The present invention relates to a method in accordance with the preamble of claim 1 for the transfer and metering of concrete mix.

Concrete mix is conventionally transferred at construction sites by means of a liftable bucket having a gate at its lower part that can be opened to meter a desired volume of concrete mix at the point of casting. Further, pumping of concrete into the point of casting has gained more frequent use, particularly when the casting operation consumes large quantities of concrete in an easy flowing form due to the large quantities of reinforcing steels used.

Other forms of concrete transport are rare, although in some factories making precast elements, the transport of concrete mix is performed by means of automatic equipment along rails in open chutes always containing premetered quantities of concrete. Due to the constant, premetered quantity of the concrete, the volume is almost invariably either too large or too small but very rarely exactly correct. Therefore, several metering steps and unnecessary transport operations are needed in a factory. Due to the insufficient control¬ lability of concrete mix metering, the planning of production is difficult and the transportation of small concrete mix quantities is expensive.

Pumped concrete may also be used at factories making precast elements in the same manner as at a construction site, whereby only such quantities of concrete mix would be metered from the pipe as necessary for each step. This transport method of concrete mix is associated with problems not solved by prior art methods.

The use of pumped concrete at a construction site in unobstructed space imposes no problems, because owing to the free upper space, the pumping pipeline can be easily transferred and lifted, and finally, the pipeline can be easily emptied and cleaned so that the concrete mix will not set in the pumping pipeline during transport and interruptions.

In factories of precast concrete products, the above- described method of utilizing pumped concrete has been impossible to apply, because the quantities to be cast and the spaces being available have been small; and further, the internal volume of the pipeline tends to become large due to the wide diameter and extensive length of the pipeline necessary for pumping, making a disproportionate volume of available concrete mix to be contained in the pipeline between the concrete mixing station and the casting site, whereby that proportion of the mix would be easily lost. Equally, due to the fact that only a small proportion of the concrete mix would be used in relation to the volume contained in the pipeline and at the concrete mixing station, the retention time of concrete mix in the pipeline would become long. Then, the concrete mix might set in the pipeline even when retarding admixtures would be added to the concrete mix.

It is an object of the present invention to provide a method which makes it possible to flexibly transfer concrete mix in a factory from the concrete mixing station to the production stations.

The invention is based on arranging the concrete mix to be pumped in the pipeline of the factory in a manner that provides for a continuous circulation of the mix. The mix recirculating in the pipeline is tapped at each point of casting for only a small portion in relation to the recirculating volume.

More specifically, the invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides significant benefits.

The disclosed invention makes it possible to achieve a cost-effective and flexible transfer method of concrete mix at a factory of precast concrete products, while simultaneously allowing the production of extremely small quantities of special mixes without waste. The disclosed method is obviously applicable to the transfer of large quantities, too, while here generally no problems exist in the fabrication and transfer operations.

According to the method it is possible to handle such mixes that are activated for extremely rapid hardening. Known from the literature and several patent publications is the accelerating effect of various alkaline compounds in all conventionally used cement grades as well as in ground iron slag. The method in accordance with our invention provides a particularly advantageous addition of such accelerating activators as admixtures just prior to the casting operation by means of a mixer/metering apparatus which is connected to the circulation system of the plain concrete mix in accordance with the invention, as well as the use of these admixtures such as water glass, soda ash, lime, silica, carbon dioxide among others for the setting and hardening of concrete. This feature is particularly important when using the method disclosed in this patent publication in the production of, e.g., front walls, whereby such architectural details as patterns, stripes, coloured areas and elevations can be cast in layers to a desired area of the precast front wall element.

Furthermore, the method is particularly applicable to production sites, where concrete mix is cast into small pieces in small series and in which the composition, colour, surface aggregate and other variables of the mix are changed at short intervals.

The method in accordance with our invention makes it possible to improve the automation level of a concrete products facility without compromising the flexibility of production. In factories of concrete products, the improvement of automation level is in general economically feasible only after the transfer and the hardening characteristics of the material have been brought to a level which allows the use of automation for an essential improvement of production capacity and profitability.

The proposed method, in which the concrete mix is continuously agitated and pumped, involves a continuous consumption of energy which may appear to be a drawback. We have in our tests, however, found that in counterweight to the energy consumption, the method uses less cement yet producing a higher-strength concrete than that available in conventional batch mixing. This result can be explained by the fact that the stone aggregate, sand and cement are intimately kneaded against each other during their continual mixing in the recirculation pipeline, whereby a good adhesion is formed between the stone aggregate and the cement paste and the mix achieves an extremely homogeneous composition owing to the long mixing time.

When the concrete mix is pumped in the apparatus described above, the pressure in the mix after the pump is at an elevated level of 30...70 bar, falling to ambient pressure level at the casting site. The solubility of air into the water of the concrete mix is increased by the elevated pressure reaching a value of

approx. 80...140 1/m at 32 bar pressure depending on the water content and temperature of the concrete mix. This air entrained in soluble form in the mix is released at the fall of the pressure, whereby the release of air always is initiated by any sharp edge of a small particle. The release of air is relatively slow, and the release of all air takes a few minutes from the pressure drop.

The disclosed method makes it possible to easily dissolve or entrap a suitable volume of air and to allow it to be released at the pressure fall in extremely small and evenly distributed microbubbles. Such a method provides for an increased porosity of the concrete without the use of additional air-entraining admixtures. Thus, the circulation pumping method is an advantageous way of achieving additional porosity, which is more homogeneous than the porosity attained by conventional methods provided that a suitable volume of air is compressed into the pressurized space.

The circulation pipeline in accordance with the invention is easy to keep clean by applying the so- called "pig" approach, in which the pipeline is brushed by continuously recirculating balls of a resilient material. In order to prevent the balls from escaping the pipeline along with the metering of the concrete mix, the exits of the discharge pipes can be formed into narrow, oblong slits having a width insufficient for the balls to penetrate.

By virtue of having a larger buffer container of concrete mix at one point of the pipeline for the addition of mix to compensate for the consumption, the method attains an extremely stable quality of the concrete mix in comparison to a mix produced in a batch mixer. Similarly, the aging of the mix prior to casting is homogenized, whereby a higher predictability of

mould stripping times is achieved in comparison to concrete mixes produced by conventional methods.

The invention is next examined with the help of the attached drawing. The drawing illustrates diagrammatically an embodiment of the method in accordance with the invention.

Shown in the drawing is a transfer system having three delivery points at the casting sites of products. The system comprises a container 1 having a funnel-shaped hopper 2, 3 at its lower part. The hopper 2, 3 has a pump 4 adapted to its lower part. Starting from the pump 4 is a recirculation pipeline 5, which returns via the delivery points to the container 1. The delivery points are connected to the recirculation pipeline 5 by branching unions 6, 8 and 9. The ends of the branching unions 6, 8, 9 are provided with mixer/metering means 7, 10, 11, to the ends of which are attached feed unions 12 of admixtures.

This invention is related to the application of patent PCT/FI87/00038. Said publication discloses a method capable of long-term storage of cement paste or concrete mix without setting. Furthermore, the publication discloses a method capable of providing a rapid stiffening of the mix after a period of storage at a required instant of time. The controllability of the setting instant of the delivered concrete mix makes it possible to utilize the transfer system of concrete mix provided by the present invention.

The method of concrete mix transfer at a factory of concrete products is applied as follows.

Together with water and a retarder, the concrete mix is stored in the feed buffer container 1. Owing to the use of a retarder, the mix can stay in the container 1 for

extended periods of time without setting. From the container l, the mix is pumped by means of the concrete pump 4 to the recirculation pipeline 5. The pressure of the circulated mix is elevated by the pump to a level of 30...70 bar as necessary for the operation of the system and the desired degree of air dissolution into the mix. The pump 4 recirculates continuously the concrete mix via the recirculation pipeline 5 back to the buffer container 1. The pipeline 5 is provided with branching unions 6, 8 and 9 at the delivery points. The end of each of the unions 6, 8, 9 is equipped with mixer/metering means 7, 10, 11, through which a desired quantity of the concrete mix can be delivered from the recirculation pipeline 5 into a casting mould or mixer. At any delivery point a maximum of 1/3 of the recirculated volume of the concrete mix can be tapped for use at the casting site according to needs. The rest of the concrete mix continues to be recirculated to the container 1 and further from there, via the pump 4 back to the recirculation pipeline 5.

The container l is filled with a ready-mixed batch of plain concrete mix whose volume is so large in relation to the combined quantity metered out at the delivery points that it corresponds to at least a half-hour consumption. This mix is then continuously circulated via the recirculation pipeline 5 and back to the container 1, whereby the mix becomes extremely effectively mixed. Due to the recirculation, also the virgin plain mix to be added to the container 1 will be mixed with the mix already placed in the container, thus ensuring a constant quality of the plain mix. Furthermore, continuous circulation prevents the mix from setting within the pipeline. The recirculation pipeline is kept clean with the help of resilient balls recirculated in the pipeline.

The homogeneous quality of the mix is additionally improved by the large volumes of the mix batches. Because the recirculation pipeline 5 is provided with a large-volume buffer container 1 of the concrete mix into which the amount of virgin mix corresponding to the consumed quantity is always added, the method provides an extremely stable mix quality in comparison with a mix produced in a batch mixer. If the mix consumption is 10 m ³ /h and the mix is produced in a batch mixer having a i m ³ mixing volume, while the accuracy of the metering means is, e.g., 3 kg for cement and 2 kg for water, the use of these same metering means in the method in accordance with the invention in conjunction with a 10 m ³ buffer container for prehydration results in a metering error of less than one-tenth of that of the batch mixer.

The present method makes it possible to provide a continuous availability of plain mix of concrete that can be quickly utilized at the casting site, yet providing a rapid hardening of the mix at a desired instant.

The concrete mix is complemented at the casting site by mixing into it with the help of a small-size auxiliary mixer a nonhydrated binder and possibly also accelerating agents such as Portland, rapid or aluminate cement or their forms of ground clinker; fine-grained silica, water glass, soda ash, lime, potash or other accelerators known in the art such as salt, calcium chloride, calcium formate or carbon dioxide that trigger the setting process and allow the hardening of the mix into the desired shape of the product. With the help of the same auxiliary mixer the mix can be complemented with colour pigments or different types of stone aggregates such as limestone, crushed marble and similar materials capable of providing colour effects or surface textures.

The operating volume of the small-size auxiliary mixer is smaller than 1 dm ³ and it has a self-cleaning ability activated as soon as the metering of the concrete mix is ended. A suitable mixer for the purpose is disclosed in parallel FI patent application filed by us under the title "Apparatus for the processing of mixes and pastes in particular". Because a part of the aggregates in the concrete mix such as reinforcing fibers increase the mix stiffness, thus causing difficulties in pumping, these aggregates are preferredly mixed into the plain mix at this stage by feeding them into the mixer/metering means 7, 10, 11 via the feed union 12. This method permits the production of extremely small casting batches of several different types of special mixes, or alternatively, to apply the techniques of continuous casting at a single operating point.

The described exemplifying embodiment has three branching unions 6, 8, 9 attached to the recirculation pipeline 5. It is obvious that the number of the metering points can be varied according to the product schedule and production volumes at the factory. When necessary, the number of buffer feed containers 1 can be increased if the recirculation pipeline 5 is long and the quantity of the needed plain mix is high. In such cases, however, the use of several shorter recirculation pipelines 5 is generally a more preferred solution. In the described exemplifying embodiment, the continuous recirculation of the mix is assured by tapping the recirculation pipeline 5 at each metering point maximally by one-third of the main mix flow recirculating in the recirculation pipeline 5. The total volume of the mix tapped simultaneously at all metering points may not exceed half, preferably not one-third, of the main mix flow recirculating in the recirculation pipeline 5. Alternatively, the total volume of mix consumed at the metering points can be

monitored with the help of, e.g., a computer to avoid the total volume of mix consumed at the metering points from exceeding not even temporarily half of the main mix flow recirculating in the pipeline 5. As a rule, it is preferred to consume a relatively small proportion, e.g., one-third of the recirculating main mix flow. Thereby, a continuous flow of the plain mix is assured in the pipeline. Provided that the mix consumption is monitored in this manner, one metering point can be allowed to consume a greater portion of the main mix flow during the time the other metering points have a period of low consumption, whereby the consumption of the main mix flow is easy to optimize.