Patent Application
20050219939
I. Field of the Invention
The present invention relates generally to concrete batching operations and, particularly, to advances in equipment, and in a method of processing or batching the ingredients used to produce concrete mixes. Specifically, this invention involves a pre-mix system which blends the wetting agents and the cementitious ingredients in relation to each other at any selected ratio in an adjustable and repeatable manner. The pre-mixer is a twin screw, preferably counter-rotating, pre-mixer agglomerator unit for pre-mixing these materials prior to combining them with aggregates in a drum of a transit mixer truck or other final mixing vessel.
II. Related Art In a typical concrete batching operation, all the ingredients are pre-measured and then all the ingredients are transferred to a mobile concrete mixing truck for mixing and transport to job sites remote from the sources of the concrete ingredients. In some batching operations, all the ingredients may be transferred to a pre-mixer, which is a permanent part of the batching operation, before being transferred to a mobile concrete mixing truck or other receiving vehicle.
Two important design criteria for concrete are (a) the rheological flow properties of fresh concrete, and (b) the compressive strength of the concrete as measured 28 days after the beginning of the hydration (hardening) process. Flow properties of concrete are typically measured by filling a 30 cm high conical cylinder with freshly mixed concrete. The conical cylinder is then removed, leaving the now conically shaped freshly mixed concrete, freestanding. The vertical distance that the concrete then drops or “slumps” corresponds to the flow property of the concrete and is known as slump. The compressive strength of concrete is typically ascertained by the load failure of concrete cylinders cured for 28 days. Strength is measured in psi (pounds/square inch) or MPa (Megapascals).
Pre-mixing of the water and cementitious materials prior to bringing them together with the aggregates is known to offer several advantages. These advantages include, but are not limited to:
In recent years, attempts have been made to design equipment that would pre-mix the water and cementitious materials as part of the batching process before combining them with the aggregates. Such devices have been only partially successful.
One such approach has employed vortex-type mixers. Vortex mixers in some ways resemble home blenders. They include a large open-face pump at the base of each unit and a drain valve at the base of the pump which is situated above a charging hopper of a transit mixing truck as a final mixing vessel. The cementitious materials, water and some of the admixtures are introduced into the top of the vortex mixer. The ingredients are blended and thereafter, the valve at the base of the pump is opened and the mixed materials are transferred to the mixer truck where they are combined with aggregates. However, these units are limited to mix designs where the water/cement ratios are relatively high: 0.38 or greater. This may be higher than allowable for mixes designed to achieve low water/cement ratios. When this occurs, additional dry cementitious material must be added, handled separately from the rest of the cementitious material that is being blended in the vortex mixer, and charged directly into a truck. This is inefficient and may result in dusting problems.
Another device that has been used is a mixing tube employing a single screw mixing auger. In the single screw mixing auger, cementitious materials can be delivered to the mixing auger by various known methods. A water injection manifold is used to introduce the liquid materials into the cementitious materials as they are being conveyed through and by the screw auger. This type of pre-mixing device has had limited success due to an inability to overcome a variety of shortcomings which include:
As is the case of the vortex-type mixers, facilities using these units must also make provisions to handle additional dry cementitious material separately from the pre-mixed cementitious material and supply it directly into a truck or other final mixer vessel.
Thus, the escape of amounts of cementitious material in the form of airborne dust has been a common problem. Accordingly, a great deal of interest has been generated with regard to reducing the escape of airborne particulate matter associated with the preparation of batches of concrete.
It is well known that concrete having a relatively low amount of water in the homogenized mix produces a stronger product than one having a higher amount of water. Sometimes, however, additional water must be used in order to produce a mix that can be pumped at a job site without difficulty. In any event, the ability to precisely control the relative amount of water added to a batch of concrete and thus, the resulting slump of the concrete in the batch produced is quite important. In this regard, the existence of a pre-mixer having the ability to pre-mix cementitious ingredients and wetting agents in ratios below 0.38 would be highly desired.
Thus, there remains a definite need in the concrete batching field to provide a concrete batching facility that includes a pre-mix arrangement that provides an accurate system to measure and control both the wetting agents and the cementitious ingredient feeds in relation to each other in any proportion so as to blend the ingredients in a known and repeatable manner over a relative wide range of ratios of wetting agent to dry ingredients.
By means of the present invention, there is provided a concrete batching system pre-mix arrangement that includes a controlled ingredient supply aspect to measure and control both wetting agent and cementitious ingredient feeds in relation to each other so as to achieve a blending of these ingredients in a known, selected, adjustable and repeatable manner that can be used to produce any desired water/cement ratio that can be therefore optimized for each mix design.
The term “cementitious” as used herein is defined to include Portland cement, fly ash silica fume and any other dry components, not including aggregate materials (sand and stone). The term “wetting agents” as used herein is defined to include water with or without other additive ingredients.
Central to the ingredient supply and pre-mixing systems of the concrete batching system of the invention is an enclosed twin screw pre-mixer agglomerator chamber which is fed the cementitious materials by a cement weigh batcher using a metering screw conveyor device and is fed one or more wetting or agglomerating agents via a liquid metering system which controls both rate and total amount of wetting agent for a batch. The metering system supplies a manifold which is provided with a plurality of spaced spray nozzles situated to infuse the liquid along a portion of the agglomerator mixing chamber. The pre-mixer agglomerator is designed to be charged with dry, cementitious ingredients at an inlet end and to discharge the agglomerated or blended materials at a discharge port during normal operation.
Mixing and material conveying in the pre-mixer agglomerator vessel is accomplished by a pair intermeshing, preferably counter-rotating, screw conveyors or augers mounted for rotation in the chamber. The augers are of varying pitch in which threads or flights of relatively fine pitch, which together act as baffles, at an input end control the feed rate to a central mixing section and also prevent material build up in that area. Coarser pitch threads provide a very aggressive and efficient kneading/squeezing mixing action and strongly convey the material through a central mixing section to specially designed discharge scoops or paddles that propel mixed material out through the discharge port or outlet at the bottom of a discharge end which is opposite to the inlet end. For the purposes of this specification, pitch is defined to mean the distance between successive convolutions of the thread of a screw conveyor or auger relative to the diameter of the auger. The terms “screw conveyor” and “auger” are used interchangeably herein.
The supply system and the construction of the pre-mixer agglomerator vessel and the mixing screw conveyors or augers allows any water/cement ratio to be selected and apportioned and mixed in the pre-mixer agglomerator. The pre-mixer agglomerator chamber is provided with a discharge chute designed to discharge mixed material into a collecting hopper which, in turn, leads into the input or charging hopper of a mobile concrete mixing truck or other receiving final mixing vessel located beneath the collecting hopper.
The batching system is designed so that the aggregate material (generally sand and stone) is measured and provided separately and fed directly through the collecting hopper to the input hopper of the mobile mixing truck or other final mixing device and is not mixed in the agglomerator.
In a preferred embodiment, counter-rotating full auger flights are used in the twin screw compulsory mixer of the pre-mixer agglomerator and, as previously indicated, they are divided into three distinct sections. The first is an inlet or receiving section that includes a short section of twin shaft counter rotating screw segments of relatively narrow or reduced pitch (such as one-quarter pitch or one-third pitch) which results in relatively small inter-flight or successive convolution gaps to regulate the delivery of cementitious materials from the discharge of a metering screw pre-feeder to the receiving or input section of the pre-mixer agglomerator and eliminate build-up in this area.
This is followed by an agglomerating or mixing section which consists of an extended length in which the twin shaft counter rotating agglomerating segments have a pitch greater than that of the inlet section (such as one-half or two-thirds pitch). This insures that the material fed from the inlet section does not completely fill the cavity of the agglomerating section thereby promoting improved mixing. Metered wetting agents are introduced into this section from a pattern of spaced nozzles located in the top of the chamber. The third and final section is a discharge section that consists of a short section of counter-rotating paddles or flat-pitch scoops that serve to eject the blended materials out of the agglomerator.
The screw pre-feeder accurately regulates the feed rate of cementitious material to the agglomerator. It is preferably a variable speed feeder which also uses reduced pitch segments (such as one-half or one-third pitch) in conjunction with multiple (double or triple) segments to create a labyrinth that eliminates the tendency of the finely divided fluidized cementitious materials to flow around and through the feeder. If desired, the system may include a by-pass line to enable the direct feed of dry powdered cementitious material through the metering screw and the agglomerator section directly into the collecting hopper to the inlet hopper of a mobile mixing truck or other final mixing vessel.
While the preferred arrangement incorporates overlapping counter-rotating twin screws that produce more vigorous kneading/squeezing mixing, a further arrangement in which both overlapping screws rotate in the same direction is also contemplated and can be used if desired.
In the drawings wherein like numerals are utilized to depict like parts throughout the same:
a and 1b are a schematic elevational views depicting a typical concrete batching facility utilizing the mixing system of the invention;
a is an enlarged fragmentary schematic elevational view of a portion of the batching facility of
b is a partial side view of components in
a is a schematic top view of one embodiment of a twin-screw agglomerator-mixer in accordance with the invention;
b is a schematic side elevational view of the agglomerator-mixer of
c is a schematic end view of the agglomerator-mixer of
a is an enlarged bottom perspective representation of an alternative embodiment of a pair of assembled counter-rotating intermeshing segmented mixing screws suitable for use in the pre-mixer agglomerator of the embodiment of
b is a perspective view of a typical removable mixer auger segment from one of the screw conveyors of
There follows a detailed description of certain embodiments which are presented as examples which capture the essence of the invention but these representations are in no way intended to be limiting with respect to the scope of the invention as it is contemplated that other embodiments using the concept will occur to those skilled in the art. For example, the concept may be used to treat other dry ingredients in other processes having flow and mixing characteristics commensurate with or similar to dry cementitious materials and wetting agents.
a and 1b are elevational views of a portion of a concrete batching facility, generally represented generally by 20, incorporating an agglomerator-mixer system in accordance with the present invention. The batching facility includes a primary Portland cement silo 22, a second silo 24 which may also contain Portland cement or other finely divided dry cementitious ingredients such as fly ash, which are typically also included in concrete mixes. An aggregate bin as shown at 26 which may have compartments containing sand and coarse stone.
Silo 24 is provided with a bottom discharge gate system 28 that is connected through gate valve 30 to a covered conveyor 32 which, in turn, discharges into a covered cement weigh batcher at 34 through a chute 36. In a similar manner (and as best seen in
The silos 22 and 24 are elevated and suitable supported on heavy steel support structures 42 and 44. A surge tank 52 is used to supply water to be mixed with the dry ingredients from the silos 22 and 24, as will be described. A collecting hopper system is shown at 54 which receives aggregates and pre-mixed cementitious material to load into a mobile mixing truck 56 via a charging hopper 58.
The aggregate bin 26 is further divided into sections addressed by mechanized swiveling loading chute 60 as at 62, 64 and 66. Chute 60 is fed normally by conveyor (not shown) which discharges material through a receiving vessel 68 and can be rotated to address any of the sections which may optionally contain sand or different sizes of coarse stone or other aggregates. The bin sections 62, 64 and 66 are provided with discharge gates 70, 72 and 74, to discharge the aggregates into a weigh hopper 76 to be discharged through gate 77 on to a loading belt conveyor 78 equipped with belt rollers 80. Belt conveyor 78 carries and discharges material into the collecting hopper 54 for direct loading into vehicle 56 where final concrete mixing occurs. The aggregate bin is also supported in an elevated disposition by a heavy structural steel support framework 82 which may be fixed to the adjacent support structure 42 to add stability to the system. A facility such as schematically shown in
a and 2b represent enlarged fragmentary schematic views of a portion of the batching facility of
Pre-feeder 90 further includes a first or normal metered feed or discharge outlet as shown at 108 which may contain an outlet shutoff valve (not shown) and which is connected by flexible conduit or chute 110 to a first inlet 152 in the pre-mixer agglomerator 100 utilized for charging dry ingredients from the metering screw of pre-feeder 90 into the pre-mixer agglomerator 100 to be mixed. This is further known as the inlet or feed end of the pre-mixer agglomerator. A further discharge arrangement 114 is provided in the metering screw 90 positioned directly below the inlet from the cement batcher suitably valved at 116 and which is connected by a flexible conduit 118 with a second or by-pass inlet 156 which is located in the pre-mixer agglomerator 100 at a point directly above discharge port 154 of the pre-mixer agglomerator with chute 157 for mixed ingredients or direct feed so that dry ingredients from the cement batcher 34 alternatively on occasion can be fed directly into the hopper 54 by-passing the metering screw system 90 and the pre-mixer agglomerator 100. More detailed aspects of the twin shaft counter-rotating agglomerating screw conveyor embodiments are discussed below.
a-3c depict one embodiment or form of an pre-mixer agglomerator 100 in accordance with the invention which includes a housing, generally at 150, a first top inlet opening 152 located toward one end of the top of the pre-mixer agglomerator 100 and a discharge opening 154 located on the bottom toward the opposite end of the pre-mixer agglomerator from the inlet opening 152 such that intended mixing takes place therebetween. The by-pass discharge arrangement 114 is aligned with by-pass inlet opening 156 to allow straight through feed of dry ingredients as discussed above.
As will also be discussed with regard to the several example embodiments of twin-screw or twin-auger conveyors, the pre-mixer agglomerator 100 is designed to pre-mix and blend the cementitious ingredients and the liquid ingredients in a known, selected, adjustable and repeatable wetting agent/cement ratio that optimizes the desired production and strength of the concrete of the mixture for each mix design. The pre-mixer agglomerator 100 is characterized functionally by three sections, namely, an inlet metering section 158, a mixing section 160 and a discharge section 162. Water or wetting agent infusion nozzle locations are shown at 130 in
A pair of generally parallel intermeshing twin screw conveyors 164 and 166 having corresponding steel shafts 168 and 170 are mounted for rotation within the housings 172 and 174 using suitable corresponding bearings 176, 178, 180 and 182. Shafts 168 and 170 are coupled to a suitable drive mechanism with intermeshing gears (not shown) so that the intermeshing screw conveyors 164 and 166 coordinate to counter-rotate at the same speed.
The water or wetting agent supply system includes four basic components. These include surge tank 52 which preferably is designed to hold enough water to produce a minimum of 1-½ batches of concrete or about 300 gallons (1272 liters). A means of refilling the surge tank (not shown) is provided with sufficient capacity to refill the surge tank 52 by the time the next batch is to be started. The system further includes a pump 120 of sufficient capacity to deliver liquid wetting agent (normally water with or without additives) to the agglomerator 100. A liquid wetting agent flow control valve 122 is provided and is one that is programmable with linear flow characteristics together with a computerized control system (
A water meter 124, which is provided with both digital and analog outputs, is also provided to measure the wetting agent supplied in two ways. The first output from the water meter is a discrete digital output which preferably produces one electronic impulse per gallon of liquid wetting agent being delivered to the agglomerator. These impulses are counted by the computer. When the total amount of water required for the batch in process is reached, the flow control valves 122, 123 and 125 are closed by the computer ending supply for that batch. The second output is a continuous analog output which is proportional to the rate at which the liquid wetting agent is flowing through the meter 124. The computer uses this output to control the setting of the water flow control valve 122 in such a manner as to deliver the liquid wetting agent to the pre-mixer agglomerator 100 at the specified ratio to the cementitious materials that are being delivered at the same time to the agglomerator by the screw feeder.
Thus, the desired water feed rate can be set in a controlling computer or CPU (See 500 in
The intermeshing, counter-rotating mixing screw conveyors of
With respect to the counter-rotating twin screw conveyors themselves, of course, it is apparent that they can be constructed to enmesh as either top converging or bottom converging combinations. In this regard, the preferred arrangement for optimum mixing in the pre-mixer agglomerator of the present invention involves configuring the twin screw conveyors as an arrangement where the flights rotate to converge together at the top so that material is slung down and away from the water inlet openings and, at the discharge end, toward the outlet. It should be noted, however, that the mixing efficiency itself is essentially equivalent either using a top or bottom converging arrangement. The advantage of the top converging arrangement, as stated, includes both prevention of buildup around the water inlet jets 130 and improved discharge of mixed materials.
In addition, as seen in
The arrangements in
a shows a schematic bottom view (or bottom diverging arrangement) of an alternate embodiment of a twin screw design for the pre-mixer agglomerator 100. That embodiment utilizes counter-rotating twin intermeshing screws 300 and 302 which are carried by shafts of a non-round (illustrated as square) shapes as at 304 and 306, respectively. The screw conveyors or augers 300, 302, themselves, include a plurality of removable auger segments or flights 308, which are sequentially slipped onto the shafts 304, 306. One of the segments is illustrated in
As illustrated in
As with those previously described, the assembled intermeshing, counter-rotating mixing screw conveyor 340 of
An important aspect of the present invention resides in a pre-mixer agglomerator design which allows mixing of dry cementitious ingredients and wetting agents in any proportion. This enables any desired wetting agent to cementitious ingredient ratio to be selected for a given batch. Unlike previous devices of the class, however, the pre-mixer agglomerators of the present invention are particularly well suited to process mixtures of relatively low wetting agent ratios which optimize high strength, low slump concrete mixes.
In the case of Portland cement and fly ash blends, it has been found with mixers in accordance with the invention that with wetting agent to cementitious ingredient ratios of less than 0.1:1 by weight, the powder is partially moistened into small rice-like beads surrounded by an amount of dusty powder indicating that the cementitious ingredients are not totally wet. At ratios in the range of about 0.1:1 to about 0.25:1, the mixture is quite flowable and there is very little dustable powder to become airborne. This is one range or realm of mixing in which the agglomerator-mixers of the invention work quite well. In any event, it is important that the agglomerated material remain free flowing in a dry sense, the water being apportioned accordingly.
Pre-mixer agglomerators in accordance with the invention, generally, are designed to operate at constant speed (although that speed can be varied if desired). The twin shaft rotating screw conveyors are specially designed for blending cementitious or other finely divided dry materials (usually Portland cement and fly ash) with liquid materials (usually water and various chemical additives) to form a pre-mixed material with a water/cement ratio that is generally designed to optimize the production and the strength of concrete produced from the mixture. The pre-mix is later combined with coarse aggregates (usually stone and sand) in the production of Portland cement concrete.
According to an aspect of the invention, as indicated in the description of representative types of twin-screw mixers, the agglomerator has been characterized as being generally divided into three distinct sections. These include an inlet section which consists of a short section of twin shaft counter rotating screw feeder segments of relatively fine or reduced pitch (such as from about one-half pitch to about one-fourth pitch) to regulate the delivery of cementitious materials from the discharge of the pre-feeder to the mixing section of the agglomerator. The mixing or agglomerating section consists of an extended section of twin shaft counter rotating agglomerating segments with a pitch greater than that of the inlet section (such as from about one-half pitch to about two-thirds pitch).
The pitch of the mixing section is made greater than that of the inlet section to ensure that the material conveyed from the inlet section does not completely fill the cavity of the mixing section. This ensures that there is sufficient empty space in the flights of the mixing or blending section to promote aggressive kneading/squeezing mixing of the cementitious ingredients and the liquids into a pre-programmed blend ratio of fully mixed material. The discharge section includes a short section of twin shaft, preferably counter-rotating scoops or paddles to help eject the blended materials out of the vessel.
Wetting agents are preferably not applied in the inlet or outlet sections to avoid undesirable buildup of materials at the inlet end of the conveyors. Clogging and material buildup has long been a problem with single screw systems which continually throw material radially away from the screw in all directions. It should also be noted with regard to single screw systems that mixing is less efficient and streaks of dry cementitious material occur generally throughout the mixture indicating a non-uniformity in combining ingredients.
Likewise, according to another aspect of the invention, the metering screw pre-feeder 90 is provided with reduced pitch segments (such as one-half or one-third pitch) in conjunction with multiple (double or triple) segments to create a labyrinth that eliminates the tendency of fluidized cementitious materials to flow around and through the feeder in an uncontrolled manner. This solves previous problems associated with attempts to use a screw pre-feeder to closely meter or regulate the feed rate of cementitious materials to a mixing or blending unit due to the fluidized nature of cementitious materials when flowing from one container such as a weigh hopper to another such as screw feeder making measurement regulation difficult.
As indicated, the most preferred arrangement of the design of the agglomerator employs intermeshing counter-rotating screws and imparts a very aggressive kneading/squeezing mixing action and strongly conveys the material through the mixing chamber and out of the outlet. As a result, it is capable of thoroughly mixing and conveying any ratio of water to dry powder materials making it possible to determine and control any selected, and preferably an optimum, water/cement ratio for each mix design and to operate the pre-mixer agglomerator at this ratio.
Components for operating the system are shown in
After all the cementitious ingredients have been emptied from the cement batcher, metering screw and agglomerator the water feed will continue until the total amount of water called for has been delivered, as determined by the digital water input from 506, at which time the CPU will close the water valves 122, 123 and 125 at 514, 520 and 522 turn off the water pump 120 at 512. This serves to flush the mixing and outlet sections of the agglomerator and to clean the spray nozzles. Control lines for water valves 123 and 125, which may be solenoids, are shown at 520 and 522.
In addition, if desired, the agglomerator can be by-passed with material directed from the weigh batcher through the screw-metering device into the inlet hopper of a mobile cement mixer directly through a connecting tube (not shown) by by-passing the metering device through a by-pass gate 116 through the outlet section of the agglomerator and directly into the collecting hopper 54 and into the inlet hopper 58 of a transit mixer truck. Valve 125, of course, can be used for direct injection of wetting agents.
The pre-mixer agglomerator augers or screw conveyors themselves may be constructed of any suitable materials including metals and non-metals and combinations thereof. Thus, the screw flights may be steel, steel coated with a polyamide material such as a nylon material or a polyurethane material or the like. They may be molded to the shafts using a relatively stiff composite elastomer material. It is desired that the flights resist abrasive wear and remain easily cleaned.
This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.
