GB2147215A - Concrete mixing system - Google Patents

Abstract

The system comprises a mixer main body with a rotational-speed controllable driving device and memory means storing various mixer driving conditions, whereby in response to the input of a driving condition selection signal and a quantity of required concrete, a predetermined driving condition is selected and retrieved from the memory means and the mixer is charged and driven accordingly in response. The load torque may be detected while the mortar (cement, sand and water) is being mixed and when the load torque is in excess of a predetermined value, gravel then charged to produce concrete. The mortar may be mixed at a high rotational speed, but when the gravel is charged, the concrete materials may then be mixed at a lower rotational speed. Other memory means may store information regarding various mixture compositions and may activate supply means which supply predetermined quantities of the components to the mixer in a predetermined sequence.

Description

SPECIFICATION Concrete Mixing System The present invention relates to a forced kneading concrete mixer and to a method for mixing concrete by using such a mixer.

In order to mix concrete at a batcher plant or the like, for instance, a two-shaft forced kneading mixer or a pan type forced kneading mixer is generally used.

Figures 1 and 2 show in elevation and plan, a conventional two-shaft forced kneading mixer. Shafts 2 and 2' carrying kneading or mixing blades 3 and 3' extend parallel to each other in a mixing pan 1. These are driven by induction motors 4 and 4' through reduction gears 5 and 5' located outside the mixing pan 1 by means of V belts 8 and 8' wrapped around V pulleys 6 and 6' carried by the output shafts of the motors 4 and 4' and V pulleys 7 and 7' carried by the input shafts of the reduction gears 5 and 5'. The output shafts of the reduction gears 5 and 5' are connected to the shafts 2 and 2' and a synchronisation coupling 9 interconnects the reduction gears 5 and 5' so that the shafts 2 and 2' are driven in synchronism with each other to enable the mixing blades 3 and 3' to mix the concrete.

Figure 3 shows in a part-cutaway elevation, a conventional pan type forced kneading mixer. A mixing blade 13 is connected to the output shaft of a reduction gear 12 within a mixing pan 11 and a V belt 18 is wrapped around V pulleys 16 and 17 carried by the output shaft of an electric motor 14 located outside the mixing pan 11 and by an intermediate shaft 15, respectively. The intermediate shaft 15 is connected to the input shaft of the reduction gear 12. Upon energization of the induction motor 14, the mixing blade 13 is rotated to mix the concrete.

Figure 4 shows the relationship between motor power output and time when concrete is mixed in a mixer of the types described above. It is apparent that the output requirement may be low when sand, water and cement are being mixed (to be referred to as the mixing of mortar), but the output must be high when gravel is added. The fact that the output must be high when gravel is added is due to the following relationship: KW=aNT where KW is the power of an induction motor; T is torque; N is a rotational speed; and a is a coefficient.

In the case of an induction motor, the rotational speed is maintained constant and will not change even when the torque required (load) increases. It follows therefore that the power of the motor to be used must be determined in dependence upon the peak value shown in Figure 4. As a resuit, the electric motor called for tends to be large in size and heavy in weight and in addition, the capital cost, the operating costs and the maintenance costs are increased.

Furthermore, the faster the rotational speed (or the peripheral velocity of a blade) during the mixing of mortar, the more the strength of the concrete would be stabilized and the shorter the required mixing time.

However, the mixing speed cannot be increased in practice beyond a certain value in order to ensure that separation of the gravel and mortar does not occur when the gravel is added, as will be considered in more detail below.

It is an object of the present invention to overcome the above and other problems encountered in the conventional concrete mixing systems.

According to the present invention, there is provided a concrete mixing system which comprises a mixer having kneading means, a variable speed driving device for the kneading means, and a control system, the control system including a memory storing data relating to relative quantities and sequences for the concrete materials, and mixer driving speeds, times and sequences corresponding to concretes having various properties, in which system, in response to an input to the control system corresponding to a particular concrete type, the mixer is charged with predetermined quantities of the concrete materials in a predetermined sequence and the driving device is driven at predetermined speeds in a predetermined sequence for predetermined times.

Preferably, in a mortar stage, water sand and cement are charged and the mixer is driven at a relatively high speed, then in a concrete stage, gravel is added and the mixer is driven at a relatively lower speed. The system may further include a torque detector arranged to provide a signal to the control system when the torque load of the driving device exceeds a predetermined value during the mortar stage, in response to which signal gravel is added.

Preferably, mixing during the mortar stage is carried out at a substantially constant power output while the mixing during the concrete stage is carried out at a substantially constant speed.

The invention may be carried into practice in various ways and some embodiments will be described by way of example with reference to Figures 5 to 9 of the accompanying drawings, in which: Figure 5 is a diagram illustrating the characteristic curve of a driving device used in the present invention; Figures 6,7 and 8 are diagrams illustrating output characteristic curves of a driving device when driven in various manners in accordance with the present invention; and Figure 9 is a block diagram of a forced kneading mixer system in accordance with the present invention.

The driving device used in the present invention is a hydraulic motor, a DC motor or the like which can change its rotational speed and has a constant output region as shown in Figure 5. The driving device is controlled by a control device to operate under certain desired conditions.

In a first driving mode, the object is to increase the time during which the driving device is operating at or near its maximum power output orto increase the capacity. The output characteristic curve of the driving device is shown in Figure 6 and as can be seen, the output of the driving device is maintained near 100% for a large proportion of the cycle.

When water, cement and sand are mixed, the torque required for mixing may be low. Therefore, when the driving device with a torque characteristic curve as shown in Figure 5 is used, the rotational speed can be increased when the mixer is driven with low torque. However, when gravel is added, the reactive torque is increased so that the rotational speed is decreased. If the water, cement, sand and gravel were mixed at a higher rotational speed, then separation between the gravel and mortar would be likely to result. Thus, when the gravel is added, the mixing must be carried out at a speed lower than a predetermined rotational speed. Consequently, when the reactive torque is increased while the rotational speed is positively decreased to a lower value, the actual output of the driving device can be satisfactorily maintained and concrete with a higher degree of quality can be obtained.

Since the mortar mixing time can be shortened and the driving device can be operated almost to its full capacity during the cycle, the mixing efficiency and the mechanical efficiency may be improved.

Furthermore, a driving device with a lower capacity can be used compared with conventional concrete mixers.

In a second driving mode, the object is to stabilize the quality of the concrete.

According to a second driving mode, and as shown in Figure 7, the mortar mixing is carried out at a high rotational speed, but when the gravel is added, the mixing is carried out at a lower rotational speed.

The second driving mode may ensure the quality of concrete.

With reference to the second driving mode, variations in compression strength 528 (kglcm2) with the rotational speed (the peripheral velocity of a mixing blade) of a mixer during the mixing of mortar are illustrated below.

In tests, two proportions as shown in Table 1 were prepared and the peripheral speed or velocity R of the mixing blade was as follows: R1=1 m/sec and R2=2 mlsec The mixing time t was 30 sec.

Tests were conducted three times under the same conditions. Six samples for measuring the strength were extracted from one batch and 528 was obtained. The test results are shown in Table 2.

TABLE 1 Proportioning

Contents kg/m3 Proportioning No. W/C% W C S G Ad 57.4 186 324 862 (927) 0.81 11 57.4 154 268 844 (1080) 0.67 where W: Water C: Ordinary Portland cement S: Sand water absorption rate=1.42% specific weight=2.65 particle size=2.68 G: Gravel Ad: admixture for reducing water, Pozzolis No.51 TABLE 2 Test Data

Proportioning No. & Flow Value Mean Strength Mean Standard Peripheral Velocity of Blade cm 28 kg/cm2 Deviation S kg/cm2 l-R1 17.5x17.8 247 12.8 l-R2 17.5x17.6 276 4.5 11R1 15x15 282 13 11R2 14.8x15 348 9.3 It is apparent from Table 2 that for the same mixing time, a higher peripheral velocity of the mixing blade results in a higher mean strength and a lower mean standard deviation, i.e. when R2=2 m/sec.

Therefore concrete of a higher quality may be obtained.

This tendency remains unchanged even when the mixing time is increased to 60 sec, therefore, it is apparent that the quality of concrete is dependent upon the peripheral velocity of the mixing blade when the mortar is mixed.

The peripheral velocity of the mixing blade and the mixing time when the gravel is added must be so selected that no separation occurs between the mortar and the aggregates so that in turn, the mortar can be uniformly attached to the aggregates.

In a third driving mode, the object is to improve the quality of the concrete.

In this mode, the time when the quality of the mortar can be most improved is detected during the mixing of the mortar and at this time, the gravel is added to produce the concrete.

With mortar, when the proportions are the same but the surface water rate of the sand varies, the fluidity varies considerably. The aforementioned time when the quality of mortar is most improved refers to the time when a hard outer shell of cement paste (capiilary paste) is formed around each particle of sand and the remaining space is filled with soft cement paste (slurry paste). Such conditions as described above are generally referred to as "the capillary state". When mortar is in the capillary state, it is easier to pump and the strength of concrete finally produced is superior.

The inventors discovered that the torque required for mixing reaches a maximum when mortar is in the capillary state. Therefore, in one preferred embodiment of the present invention, the drive torque for the mixer is monitored and the point at which torque reaches a maximum is detected. At that point, gravel is added to produce concrete. Figure 8 shows the output characteristic curve of a driving device when the concrete mixing is carried out in this manner.

Various methods may be used to detect the maximum torque of the driving device. In one method, a torque detector is attached to the driving device and the driving device is rotated at a predetermined constant rotational speed lowerthan its maximum rotational speed.

The torque detector detects the maximum torque and in response to an output signal from the torque detector which represents the maximum torque detected, gravel is added to produce concrete. In this way concrete of high quality can be obtained.

Another method for detecting the maximum torque is to derive this from the maximum power consumption of the driving device.

As shown in Figure 8, after the gravel is charged, the mixer is driven so as to maintain the highest efficiency, as far as possible. Thus, the driving device can be used to its capacity.

As described above, various driving methods have been proposed using the control systems of the driving devices. Various driving programmes are entered into the control system so that the best driving system may be selected by an operator to achieve the desired working conditions.

Referring now to Figure 9, a driving system is shown which is capable of accomplishing the various driving programmes discussed above.

In Figure 9, reference numerals 20 denotes an input device; 21, a plant computer; 22, a mixer control computer; and 23, a mixer. The plant computer 21 comprises three interfaces 24, 25 and 26, a processor 27 and a memory 28. The processor 27 is connected through the interface 24to a display device 29 and a recorder 30 such as a printer.

The processor 27 is further connected through the interface 25 to a device 32 for selectively opening and closing bins containing sand, cement, etc., a weighing device 33 and a device 34 for selectively opening and closing a gate of a weighing hopper.

TABLE 3 Proportioning No.

Unity Quantity kg/m Water/ Coarse Fine Concrete Aggregate Aggregate Coarse Fine Strength Slump Ratio Max. Size Ratio Water Cement Aggregate Aggregate ...

kg/cm cm % cm % The mixer control computer 22 comprises two interfaces 35 and 36, a processor 37 and a memory 38.

The processor 37 is connected through the interface 36 to a device 39 for selectively opening and closing a gate of the mixer 23, a mixer driving device 40, a torque detector 41 and a tachometer 42. The main body of the mixer 23 is indicated by a reference numeral 43.

Various previously determined data required for charging concrete materials into the mixer 23 are stored in the memory 28 of the plant computer 21. Among these data are the proportions of the materials depending upon the required properties of concrete to be produced, as shown in Table 3. Such proportioning data is tagged with a proportioning number identifying the type of concrete required. In addition, the memory 28 has stored data against the proportioning numbers the data required for charging the concrete materials in a predetermined sequence and at predetermined times.

When a proportioning number and a required concrete quantity (the quantity of concrete to be mixed) are entered into the input device 20, the processor 27 of the plant computer 21 retrieves the data corresponding to the entered proportioning number from the memory 28. In response to the retrieved data, the quantity of each material to be charged is obtained based upon the required concrete quantity. When the output from the weighing device 33 coincides with the quantity of a material to be charged derived from the processor 27, signals for driving the bin gate opening-closing device 32 and the hopper gate opening-closing device 34 are generated so that the material is charged under predetermined conditions into the mixer 23 at a predetermined time.

The mixer driving conditions, i.e. the mixing time, the mixing speed, the final rotational speed (i.e. the rotational speed of the mixer 23 immediately before the concrete is discharged therefrom), the mixing time in response to the rotational speed when the latter is varied, and the driving system or method as shown in Figure 6,7, or 8 are stored in the memory 38 of the mixer control computer 22. These data are tagged with a proportioning number. In response to a proportioning number and a quantity of required concrete entered into the input device 20, the driving conditions are retrieved from the memory 38 and a start-stop signal 44 is applied to the mixer driving device 40. A rotational speed setting signal 45 is also applied to the driving device 40. Thus, the driving device of the mixer is controlled in response to the selected driving system or method.

The output of the tachometer 42 representing the rotational speed of the mixer driving device (or the rotational speed of a mixer blade) is fed back to the processor 37 so that the rotational speed of the mixer driving source is maintained at the required rotational speed.

After a predetermined mixing time has elapsed, the mixer 23 is rotated at a predetermined final rotational speed and then the mixer gate opening-closing device 39 opens the gate of the mixer 23 so that the mixed concrete is discharged.

When the concrete mixing is carried out as shown described with reference to Figure 8, a load torque signal 46 is applied from the torque detector 41 to the processor 37 which in turn detects the maximum load torque; which corresponds to the capillary state. A signal corresponding to the maximum load torque is then applied to the computer 21 and the gravel is charged into the mixer 23.

As described above, when a rotational-speed controllable driving device is used and the concrete materials are mixed in response to preselected driving conditions, they can be mixed with a higher efficiency and concrete of higher quality can be obtained.

Claims (8)

1. A concrete mixing system which comprises a mixer having a kneading means, a variable speed driving device for the kneading means, and a control system, the control system including a memory storing data relating to relative quantities and sequences for the concrete materials, and mixer driving speeds, times and sequences corresponding to concretes having various properties, in which system, in response to an input to the control system corresponding to a particular concrete type, the mixer is charged with predetermined quantities of the concrete materials in a predetermined sequence and the driving device is driven at predetermined speeds in a predetermined sequence for predetermined times.

2. A system as claimed in Claim 1 in which in a mortar stage, water sand and cement are charged and the mixer is driven at a relatively high speed, then in a concrete stage, gravel is added and the mixer is driven at a relatively lower speed.

3. A system as claimed in Claim 2 further including a torque detector arranged to provide a signal to the control system when the torque load of the driving device exceeds a predetermined value during the mortar stage, in response to which signal gravel is added.

4. A system as claimed in Claim 2 or Claim 3 in which mixing during the mortar stage is carried out at a substantially constant power output while the mixing during the concrete stage is carried out at a substantially constant speed.

5. A concrete mixing system constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 5 to 9 of the accompanying drawings.

6. A forced kneading mixer characterised by a mixer main body with a rotational speed controllable driving device and memory means which store various mixer driving conditions, whereby in response to the input of a driving conditions selection signal and a quantity of required concrete, a predetermined driving condition is selected and retrieved from said memory means and said mixer is driven in response to thus selected and retrieved driving or mixing conditions.

7. A concrete mixing method characterised in that mortar is mixed at a constant speed, and when a load torque is detected in excess of a predetermined value, gravel is charged into a mixer and mixed with other concrete materials.

8. A concrete mixing method characterised in that mortar is mixed at a high speed while concrete is mixed at a lower speed.