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A Portable concrete mix batching plant

2009-09-01 16:35   Article Source:N/A   View Times:

A portable concrete mix batching plant capable of producing up to 240 to 300 cubic yards or more of concrete per hour is disclosed in which two hauling trailers, a mixer trailer and an aggregate trailer, both serve as the transport vehicles and as foundation for the completely erected plant.

concrete mix batching plantThe aggregate trailer includes three bins used respectively for sand, fine aggregate and course aggregate. Weigh belts convey weighed and humidity measured amounts of sand and aggregate from discrete open top bins to a batch aggregate loading belt for batch loading of a compulsory mixer on the mixer trailer. The side of the aggregate trailer is provided with a bulkhead for an earthen or gravel ramp with large width bins enabling a single loader to supply the high volume of sand and aggregate required. The mixer trailer supports a twin shaft compulsory mixer on a foundation pad which is top loading for concrete constituents and bottom emptying to an elevating and concrete discharge point to awaiting trucks. Pivotally mounted to the mixer trailer from a horizontal transport disposition to a vertically erect operating position are a multi-compartment air loaded, gravity feed cement silo, and weigh hopper for batch weighing of concrete and discharge to the underlying compulsory mixer. Provision is made for the axles located at the rear of the trailer to steer so that precision joining of the mix batching plant components can easily occur.

Description:

This invention relates to a portable and modular mix batching plant or concrete, soil cement, cement stabilize base course and the like. Specifically, a two trailer portable plant is disclosed including a mixing trailer and an aggregate trailer. This two trailer plant is complete with aggregate storage, water tank, cement silo, admixture tanks, dust collection system and generator set. The two trailer configuration enables assembly and disassembly to and from the transport mode with three to four men in one 8 hour shift to enable portable mix batching plant assembly and disassembly time and transport to match that of a portable slipform paver. Thus, there results a highly portable and high capacity 240 to 300 cubic yard per hour or more mix batching plant results which can deliver uniformly mixed, low moisture soil cement, and cement stabilized base courses or low slump (low moisture content) concrete.

Background of the mix batching plant

In the discussion that follows, the prior art is set forth in terms of the need for this invention. It is to be understood that we claim invention both in the recognition of that need as well as the solution that follows.

Modern concrete paving practices impose more severe constraints on concrete quality every year. Specifically, concrete when freshly mixed is tested and measured for different desired qualities and standards pursuant to imposed and specified quality control standards. These standards include moisture content (or slump), both compressive and flexural strength after a prescribed number of days, aggregate shape, air content, and uniformity, to name a few. If the quality standards of the concrete produced vary statistically above or below the prescribed standard mean, then the concrete producer is penalized financially.

Exemplary of these standards would be concrete compressive strength where the concrete strength is to reach say 3,500 psi in 28 days. The specification might allow a variation of this standard of 5% above or below this mean or the contractor would be penalized.

It is generally agreed that higher strength concrete can be reached in a shorter period of time by better mixing action and lower water/cement (W/C) ratios. Thus the lower the concrete slump, the easier it is for the contractor to reach the specified strengths. The trend in the industry is toward lower W/C ratios. Low W/C ratio concrete mixed in conventional tilting drum mix batching plants do not reach uniformity as quickly as the mixer used in this invention.

The cost of the concrete makes up the majority of the cost of the road being built. Given the large volumes of concrete processed in such paving contracts, supervisory and specifying authorities such as state and federal inspectors can only statistically sample the loads of concrete to determine the quality of the concrete delivered by the contractor. Because of the large quantity of concrete that can be produced by the contractor in a day, the contractor faces great financial risk if many days pass before he realizes the concrete he is producing is testing outside of specification mean. The above example is intended to show how important it is for the contractor to maintain quality control on the concrete he produces. It is imperative that the contractor use batching and mixing equipment capable of delivering uniformly mixed concrete of the low slump variety to precision construction specifications.

Modern paving practices also call for the use of slipform pavers, which in operation consume relatively large amounts of concrete. On a typical urban size paving job, where the total cubic yards of concrete to be used on the job is relatively small, a modern paver can consume concrete in the range of 240 to 300 cubic yards per hour. On larger jobs the contractor may choose to mobilize, produce and deliver concrete to the slipform paver at a higher rate with a larger plant with higher capacity. Exemplary of such a paver is that Slipform Paver sold under the designation of model S850 built by Guntert and Zimmerman of Ripon, Calif. The fundamental design of this model was pioneered by the late Ronald M. Guntert, Sr. of Stockton, Calif. as set forth in U.S. Pat. Nos. 4,493,584 and 5,135,333.

Other more recent examples of pavers consuming high volumes of concrete can be found in U.S. Pat. No. 5,590,977 entitled Four Track Paving Machine and Process of Transport by Ronald M. Guntert (herein) et al. And U.S. Pat. No. 5,615,972 entitled Paving Machine with Extended Telescoping Members by Ronald M. Guntert (herein).

As cement in the concrete starts to hydrate during transport to a paving site, portable concrete mix batching plants have been developed for mixing concrete adjacent the paving site. This reduces the hauling distance to where the concrete is being used and to reduce the number of concrete hauling units required. Simply stated, from a plant which mixes concrete to the site where such mixed concrete is placed, most contract specifications set a time limit of 30 minutes for non-agitating trucks, which is about a 12 mile transport limit. This practical transport limit is reduced in high traffic areas or other situations where the average speed at which the hauling unit can travel is reduced. If the time limit is exceeded, the concrete that is hauled will start to set before the paver places it and the paver placed concrete will not meet the required contract standards.

Secondly, and given the high quality constraints placed on the paved and/or placed concrete product, so-called continuous flow concrete batching plants have proven inadequate. Such plants are capable of delivering large volumes of concrete batching but do so on a continuous flow basis. The exacting standards of thorough mixing covered by precise constituent proportion make the continuous flow adjustment of such plants hazardous from the quality control standpoint. As a result, such continuous flow concrete batching plants have not been accepted in modem paving practice, at least in the North American paving market. It is only the processing of specific "batch" quantities of cement, water and aggregates that constitute concrete that enables the relatively high quality requirements to be maintained and conventional calibration and quality assurance measures to be used.

Prior art portable modem batching and mixing concrete plants are large, require concrete foundations and are difficult to erect, often consuming three to five days in assembly. Frequently, these plants require special rigging equipment, such as cranes to accomplish erection. Specifically, it is not uncommon for such plants to occupy 3 or more (sometimes as many as 7) transporting trailers. Further, such plants utilize rotating and tilting drum mixers located high overhead so they can tilt and gravity feed the mixed concrete into the hauling units. The mix batching plant itself is belt fed with aggregates that are gravity fed through weighing hoppers to maintain precise concrete constituent proportions. This produces several undesirable features, which complicate the erection and subsequent operation of such plants:

First the feeding belt is usually gravity fed from overlying storage bins and weighing hoppers. Thus, considerable weight must be supported at substantial heights from the ground on such portable plants. Using weighing belts instead of weighing hoppers is novel in the U.S. for mixing concrete. In order to load the overlying storage bins that cannot be reached directly by a front end loader, separate charging conveyors with charging bins are used for each aggregate and sand. The charging bins are at an elevation that can be reached by a front-end loader. Because of the requirement of these charging conveyors and bins, the plant site required is quite large limiting the number of places the plant may be set up.

Second, such rotating drums must be tilted, and in a few cases, reversed in rotation for discharge. This tilting superimposes a moment requirement upon the weight support requirement of the rotating drum. As a result of the weight and moment requirements, most so-called portable concrete mix batching plants require concrete foundations. Further, in a few cases, reversing the mixing drum rotation not only interrupts mixing, but also consumes momentum, and utilizes heavy reversible drives.

Third, because the rotating mixer drums are supported high in the air, if the more desirable gravity feed of cement is used with the rotating mixer drum, the cement silo must be elevated even higher in the air. The resulting silo and structure requires concrete foundations. To save height, and in lieu of gravity feed from the silo to the cement batcher, many manufacturers of conventional concrete batching plants use cement screws or air slides to convey the cement into the mixer. Most contractors agree these cement-conveying schemes are undesirable although many times tolerated to minimize the silo height.

Four, because tilting drum mixers are open in front for discharge and open in the back for loading the concrete constituents into the mix batching plant, it is very difficult to suppress the dust that results from the loading operation. The inability to adequately suppress the dust coming out of the mixers limits the use of the plant in many urban settings.

Five, because the tilting/rotating drum mixer rotates on rollers, can be driven by chain drives or gearbox driving gear on drum. The mixer drum is essentially open during the mixing process. As a result, these conventional mixers are very noisy which limits the use of this plant in many urban settings because of the high decibel readings produced.

Six, conventional mix batching plants are highly specialized. A contractor will own one plant for his jobs requiring concrete production of 200 to 300 cubic yards per hour and another complete plant when his concrete production needs are 400 to 500 cubic yards per hour. Generally, the larger the plant production capacity per hour the more cumbersome the plant is to transport, set-up and tear down.

Finally, rotating/tilting drum mix batching plants are relatively slow in delivering desired amounts of thoroughly and uniformly mixed low slump concrete, base courses and soil cement. Rotating/tilting drum mixer has paddles affixed to the rotating drum wall. The limitation of this design is that dry material bridges in the mixer and does not discharge out of the drum readily. Moreover, when cement substitutes are used such as slags, the concrete tends to be sticky which again impedes rapid discharge. With low slump concrete or soil cement, this problem is amplified. As compared to contemporary twin shaft, compulsory batching plants now utilized in Europe, longer mixing cycles are generally required for the same material in rotating/tilting drum mixers. With low slump or difficult mix designs, rotating/tilting drum mixers produce less than thorough mixing with resultant "ribbons" of less than homogeneously mixed concrete when compared to a compulsory mix batching plant. As a result, considerable additional mixing time or "dwell time" of the concrete in the rotating/tilting drum mixer is required resulting in fewer loads of concrete being produced in an hour.

It should be understood that so-called compulsory mixers are now in use in Europe and in limited use in North America for mixing soil cement. These mixers include a top loading, parallel rotating shafts with interval and paired counter-rotating paddles, and a bottom discharge feature. In the past, such compulsory mixers have been used in the European market where the total transport envelope allowed is small when compared to North America. As a consequence, such compulsory mix batching plants have not been adapted to high volume portable concrete mix batching plants used in North America. A plant of the dimensions of this invention would not have been conceived for the European market (or other markets which have adopted European transport standards) because it would not be allowed to be transported on the highways. It should also be noted that the majority of the compulsory mixers used in North America today are foreign made and are used in a continuous mixing process as opposed to a batch mixing process that this invention utilizes.

In understanding the background of this invention, attention should be directed to the practical consequences of having long erection times for portable concrete mix batching plants. First, modern slipform pavers can be moved to a new paving site and set-up within one working day (when short transport distances are involved, transport and set-up of the slipform in a day is feasible). Second, current "portable" concrete mix batching plants require between three and five days for an equivalent move with 300 to 400 man hours being devoted to each set-up and tear down. The practical result of the time differential between the movement of the slipform paver and the movement of the current so-called portable mix batching plant is interesting to understand.

Taking the case of roadway paving of a four lane divided highway, both directions of traffic are diverted to one side of the highway while concrete placement, paving and curing occurs on the opposite side of the highway. Traffic must be maintained while rehabilitating the concrete road. Curing of newly placed concrete on a highway occupies up to 28 days before traffic is allowed on the highway. There is a considerable interval of time where the nearby mix batching plant--required to be nearby to reduce the transport interval--will normally remain idle given the total time interval for plant moving. Moving requires 3 to 5 days to set up and 3 to 5 days to dismantle. Thus the decision is frequently made to leave an erected plant idle and in place for paving the opposite side of a highway. Considered from the standpoint of the contractor, the operating hours of a current portable batch plant are about half the operating hours of modern slip form pavers. Stated in other terms, the contractor must either own an additional mix batching plant or lose the opportunity to use the slip form paver in performing other work. Given modern capital requirements (including about $850,000 for a "portable" batch plant and $600,000 for a modern slipform paver), neither alternative is desirable.

Finally, there must be considered the dimension of the North American road transport envelope used in Canada, USA, Mexico, and Australia. Maximally, transported loads over high quality highways are normally limited to trailer vehicles having less than 85 feet length overall, 13 feet 6 inches in height, and under 12 feet in width. It will thus be immediately understood that in producing a high capacity batch plant, the size of the transport envelope works against the design. While relative size is not normally a consideration in determining invention, in what follows transport envelope size is a critical design factor in the design of the two trailer transportable, high capacity concrete mix batching plant of this invention.

SUMMARY OF THE INVENTION

A portable concrete mixing plant capable of producing up from 240 to 300 cubic yards or more of concrete per hour (depending on the mixing time required) has two hauling trailers. These trailers are a mixer trailer and an aggregate trailer. Both serve as the transport vehicle chassis and the foundation for the completely erected plant. The aggregate trailer includes three bins (up to four if the trailer length can be legally transported in the state of use), these bins being used respectively for sand, fine aggregate and course aggregate. Weighing belts, one located at the outlet of each bin, convey weighed and humidity measured (if so desired) amounts of sand and aggregate from open top bins to a batch aggregate loading belt for batch loading of a compulsory mixer on the mix batching plant trailer. The side of the aggregate trailer is provided with a bulkhead for a relatively low earthen or aggregate ramp with large width bins enabling a single loader of high capacity to supply the high volume of sand and aggregate required. The aggregate trailer also holds two to three admixture tanks, the dust collection filter for the cement weigh batcher/hopper, a 5,000 gal water tank and the operator batching control cabin for the entire plant. The mixer trailer supports a twin shaft compulsory mixer on a foundation pad which is top loading for concrete constituents and bottom emptying to an elevating and concrete discharge point to awaiting trucks. Pivotally mounted to the mixer trailer from a horizontal transport disposition to a vertically erect operating position are a multi-compartment air loaded, gravity feed cement silo, and weigh hopper for batch weighing of cement and discharge to the underlying compulsory mix batching plant. The dust collection system for the cement silo is built into the top of the silo. The mixer trailer contains a large diesel generator set, which supplies the required power requirements for the plant and the cement storage guppies. The plant is capable of erection from the transport mode to the mixing mode in less than 8 hours by a three to four man crew without the need for a crane or concrete foundations. Provisions are made for rear wheel steering of the trailers so that precision joining of the plant components can easily occur. This trailer rear wheel steering ability also allows these long trailers to maneuver more easily around corners in confined urban areas and tight radius ramps on highways without scuffing the tires.

Because the plant is modular, the addition of another mixer trailer is possible. Further another ramp bulkhead may be placed on the opposite side of the aggregate trailer. The aggregate bins can be loaded from either or both sides. This modular concept also allows a second mix batching plant trailer to be added to supplement the first mixer trailer. By adding one additional mixer trailer, a contractor can significantly increase the capacity of his plant, delivering uniformly mixed low slump concrete at a rate of over 400 cubic yards per hour depending on the required mixing time to achieve uniformity. This higher concrete plant capacity is sometimes required to be competitive on certain larger highway and airport paving projects. This feature eliminates the need for a contractor to own one complete plant for concrete production in the 240 to 300 cubic yards per hour range and another separate plant for projects requiring concrete production in excess of the 400 cubic yard per hour range.

OBJECTS AND ADVANTAGES OF THE PLANT

An object of this invention is to provide a high capacity portable batch or concrete batching plant, which can be moved in just two transporting trailers. The two trailers fit within a North American transport envelope where the trailer is less than 85 feet in length, 13 feet six inches in height, and less than 12 feet in width.

A further object of this invention is to provide a portable high capacity concrete plant, which can be moved with approximately the same speed as a modern slipform paver. The disclosed plant can thus follow the paver and supply the paver with concrete from a plant site within a relatively short distance from where the paver is working (normally within a 12 mile or 40 minute concrete transport interval.) The closer the concrete batching plant is located to the paver, the fewer concrete hauling trucks are required on the road. This allows the plant to provide the contractor with higher utilization.

An additional object of this invention is to provide a concrete plant that can be erected from the transport envelope or disassembled to the transport envelope in one shift by three to four men without the necessity of assisting erection rigging such as cranes and the like. The plant also eliminates the need for concrete foundations.

An additional object of this invention is that both the mixer and aggregate trailer are provided with steerable air ride axles that allows the plant to be transported at higher transport speeds. These axles allow the trailers to be maneuvered more easily in the tight confines of a roadside plant site or confined urban paving location. Finally, once the plant is on site, the steerable axles allow precision joining of the aggregate trailer to the mix batching plant trailer. Without these axles, a trailer of this length would require multiple and time consuming attempts to join the two trailers together.

An additional object of this plant is to create a modular plant, which can be reconfigured. This reconfiguration can be customized to match the contractor's needs and for minimizing or optimizing the size of the plant site required. In the standard configuration, the plant is arranged with the two trailers oriented 90 degrees to each other. The two trailers can also be arrange side by side or end to end with the addition of auxiliary belts so aggregates can be fed into the mixer. This may be necessary if the plant site is small or the only available site dimensions are limited.

Another object of this invention is to provide a concrete mixing plant that will be accepted in the market unlike the continuous mixing plants. In a continuous plant, cement, water, aggregates and sand are weighed continuously and delivered to the compulsory mixer, which mixes and discharges continuously. This is unconventional and presents a difficult challenge to monitor and assure the concrete quality. The plant of this invention differs from a continuous mixing plant in that it has all the advantages of a continuous plant. These advantage include low profile, and highly transportable. At the same time, the plant is easy to set-up and tear down. The erected plant batches and weighs the cement, water, aggregates and sand and delivers them to the compulsory mix batching plant which mixes these components in a batch in a conventional manner.

It will be noted that this plant uses weighing belts in lieu of using a weigh batcher hopper to weigh each batch of aggregates and sand discharged from the storage bins. Although weighbelts are unconventional for concrete batching plant (and more commonly found on an asphalt plant), they are as accurate as batch weighing of aggregates yet allow a significantly lower profile plant. Of course, as previously described, this plant mixes the batch of concrete constituent materials in a compulsory mixer in lieu of a tilting drum type mixer.

Another important object of this invention is because conventional weigh/batch bins of the aggregate and sand bins have been eliminated by using weighing belts, this invention's lower height storage bins can be reached by a front end loader with the aid of a ramp. This feature eliminates the loading bins and loading conveyors thus improving mobility and speeding up erection or disassembly time. The width and capacity of this invention's bins allow a single loader operator to feed sufficient material to the plant to sustain the design production capacity. If increased plant throughput is desired, the aggregate trailer can have a second ramp bulkhead on the opposite side of the bins so a second front-end loader can be used. For example, such a second loader can be dedicated to filling the sand bin while the other loader concentrates on charging the aggregate bins.

Another advantage of this invention is that the enclosed mixer design is quieter and produces less dust than a conventional plant. This allows this plant to be utilized in a more urban setting. Because of this plant's design, it is more environmentally friendly than conventional plants. It is a quieter plant to run than conventional plants thus making it more practical to use in urban areas where high decibel readings could prevent a plant from being used. Because of the plant enclosed mix batching plant design, dust problems are significantly less than a conventional plant where open tilting mixing drums allows dust to escape into the environment.

An advantage of the disclosed design is that for the first time, a high capacity batching and mixing concrete plant is disclosed. This plant can be set up and torn down in 1/3rd to 1/5th the time of a conventional concrete batching plant of the same capacity and can follow a modem paver with the same transport time, erection time and tear down time. In short, the disclosed portable concrete plant is capable of both following and supplying its slipform paving counterpart in timing and capacity resulting in tandem economic packaging control for the paving contractor.

An additional object of this invention is to be able to mix a greater variety of concrete material mixtures at various slumps and consistencies.




Respective topics: Concrete maker cements growth with fundin   Concrete Batching Plant   Batching Equipments  
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