METHOD AND DEVICE FOR COLD MIXING OF ROAD SURFACING MATERIAL

Abstract: The invention relates to a method and a device for continuous cold mixing of road surfacing material. Aggregate is supplied to a hopper (1), is weighed on a belt weigher (3) and conveyed to a screen (6) for separation into different size ranges. The different portions are then supplied to a compulsory mixer (7), in which they are cold mixed with bitumen emulsion. According to the invention, the aggregate is continuously separated into at least three portions which are supplied to the compulsory mixer (7) via different inlets (9a, 9b). The portion having the greatest particle size is first mixed with an excess of bitumen emulsion for this portion. Each aggregate portion of successively decreasing particle size is then separately mixed with an excess of bitumen emulsion for each portion, while they are successively mixed with the already bitumen-mixed aggregate portion/s having a greater particle size. Last, the aggregate portion/s having the smallest particle size/s are mixed with the already bitumen-mixed aggregate portions without additional bitumen emulsion being supplied separately to this or these aggregate portions.

Description: METHOD AND DEVICE FOR COLD MIXING OF ROAD SURFACING MATERIAL The present invention relates to a method and a device for cold mixing of road surfacing material. The methods used today when mixing road surfacing material are hot mixing and cold mixing. Hot mixing requires stationary plants, in which a bituminous binder and gravel material are mixed during heating. The road surfacing material must then be trans¬ ported to the road which is to be coated. WO Al 83/00700 discloses a batchwise method and an apparatus for hot mixing of asphalt concrete. In this method and in this apparatus, heated aggregate is divided into three different particle sizes by screening. The coarse-grained portion (> 3 mm) is mixed with a hot bitu¬ minous binder. The hot bituminous binder covers the sur- face of the particles of the coarse-grained portion. After thorough mixing of the bituminous binder and the coarse-grained portion, a filler (< 0.1 mm) is added to increase the thickness of the bituminous film on the par¬ ticles of the coarse-grained portion. Only then is the fine-grained portion (< 2 mm) added, which thus has a greater particle size than the filler and which is added to the thickened layer of bitumen on the particles of the coarse-grained portion. The adding of the filler results, in hot mixing, in the particles of the fine-grained por- tion more easily adhering to the particles already coated with the bituminous binder. A problem in today' s hot mixing is the large amounts of diesel oil, solvent and dust emitted to the surround¬ ings during heating and working of the road surfacing ma- terial. The increasing cost for transporting the heated road surfacing material is an additional problem. In order to avoid these problems of emission and heavy transport expenses, one has tried more and more to switch over to cold-mixed road surfacing materials which can be mixed in mobile plants on the site of spreading. In cold mixing, it is thus possible to avoid the problem of various emissions and to reduce the transport expenses in connection with the spreading of the road surfacing material. US-A-4, 974, 993 discloses a method and a device for renewing a road surface. In this method and in this de¬ vice, the road material is first scarified and screened in order to separate particles > 35 mm from particles having the size of 0-35 mm. The reason for such screening is that subsequently the material having a particle size of > 35 mm is to be crushed to a smaller size. The par¬ ticles of the two different portions, which are then united, thus have essentially the same size range. A sol¬ vent and a bitumen emulsion are then added to the crushed portion, which is then mixed with the portion that has not been subjected to crushing. WO Al 87/03317 discloses a method and an apparatus for mixing gravel material and a bitumen emulsion. In this method and in this apparatus, the gravel material is separated into two portions. The two different portions are separately sprayed with the bitumen emulsion and are then united. The two portions sprayed with bitumen emul- sion are then mixed together by letting the material fall freely onto strongly inclined baffles. Any remaining bi¬ tumen emulsion is added to the total mix of the two por¬ tions before being discharged from the apparatus. Cold mixing has, in turn, caused new problems which are connected with the final road surfacing. One problem is that a separation occurs between the large and small particles of the gravel material, and this results in a non-uniform road surfacing material and stones co ing loose from the finished road surface. Owing to the form- ing of lumps and stones coming loose, the road surface will be of non-uniform, unsatisfactory quality for coat¬ ing busy roads. One object of the present invention is to provide a road surfacing material without any problems of emission of various chemicals and dust. A further object is to provide a road surfacing ma- terial which is of uniform composition after cold mixing and which, when being spread, does not cause the forming of lumps and the coming-loose of stones. Other objects as well as features and advantages of the present invention will be apparent from the following description. These objects are achieved by a method and a device according to the accompanying independent claims. Par¬ ticularly preferred embodiments are defined in the sub- claims . Briefly, the present invention relates to a method and a device for continuous cold mixing of road surfacing material. Aggregate is supplied to a hopper, weighed on a belt weigher and conveyed to a screen to be separated into different size ranges. The different portions are then supplied to a compulsory mixer in which they are cold mixed with bitumen emulsion. According to the inven¬ tion, the aggregate is screened continuously into at least three portions which are supplied to the compulsory mixer via different inlets. The portion having the greatest particle size is first mixed with an excess of bitumen emulsion for this portion. Each aggregate portion of successively decreas¬ ing particle size is then separately mixed with an excess of bitumen emulsion for that aggregate portion at the same time as it is successively mixed with the aggregate portion/s already mixed with bitumen and having a greater particle size. Last of all, the aggregate portion/s hav¬ ing the smallest particle size/s is/are mixed with the aggregate portions already mixed with bitumen, without additional bitumen emulsion being supplied separately to this/these aggregate portion/s having the smallest par¬ ticles . Bitumen emulsion is supplied via nozzles which are arranged in the vicinity of the inlets for the aggregate portion/s having the greatest particle size/s, no nozzle being arranged adjacent the inlet for the aggregate por- tion/s having the smallest particle size/s. The advantage of the present invention is that a road surfacing material of uniform composition is ob¬ tained, which, when being spread, yields a road surface of high and uniform quality, without problems of lumps being formed and stones coming loose. By the term aggregate is meant new aggregate in the form of crushed stones and recovery masses, i.e. scari¬ fied masses, having a predetermined size distribution. Preferred embodiments of the present invention will now be described with reference to the accompanying draw¬ ings. These embodiments must not be considered restric¬ tive to the scope of the present invention. Fig. 1 is a principle sketch of a preferred embodi¬ ment of the entire inventive device, and Fig. 2 is a principle sketch of an enlarged portion of a preferred embodiment of the screening and mixing de¬ vice according to the invention. The embodiment of the inventive device as illus¬ trated in Fig. 1 comprises a hopper 1 whose lower part is provided with a conveyor belt 2. Under the discharge end of the conveyor belt 2, a belt weigher 3 is arranged. Ad¬ jacent the belt weigher 3, there is a further conveyor belt 4. The discharge end of this conveyor belt 4 opens into an inlet 5 of a screen 6. Under the screen 6, a com- pulsory mixer 7 is arranged. Fig 2 is an enlarged longi¬ tudinal section of the screen 6 and the compulsory mixer 7. The screen comprises two different grading screens 8a, 8b which open into different inlets 9a, 9b of the compul¬ sory mixer 7. In the compulsory mixer 7 there are a num- ber of nozzles 10a, 10b, 10c and an agitator 11, in which three of four agitator blades 12 propel the material in the direction of arrow 13, whereas the fourth agitator blade 12 propels the material in the opposite direction. In the continuous cold mixing of the road surfacing material, the aggregate is first supplied to the hopper 1. Then the aggregate is conveyed continuously to the belt weigher 3 which registers the weight of the aggre¬ gate which is to be supplied to the screen 6 and the com¬ pulsory mixer 7. Subsequently, the aggregate is conveyed to the inlet 5 of the screen 6. Now the aggregate is dis- charged and falls onto the grading screen 8a which sepa¬ rates the smallest particles. The largest particles thus do not pass through this grading screen 8a but roll along this screen into the inlet 9a, in which the particles are mixed with an excess of bitumen emulsion for this por- tion, said bitumen emulsion being discharged from the nozzle 10a in the vicinity of the inlet 9a. The particles which pass through the first grading screen 8a then impinge upon a new grading screen 8b, and the particles having a greater particle size than allowed by the grading limit of the grading screen 8b roll along the grading screen 8b down into the inlet 9b of the com¬ pulsory mixer 7, while an excess of bitumen emulsion for this portion is supplied via the nozzle 10b. The particles that have passed through the two grad- ing screens are supplied to the compulsory mixer over a longer distance and are mixed with the other portions of greater particle size, which have already been mixed with an excess of bitumen emulsion. From the nozzle 10c, the remaining bitumen emulsion is then supplied that is re- quired for the total road surfacing material to have the correct consistency. It is not always necessary to add any remaining bi¬ tumen emulsion via the nozzle 10c. It may also happen that all the bitumen emulsion has been supplied before adding the aggregate portion/s having the smallest par¬ ticle size. The added quantity of bitumen emulsion in the dif¬ ferent nozzles 10a, 10b, 10c is controlled by the meas¬ ured input weight of the aggregate that is measured on the belt weigher 3. The proportions of the output quanti- ties in the different nozzles are controlled on the basis of the known size distribution of the aggregate. When the largest particles are first mixed with an excess of bitumen emulsion, bitumen emulsion is immedi¬ ately deposited and adheres to the large particles. Since there is an excess of bitumen emulsion around the large particles, the fine material adheres to these stones and there will be a minimum of separation. The ready-mixed road surfacing material is uniform, and the quality of the final road surface will be uniform and high.

Claims: CLAIMS 1. A method for cold mixing of road surfacing mate- rial, comprising the steps of supplying aggregate to a hopper, weighing the aggregate on a belt weigher and con¬ veying it to a screen to be separated into different size ranges, whereupon the different portions are supplied to a compulsory mixer in which they are cold mixed with bitumen emulsion, c h a r a c t e r i s e d by the steps of continuously separating the aggregate, before mixing with the bitumen emulsion, into at least three portions in the screen, subsequently mixing the portion having the largest particles with an excess of bitumen emulsion for this por¬ tion, separately mixing each aggregate portion of succes¬ sively decreasing particle size, except the aggregate portion/s having the smallest particle size/s, also with an excess of bitumen emulsion for that aggregate portion, while successively mixing it with the already bitumen- mixed aggregate portion/s having a greater particle size, and subsequently mixing the aggregate portion/s having the smallest particle size/s with the previously bitumen- mixed aggregate portions, without additional bitumen emulsion being separately supplied to this or these ag¬ gregate portions having the smallest particles. 2. The method as claimed in claim 1, c h a r a c - t e r i s e d in that the remaining bitumen emulsion, which is required to obtain the correct consistency of the total road surfacing material, is added and mixed with the road surfacing material after the aggregate por¬ tion/s having the smallest particle sizes being supplied. 3. The method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that all the bitumen emulsion is added before admixing the aggregate portion/s having the small¬ est particle size. 4. A device for continuous cold mixing of road sur¬ facing material containing aggregate and bitumen emul- sion, said device comprising a hopper (1) for the aggre¬ gate, a screen (6), a compulsory mixer (7), a conveyor belt (2) for conveying the aggregate from the hopper (1) to the screen (6) , and a belt weigher (3) for weighing the aggregate supplied from the hopper (1) to the screen (6), the screen (6) being arranged over the compulsory mixer (7), c h a r a c t e r i s e d in that the screen (6) is arranged to separate the aggregate discharged from the hopper (1) into at least three portions having different particle sizes before supplying the aggregate to the compulsory mixer (7), that the compulsory mixer (7) has different inlets (9a, 9b) for the portions sepa¬ rated in the screen (6) according to particle size, and that nozzles (10a, 10b, 10c) for bitumen emulsion are ar¬ ranged in the vicinity of at least the inlets (9a, 9b) for the portion/s having the greatest particle size/s, while no nozzle is arranged adjacent the inlet for the portion/s having the smallest particles size/s. 5. The device as claimed in claim 4, c h a r a c ¬ t e r i s e d in that a nozzle (10c) is arranged after the inlet for the portion having the smallest particle size.

Inventors: PERSSON LEIF LENNART, PERSSON, LEIF, LENNART
Applicants: PERSSON LEIF LENNART, PERSSON, LEIF, LENNART
Application year: 1995
Main classification: E01C19
IPC codes: E01C19