing procedure is stopped until said cells are filled again from the sorter device. The consistency of the mixture is continuously controlled through the addition of water and superplasticizers. This can be done by meas- uning the change in power of a motor driving a mixer in the mixer drum, on the one hand for a dry mixture and on the other hand for a moist mixture, and by measuring in the mixer drum the conductivity, the electrostatic voltage drop and the resistance of the mixture. When the desired consistence is reached, the supply of water or superplasticizers is interrupted by activating magnetically operated valves for said supply by computer control.
The invention will be further explained in the following in connection with Figs. 1 to 3, attached hereto.

Fig. 1 shows in the upper part a perspective view of a m0bil£#1 concrete station according to the invention with a collapsible transport device for aggregate material, and in the lower part of the Figure three different stages of folding said collapsible device.
Fig. 2 shows schematically the concrete station in a longitudinal section.
Fig. 3 shows a curve of the ZETA-potential difference in the cement paste being mixed in the mixer drum as a function of the weight of the added superplasticizers in per cent of the weight of added cement.

The upper part of the concrete station according to Fig. 1 consists of a trailer 22, transported to and trestled close to a building site, where a transport device 23 supplies aggregate material to an intake in the roof of the trailer. The lower part of Fig. 1 shows the transport device, being part of the concrete station, in differently folded stages, the fully unfolded stage being shown at the left. In the middle, the device is under folding, and at the right it is collapsed and is in a transportable state 25, resting on transport wheels. It is also shown that the transport device comprises a screw tube conveyor 24 for cement.
Fig. 2 shows the concrete station in a longitudinal section. At the top there is an intake 1 for aggregate material. This is received by a telescopic oscillating sorter device 2, screening the aggregate material and delivering it according to size to a plurality of collecting cells 3. Such cells, in a requisite number, are arranged behind each other transversely to the plane of the Figure, and contain aggregate of different particle size. Each cell has a trap door 4. Said doors are so situated that the aggregate material can be supplied from all trap doors to an intake opening of a weighing drum 6. The concrete station is equip- ^ped with a computer, controlling the different functions determining the composition of the concrete mixture. The computer controls the supply of aggregate material of different particle size to the collecting cells 3. Overfilling of any cell is indicated by means of a level indicator 16 at the upper edge of the cell, and causes a trap door to be opened by a valve 5 to a discharging conveyor belt 15, which is started and discharges the surplus material. If one of the cells is emptied during the delivery of aggregate material before the required amount has been taken out, the delivery from all other cells is stopped and the filling of aggregate materiel in them is continued. When the cell is again filled completely or to a predetermined level, the delivery (tapping) of aggregate material to the weighing drum 6 is started again.
With due observance of preprogrammed weights of aggregate material, the computer controls the opening and closing of valves, preferebly such of a pneumatic type, said valves actuating the trap doors 4 for delivering preprogrammed amounts of aggregate material from the collecting cells 3 to the weighing drum 6. The moistness of the aggregate material is being continuously detected by means of moisture detectors 13, connected to the computer. It will thus be possible to determine the amount of water in the aggregate material and to let the computer correct the amount of such material supplied to the weighing drum 6. Said drum is equipped with means for turning or rotating it, for instance by pneumatic means, so that the drum, as response to a signal from the computer, is brought to an emptying position, where its content is discharged in an intake opening of a mixer drum 7, having continuously driven mixer blades.
Simultaneously with the supply of aggregate material to the weighing drum 6, cement is being transported by a screw tube conveyor 10 to a cement weighing vessel 11 above the mixer drum 7. The conveyor is- stopped when the. prescribed amount of cement has been transported to the cement weighing vessel 11. Said vessel is equipped with a shutable conduit leading to the intake opening in the mixer drum 7. When the aggregate material has been supplied to the weighing drum 6 from a preprogrammable number of collecting cells 3 and the content of the weighing drum, under control of the computer, has been delivered to the mixer drum and has been mixed by the mixer blades, cement is supplied to said drum after a predetermined number of revolutions of said blades. After an additional number of revolutions, signals are supplied from the computer to water tanks and to superplasticizer tanks (commonly designated 12, as they, according to the illustrated form of the invention, are thought to lie behind each other).
If concrete without the admixture of superplasticizers shall be produced, a preprogrammable amount of water is supplied in a large dose to the mixer drum 7 through a conduit. This supply can be choked to a fine dose or can be stopped completely. Before that the preset amount of water has been reduced by the amount of water in the aggregate material that has been determined by the moisture detectors. As a control quantity for changing the dosage of water, the change of power of the motor driving the mixer blades can be used, the change quantity being supplied to the computer. The addition of water influences the consistency of the mixture. A control quantity for controlling the mixing procedure can also consist of the electrical conductance of the concrete paste, said conductance being measured in a convenient way between the ends of the mixture in the mixer drum. It is also possible to measure the ZETA-potential difference between the said ends, or to perform a voltage measurement, i.e. a measurement of the ZETA-potential difference, in the drum.
The addition of superplasticizers is performed in a similar way by means of a large dose, later on changed into a fine dose. The effect of said addition will be explained in the following in connection with Fig. 3.
When the preprogrammed mixing time has lapsed, the computer actuates the emptying of the mixer drum 7, for instance by means of a pneumatic turning device, positioning the intake opening of the drum above a receiving hopper 8, from which concrete can be emptied by manual means 9.
During the mixing cycle a new weighing phase may have been started, upon which the mixing procedure is automatically repaa- ted. The process can be supervised at a control panel 20, situated outside the mixing zone. The station can also be equipped with a device 21 for evacuating dust and whirling around cement particles to a collecting cell.
Fig. 3 illustrates the effect of the addition of water and superplasticizers. The curve in the diagram shows the ZETA-potential change in the cement paste in the mixer drum (in millivolts along the y-axis) as a function of the weights of added superplasticizers in per cent of the weight of cement (as a decimal number along the x-axis). The measurement of the ZETA-potential difference eliminates the influence of impurities in the added super- ^plasitcizers and a dilution thereof. When only mixing aggregate material and cement, the ZETA-potential difference zero is obtained. By adding only water to aggregate material and cement, a ZETA--potential difference having positive sign is obtained (downwards from zero). The lowest point (about 9.8 mV) is possible to reach when the most suitable proportions of the components are chosen. (If another type of cement or other quantities of the components, especially of water, is used for the mixture, it is possible to reach another value, below 9.8 mV.)
Through the addition of superplasticizers, the highest point of the curve may be reached (from about 9.8 mV to about -38.0 mV). Two nominal values 26 and 27 are indicated on the curve. The nominal value 26 is reached by the programmed control and by the addition of a large dose of superplasticizers. Switching to a fine dose of superplasticizers is suitable at 85 to 90 % of the programmed (maximal) nominal value 27 of the voltage. The supply of superplasticizers is stopped when the last-mentioned nominal value is reached. The nominal value 27 is depending on the type of superplasticizers and gives a concrete mixing program resulting in the desired consistency of every mixture. The curve shows that all kinds of superplasticizers give a change in sign from a positive to a negative voltage value when supplied to cement (or even to gypsum). This change in sign has a limited duration and reverts after about 45 or 60 minutes, depending on the type of superplas-- ticizers, to positive values when the hardening of the concrete starts.

Claims

1. Method of producing concrete in a mobile, automatically controlled concrete station for the production of concrete mixtures, comprising cement, aggregate material of gravel and sand, and water, with or without superplasticizers, characterized in that the aggregate material is added by means of a telescopic sorter device (2) comprising a plurality of collecting cells (3) and that a programmable amount of aggregate material is delivered from preselected cells through automatically controlled trap doors (4) to a weighing drum (6), and from that drum to a mixer drum (7), to which a preselected amount of water is added, and that the consistency of the concrete paste in the mixer drum is determined by directly measuring in said drum the electrostatic voltage drop and the resistance between the ends of the concrete paste, and that the consistency is controlled by adding an additional amount of water, and possibly of superplasticizers by presetting in the computer a nominal value of the amount of water and one or more nominal values (26, 27) of the amount of superplasicizers for said voltage drop value and said resistance value, and by first adding water, until the nominal value of water is reached and after that adding superplasticizers until the nominal values (26, 27) of superplasticizers are reached, said latter values being negative with respect to the nominal value of water, and thereafter discharging the mixed concrete for use at a building site.

2. A method according to claim 1, characterized in that the amount of water added to the mixture of cement and aggregate material is reduced by the amount of moisture water determined by means of moistness detectors (13) in the aggregate material.

3. A method according to claim 1, characterized in that the amount of aggregate material supplied is corrected with respect to the determined weight of moisture water in the aggregate material.