FR2570639A1 - Methods and devices for accelerating the setting of concrete - Google Patents

Abstract

The subject of the invention is methods and devices for accelerating the setting of concrete. A device according to the invention comprises a concrete station equipped with vertical silos 1, 2, 3, 4 in which the aggregates used in the concrete mix are stored. The lower part of these silos is equipped with heating elements 5 which are inserted through the mass of the aggregates and which are distributed in staggered rows with each element separated from its neighbours by a distance of approximately 500 mm. Each element 5 has a power of the order of 1.5 kW. The aggregates are heated during the night preceding their use to a temperature of the order of 50 DEG C +/- 5 DEG C. The mixing water is heated in electrical water-heaters to approximately 65 DEG C. The mixed concrete is poured into the moulds at a temperature of the order of 50 DEG C and may be removed from the mould after 4 hours of self-thermomaturing without any other additional energy. One application is the equipment of factories for precasting concrete elements.

Description

 Methods and devices to accelerate the setting of concrete.

 The present invention relates to methods and devices for accelerating the setting of concrete.

 The technical sector of the invention is that of the construction of structures and precast concrete elements.

 One of the problems in concrete element prefabrication factories, as well as on certain concrete construction sites, is the acceleration of the setting of concrete to reduce the waiting time required before demolding or formwork and to accelerate the rotation of formwork or molds.

 In most factories for the production of reinforced concrete or prestressed concrete, concrete sets are accelerated by heating the molds into which the concrete has just been poured. This heating operation is carried out either by means of heating molds which require expensive equipment, or by passing the molds in heating ovens or tunnels, which requires heating equipment and handling of large loads.

 Sometimes the molds are heated in place by covering them with a tarpaulin under which steam is injected.

 The setting of concrete is a hydration of cement which is an exothermic reaction whose speed increases with temperature.

 The temperature of a concrete part being set in a mold or in a formwork tends to stabilize at an equilibrium temperature when the heat released by the exothermic reaction balances the loss of calories. This equilibrium temperature and the speed at which it is reached depend on the ambient temperature, the shape of the part, the insulation of the mold and the initial temperature of the concrete.

 We know that it is necessary to wait two days to remove the form if the setting takes place at 200C, 12 hours if it takes place at 400C and only 3 hours if it takes place at 60 "C.

 In the traditional process, the concrete is heated during setting, so that calories from the outside add to the heat due to the exothermic hydration reaction. However, one should not heat too quickly to avoid creating internal tensions which would reduce the strength of the concrete.

 Generally, the temperature is raised from 200 per hour to 600C and this temperature is then maintained, which makes it possible to remove form after 4 or 5 hours and to reuse the molds during the following work station.

 Traditional techniques use a lot of energy to heat the concrete after it has been poured into the molds.

 In order to reduce energy consumption, a method of preheating concrete in an electrode bucket has been proposed.

 According to this process, the concrete once kneaded at room temperature is heated to a temperature of the order of 500C, thanks to electrodes which are immersed in the liquid concrete contained in the bucket, then it is poured hot in the molds and its temperature is then maintained at a temperature between 500C and 600C for a solid part poured into a thermally insulated mold thanks to the heat due to hydration, which makes it possible to form after 4 to 5 hours of setting and to re-use the molds at reason 8 hour shift rotation.

 This process results in energy savings and handling of the molds but it requires a high electrical power because it is necessary to heat the concrete in a few minutes before pouring it into the molds and this consumption of electricity takes place during working hours therefore at the most expensive rate.

 An object of the present invention is to provide methods and devices for accelerating the setting speed of concrete by reducing energy consumption.

 Another object of the present invention is to provide methods and devices which make it possible to accelerate the setting of concrete by using off-peak electricity as an energy source in order to benefit from the relatively reduced cost of this energy and the ease of use thereof.

 Another object of the present invention is to provide methods and devices which make it possible to pour already hot concrete into molds or formwork, without having to make significant modifications to existing concrete plants and without having to interpose installations and new operations in a prefabrication chain.

 Another object of the present invention is to provide methods and devices which make it possible to pour into the molds or into the usual formwork a concrete sufficiently hot for the phenomenon of self-thermomatization of the concrete to start immediately, that is to say -to say that the temperature is maintained at an equilibrium value of the order of SOC at 600C, such that it can be removed from the mold or stripped after four hours of setting.

 The objectives of the invention are achieved by a process according to which, before kneading the concrete components, one or more of these components are heated to a temperature such that the kneaded concrete is poured at a temperature of the order of SO0C.

 Preferably, only the kneading water is heated to a temperature of 650C + 30C and the aggregates to an average temperature of 500C + 50C.

 According to a preferred embodiment, the components are heated by means of electrical resistors during the night hours.

 According to a preferred embodiment, the aggregates contained in the lower part of the silos equipping a concrete plant are heated.

 Advantageously, the volume of aggregates which is between 1.1 times and 2.2 times the daily consumption is heated in the lower part of each silo, during each night preceding a working day.

 Advantageously during the night preceding a working day, a first volume of aggregates slightly higher than the daily consumption is heated in the lower part of each silo at an average temperature of 50 ° C. and 50 ° C. and the medium part of each silo is preheated a second volume of aggregates which corresponds substantially to a daily consumption at an average temperature of the order of 35 "C + 50C.

 A device according to the invention is part of a concrete plant equipped with vertical silos in which the aggregates used in the composition of the concrete are stored.

 According to the invention, said aggregate silos are equipped, in their lower part, with heating resistances which are immersed in the mass of the aggregates and which are arranged in staggered rows with intervals between resistances of the order of 500 mm.

 According to a preferred embodiment, each heating resistor is placed inside a steel tube which passes through said silo right through, the rear end of which is closed and is engaged in a socket welded to the rear wall of the silo and the front end of which, which is located outside the front wall of the silo, has one or more orifices.

 According to a preferred embodiment, the heating resistors have the shape of a pin and the two branches of said pin are clamped in metal clips which are distributed over the length of pin and which can slide freely inside said tube.

 The invention results in the acceleration of the setting of the concrete making it possible to unmold or to release the mold approximately four hours after pouring, with a reduced energy expenditure.

The amount of energy consumed per ton of concrete, taking
3 accounts for losses, is around 30 KWH / m3, or 12.5 KWH / ton, while energy consumption with hot shuttering
3 fairies per lost steam is around 250 KWH / m3 and gold
3 dre of 1DO K / m, in the case where molds heated by a recycled heat transfer fluid are used.

 The amount of energy consumed is of the same order of magnitude as that consumed in the process of preheating concrete in skips with electrodes, but compared to this process, the present invention has the advantage of using electricity. during off-peak hours, therefore with a much lower cost and without any additional operations or installations in the prefabrication line.

 The only modification is the equipment of the silos with heating resistance aggregates, the possible thermal insulation of these silos and the need to have large capacity electric water heaters to heat the mixing water.

 The energy saving results from the fact that the auto-thermomaturation of the concrete begins as soon as it is poured into the molds at a temperature of the order of 50 ° C and the temperature is maintained above 500C throughout the maturing without any external energy input, the only energy expenditure is that which is consumed during off-peak hours to heat aggregates, water and possibly cement.

 Hot mixing water has already been used in very cold countries to avoid the risk of freezing of the concrete, that is to say, to perform a function other than accelerating setting by self-thermomatization.

 In these previous applications, the temperature of the kneaded concrete is much lower than 50 ° C., so that the setting of the concrete does not start with a sufficient speed to obtain rapid hardening by self-thermomatization. The function of heating the water is therefore different and the process according to the invention does not clearly follow from these previous applications.

 An important advantage of the methods and devices according to the invention resides in the fact that they require only relatively limited modifications to the concrete plant equipping construction sites or a prefabrication plant, that they eliminate any handling of molds filled with concrete. and that they also eliminate the need for a boiler to provide a hot fluid to heat the poured concrete to speed up setting.

 Compared to the traditional technique of heating concrete in ovens or in molds heated by steam or hot water, the methods according to the invention make it possible to accelerate the rotation of the molds and they eliminate the need to have a boiler, which saves equipment and labor and cleaner working conditions.

 They improve the mechanical strengths of concrete thanks to better temperature control during maturation and the absence of a thermal gradient in the mass. They improve the safety of working conditions by eliminating the risk of accidents due to the use of steam or hot water.

They eliminate gaseous and liquid pollutant discharges (boiler fumes, purge water, fuel oil).
The following description refers to the accompanying drawings which show an embodiment of a device according to the invention.

 Figure 1 is a horizontal section of a group of four vertical silos of a concrete plant.

 Figure 2 is an elevational view of a silo in Figure 1.

 Figure 3 shows an elevational view of the same silo perpendicular to the previous one.

 Figure 4 is an axial section and Figure 5 a cross section of a heating resistor fitted to an aggregate silo.

 A method according to the invention can be applied in all prefabrication factories or on all sites equipped with an automatic concrete plant comprising one or more concrete silos, and one or more silos in which the sand and the gravel used in the composition of concrete and hereinafter referred to as aggregates.

The method consists in preheating at least part of the concrete components before mixing, this preheating being carried out, preferably, by means of electrical resistances during the night preceding a work station, in order to benefit from an hourly rate. hollow. As it is heated to a relatively low temperature, of the order of 500 ° C. to 650 ° C. and which can be heated during the whole period of off-peak hours, the installed electrical power necessary for this heating is relatively low and the cost of the electric subscription, which depends on the installed power, is moderate,
According to a preferred embodiment, only the mixing water and the aggregates are heated and the cement is avoided. Alternatively, however, the cement can also be preheated.

 The mixing water is heated in heating tanks of the electric water heater type, having a capacity equal to or slightly greater than the consumption of a working day. It is heated during off-peak hours during the night preceding its use.

 The heating temperature is regulated to around 650C. The construction of the water heater and the regulation of the water temperature are well known to those skilled in the art and need not be described in detail. We will only note the choice of the temperature around 650 C. Taking into account the specific heat of water which is five times higher than that of cement and the respective proportions of water and cement, which are for example of 190 liters of water for 420 kg of cement, by heating to 650C + 30C the water which is then mixed with the cement, one obtains a temperature of the mixture close to 5O0C, without the need to equip a cement silo heating means.

 According to a characteristic of a process according to the invention, the aggregates used in the composition of the concrete are also preheated and this preheating is carried out by means of electrical resistances, which are embedded in the mass of the aggregates contained in the silos of the plant. to concrete, so that this preheating is carried out without requiring any additional handling or any new installation other than the installation of heating resistors in existing aggregate silos.

 It is also possible to insulate the walls of these silos to reduce the loss of calories, which are very low given the relatively low temperature.

 The aggregates are heated during off-peak hours at night preceding the use station to obtain an average temperature of around 500C + .50C.

 Preferably, the aggregates are preheated, in the middle part of the silo, to an average temperature of the order of + 350C during the penultimate night preceding their use, so that the temperature has time to become uniform during the day before use, then they are brought to an average temperature of 500 + 5 "C during the second night.

 Heating the aggregates poses problems that a device according to the invention had to solve. In fact, aggregates are made of materials that are poor conductors of heat. In addition, the gravel has many voids between the grains which prevent heat exchange.

 It is therefore not possible to heat aggregates by means of a few high-power resistors, embedded in the mass because there would then be a strong accumulation of heat around the resistors and the temperature of the heating resistors would reach values that these do not can bear. In addition, the temperature of the aggregates would not be uniform, which could create internal stresses in the concrete.

 The accompanying drawings show a device according to the invention for heating aggregates.

 FIG. 1 represents a view in horizontal section of a group of four vertical silos 1, 2, 3 and 4 which are part of an automatic concrete plant and which contain one of the sand and the others of gravel or gravels of determined particle size.

 Preferably, silos 1, 2, 3 and 4, which have a square section, are grouped back to back, so that each silo has two dividing partitions with two neighboring silos, which has the advantage of halving the external surface of the silo and therefore also the loss of calories.

 Each silo has a capacity which corresponds to the consumption of several days. For example, each silo has a square section of 3 m x 3 m and a height of the order of 10 m.

 The lower part of each silo is equipped with horizontal heating resistors 5 which pass right through the silo.

In order to distribute the heat in the mass of the aggregates, the number of resistors which are arranged following a mesh of the order of 50 cm is multiplied. Preferably, the resistors 5 are staggered. Gloves 6a, 6b are placed in the mass of the aggregates and these gloves contain a temperature measurement probe.

 As can be seen in Figures 2 and 3, the heating resistors 5 are arranged in the lower part of the silos.

 Generally, a silo of this type is equipped with three level probes, a low level probe 7, a middle level probe 8 and a high level probe 9. The low level probe is placed at a height such that the volume of the silo located below this level corresponds to one day of consumption and the mid level probe to two days of consumption. In this case, the heating resistors are arranged in the entire volume below the middle level 8 and they are divided into two groups separated by the low level.

 The group of resistors located between the low levels 7 and the medium level 8, is used to preheat the aggregates during the penultimate night preceding their use. The amount of heat supplied to them is calculated to obtain an average preheating temperature of the order of 350C + 50C. A thermostatic probe 6a makes it possible to regulate the temperature. In fact, as a result of the low thermal conductivity of the aggregates, the temperatures obtained at the end of the night are dispersed around the average value but during the following day, they equalize.

 During the second night, the aggregates are lowered into the volume located between the middle level 7 and the bottom of the silo and this volume is equipped with a second group of heating resistors, controlled by a second thermostatic probe 6b, which are put in service during the second night and which heat the aggregates to an average temperature of the order of 5O0C + 50C.

 Although the aggregates are heated only during off-peak hours overnight, the temperature remains substantially constant throughout the following day, since the ratio between the external surface and the volume of aggregates is low, the temperature of the aggregates is relatively low. and they are poor conductors of heat.

 During the working day following the second night of heating, the aggregates contained below the low level 7 are used, which are at an average temperature of 50 "C + 50C.

 By mixing these with cement at room temperature and with water at 65 "C., after mixing, a concrete is obtained which is poured into the molds at approximately 50 ° C. At this temperature, the concrete self-thermomatization takes place. produced without requiring any other energy input.

 The losses through the walls of the molds are compensated by the heat given off by the exothermic reaction of hydration of the concrete and the temperature is maintained above 50 "C, so that the release can take place after four hours of maturation.

 In addition to the savings in energy and handling, the method according to 1 invention has the advantage that the temperature of the concrete remains substantially uniform throughout the maturation, throughout the thickness of the prefabricated parts so that there is no risk that internal tensions arise under the effect of temperature gradients and the mechanical strengths of the concrete after demolding are very good.

 Figures 2 and 3 show a preferred arrangement of heating resistors which equip a silo volume substantially equal to twice the daily consumption of aggregates.

Of course, we can adopt other arrangements and heat only in the lower part of each silo a volume corresponding to a little more than daily consumption.

 According to a characteristic of the invention, the volume of each heated silo is between 1.1 and 2.2 times the daily consumption.

 To heat the aggregates in a substantially uniform way, it was necessary to determine the calorific power and the particular arrangement of the heating resistors 5.

 FIGS. 4 and 5 respectively represent an axial section and a cross section of a resistor 5.

 Each resistor is composed, in known manner, of a pin 10 composed of a conductive wire 11 placed in a metal sheath 12 which is, for example, a sheath made of stainless steel.

 A powdery material 13 which conducts heat and insulates electrically, fills the sheath 12. The power of each heating resistor is of the order of 1.5 KW.

 Given that the resistors are distributed in a mesh of 500 mm on one side and that the silos have a width of 3 m, this corresponds to an installed power of 2 KW per tonne of aggregates, so each resistance can supply during the eight hours hollow of a night station 16KWH per ton of aggregates, which corresponds to a temperature rise of around 60, taking into account the losses.

 Each heating resistor 10 is placed inside a mild steel tube 14 which passes right through the silo.

The rear end of the tube 14 is sealed off by a plate 14a and it is engaged in a tubular sleeve 15 welded to the rear wall 16 of the silo. The tube 14 has for example an external diameter of 90 mm, so that the contact surface between the tube and the aggregates is sufficient to ensure good heat exchange and that the tube 14 has sufficient bending strength to support the thrust of the aggregates . The front end of the tube 14 emerges outside the front wall 17 of the silo and carries a square flange 18 on which is fixed a solid flange 18a and the head 19 of the heating resistor.

 The tube 14 has one or more holes 20 which are located in the part of the tube external to the silo in order to prevent the air contained in the tube from risking pressure under the effect of 1 heating.

 It can be seen in FIGS. 4 and 5 that the two branches of each heating pin 10 are placed between the two branches of a clamp 21 which are tightened against the sheaths 12 by a bolt 22 and a nut 23.

 The clamps 21 are made of stainless steel. They are distributed over the entire length of needles 10 at the rate of a pliers every 50 cm approximately. The length of the clamps 21 is such that they slide freely in the tube 14 with little play. The clamps 21 serve to center the heating resistors in the tube 14 and they also serve as a thermal bridge to transmit by conduction to the tube 14, the heat given off by the resistors 10 which are in the air. Given the dimensions and the relatively low power of the heating resistors, they emit around 0.5 Watt / cm2 whereas it is commonly accepted to dissipate 2 to 7 Watts / cm2 of resistance surface.

Claims (10)

R E V E N D I C A T I O N S  1. Method for accelerating the setting of concrete, characterized in that, before kneading the components of the concrete, one or more of these components is heated to a temperature such that the kneaded concrete is poured at a temperature of the order of 500 .  2. Method according to claim 1, characterized in that only the mixing water is heated to a temperature of 650C + 3 "C and the aggregates to an average temperature of 500C + 50C.  3. Method according to any one of claims 1 and 2, characterized in that lton heats said components by means of electrical resistors during the night hours.  4. Method according to any one of claims 1 to 4, characterized in that lton heats the aggregates contained in the lower part of the silos equipping a concrete plant.  5. Method according to claim 4, characterized in that one heats in each silo, during each night preceding a working day, a volume of aggregates of between 1.1 and 2.2 times the daily consumption.  6. Method according to claim 5, characterized in that during the night preceding a working day, is heated in the lower part of each silo a first volume of aggregates slightly greater than the daily consumption at an average temperature of 500C + 50C and a second volume of aggregates is preheated in the middle part of each silo which corresponds substantially to a daily consumption at an average temperature of the order of 350C i50C.  7. Device for accelerating the setting of concrete of the type comprising a concrete plant equipped with vertical silos (1, 2, 3, 4) in which the aggregates used in the composition of the concrete are stored, characterized in that said silos are equipped in their lower part, heating resistors (10) which are immersed in the mass of aggregates and which are staggered with intervals between resistors of the order of 500 mm.  8. Device according to claim 7, characterized in that each heating resistor (10) is placed inside a steel tube (14) which passes through said silo right through, the rear end of which is closed and is engaged in a sleeve (15) welded to the rear wall of the silo and the front end of which, which is located outside the front wall of the silo, has one or more orifices (20).  9. Device according to claim 8, characterized in that said heating resistors (10) have the shape of a pin and the two branches of said pin are clamped in metal clips (21) which are distributed over the length of the pin and which can slide freely inside said tube (14).  10. Device according to any one of claims 7 to 9, in which said silos are equipped with a heating resistance over a height which corresponds to a volume of aggregates of between 1.1 and 2.2 times the daily consumption extracted from said silo.