Technical field:

The present invention relates to a process for heating and drying of reinforced concrete products in which is arranged a reinforcement of electrically con- due ting material.

Background:

The firing time and the drying time for concrete casting work, for example in the construction of houses, have long been a dela ing factor since the following stage of the work cannot generally begin before the concrete has fired. In order to shorten the drying and firing time for concrete, it is nowadays preferable to use electrical _^ warm-air fans wh ch blow warm air over the concrete sur¬ ace.

Technical problem:

T is certainly shortens the drying and firing time for concrete but often results in problems. The surface layer alone is affected by warm air which results in uneven heating. In order to obtain a more even heating over the whole surface in the case of large surfaces, several warm-air fans must be used. This, however, results in considerable expense and loss of energy. Furthermore, it is difficult to check and control the heating to an adequate temperature for the heated concrete, which can result in damage to the concrete, for example in the form of cracks, wh ch impairs the concrete product.

Solution:

According to the invention a process is applied in which it is provided that parts of the reinforcement can be reached from the outside, wherein at least two of the parts of the rein orcemen which are located at a

distance from each other and can be reached from the out¬ side form two poles which are in electrical contact with each other by means of the reinforcement, that the poles are connected to a current supply source so that at least one closed circuit is formed and, with such adjustment of the current from the current supply source to the size of the concrete produ es together with the resistance in the closed circuit, that heat is generated in the re¬ inforcement, which heat is conveyed to the concrete pro¬ ducts by means of heat conduction to the concrete body surrounding the reinforcement, that the heat in the con¬ crete products is detected, and that the current is con¬ veyed at selected periods depending on the detected heat in the concrete products-

Advantages:

The present invention provides a process for heating and drying of reinforced concrete products which eliminates the said disadvantages. The concrete products obtain an even heating and the heat conveyed can. be regulated.

Description of the drawings:

The invention will be described in greater detail below by referring to five exemplary embodiments, with reference to the attached drawings in which Figure 1 shows a block diagram for heating of a rectangular concrete product. Figure 2 shows a block diagram for heating of a square concrete product, Figure 3 shows a block diagram for heating of a frame-shaped square concrete product by means of one current supply source, Figure 4 shows a block diagram for heating of a frame-shaped square concrete product by means of two current supply sources, and Figure 5 shows a concrete product with devices for measuring the temperature .

Preferred embodiments:

The process according to the invention will be described with reference to Figure 1 by way of a device which is connected to a source for generating electrical current with controllable voltage and current, preferably direct current, but also alternating current. The cur¬ rent supply source is designated below as 1 and is con¬ nected to two terminal blocks 2, 3 each around three outlets 4 and 5 respectively, where, preferably with direct current, the one ter inal block 2 has three positive poles 4 and the other terminal block 3 has three negati e poles 5.

Electrical cables 6 can be connected at one of their ends to each of poles 4, 5 of the terminal blocks 2, 3. The other ends of the cables 6 are connected by means of couplings 7 to parts of a reinforcement 9 which extend out from a concrete product 8, which re nforcement preferably consists of elongate rods which can be bent to the desired shape to fit the concrete product, which rods are in contact with each other in that they are con¬ nected to each other, for example by being threaded to¬ gether by means of steel wire, to the desired reinforcement pattern, especially a checked pattern, and are introduced, on casting, into a concrete mass which forms the product 8, it being ensured that the parts mentioned can be reached from the outside. It is also possible, on casting, to use finished nets as the reinforcement wh ch are in¬ troduced into the concrete.

The number of cables 6 depends on the number of coupling possibilities on the reinforcement. Under all circumstances at Least two cables must be connected between two, of the said parts of the reinforcement 9 which are located at a distance from each other and each to its pole 4, 5 in the terminal blocks 2, 3.

As shown in Figure 5, four temperature sensors 10, 11, 12,13 are arranged in the concrete product 8 at a mutu¬ ally varying distance relative to each other and relative to the reinforcement and the concrete product's outer edges.

The temperature sensors are connected by means of cables, which are not shown, to any type of known measuring in¬ strument, which is not shown either. In Figure 5 the concrete product has been di agrammat i cal ly divided into four temperature zones 14, 15, 16 and 17 of identical size the said temperature sensors being placed in section 14 in accordance with a certain pattern which is chosen with great care. The concrete product is, as has been mentioned, divided nto the temperature zones 14, 15, 16 and 17, it being sufficient for the temperature sensors to be placed in one of the zones in order to detect the heat in the whole concrete product. Since the zones are of similar shape and the cable couplings therein are of simi¬ lar shape, the temperatures on the whole in all of the zones will be alike at all times. The temperature sensors are set out at measuring points where it is supposed different temperatures will occur dur ng heating and dry ing, preferably such that one- of the sensors 10 is placed at the point which it is thought will ^' - be warmest ini¬ tially, i.e. close to one of the reinforcement parts 9 at one of the cable couplings, and another of the sen ^' sors 11 is placed at 'the point which it is thought will be coldest initially,, i.e. halfway between two of the reinforcement part couplings. The remaining two sensors 12, 13 are set out at two points which it may initially be assumed will have a mean temperature relative to the temperatures at the two first-mentioned measuring points. To prevent the sensors from being affected by any undesirable factor, such as for example the heat from the surrounding air, the sensors are introduced into bores in the concrete product at the said measuring points, and these sensors, when they are introduced into the bores, are covered with sand or cement, whereupon the heat de-tected by the sensors at the measuring points can be read off by means of a measuring instrument at the current supply source. This measuring instrument is coupled with a relay unit at the current supply source which controls the current supply to the concrete product, depending on the detected heat.

The process according to the invent i n is based on the principle that when the current is sent through the reinforcement, the rods of which are in electrical contact with each other, heat is generated in the rein¬ forcement due to its resistance. In this connection it can generally be said that for heat to be generated to a sufficient extent a certain ratio is required between the resistance in the reinforcement and the current intensity, in the current which is sent through the reinforcement. The customary reinforcement in use nowadays in the build¬ ing industry has a relatively large diameter and is not particularly long, which results in the resistance being low. Therefore, according to the invention, a high cur¬ rent intensity is used in order to heat the reinforcement, and this current intensity can be controlled preferably up to 2000 A. Beyond the mentioned adjustment of the current inten¬ sity the current can be cut off at selected time intervals, the length of which depends on the dampness of the concrete and the reinforcement introduced into the concrete. These intervals, during which the current is cut off can be preset to preferably 1 , 2 or 4 minutes.

On coupling, the cables 6 are connected, as shown in Figures 1 - 4, partly to the desired number of poles 4, 5 in the terminal blocks, partly to the parts of the reinforcement 9 which extend out from the concrete pro¬ duct 8 by means of the couplings 7. In order to make maximum use of the heat generated at the reinforcement, it is of particular importance here how the coupling takes place with regard to the concrete product's geometry and the reinforcement arranged in the concrete product for maximum utilization of its resistance. Here, the cables are connected symmetrically to the projecting parts of the reinforcement 9 in such a way that an equal number of cables 6 are connected to opposite sides of the con¬ crete product 8. It is of particular importance here that if only two sides of the concrete product are used for connecting of the cables, these are opposite so that the current is conducted through the whole concrete product. In order to achieve an even distribution of

the heat generated at the reinforcement over the whole concrete product, it is required that the current in several tracks shall meet appro imately the same resis¬ tance in all the tracks. The cables 6 shall then be connected to the parts of the reinforcement 9 at similarly mutual distances when the reinforcement is uniform in shape.

Before the cables are connected at the parts of the re nforcement which extend out from the concrete product, rust or other insulating material should be removed from the reinforcement in order to improve the electrical contact between the couplings 7 and the re¬ inforcement parts 9.

As shown in Figures 1 - 4, the connection of the cables to the reinforcement parts extending out from the concrete product depends partly on the shape of the con¬ crete product, partly on the number of projecting rein- . forcement parts and how they are distributed over the surface of the concrete product. In the case of long, narrow concrete products, as shown in Figure 1, it is preferable to connect six of the cables, three from each of the poles, to the concrete product's opposite short sides so that the reinforcement arranged in the concrete product is used to the maximum by the current being con¬ ducted through the longest part of the concrete product's reinforcement, by which means the resistance increases and with it the generation of heat.

If, however, the concrete product has a square or a substantially square shape, as shown in Figure 2, four of the cables are connected, two from each of the* poles, into the pro ecting parts of the reinforcement, so that each of the four cables is uniformly connected to each of the concrete product's sides, wherein each side's polarity is different from the polarity of the next side's coupling. In this way, a more even circu¬ lation of the current is achieved by means of t ^' he rein¬ forcement through the concrete product, and the generation of heat at the reinforcement becomes more even.

If, however, the concrete product has a cavity, as shown in Figures 3 and 4, only one part of the re¬ inforcement passes through between the opposite outer sides of the concrete product, i.e. that part of the re¬ inforcement which runs parallel to the sides and which can be reached at each of the corners and a part from the same corresponding thickness of the concrete product. In this connection, the coupling takes place advantageously at the two diagonally opposite corners, as shown in Figure 3, the current is on the whole conducted in equal parts by means of the reinforcement around the concrete product, and the heat is generated. In the case of very large concrete product.s it may be necessary to use two current supply sources, as shown in Figure 4, with mutually similar current intensity conveyed to the concrete pro¬ duct. In -this connection, the cables 6 are connected to each of the concrete product's corners, and the connections at the diagonally opposite corners have identical polarity, which gives an. increased through-flow of current by means of the reinforcement in the concrete product.

After the cables 6 have been connected by means of the couplings 7 to the reinforcement parts 9, the temperature sensors 10, 11, 12, 13 are set out at said measuring points 10 - 13. Thereafter, the process using the device is as follows:

1. A current is sent through the reinforcement and the resistance in the latter can be measured by means of measuring nstruments at the current supply source so that, where necessary, current intensity during heating can be adjusted.

2. The desired current intensity to the concrete product's reinforcement is set at the current supply source.

3. The temperature sensors are connected and the desired cut-off interval is set for the current.

4. The current is switched on at the current supply source and heating begins.

The current is conducted from the current supply source 1 via the positive poles through the cables 6,

which have a Cower resistance than the reinforcement, to the reinforcement in the concrete product. Due to the resistance which the current meets when it is conducted through the reinforcement to the cables 6 back to the current supply source 1 via the negative poles, heat is generated in the reinforcement. The heat is transmitted by means of conduction to the concrete product which is heated. Those parts of the concrete product w ch are n direct contact with the reinforcement are first to be heated in this case. So as not to stress the concrete on rapid heating and drying, where risk of damage exists, for example, in the form of cracks, the first temperature sensor is placed on what is thought ill be the warmest point close to one of the reinforcement parts at one of the cable connections and measures the temperature. When the temperature in the concrete product, which lies in direct contact with the reinforcement, reaches the pre¬ determined temperature, the current is cut off for a period which corresponds to the preset cut-off interval mentioned earlier. During the time when the current is cut off, heat is transmitted from the reinforcement by means of conduction to the remaining parts of the con¬ crete product, and an even distribution of heat takes place over the whole concrete product. After the said period, the current is switched on so that the reinforce¬ ment is heated once again and the temperature sensors measure the temperature so that, when the desired tem¬ perature is reached once again, the current is cut off and a new cycle is begun. The period of time during which the current is switched on between two of the said cut-off intervals varies and depends on how quickly the concrete products are heated at the measuring points. The temperature is detected continuously by the temperature sensors at all the measuring points, and the current is conveyed to the reinforcement intermittently in the said cycles.

The heating of the concrete product is complete when, dur ng a cycle, the detected temperature in all the

temperature sensors at all the measuring points is equal to or greater than the desired temperature which was preset. The temperature in the concrete product is, then, equal to the desired temperature, and the temperature is also, then, even over the whole concrete product.

As described above, the process according to the invention comprises a current, preferably with high cur¬ rent intensity, being conducted through the reinforcement in the concrete product, heat being generated at the re¬ inforcement due to its resistance, and the heat being detected by means of temperature sensors in the concrete product, these temperature sensors being set out n a certain pattern depending on the varying temperature in the concrete product initially, and these temperature sensors continuously measuring the temperature in the concrete product, that the current is cut off at intervals of preferably one, two or four minutes, during which the . heat is transmitted to the concrete product by means of conduction, and, when the temperature in all the tem¬ perature sensors during a cycle is equal to or greater than the desired temperature which was preset, the heat¬ ing process is complete.

The invention is not limited to the exemplary em¬ bodi ents described above and shown in the draw ngs, but can be varied within its scope for subsequent patent claims. Thus, for example, the number of cables for con¬ nection to the reinforcement parts of the concrete product can be increased in order to obtain a more even heating of the concrete product. Likewise, the periods when the current is cut off can additionally vary.