WO1993017848A1

WO1993017848A1 - Apparatus and method for measuring moisture in the manufacture of concrete

Info

Publication number: WO1993017848A1
Application number: PCT/SE1993/000189
Authority: WO
Inventors: Ingemar Dahl
Original assignee: Ingemar Dahl
Priority date: 1992-03-13
Filing date: 1993-03-04
Publication date: 1993-09-16
Also published as: SE9200768D0; SE500831C2; EP0648156A1; SE9200768L

Abstract

A moisture measurement device in a concrete manufacturing system, where a concrete mixer mixes gravel, cement, additives and water, comprises a control means (8, 30) for selectively opening a measurement door (51) in a measurement opening (52) in a gravel bin (1). The moisture contents in a gravel sample (55), which is fed from the measurement opening to a measurement container (41), is calculated in the control means by resistance measurement in the gravel sample in the measurement container. The water supply to the concrete mixer is thereafter adjusted in accordance with the measured moisture contents.

Description

APPARATUS AND METHOD FOR MEASURING MOISTURE IN THE MANUFACTURE OF CONCRETE

The present invention relates to a device and method for measuring moisture in the manufacture of concrete, and parti¬ cularly it relates to an automatic system for measurement of moisture in the manufacture of concrete in a concrete factory.

Concrete is in general manufactured according to carefully defined recipies and proportions of gravel, sand, cement, additives and water. The moldability of the concrete is of central importance for its quality and is strongly influenced by the so called vtc-number, i.e. the ratio between the amount of water and the amount of cement. It is therefore very impor¬ tant that one be able to continuously and accurately measure and control the moisture contents in the manufacture of con¬ crete.

In the manufacture of concrete it is known to measure the moisture content in a gravel bin by measuring the resistance between two electrodes, placed on opposing sides in a gravel bin. ,A disadvantage with this system for measuring moisture is that coatings on or other damage to the electrodes gradually occur, such that the moisture measurement step by step becomes unreliable.

Another known technique for determining the moisture content in a gravel bin is to place a radioactive probe in the bin and measure the number of hydrogen ions. The method gives good accuracy but the disadvantage is high costs and an environ¬ mentally hazardous technique. The invention is defined in the attached claims.

An advantage with the present invention is that the moisture measurement in the manufacture of concrete can be carried out in an accurate and reliable way.

Another advantage with the invention is that the moisture measurement can be carried out automatically during the course of manufacture of concrete.

An additional advantage with the invention is that the moistu¬ re measurement becomes economically profitable compared with previously known systems.

Still another advantage with the invention is that the risk for the occurance of coatings on and other damage to measure¬ ment electrodes is minimized.

A further advantage with the invention is that the moisture measurement can be controlled by the same control system that controls the manufacture of the concrete.

A still further advantage with the invention is that the influence of particle size in the gravel is taken into ac¬ count.

The invention will now be described in detail with reference to the appended figures, wherein

Fig. 1 shows the principle of material flow in a concrete factory.

Fig. 2 shows measurement of moisture contents by known met¬ hods,

Fig. 3 shows a moisture measuring device in a concrete fac¬ tory, in accordance with the present invention, Fig. 4 shows in detail the moisture measurement device of Fig. 3, and

Fig. 5 is diagram showing the moisture content as a function of resistance.

According to Fig. 1, showing previously known techniques, ballast material in the form of gravel and stone are fed from the gravel bin 1 and stone bin 2 respectively, to a scale 6. In the same way cement from a container 3, water from a water source 4 and additive 5 are fed to respective scales 6, for dosing outgoing material 17 to a concrete mixer 10, and for further conveying with conveyer means 14.

Fig. 2, which essentially is a magnification of the encircled part 11 of Fig. 1, shows two previously known methods for measuring the contents in a gravel bin 1. The first method comprises moisture determination by measurement of resistance, and the other method-relates to moisture measurement utilizing a radioactive probe.

The plant for concrete manufacture usually comprises a process computer 18 for controlling the dosage of material from the different material sources 1-5 to the concrete mixer 10. Process computer 18 controls a bottom door or shutter 7 in the gravel bin 1 by means of a manipulator means 9, such that the gravel material 15 flows out of the bin to a scale 6 placed beneath. In the same way the process computer 18 can control the material flow from the other sources 2-5 to the respective scales 6 for correct dosage to the mixer 10.

In order to achieve the correct dosage of ballast material and water, it is important that the values of the moisture con¬ tents in the gravel bin 1 continuously or often enough are fed into the process computer 18. For this purpose two electrodes 12 and 16 have been mounted on the inner side of bin 1 for measuring the resistance in the gravel material 15 in the bin. The signal from the electrodes 12 and 16 are passed via signal conductors to a resistance measurement circuit or an instru¬ ment 30, the output of which is connected to the computer 18. From the measured resistance value the computer then calcula¬ tes the corresponding moisture content of the gravel in the bin 1. This enables the computer to optimally dose the source material from the various sources 1-5, including the water addition from the source 4 to the mixer 10.

A problem with this known system, as previously noted, is that the electrodes 12 and 16 tend to be exposed to coatings and other damage, such that the resistance value becomes erro¬ neous. Another problem is that electrodes 12 and 16 do not measure the resistance of the gravel near the outlet 7 of the bin, but rather higher up in the gravel, where the moisture conditions can be different compared to the actual gravel batch to be weighed on the scale 6.

This problem has therefore been solved with another embodi¬ ment, e.g. by measuring the moisture content by means of a radioactive probe 13 instead of resistance measurement between electrodes 12 and 16.

According to said second embodiment of the technique shown in Fig. 1, the radioactive probe 13 is placed in a gravel pocket in the bin 1, and is connected by means of a signal conductor to an instrument circuit 30 for measurement of hydrogen ions. The output of the circuit is connected to the input of the computer 18. According to experience this gives a reliable method for determination of moisture contents of the material 15 in the bin 1. However, the problem is that the method becomes relatively expensive and the use of a probe which irradiates radioactivity is from an environmental viewpoint disturbing.

Fig. 3 shows a system for the manufacture of concrete accor¬ ding to Fig. 1, with a modified moisture measurement system in accordance with the present invention. The same reference numerals as in Fig. 1 and 2 have been used for the same means.

A measurement opening 52 with a selectively openable shutter 53 has been arranged in the side of the gravel bin 1, close to but above the emptying door 7, for sampling material 55 from the bin. When the shutter 53 has been opened the material sample 55 is conveyed over a grating 54, which removes coarser pieces of stone, and to a measurement container 41. The measu¬ rement container is provided with two electrodes 45 and 46, which via signal conductors are connected to a measurement unit 30.

A control conductor is connected between the measurement unit 30 and a means 50 for manipulation of the measurement shutter, which means via a manipulating rod influences the shutter 53.

In operation a control unit 8, which preferably comprises the process computer 18 with an I/O-unit 20 and a display 27, controls the material dosage from the sources 1-5 to the mixer 10, by selectively opening and closing the emptying door 7 by means of the manipulator means 9, and the corresponding con¬ trol means for the additional sources 2-5. In order to monitor the measurement contents of the material 15 in the gravel bin 1, the measurement unit 30 which is connected to the control unit 8, periodically measures the moisture contents in samples that .has been taken out from the bin. For such a sampling the measurement unit 30 temporarily opens the shutter 53 by means of the manipulating means 50, whereby the material sample 55, after sieving on the grating 54, is brought to the measurement container 41. The measurement unit 30 measures a resistance value for the sample in the container 41, and calculates its moisture value which is fed into the computer 18. If the moisture contents is too low the control unit 8 increases the supply of water from the water source 4. If on the other hand the moisture contents is too high the water flow is reduced. In this way the moisture contents of the material to the mixer 10 is measured and controlled automatically during the course of the process.

The display screen 27 which is comprised in the control unit 8 enables an operator to supervise the operation in the concrete mixer 1. The operator has thereby a possibility to manually influence the process control. In certain applications it can also be advantageous to manually carry out some or most of the process controll functions for feeding the material, dosage and delivery. The control unit 8 then primarily functions as a computing unit which indicates to the operator the process state.

Fig. 4 shows in detail the moisture measurement system accor¬ ding to the invention. The measurement container 41, which preferably is of cylindrical shape, has a first electrode 46 for the measurement of resistance arranged on an opening door 73 in the bottom of the container. The door 73 is opened by turning about a pivot point 74 after influence from a manipu¬ lating means 43. A second electrode 45 is arranged on a carri¬ age 61, which is displaceable via a manipulator rod 65 by means of another manipulating means 64. The carriage 61 is provided with a gravel scraper 63 and a rolling surface com- pacter 62. The scraper 63 is attached to the carriage 61.

When the gravel sample for measurement of moisture is to be taken out of the gravel bin 1, the shutter 53 of the measure¬ ment opening 52 is opened by means of a signal from the measu¬ rement unit 30 to the manipulator means 50 for the shutter 53, and the gravel sample 55 flows through the grating 54 into the measurement container 41, such that a gravel hill 75 results on top of the container 41. The measurement unit 30 then activates the manipulator means 64 such that the carriage 61 is pushed towards the container 41. The gravel scraper 63 scrapes away the gravel hill 75 and a compactor 62 presses the gravel sample down into the container 41. When the carriage 61 reaches the other edge of the container, the compactor 62 rolls down a sloping plane 76 such that the electrode 45 can be pressed down against the gravel sample in the upper part of the container 41. Now resistance measurement of the sample can be carried out between the two electrodes 45 och 46, when the sample is located under constant pressure in the measurement container 41 having a predetermined volume. The actual resis¬ tance measurement is carried out by the measurement unit 30 to which the electrodes 45 and 46 are connected. The measurement unit 30 then converts the resistance value to a moisture percentage, which is fed into the computer 18 of the control unit 8.

When the resistance and moisture measurement is completed, the measurement unit 30 influences a manipulator means 43 for opening the bottom door 73 in the bottom of the measurement container 41. After the sample has been emptied from the measurement container 41, and its bottom door 73 has been closed again, the measurement container is ready to recieve a new sample.

It has been shown that the measurement result for the resis¬ tance value of the gravel sample in the measurement container is best when the two measurement electrodes 45 and 46 are placed in the upper side and the bottom side respectively, of the container when carrying out the measurement. Likewise it has shown to be advantageous to carry out the resistance measurement when the gravel sample is under constant pressure.

Fig. 5 shows two experimental curves for moisture measurement of a gravel sample with resistance measurement in the manufac¬ ture of concrete in accordance with the present invention. Curve A shows a relationship between measured resistance R in kΩ and the corresponding moisture content in percent, when the sample has been exposed to a pressure of 10 kg. Curve B shows corresponding values when the sample has not been exposed to any pressure. The above described method for determination of moisture contents in gravel by resistance measurements gives a satis¬ fying result, provided that the particle size of the gravel is constant. If however the particle size varies, this parameter should be taken into account when calculating the moisture contents of the sample. According to the present invention this is achieved by means of an instrument 31 in the measure¬ ment unit 30, which measures the capacitance in the gravel sample in the measurement container 41. The measurement unit 30 can then calculate the true moisture content in the gravel sample by taking into account both the resistance value and the capacitance value, which has been measured by the instru¬ ment 31.

The present invention for determination of moisture contents in gravel is usable both for semi-automatic and automatic systems for controlling concrete manufacture.

In a semi-automatic system a control unit 8, which can be manipulated by an operator, which is connected to a resistance measuring measurement unit 30 is used. The operator first gives a command to the control unit 8, see Fig. 3, such that the control unit by means of a signal initiates the measure¬ ment unit 30 to influence the manipulator means 50 for opening the shutter 52. Then the measurement unit receives an operator command for displacing the carriage 61 to a measurement posi¬ tion, see Fig. 4. Thereafter the measurement unit 30 measures the resistance value for the gravel sample, and optionally the instrument 31 measures the capacitance value of the sample. The operator now inputs the measurement values measured by the instruments to the control units calculating means. Based on the calculation result for the moisture contents of the gravel sample the operator decides whether he shall increase or decrease the water supply from the source 4, see Fig. 1. The manipulation of water supply from source.4 can thereby either be controlled directly from the control unit 8 or from another control means which can be influenced by the operator. In a fully automatic system a control computer 18 is included in the control unit 8, which computer is a process computer for the concrete manufacture. This process computer automati¬ cally controls material dosage from sources 1-5 to the scales 6, and further to the mixer 10. The measurement unit 30 howe¬ ver controls the manipulator means 50, 43 and 64 for the gravel sample, and the calculation of the moisture contents of the sample. Alternatively one can use only one process compu¬ ter replacing both the control unit 8 and the measurement unit 30, and which functions as a control means for controlling both the moisture measurement and the concrete manufacturing in process. The concrete manufacturing process operates in both cases fully on-line.

It is obvious that the present invention can be used for various degrees of automatic process control in the manufac¬ ture of concrete, within the range of the above described semi and fully automatic process control systems.

The above described manipulating means 9, 43, 50 and 64 are conventional, preferably pneumatically, hydraulically or electrically driven manipulator means.

Claims

1. Moisture measurement device for a concrete manufac¬ turing system, said system comprising a gravel bin (1), an adjustable water source (4) and a plurality of other material sources (2, 3, 5), preferably for stone, cement and additives, and means (6, 7, 9) for feeding material from the various material sources to a concrete mixer (10), c h a r a c t e ¬ r i z e d by a control means (8, 30) arranged to selectively control the opening of a measurement shutter (53) in a measu¬ rement opening (52) in the gravel bin (1) for feeding a gravel sample (55) to a measurement container (41), to measure the resistance value of the gravel sample in the measurement bin, and to calculate the measurement content in the gravel sample, based on the measured resistance value.

2. Moisture measurement device according to claim 1, c h a r a c t e r i z e d by two electrodes, one of which (45) is arranged on a displaσeable carriage (61), and the other (46) on the bottom (73) of the measurement bin (41), said bottom being openable.

3. Moisture measurement device according to claim 2, c h a r a c t e r i z e d by the carriage (61) being displace- able by means of the manipulating means (64, 65) controlled by the control means (3), to a measurement position on top of the measurement container ( 1).

4. Moisture measurement device according to any of claims 1-3, c h a r a c t e r i z e d in that the carriage

(61) is provided with a gravel scraper (63) which scrapes away a gravel hill (75) on the overside of the measurement contai¬ ners when the carriage is displaced.

5. Moisture measurement device according to any of claims 1-4, c h a r a c t e r i z e d in that the carriage (61) is provided with a rolling surface compactor (62), which compresses the gravel sample (55) in the measurement contai¬ ner, when the carriage is brought to a measurement position.

6. Moisture measurement device according to any of claims 1-5, c h a r a c t e r i z e d in that the measurement container (41) exhibits a sloping plane (76) in the end posi¬ tion of the carriage (61), whereby said first electrod (45) is pressed against the overside of the gravel sample.

7. Moisture measurement device according to any of claims 1-6, c h a r a c t e r i z e d in that the control means comprises a measurement unit (30) and a control unit (80) said control unit comprising a computer (18) with a display screen (27), said computer being arranged to control the dosage of the supply source material (17), including the water supply, to the concrete mixer (10) whereby the measure¬ ment unit (30) controls the removal of the moisture sample (55) from the gravel bin (1) and the measurement of the mois¬ ture contents.

8. Moisture measurement device according to any of claims 1-7, c h a r a c t e r i z e d in that the measurement unit (30) comprises a circuit (31) for measuring the capaci¬ tance value of the gravel sample in the measurement container (41) in order to take into account the particle size of the gravel for calculation of the moisture contents.

9. Moisture measurement device according to any of claims 1-8, c h a r a c t e r i z e d in that the measurement is arranged to take place with a constant pressure in the measurement container (41) with a predetermined volume.

10. A method for the measurement of moisture in the manufacture of concrete, comprising feeding and dosing materi¬ al from a gravel bin (1), an adjustable water source (4) and a number of other material sources (2, 3, 5), preferably for stone, cement and additives, to a concrete mixer (10) under control of a control means (8, 18, 30) c h a r a c t e r ! - z e d in that the control means also selectively controls the opening of a measurement door in the gravel bin (1), for taking out a gravel sample (51) to a measurement container (41), measures the resistance value for the gravel sample in the measurement container, calculates the moisture contents in the gravel sample based on the measured resistance value, and adjusts the water dosage from the water source (4), based on the calculated measurement content.