Method of manufacturing concrete and apparatus therefor

Abstract: Method and apparatus for manufacturing concrete by mixing concrete materials including a cement, aggregate, admixture and at least one of water and ice. The aggregate is moved prior the mixing and a low-temperature liquid is sprayed on the aggregate for cooling while the aggregate is being moved.

Description: The present invention relates to a method of manufacturing concrete and an apparatus therefor, and particularly but not exclusively relates to a method of manufacturing mass concrete and an apparatus therefor. When mass concrete members used for dams, bridges, reactor facilities, or the like are constructed, cracking tends to occur in mass concrete members due to temperature stresses caused by heat of hydration of the cement. Therefore, it is important to prevent such cracks in terms of quality control. In order to prevent the cracks in mass concrete members, the following methods have been employed. For example, according to a so-called pre-cooling method, a variation amount and difference in concrete temperature, which are caused by heat of hydration of cement, are restricted by decreasing a temperature of the concrete at the end of mixing. According to another typical conventional method, use is made of a concrete of enhanced strength, which provides a temperature rise due to heat of hydration at a degree equal to ordinary concretes, for increasing the resistance to cracks. In the pre-cooling method, prior to the mixing of components of concrete, each component is pre-cooled, using cool water, cool air, or ice so as to lower a temperature of the concrete at the end of mixing. The cooled components are then mixed so that cracks caused by thermal stresses can be prevented by erecting the concrete thus obtained. Recently, particles of ice are used for mixing concrete in place of water so as to uniformly disperse components of the concrete, thereby enhancing strength of the concrete while lowering a temperature of the concrete at the end of mixing by latent heat of the ice in the same manner as in the pre-cooling method. However, in the above conventional methods of preventing cracks in mass concrete, the following problems are posed. In the pre-cooling method, cool water, cool air, or ice is used as a refrigerant for cooling each component of concrete. When a temperature of the concrete at the end of mixing need be lowered to a great extent, components may not be sufficiently cooled to a predetermined temperature. Besides, a considerably long period of time is required to cool them. In the method wherein ice is used for mixture in place of water, particles of ice may remain in concrete after mixing and a mixing time may be prolonged depending on the size and amount of ice, and hence the amount of ice is limited. Therefore, a temperature of the concrete may not be sufficiently lowered as in the case of using cool water or cool air. It is known from DE 2438969 to spray liquid nitrogen onto aggregate before mixing in order to cool the aggregate to a predetermined temperature. The liquid nitrogen is sprayed downwardly onto the surface of the aggregate. Mixing drums or shaking troughs are also disclosed to avoid formation of lumps. In US patent 3 583 172 there is disclosed a mixer for concrete in which liquid nitrogen is introduced into the concrete mixture at a position below its surface to enable use of both the latent heat and sensible heat of the liquid nitrogen for cooling. In accordance with one aspect of the present invention there is provided a method of manufacturing concrete by mixing concrete materials including a cement, aggregate, admixture and at least one of water and ice, including the steps of: agitating the aggregate prior to the mixing; and spraying the aggregate using a low temperature liquid in order to to cool the aggregate to a predetermined temperature while the aggregate is being agitated, characterized in that the liquid is sprayed directly into the aggregate to pass therethrough upwardly so that surface water of the aggregate is frozen and ice layers are formed on surfaces of all the grains of the aggregate without fusing together of the grains. In accordance with another aspect of the present invention there is provided an apparatus as defined in claim 7. In accordance with another aspect of the present invention there is provided an apparatus as defined in claim 8. Brief description of the Drawings Figure 1 is a schematic longitudinal sectional view of an aggregate cooling apparatus which is not comprised in the claimed invention; Fig. 2 is a cross-sectional view thereof taken along a line II - II of Fig. 1; Fig. 3 is a schematic enlarged sectional view of a grain of sand cooled in accordance with the invention; Fig. 4 is a cutaway side view of an aggregate cooling apparatus for performing the method of the present invention; Fig. 5 is a schematic view illustrating a concrete manufacturing apparatus using the aggregate cooling apparatus in Fig. 4; Fig. 6 is a vertical sectional view of an aggregate cooling apparatus according to a first embodiment of the present invention; Fig. 7 is a schematic view of a concrete manufacturing apparatus using a concrete mixer according to a second embodiment of the present invention; Fig. 8 is an enlarged sectional view of the concrete mixer of Fig. 7 along the axial direction; Fig. 9 is a sectional view showing another operation of the concrete mixer of Fig. 8 along the axial direction; Fig. 10 is a schematic reduced sectional view showing a concrete manufacturing apparatus using a pair of concrete mixers in Fig. 8 according to a modification of the present invention; Fig. 11 is a graph showing a relationship between a temperature of sand and a temperature of concrete at the end of concrete mixing in an experiment; and Fig. 12 is a graph showing a relationship between a water content of sand and an amount of liquid nitrogen used for cooling the sand in an experiment. Detailed Description of the Preferred Embodiments Preferred embodiments of the present invention will be described with reference to the accompanying drawings. The same reference numerals denote the same parts in the drawings, and hence a description thereof will not be repeated after once given. Figs. 1 and 2 show an aggregate cooling apparatus which sprays liquid nitrogen onto the surface of the aggregate in a conventional manner as known. This manufacturing apparatus serves to manufacture frozen sand (fine aggregate), on each surface of grains of which an ice layer is formed. Referring to Figs. 1 and 2, reference character A denotes a vibrating chute system mounted on base G. The vibrating chute system A is constituted by a substantially U-shaped chute 1 for transferring sand 4, a vibrating mechanism 2, and three pairs of coil springs 3. Two pairs of the coil spring 3 are arranged on the base G and support the chute 1 from the lower side. The chute 1 is tilted so that the start position of the chute is slightly higher than the end position thereof. In addition, the chute 1 is designed to be vertically vibrated by the vibrating mechanism 2 and the coil springs 3. Silos 5 and 6 are arranged above the start position of the chute 1 and below the end position thereof, respectively. Covers 7 and 8 which can be opened/closed are arranged on lower end opening portions of the silos 5 and 6. A cover 9 is arranged on an upper end portion of the silo 6 located at the end position of the chute 1 so as to enhance the cold insulation effect. The cover 9 is opened by the weight of the sand 4, and automatically closed when the sand 4 is not supplied. The cover 9 is preferably made of, e.g., hard rubber. The silo 6 is preferably a silo whose heat insulation effect is enhanced by, e.g., forming a known heat-insulating material on a wall surface and the like of the silo. A cylindrical freezing duct 16, both ends of which are opened, covers the entire chute 1 except for the start and end positions thereof, i.e., portions for receiving and discharging the sand 4 from/to the silos 5 and 6. The freezing duct 16 communicates with a cool air duct 18 at the end position of the chute 1 through a communicating duct 17. Blowers 19 and 21 are arranged inside the cool air duct 18. The cool air duct 18 branches into two paths midway along the duct. One path serving as a branch pipe 20 is connected to a silo (not shown) storing coarse aggregate, while the other path is directed downward to the silo 5 located at the start portion of the chute 1. The end portion of the chute 1 extends through the start portion of the communicating duct 17 so as to transfer the sand 4 to the silo 6. A cooler B is arranged near the chute 1 to cool the sand 4, transferred along an upper surface of the chute 1, by spraying liquid nitrogen on the sand 4. The cooler B is constituted by a liquid nitrogen tank 10 located on the base G or another place, a controller 11 for controlling a supply amount of liquid nitrogen supplied from the tank 10, a supply pipe 12 for supplying the liquid nitrogen from the controller 11 to the chute 1, pipes 14 communicating with the supply pipe 12 through a flexible joint 13 and arranged above the chute 1, located in the freezing duct 16, to extend in a longitudinal direction thereof, and spray nozzles 15 for spraying the liquid nitrogen on the sand transferred in the chute 1. The spray nozzles are formed in the pipes 14 at predetermined intervals in a longitudinal direction thereof. An operation of the above-described aggregate cooling apparatus will be described below. The vibrating mechanism 2 of the vibrating chute system A is driven to vertically vibrate the chute 1 in advance. A vibration frequency and a stroke of the chute 1 can be arbitrarily set. Time for conveying the sand 4 along the chute 1 can be controlled by appropriately adjusting these parameters. Subsequently, the sand 4 stored in the silo 5 is dropped on the start-position portion of the chute 1 by opening the cover 7. The sand is then transferred along the chute 1 while vibrating, jumping, and rotating, and charged into the silo 6 from the end position portion of the chute 1. The liquid nitrogen is sprayed on the sand 4 by the cooler B while the sand 4 is transferred along the chute 1. More specifically, the controller 11 controls to supply the liquid nitrogen from the tank 10 to the pipes 14 through the supply pipe 12. Subsequently, the liquid nitrogen is sprayed on the sand 4 through the spray nozzles 15, thereby cooling the sand 4 below 0° C. As a result, water on surfaces of a grain of the sand 4 is frozen and an ice layer is formed on the surfaces thereof. The sand 4 used for normal concrete manufacture includes 5 to 10% of the surface water. If the amount of surface water is regarded to be insufficient, the amount of surface water is preferably controlled by, e.g., sprinkling the sand with water in advance. Any discharge rate of the liquid nitrogen from the spray nozzles 15 can be set so that a desired cooling temperature can be obtained in accordance with a type of material to be cooled by controlling the time for conveying the sand 4 along the chute 1. This conveying time controlling is achieved by appropriately selecting the vibration frequency and the stroke of the chute 1. When sand and gravel are cooled below 0° C, water on their surfaces may form ice layers or may partly become fine ice grains, separate from the sand and the gravel, and exist in a state mixed with them. At the same time, blowers 19 in the cool air duct 18 are driven to generate air flowing in a direction indicated by arrows in Fig. 1, i.e., from the freezing duct 16 to the cool air duct 18. As a result, low-temperature air which has cooled the sand 4 flows through the freezing duct 16, communicating duct 17, and the cool air duct 18 in the order named, and part of the air is supplied to the silo storing the coarse aggregate through the branch pipe 20, thereby cooling the coarse aggregate, while the rest of the air is supplied to a lower portion of the silo 5 to pre-cool the sand 4. According to the above-described method, the sand grains, on each surface of which an ice layer is formed, can be produced. Thereafter, concrete is manufactured by mixing the sand 4 with gravel (coarse aggregate), cement, and water or particles of ice, and if necessary, mixing various types of admixtures. Although a method of mixing these components of the concrete can be arbitrarily selected, it is preferable to select, e.g., a method wherein the sand 4 having an ice layer formed on each surface of the grains thereof is supplied into a mixing device such as a concrete mixer, and then cement, gravel, and water or ice are sequentially supplied into the mixing device in the order named, and these components are mixed together, thereby manufacturing the concrete. These components may be simultaneously supplied into the mixing device. It is also preferable to select a method wherein the sand 4 is mixed with cement such that grains of the cement are evenly covered on each surface of the ice layer of the sand 4, and then the sand 4 thus processed, gravel, and water or particles of ice are simultaneously supplied into the mixing device. In this case, a temperature of the concrete at the end of mixing can be lowered in the same manner as in the conventional method to enhance strength of the concrete by replacing same water with particles of ice. Therefore, according to the aggregate cooling apparatus described above, the sand 4 can be cooled below 0° C by spraying the liquid nitrogen on the sand 4 while the sand 4 transferred along the surface of the chute 1 is vibrated by vibrating the chute 1 using the vibrating mechanism 2. As a result, water on each surface of grains of the sand 4 can be frozen, and hence an ice layer can be formed on each surface thereof. In addition, since cooling of the sand 4 is performed using the liquid nitrogen having a very low temperature, it can be performed within a short period of time, and moreover, since cooling is performed while the sand 4 is vibrated, the ice layers can be easily and reliably formed. Furthermore, since the sand 4 is quickly cooled by the liquid nitrogen to a very low temperature, after the ice layers are formed, the grains of the sand 4 are not fused together. Accordingly, concrete can be manufactured by the above-described aggregate cooling apparatus using the sand 4, on each surface of the grains of which an ice layer is formed while the following effects concerning a decrease in temperature of the concrete at the end of mixing, an increase in strength of the concrete, and the like can be obtained according to the same steps as described below. (1) Since the ice layers are formed on the grains of the sand 4, the temperature of the concrete at the end of mixing can be lowered due to latent heat of the ice and cooling heat of the sand 4. As a result, the same effect as in the conventional pre-cooling method can be obtained. For example, assume that the sand 4 includes 5 weight % of the surface water and a unit amount of the fine aggregate is 850 kg/m³ (20° C). The effect obtained by freezing the all surface water is equivalent or superior to the effect obtained by using ice of 42.5 kg/m³ in place of water used for mixture, or the effect when water of 170 kg having a temperature 20° C is cooled to 0° C. (2) When various components of concrete are mixed together, a large number of cement grains 22 are adhered to grains of the sand 4 through layers of ice 21, as shown in Fig. 3. Accordingly, in the mixture of the components of the concrete, since the grains of the cement are evenly dispersed and so-called bearing effect is obtained, a unit amount of water for obtaining a desired physical value can be reduced. Therefore, if the unit amount of cement is not changed, strength of the concrete can be increased. In addition, when the components are mixed, a dense cement paste can be formed on a surface of the grains of the sand 4 because of the presence of the grains 22 of the cement. As a result, adhesion strength between the grains of the sand 4, or between other coarse aggregate such as the gravel and the sand 4 of the concrete can be increased. According to the method of manufacturing concrete, a temperature of the concrete at the end of mixing can be lowered while strength of the concrete can be increased without using ice for mixture in place of water. Therefore, unlike the conventional method wherein part of water used for mixture is replaced with particles of ice, even when the temperature of the concrete is not relatively high during the seasons excluding the summer season, ice is not left in the resultant concrete. Thus, the same effect as described above can be obtained even in severe construction conditions during a period of the fall, the winter, and the spring. Although in this embodiment surface water of aggregates are frozen, this is not necessarily done. Figs. 4 and 5 show a first embodiment of the present invention. Referring to Fig. 5, reference numeral 101 denotes a sand stocker for storing sand (fine aggregate) 103; 102, a sand stocker for storing gravel (coarse aggregate) 104; 105, a sand weighing device for weighing the sand 103 supplied from the sand stocker 101; and 106, a gravel weighing device for weighing the gravel 104 supplied from the gravel stocker 102. An aggregate hopper 107 for temporarily storing the weighed sand 103 and gravel 104 is arranged under the sand and gravel weighing devices 105 and 106, and an aggregate cooling apparatus 108 according to this embodiment is interposed between the sand weighing device 105 and the aggregate hopper 107. Although a known heat-insulating material is preferably formed on, e.g., a wall surface of the aggregate hopper 107 to enhance the heat-insulating effect, if the sand 103 or the gravel 104 need not be temporarily stored, the aggregate hopper 107 is omitted. Reference numeral 109 denotes a cement stocker for temporarily storing cement. A cement weighing device 110 is located under the cement stocker 109 and coupled thereto. Concrete mixer 111 for mixing components of concrete such as cement and aggregate is arranged under a supply port of the cement weighing device 110 and a supply port of the aggregate hopper 107. A heat-insulating material is preferably formed on, e.g., a wall surface of the concrete mixer 111 in the same manner as in the aggregate hopper 107 to enhance the heat-insulating effect. Note that supply devices for supplying water, admixture, etc., used for mixture, into the concrete mixer 111 are omitted for the sake of a simple explanation. The aggregate cooling apparatus 108 is constituted by an aggregate mixer 112 for mixing the sand 103, and a cooler 113, provided to the aggregate mixer 112, for cooling the sand 103 by spraying liquid gas on the sand 103 in the aggregate mixer 112. The aggregate mixer 112 is constituted by a tub-like drum 114, a substantially disk-like cover 115 for covering an upper opening of the drum 114, and support legs 116 arranged on a bottom portion of the drum 114. The cover 115 has an aggregate charge port (not shown) through which aggregate is charged into the drum 114, while an aggregate discharge port (not shown) is formed in the bottom portion of the drum 114. A column-like support cylinder 117 is vertically fixed at the center of the drum 114, while a rotor 118 is mounted on an upper portion of the support cylinder 117 to be pivotally supported around a vertical axis Z. A plurality of arms 119 radially extend from a circumferential portion of the rotor 118, while scrapers 120 for mixing the sand 103 and the like stored in the drum 114 extend from distal end portions of the arms downwardly. A rotary shaft 161 extends from the bottom portion of the drum 114 to penetrate the support cylinder 117 along the axis Z. An upper end of the rotary shaft 161 is fixed to the rotor 118, and a pulley 121 is fixed to a lower end of the rotary shaft 161. Reference numeral 122 denotes a motor for rotating the rotor 118. A drive shaft of the motor 122 is coupled to a pulley 123 arranged on a lower end of the motor 122. A V belt 124 is arranged around the pulleys 121 and 123 to transfer a rotating force of the motor 122 to the rotor 118. Note that in the aggregate mixer 112, a heat-insulating material is preferably formed on a wall surface of the drum 114 in the same manner as in the concrete mixer 111 so as to enhance the heat-insulating effect. The cooler 113 is constituted by a liquid gas or cool air tank 124 arranged near the aggregate mixer 112 or in another place, a controller 125 for controlling a supply amount of liquid gas supplied from the tank 124, supply pipes 126 for supplying the liquid gas from the controller 125 toward the aggregate mixer 112, and a plurality of spray nozzles 127, provided to distal ends of the supply pipes 126 and arranged on a lower end portion of side plate 163 on the drum 114 side and a bottom plate 165, for spraying the liquid gas toward the bottom portion of the drum 114. As a result, the liquid gas is directly sprayed from the spray nozzles 127 on the aggregate (sand 103) stored in the bottom portion of the aggregate mixer 112. Reference numeral 128 denotes an exhaust duct, mounted on the cover 115 of the aggregate mixer 112, for discharging a gas derived from the liquid gas supplied into the aggregate mixer or cool air supplied into the aggregate mixer 112 outside the system. Reference numeral 129 denotes a screen for adjusting a grading of the sand discharged from the aggregate discharge port (not shown). The low-temperature gas exhausted from the exhaust duct 128 is supplied to the sand and gravel stockers 101 and 102, or the concrete mixer 111 as needed, and is used for pre-cooling the sand 103 and the gravel 104, or cooling during mixing of the concrete. A method of cooling aggregate using the aggregate cooling apparatus 108 with the above-described arrangement will be described below. The sand 103 is transferred into the sand stocker 101 in advance using conveyor (not shown) or the like. When concrete is to be manufactured, the sand 103 is appropriately supplied from the sand stocker 101 to the sand weighing device 105 to weigh the sand 103 according to a predetermined mixing ratio for the concrete. The weighed sand 103 is charged into the drum 114 of the aggregate mixer 112. After the sand is charged into the drum 114, the scrapers 120 arranged in the drum 114 are rotated inside the mixer 112 by driving the motor 122, thereby mixing the sand 114 inside the drum 114. The controller 125 controls to supply the liquid gas or the like from the tank 124 to the spray nozzles 127 through the supply pipes 126 so that the liquid gas or the like is sprayed inward from the spray nozzles 127 toward the bottom portion of the drum 114, thereby directly spraying the liquid gas or the like on the bottom of the sand 103. As a result, the sand 103 is instantly cooled below 0° C so that water on surfaces of grains of the sand 103 is frozen and ice layers are formed on the grains. If the sand 103 is cooled to -5° C to -10° C or below, the ice layers on the surfaces of the grains of the sand 103 are separated from each other, and hence a large number of the grains of the sand 103 are rarely fused into a mass of frozen. The sand 103 used for a normal concrete manufacture contains 5 to 10 wt.% of the surface water. If, however, the amount of surface water is regarded to be insufficient, the amount of surface water is preferably adjusted in advance by sprinkling the sand 103 with water. With surface water ratio of more than about 15 wt.%, water is liable to separate from the aggregate and is not preferable. Concrete is efficiently cooled with an aggregate having surface water ratio of more than about 3 wt.%. A discharge rate of the liquid gas from the spray nozzles 127 can be arbitrarily set. A desired cooling temperature can be obtained in accordance with a type of material to be cooled by appropriately adjusting and selecting the discharge rate, a rotating speed of the scrapers 120 of the aggregate mixer 112, and the time for keeping the liquid gas in the aggregate mixer 112. After the sand 103 is discharged from the aggregate discharge port (not shown) while the sand 103 is kept mixed by the scrapers 120 and the grading of the sand 103 is adjusted by filtering the sand 103 through the screen 129, the sand 103 is charged into the aggregate hopper 107. As a result, ice layers can be formed on the surfaces of the grains of the sand 103 prior to mixing. When the cooling operation of the sand 103 in the aggregate mixture 112 is not performed, the spray nozzles 127 are kept spraying nitrogen gas, air, or the like to prevent the distal ends of the nozzles 127 from clogging or freezing. Subsequently, concrete is manufactured as follows. The gravel is appropriately supplied from the gravel stocker 102 to the gravel weighing device 106 and weighed thereby, and then charged into the aggregate hopper 107. The low-temperature sand 103, the gravel 104, the cement, and the water or the particles of the ice are mixed together, and various admixtures are mixed as needed, thereby manufacturing the concrete. Although a method of mixing these components of the concrete can be arbitrarily selected, it is preferable to select, e.g., a method wherein the low-temperature sand 103 and the gravel 104 are charged from the aggregate hopper 107 into the concrete mixer 111, and then the cement, the gravel, and the water or the particles of ice are sequentially supplied into the concrete mixer 111 in the order named, and these components are mixed together, thereby manufacturing the concrete. These components may be simultaneously supplied into the concrete mixer 111. According to the aggregate cooling apparatus 108 described above, the surface water of the sand 103 can be frozen by cooling the sand 103 below 0° C, and hence ice layers can be formed on the surfaces of the grains of the sand 103 prior to mixing of the components of the concrete such as cement and aggregate. In addition, since the sand 103 is cooled by spraying the liquid gas or the like thereon while the sand 103 is mixed by the aggregate mixer 112, the liquid gas or the like can be uniformly sprayed to the grains of the sand 103. Therefore, since cooling efficiency of the sand 103 using the liquid gas or the like is improved compared with that using the cooler shown in Fig. 1, the cost required for cooling the sand 103 can be reduced. Fig. 6 shows an aggregate cooling apparatus 208 according to still another embodiment of the present invention. Similar to the aggregate cooling apparatus 108 in the above embodiment, the aggregate cooling apparatus 208 is constituted by an aggregate mixer 212 and a cooler 213 provided to the aggregate mixer 212. A housing 230 of the aggregate mixer 212 includes substantially cylindrical upper and lower chambers 234A and 234B. An aggregate charge port 231 is formed at one end of an upper wall 251 of the upper chamber 234A and an aggregate discharge port 232 is formed at one end of a lower wall 253. An opening 257 is formed at the other end of a wall 255 of the chambers 234A and 234B so that the chamber 234A communicates with the chamber 234B. Screw conveyors 235 are arranged inside the chambers 234A and 234B. Rotary shafts 236 of the screw conveyors 235 extend in a longitudinal direction of the chambers 234A and 234B and are concentrical therewith, respectively, while both ends of the rotary shafts 236 are pivoted to the housing 230. As a result, the screw conveyors 235 are pivotally supported by the housing 230. One end (right end in Fig. 6) of each of the rotary shafts 236 extends outwardly from the housing 230, while gears 237 are coaxially mounted on the extending portions of the rotary shafts 236, respectively. These gears are meshed with each other, while a gear 239 coupled to a drive shaft of a motor 238 is meshed with the lower gear 237. More specifically, a rotating force of the motor 238 is transmitted to the screw conveyors 235 through the gears 237 and 239, while the screw conveyors 235 are rotated in the opposite directions. Similar to the embodiment shown in Fig. 4, a heat-insulating material is preferably formed on, e.g. a wall surface of the aggregate mixer 212 in this embodiment to enhance the heat-insulating effect. Similar to the cooler 113 in the embodiment of Fig. 4, the cooler 213 is constituted by a liquid gas or cool air tank (not shown), a controller (not shown) for controlling a supply amount of liquid gas or the like supplied from the tank, a supply pipe 240 for supplying the liquid gas or the like from the controller to the aggregate mixer 212, and a spray nozzle, provided at a distal end of the supply pipe 240 and arranged on a bottom portion of the chamber 234A of the aggregate mixer 212, for spraying the liquid gas or the like inwardly from the bottom portion of the chamber 234A. Reference numeral 242 denotes an exhaust duct arranged on the housing 230 to communicate with the lower chamber 234B; and 243, a screen. A method of cooling the sand on surfaces of grains of which ice layers are formed using the aggregate cooling apparatus 208 with the above described arrangement is substantially the same as that using the aggregate cooling apparatus described in the embodiment of Fig. 4. The sand 103 is supplied from the sand stocker 201 to the aggregate cooling apparatus 208. Ice layers are formed on the surfaces of the grains of the sand 103 by cooling the sand 103 using the aggregate cooling apparatus 208. More specifically, after the sand 103 is charged into the housing 230 through the aggregate charge port 231, the screw conveyors 235 inside the housing 230 are rotated by driving the motor 238 so that the sand 103 is conveyed from the aggregate charge port 231 to the aggregate discharge port 232, while the sand 103 is mixed by the screw conveyors 235. Thereafter, the controller controls to supply the liquid gas or the like from the tank to the spray nozzle 241 through the supply pipe 240 so that the liquid nitrogen is sprayed by the spray nozzle 241 inwardly from the bottom portion of the chamber 234A, thereby directly spraying the liquid gas or the like on the bottom of the sand 103. As a result, the sand 103 is instantly cooled below 0° C so that water on the surfaces of the grains of the sand 103 is frozen and ice layers are formed thereon. Figs. 7 to 9 show a concrete manufacturing apparatus according to still another embodiment of the present invention. This concrete manufacturing apparatus is different from that in Fig. 5 in that sand 103 supplied from a sand weighing device 105 is directly supplied to an aggregate hopper 107, and a concrete mixer 308 having an aggregate cooling apparatus is provided. In order to simplify the explanation, feeders for supplying cement and water used for mixing to the concrete mixer 308 are omitted. A heat-insulating material is preferably formed on, e.g., a wall surface of an aggregate hopper 107 to enhance the heat-insulating effect. The concrete mixer 308 is constituted by a tub-like drum 311, a substantially disk-like cover 312 for covering an upper opening of the drum 311. The cover 312 has a material charge port 312a through which each component of concrete is charged into the drum 311, while an aggregate discharge port 311a is formed in a bottom portion of the drum 311. A cylindrical rotor 313 rotatably and vertically extends through the drum 311 and is coaxial therewith. A plurality of arms 314 radially extend from an upper portion of the rotor 118, while scrapers 315 for mixing the sand 103 stored in the drum 311 extend from distal end portions of the arms downward. Reference numeral 316 denotes a motor for rotating the rotor 313. A drive shaft of the motor 316 is coupled to a pulley 317 arranged on a lower end of the motor 316. A V belt 318 is wound around the pulleys 317 and a lower circumferential surface of the rotor 313 to transfer a rotating force of the motor 316 to the rotor 313. Note that heat-insulating materials 319 are adhered to outer surfaces of the drum 311 of the concrete mixer 308 and cover 312 so as to enhance the heat-insulating effect of the mixer 308. The concrete mixer 308 is provided with an aggregate cooling apparatus 320 for cooling aggregate stored in the mixer 308 by spraying the liquid gas such as nitrogen gas. The aggregate cooling apparatus 320 is arranged on a lower side wall portion of the drum 311 of the concrete mixer 308. The aggregate cooling apparatus 320 includes nozzles 321 (only one nozzle is shown in Fig. 8) for spraying the liquid gas or the like inward from a bottom portion of the mixer 308, a moving mechanism for mounting the nozzles 321 on the concrete mixer 308 such that the nozzles 321 can extend or can be retracted with respect to the concrete mixer 308, and a cooler 323 for supplying the liquid gas or the like to the nozzles 321. The moving mechanism 322 is constituted by a box-like cylinder 324 arranged near the nozzles 321 and extending along a vertical direction of a wall of the drum 311, a piston 325 fitted to an inner surface of the cylinder 324 and having the nozzle 321 mounted on one end thereof, and a hydraulic jack 326 for sliding the piston 325 inside the cylinder 324. A nozzle hole 327 for spraying the liquid gas or the like is formed in a side wall of the drum 311 at a position opposite the nozzle 321 when the hydraulic jack 326 contracts. According to the above arrangement, the nozzle 321 communicates with the nozzle hole formed in the drum 311 of the concrete mixer 308 upon operation of the hydraulic jack 326. Similar to the drum 311, the heating material 319 is adhered to an outer surface of the cylinder 324. The cooler 323 is constituted by a liquid gas or cool air tank 328 disposed near the concrete mixer 308 or in another place, a known controller 329 for controlling a supply amount of liquid gas or the like supplied from the tank 328, and a supply pipe 330 for supplying the liquid gas or the like from the controller 329 to the nozzle 321. A high-pressure flexible pipe 331 is mounted midway along the supply pipe 330 near a pipe portion to which the nozzle 321 is mounted. Reference numeral 332 denotes an exhaust duct, formed on the cover 312 of the concrete mixer 308, for exhausting a gas derived from the liquid gas supplied into the concrete mixer 308 or cool air supplied into the concrete mixer 308 outside the system. The low-temperature gas or cool air exhausted from the exhaust duct 332 flows into the sand and gravel stockers 101 and 102 to pre-cool the sand 103 and the gravel 104. A method of manufacturing concrete using the concrete manufacturing apparatus with the above-described arrangement will be described below. The sand and gravel 103 and 104 are transferred into the sand and gravel stockers 101 and 102 using conveyors (not shown) in advance. When concrete is to be manufactured, the sand and gravel 103 and 104 are supplied from the sand and gravel stockers 101 and 102 into the sand and gravel weighing devices 105 and 106 so as to weigh the sand and gravel 103 and 104 according to a predetermined mixing ratio for the concrete. The weighed sand and gravel 103 and 104 are charged into the aggregate hopper 107, and then charged into the concrete mixer 308 from the material charge port 312a. When the sand and gravel 103 and 104 are charged, since the hydraulic jack contracts, the nozzle 321 is ready for spraying the liquid gas or the like into the drum 311 as shown in Fig. 8. In this state, the scrapers 315 inside the drum 311 are rotated in the mixer 308 by driving the motor 316, thereby mixing the sand and gravel 103 and 104 inside the drum 311. Subsequently, while the sand and gravel 103 and 104 are mixed, the controller 329 controls to supply the liquid gas or the like from the tank 328 to the nozzles 321 through the supply pipe 330 so that the liquid gas or the like is sprayed inward from the bottom portion of the drum 311 through the nozzle hole 327, thereby directly spraying the liquid gas or the like on the sand and gravel 103 and 104 from the bottom of the drum 311. As a result, the sand and gravel 103 and 104 are instantly and evenly cooled. A discharge rate of the liquid gas from the spray nozzles 321 can be arbitrarily set. A desired cooling temperature can be obtained in accordance with a type of material to be cooled by appropriately adjusting and selecting the discharge rate, a rotating speed of the scrapers 315 of the concrete mixer 308, and a time for cooling the sand and gravel inside the mixer 308. Thereafter, the nozzles 321 are moved to a lower position of the drum 311, as shown in Fig. 9, by extending the hydraulic jack 326, and then cement and water or particles of ice are charged into the concrete mixer 308. In addition, various admixtures are mixed with them as needed, thereby manufacturing concrete. Although a method of mixing these components of the concrete can be arbitrarily selected, it is preferable to charge the cement and water or particles of the ice into the concrete mixer 308 in the order named, and these components are mixed together, thereby manufacturing the concrete. These components may be simultaneously supplied into the concrete mixer 308. When the liquid gas is used for cooling the sand and gravel 103 and 104, the spray nozzles 321, which are located at a position where the liquid gas is not sprayed into the drum during mixing of the components as shown in Fig. 9, are kept: spraying the liquid gas or air to prevent the distal ends of the nozzles 321 from clogging and freezing. Similarly, as shown in Fig. 9, a spraying pipe 340 having an opening may be arranged near the distal ends of the nozzles 321 so that air having a room temperature or hot air can be sprayed on the distal ends of the nozzles 321 through the spraying pipe 340, thereby preventing them from clogging and freezing. Fig. 10 shows a modification of the concrete manufacturing apparatus in Fig. 7. In this modification, the aggregate supply port of the aggregate hopper 107 is branched into two ports, while the concrete mixers 308 are disposed under the ports, respectively. That is, the concrete manufacturing apparatus in this modification includes two concrete mixers 308. The aggregate is supplied from the aggregate hopper 107 to one or both of the mixers 308. The method of manufacturing concrete using the concrete manufacturing apparatus shown in Fig. 10 is the same as that using the apparatus shown in Fig. 9. However, since the concrete manufacturing apparatus in this modification includes a plurality (two) of mixers 308, concrete can be alternately or simultaneously manufactured using the concrete mixers 308, thereby improving productivity of concrete. More specifically, according to the method of manufacturing concrete, the step of cooling aggregate is added to the steps of manufacturing concrete. Therefore, if the concrete is manufactured by facilities having a size similar to that of a normal concrete plant, a cycle time of concrete manufacture is inevitably prolonged by the step of cooling the aggregate. However, by arranging a plurality of concrete mixers 308, high productivity of concrete which is equal to that of the normal concrete plant can be assured. The method of manufacturing concrete and an apparatus therefor according to the present invention are not limited to the above-described embodiments. Various changes and modifications can be made within the scope of the present invention. For example, the low-temperature liquid and air for cooling the aggregate (sand and gravel) are not limited to liquid nitrogen or the like, as described in the embodiments. If liquid helium having a low boiling point is used, the aggregate can be more efficiently cooled. Furthermore, in the embodiments, the sand and the gravel need not be simultaneously cooled, only the sand or the gravel may be cooled depending on a degree of drop in temperature of the concrete at the end of mixing. The forms and shapes of the vibrating chute system A, the aggregate mixers 112 and 212, and the concrete mixer 308 are not limited to those described in the embodiments. Known devices can be used in place of them. Example 1 Concrete was mixed according to the mixing amounts shown in Table 1 using the apparatus in Fig. 5. The temperatures of the components were: gravel temperature T g = 30° C, water temperature T w = 20° C, cement temperature T c = 60° C, and sand temperatures = -30° C, 0° C, and 30° C. The above sands, which had temperatures of -30° C and 0° C, were cooled by the apparatus shown in Fig. 4 using the liquid nitrogen. Fig. 11 shows a relationship between the sand temperatures and the temperatures of the concrete at the end of mixing when a surface water ratio S r of the sand was 0.0 wt.%, 5.0 wt.%, and 10.0 wt.%. It was confirmed in the experiment that the temperature of the concrete of 1 m³ at the end of mixing was lowered by about 1° C using the liquid nitrogen of 10 ℓ. Example 2 Sand having a temperature of 30° C was cooled by the aggregate cooling apparatus in Fig. 4 using liquid nitrogen. Fig. 12 shows a relationship between a water content γ m and an amount of liquid nitrogen used in the experiment wherein a temperature drop ΔT of the sand of 100 kg = 30° C, 50° C, and 70° C. It was confirmed in the experiment that the sand could be efficiently cooled using a relatively small amount of liquid nitrogen.

Claims: A method of manufacturing concrete by mixing concrete materials including a cement, aggregate, admixture and at least one of water and ice, including the steps of: agitating the aggregate prior to the mixing; and spraying the aggregate using a low temperature liquid in order to cool the aggregate to a predetermined temperature while the aggregate is being agitated, characterized in that the liquid is sprayed directly into the aggregate to pass therethrough upwardly so that surface water of the aggregate is frozen and ice layers are formed on surfaces of all the grains of the aggregate without fusing together of the grains. A method according to claim 1 wherein the low-temperature liquid is selected from a group consisting of liquid nitrogen and liquid helium. A method according to claim 1 or 2, wherein the predetermined temperature is -5°C to -10°C or below. A method according to any of claims 1,2 or 3 wherein the agitating step includes the step of agitating the aggregate by the gravity. A method according to any of claims 1, 2 or 3 wherein the agitating step includes the step of rotating the aggregate around at least one axis. A method according to any of claims 1 to 5, further comprising the step of adjusting the aggregate in surface water ratio prior to the agitating step to provide a predetermined amount of surface water to the aggregate. A concrete manufacturing apparatus for performing the method of claim 1, comprising a mixer (112) with a container (114) for aggregate, spraying means (127) for spraying low-temperature liquid into the aggregate contained in the container and agitating means (120) for agitating the aggregate whilst spraying said liquid, and supplying means for cement, admixture and water working after the aggregate have been agitated and cooled, the spraying means being located at a lower region of the container covered by said aggregate to spray said liquid directly into the aggregate to pass therethrough upwardly, said container (114) comprising a stationary drum (114) for storing the aggregate, and the agitating means, including a rotor (118), rotatably arranged around a shaft of said drum, for mixing the aggregate, rotating means (122) for rotating said rotor, and injection means (127), mounted on said drum, for injecting the low-temperature liquid into said drum, said injection means (127) including a low-temperature liquid injection port (327) formed in a lower portion of a side wall of said drum, a nozzle (321) for injecting the low-temperature liquid, and shift means (326) for selectively shifting said nozzle to a position to allow said nozzle to communicate with said injection port and to a position to allow said nozzle not to communicate with said injection port. A concrete manufacturing apparatus for performing the method of claim 1, comprising a mixer (112) with a container (114) for aggregate, spraying means (127) for spraying low-temperature liquid into the aggregate contained in the container and agitating means (120) for agitating the aggregate whilst spraying said liquid, and supplying means for cement, admixture and water working after the aggregate have been agitated and cooled, the spraying means being located at a lower region of the container covered by said aggregate to spray said liquid directly into the aggregate to pass therethrough upwardly, said container comprising a hollow cylindrical casing having a charge port (231) for charging the aggregate and a discharge port (232) for discharging the aggregate, a screw conveyor (235) rotatably arranged inside said casing and concentrical with said casing, and rotating means (238) for rotating said screw conveyor to transfer the aggregate from the charge port to the discharge port, and said spraying means (241) being mounted on said casing so as to spray the low-temperature liquid into said casing.

Inventors: GOTO SADAO, ISHIKURA DAISUKE, KAJIOKA YASUO, KAMEZAKI KAZUYA, KIMURA KATSUHIKO, KUWAHARA TAKASHI, MINEGISHI KOJI, NEGAMI YOSHIAKI, ONO SADAMU, OSHITA KENICHI, GOTO, SADAO, ISHIKURA, DAISUKE, KAJIOKA, YASUO, KAMEZAKI, KAZUYA, KIMURA, KATSUHIKO, KUWAHARA, TAKASHI, MINEGISHI, KOJI, NEGAMI, YOSHIAKI, ONO, SADAMU, OSHITA, KENICHI
Applicants: SHIMIZU CONSTRUCTION CO LTD, SHIMIZU CONSTRUCTION CO. LTD.
Application year: 1987
Main classification: B28C5
IPC codes: B28C5