Method and device for the production of concrete

Abstract

The water necessary for the production of concrete is replaced by a super-cooled, cooling transfer agent, comprising snow crystals. The cooling transfer agent is produced by mixing water with air and a cooling medium, such as liquid nitrogen, to give a cold gas mixture. The cold gas mixture is sprayed in the form of a cold gas stream. The snow crystals thus formed from the water are brought to a low temperature, of, for example, below minus 30° C. and added to the mixture of binding agents.

Description

[0001] The invention relates to a method and to a device for the production of concrete.

[0002] In order to avoid the formation of cracks and the occurrence of stresses in the construction of large concrete elements such as bridge abutments, concrete dams, pylons and the like, it is common practice to employ cooled fresh concrete for these structures. With this technique, the heat released during the hydration of the cement can be better compensated for and dissipated more evenly, thus avoiding stress differences in the concrete. Moreover, processing uncooled fresh concrete at outdoor temperatures above 25° C. [77° F.] is problematic because, at such high outdoor temperatures, the resultant peak temperature of the concrete while it is setting lies above 60° C. [140° F.], and this can lead to losses in strength. Cooling prevents this limit from being exceeded.

[0003] In order to produce cooled fresh concrete, it is a known technique to cool the already freshly mixed concrete to the desired temperature by adding chip ice or liquid nitrogen in the stationary or mobile mixer or in the rotary drum of a concrete-mix truck. A summarizing overview of the known cooling methods can be found in the articles titled “Kuhlen von Frischbeton mit flussigem Stickstoff” [Cooling fresh concrete with liquid nitrogen] by W. Trappman and W. Duesberg in the German publication “gas aktuell”, 25 (1983), page 15, and “Frischbeton mit flüssigem Stickstoff” [Fresh concrete with liquid nitrogen] by D. Rebhan in the German publication “Betonwerk+Fertigteil-Technik”, issue 8,1981, page 507.

[0004] When ice is employed as the coolant, there is a risk of inhomogeneities caused by water pockets in the concrete. Furthermore, the cooling effect is extremely limited since cooling chip ice to temperatures T<0° C. [32° F.] can only be achieved with a great deal of effort, and this is not economically warranted in most cases. Moreover, chip ice does not dissolve well and it prolongs the mixing times, an aspect that considerably reduces the production output, particularly in the case of mixers.

[0005] A drawback of cooling by means of cryogenic nitrogen (LN2) is the fact that this method is not very efficient since relatively large amounts of LN2 have to be fed into the batch in question within a short period of time in order to achieve the desired cooling effect. The poor efficiency of this cooling method lies in the fact that primarily only the evaporation heat of the liquid nitrogen can be utilized and that the surface area available for the heat exchange is limited. The fast evaporation of the nitrogen causes large volumes of gas to be released within the shortest of times, and this can lead to local explosions and ejections from the mixer. Besides, owing to localized critical sub-cooling, the fresh concrete can suffer frost damage.

[0006] JP 61201681 A1 suggests adding a liquefied gas, such as nitrogen or carbon dioxide, in order to cool down the starting materials prior to adding cement and water. This method, however, can only be employed with certain restrictions since the cooled medium has to be kept insulated against heat and moisture. Moreover, substances with a high moisture content cannot be cooled to temperatures below 0° C. [32° F.].

[0007] Therefore, the objective of the present invention is to put forward a method as well as a device for the production of concrete which avoid the disadvantages of the prior-art cooling methods and which particularly allow a cost-effective use of the coolant while concurrently providing a high cooling output.

[0008] This objective is achieved, on the one hand, by means of a method having the features cited in Patent claim 1 and, on the other hand, by means of a device having the features cited in Patent claim 8.

[0009] Thus, with the method according to the invention, either some or all of the water added to the binder mixture consisting of a binder, such as cement, and of aggregates, such as gravel, sand, fly ash and the like, is in the form of a previously prepared cold transfer agent made up of snow crystals.

[0010] The term “snow crystals” as employed throughout the text is to be understood as particles of frozen water that are generated in a cold atmosphere.

[0011] In comparison to chip ice, such snow crystals entail the advantage that they have a larger surface area while at the same time having a lower specific weight. As a result, the process heat generated during the setting phase is transferred much faster than with chip ice. Moreover, inhomogeneities due to water pockets can be largely avoided.

[0012] In a preferred method for generating snow or ice crystals, a cold gas is made from water, a propellant as well as a coolant and this gas is then sprayed in the form of a cold gas stream into a spraying chamber. This cold gas stream is at a temperature that is clearly below the freezing temperature of water. The water present in the cold gas stream freezes to form crystals that are subsequently admixed with a binder mixture in order to produce fresh concrete. The size, temperature and surface characteristics of the crystals are decisively determined by the composition of the cold gas and by the temperature of the cold gas stream. The production of crystals at temperatures well below 0° C. [32° F.] is likewise possible without any problems. In particular, at temperatures below −30° C. [−22° F.], the crystals formed exhibit especially good transport properties since, at these temperatures, microscopic domains of liquid water that could cause the snow crystals to adhere to each other are no longer present on the surface of the snow crystals. Therefore, the cooling action of the coolant is utilized to a far greater extent than in conventional cooling methods.

[0013] Advantageously, the cold gas stream is made to rotate. This lengthens the stream path in the stream chamber, thus achieving greater homogeneity of the crystals formed.

[0014] In order to quickly reach the target temperature of the fresh concrete, the cold transfer agent generated in the spraying chamber is subjected to an after-cooling procedure. When a suitable coolant is selected, for instance, liquid nitrogen, snow crystal temperatures as low as −190° C. [−310° F.] can be generated. A cryogenic gas is the preferred coolant for the production of the cold gas as well as for the after-cooling procedure. In this context, the use of liquid nitrogen or liquid carbon dioxide is particularly recommended from an environmental and cost standpoint.

[0015] Gaseous nitrogen is advantageously employed as the propellant for the production of the cold gas or of the cold gas stream. The use of nitrogen—which is itself hardly water-soluble—prevents oxygen from dissolving in the water.

[0016] The liquid coolant used preferably also serves to transport the cold transfer agent from the spraying chamber to the binder mixture. Thus, the cold transfer agent can be discharged virtually without compressing, clumping or altering the crystal structure of the snow or ice crystals.

[0017] The device according to the invention as cited in claim 8 comprises a mixing device in which water, a propellant, for instance, nitrogen, and a coolant, for example, liquid nitrogen, are mixed together to form a cold gas, said mixing device being flow-connected to a spraying device housed in the spraying chamber. When the cold gas is sprayed, its water content forms snow or ice crystals in the spraying chamber. The spraying of the cold gas into the spraying chamber gives rise to an inert and sub-cooled atmosphere therein and this promotes the formation of snow crystals having a large surface area and low specific weight. The cold transfer agent generated in the spraying chamber is fed to a mixing chamber, where it can be mixed together with a binder mixture to form fresh concrete.

[0018] Advantageously, the spraying chamber is associated with a cooling unit with which the snow created in the spraying chamber can be further cooled to a specified temperature. By choosing a suitable coolant in the cooling unit, temperatures of −30° C. to −190° C. [−22° F. to −310° F.] can be attained.

[0019] In an advantageous embodiment of the invention, the mixing device for generating the cold gas and/or the mixing chamber for making the fresh concrete are connected to a control unit by means of which the composition of the cold gas and/or of the fresh concrete can be set in accordance with a specified program. Aside from the geometry of the spraying nozzle, it is the composition of the cold gas that decisively determines the consistency and temperature of the generated cold transfer agent. The feed is regulated by suitable valves on the mixing device or mixing chamber that are operated by the control unit.

[0020] The drawing will serve to explain an embodiment of the invention in greater detail below.

[0021] The single drawing (FIG. 1) schematically shows the mode of operation of a device according to the invention for the production of fresh concrete.

[0022] The device 1 has a conventional mixing chamber 2 which receives the aggregates needed for the production of fresh concrete such as sand, gravel, fly ash as well as cement, and these are mixed to form a binder mixture Z. The mixing chamber can be, for instance, a mobile or stationary mixing installation. Instead of the water in liquid form employed in common production methods, a sub-cooled cold transfer agent S is added to the binder mixture in the mixing chamber 2; the preparation of said cold transfer agent will be described below.

[0023] In order to prepare the cold transfer agent S, water as well as gaseous and liquid nitrogen are fed in via the appertaining lines 4, 5, 6, then thoroughly mixed in a mixing segment 7 to form a cold gas mixture, and subsequently conveyed to a spray nozzle 8 situated in a spraying chamber 9. Instead of having a mixing segment of a certain length, the lines 4, 5, 6 can also open up directly into the spraying nozzle 8, which is then configured as a three-component nozzle for this purpose. Moreover, the invention is not limited to liquid nitrogen as the coolant for the production of the cold gas mixture, but rather, other known coolants, especially other liquefied gases, can also be employed for this purpose. Moreover, a different multi-nozzle system can also be used instead of a three-component nozzle.

[0024] By means of the spraying nozzle 8, the cold gas mixture is emitted in the form of a cold gas stream aimed at the interior of the spraying chamber 9, whereby the cold gas stream is made to rotate around its own axis in order to lengthen the stream path. The feeding of the cold gas stream causes a sub-cooled atmosphere to form inside the spraying chamber 9 already after a short time. The water contained in the cold gas stream freezes and precipitates inside the spraying chamber 9 in the form of snow crystals, i.e. the cold transfer agent S. The cold, inert atmosphere inside the spraying chamber 9 promotes the formation of crystals having a large surface area and a low specific weight. In this context, the size, consistency and temperature of the crystals are especially determined by the mixing ratio of gaseous and liquid nitrogen as well as water in the cold gas mixture.

[0025] In the embodiment presented, the snow crystals formed in the spraying chamber 9 are fed into an after-cooling unit 10, where the cold transfer agent S is cooled down further. The after-cooling unit 10 consists of a cooling chamber 11 for the material to be cooled, which is in thermal contact with a cold transfer agent 12. Within the scope of the invention, it is likewise possible to directly employ the spraying chamber 9 with the liquid nitrogen that has been fed into it as the cooling chamber 11 of the after-cooling unit 10 and/or to also use liquid nitrogen from the cold gas stream as a transport medium for the snow crystals. If the after-cooling unit 10 is operated with liquid nitrogen as the cold transfer agent 12, then temperatures as low as −190° C. [−310° F.] can be reached. The snow crystals can be transported very well at temperatures below −30° C. [−22° F.], for instance, −40° C. [−40° F.]. The cold transfer agent S thus cooled is conveyed to the mixing chamber 2, where it is admixed with aggregates and with cement in a known manner to form fresh concrete.

[0026] The large surface area of the snow crystals of the cold transfer agent S allows an effective and fast absorption of the process heat generated during the setting process of the cement. By varying the temperature and the amount of cold transfer agent S employed, fresh concrete temperatures as low as 0° C. [32° F.] can be reached.

[0027] An electronic control unit 13 allows the production of the cold transfer agent S or of the fresh concrete according to a specified program. The electronic control unit 13 is connected to actuatable valves 14, 15, 16, for instance, solenoid valves, in the lines 4, 5, 6, by means of which the mixing ratio and/or the appertaining pressure in lines 4, 5, 6 can be set. A control line 17 serves to regulate the temperature in the after-cooling unit 10. The feed line 18 for the cold transfer agent S to the mixing chamber 2 is likewise fitted with a solenoid valve 19 that can be actuated by the control unit 13. In this manner, the temperature, consistency and amount of the cold transfer agent S fed to the binder mixture Z can all be precisely and reliably set and, for example, selected in such a way that the fresh concrete made has a certain temperature, for instance, 0° C. [32° F.]. In order to maintain the specified temperature for the duration of the production of a batch of concrete, the temperature of the fresh concrete, which is continuously or regularly measured with an appropriate measuring device, is utilized as the manipulated variable to which the value is constantly regulated by setting the temperature of the added cold transfer agent S.

[0028] The method according to the invention makes it possible to convert the feed water into cold transfer agent S within a few minutes. Due to the large surface area of the snow crystals, the transfer of cold during the after-cooling procedure is very effective, so that this procedure, too, only takes a few minutes.

List of Reference Numerals and Letters

[0029] 1 device

[0030] 2 mixing chamber

[0031] 3 -

[0032] 4 line

[0033] 5 line

[0034] 6 line

[0035] 7 mixing segment

[0036] 8 spraying nozzle

[0037] 9 spraying chamber

[0038] 10 after-cooling unit

[0039] 11 cooling chamber

[0040] 12 cold transfer agent

[0041] 13 electronic control unit

[0042] 14 valve

[0043] 15 valve

[0044] 16 valve

[0045] 17 control line

[0046] 18 feed line

[0047] 19 solenoid valve

[0048] S cold transfer agent

[0049] Z binder mixture

Claims

1. A method to produce concrete, in which a binder, such as cement, is mixed together with aggregates to form a binder mixture (Z) and then fresh concrete is made by adding water to the binder mixture (Z),

characterized in that

the water that is added to the binder mixture (Z) is at least in part in the form of a cold transfer agent (S) consisting of snow crystals.

2. The method according to claim 1, characterized in that, in order to prepare the cold transfer agent (S),

water is mixed together with a propellant and a coolant to form a cold gas mixture,

the cold gas mixture is then sprayed in the form of a cold gas stream into a spraying chamber (9), a process in which the water freezes to form snow crystals.

3. The method according to claim 2, characterized in that the cold gas stream is made to rotate when it is sprayed into the spraying chamber (9).

4. The method according to claim 2 or 3, characterized in that, prior to being added to the binder mixture (Z), the cold transfer agent (S) is brought to a specified temperature of less than −30° C. [−22° F.] in an after-cooling process.

5. The method according to one of claims 2 to 4, characterized in that a cryogenic gas, for instance liquid nitrogen or liquid carbon dioxide, is employed as the coolant for the cold gas mixture and/or for the after-cooling of the cold transfer agent (S).

6. The method according to one of the preceding claims, characterized in that nitrogen and/or air is employed as the propellant for the cold gas mixture.

7. The method according to one of claims 2 to 6, characterized in that the coolant is employed to transport the generated cold transfer agent (S) to the binder mixture (Z).

8. A device to prepare concrete, comprising

a mixing device (7) to mix the water, the propellant and a coolant to form a cold gas mixture,

a spray nozzle (8) that is housed in a spraying chamber (9) and that is flow-connected to the mixing device (7), said spray nozzle (8) being used to spray the cold gas mixture so as to form a cold transfer agent (S) that at least largely consists of snow crystals, and

a mixing chamber (2) in which the cold transfer agent (S) generated in the spraying chamber (9) can be mixed together with a binder mixture (Z).

9. The device according to claim 8, characterized in that the spraying chamber (9) is linked to a cooling unit (11) for the after-cooling of the cold transfer agent (S) generated in the spraying chamber (9).

10. The device according to one of claims 8 or 9, characterized by a control unit (12) that is effectively connected to the mixing device (7) and/or to the mixing chamber (9), by means of which control unit (12) the temperature and/or the composition of the cold gas mixture and/or of the fresh concrete can be set.