US 3418900 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
Dec. 31, 1968 u. B. BRAY ET L COLD ASPHALT PAVING PROCESS Filed May 29, 1967 ASPHALT LJ M/LL STONE jA/VEIVTORS 1.2/6 5. BRAY (/4 CK E. W000 ASPHALT POWDER a DUMP TRUCK Fl UX O/L United States Patent 3,418,900 COLD ASPHALT PAVING PROCESS Ulric B. Bray, Pasadena, and Jack E. Wood, Upland, Calif., assignors to Bray-Wood Company, Upland, Calif., a partnership of California Filed May 29, 1967, Ser. No. 642,050 6 Claims. (Cl. 94-23) ABSTRACT OF THE DISCLOSURE A method of paving with asphalt and stone without heating, wherein the stone is first uniformly coated in a mixing machine with a powdered, hard asphalt, then mixed with a flux oil and subsequently spread onto the pavement where it is compacted, as by rolling. Excellent distribution of the asphalt and flux on the stone is obtained in a minimum time of mixing.
This invention relates to a process of paving with asphalt and stone, applicable to roadways, parking areas, tennis courts and other playing fields, revetments and to any suitation where the familiar hot mix asphalt is now used. One object of the invention is to avoid the need of heating the stone aggregate and spreading it before it becomes cold as in the usual hot mix process. Another object of the invention is to provide a simple, low cost portable machine for preparing asphalt paving mix which can be located near the site of the paving project, thereby reducing the travel time for transporting the mix to the job. Still another object of the invention is to provide a method of mixing asphalt and stone aggregate accurately and rapidly to give a uniform product meeting all highway specifications without the need of stock pilin and rehandling.
Our invention employs the principle of blending hard asphalt in the form of a fine powder with a flux oil and stone aggregate in the cold, that is, at temperatures prevailing at the job site, whereupon the powdered, hard asphalt and the flux oil, more or less slowly, depending on temperature, particle size, etc., combine or coalesce into a strong, adherent binder to hold the stone together in the pavement. This principle has been the subject of numerous attempts heretofore to capitalize on this inherent advantage of a cold asphalt paving process as shown by such patents as Amies, 945,071; Radcliffe, 1,655,240; Beckwith, 2,220,670; and Mollring, 2,785,163.
However, the process has been beset with numerous difiiculties viz: (1) moisture in the stone has interfered with mixing stone and oil. Pretreating the stone with a volatile solvent or adding a wetting agent to the oil has been proposed as a partial solution. Ideally, the stone should be dry but if it is heated to dry it, much of the economic advantage of the process is lost. (2) Powdered asphalt, owing to cohesion of particles, electrical charges on the particles, etc., has a strong tendency to clump into aggregates which are not easily dispersed in the oil flux, either when mixed separately or in presence of stone aggregate. The result is a non-uniform product which produces a poor pavement, soft in some spots and brittle in others. Attempts to correct this difficulty by more extensive mixing results in reduced output of the plant which cannot be tolerated. (3) Applying the oil to stone first, then adding asphalt powder tends to localize the powder in areas where first brought into contact. Premixing asphalt and oil gives a viscous slurry or partial solution which is difficult to mix with stone, especially when much fines are present, tending to form pasty masses difiicult to distribute.
We have now discovered that the foregoing problems can be solved by the expedient of first combining the asphaltpowder and stone aggregate thoroughly mixing and gr nding the two together then adding the flux oil to the mixture and continuing the mixing. By this technique, the powdered asphalt is dispersed in an amazingly short time as will be shown hereinafter. Dispersion is facilitated by the grinding action of the stone as in a ball mill, breaking up all lumps and even further reducing particle size of this brittle asphalt. The presence of dust in the aggregate, for example 2 to 10% passing 200 mesh, combines to aid in the distribution of the asphalt, partly by coating the particles with rock dust which prevents cohesion and partly by neutralizing electrostatic charges. As a result all surfaces of the stone are evenly coated with a gray powder of asphalt and stone dust. When oil is later added, this coating of asphalt dust on the stone speeds the wetting by the oil, thus decreasing the time required to produce a satisfactory product of uniform composition throughout the batch. Even when considerable moisture is present in the stone, e.g.: 1 to 3% by weight, the presence of the rock dust serves to distribute the asphalt particles, and subsequently the flux oil, over the surface of the stone without stripping.
Mixing can be effected in a typical, drum type mixer common to the portland cement industry, the stone and powdered asphalt being charged and mixed until uniform, then the oil flux added and mixed again until the oil is completely dispersed. This operation can be carried out in a transit mixer receiving the weighed charges of aggregate and asp-halt powder at the stone pile, mixing thoroughly on the way to the job and introducing the flux oil to complete the mixture before discharging. After the mixture is spread evenly on the roadway or other surface it is compacted by rolling or beating into the desired form. On ageing, the particles of asphalt gradually absorb the oil to give a strong, adherent cement holding the stone in place. Vibration from traflic-serves to speed the hardening process. It is advantageous to spread the mix within an hour or two from the time of mixing, although this time can be extended to a day or more under favorable conditions such as low temperatures, coarser grind of the asphalt and lower asphalt solubility as in the case of hard blown asphalts and gilsonite. Flux oil composition is also important.
We prefer to prepare the asphalt, stone and flux oil mixture in a mixing plant specially designed for the purpose, in which we have provided a rapid mixing machine into which the ingredients are introduced in proper sequence. Such a plant is shown schematically in the drawing which accompanies this application.
Referring to the drawing, aggregate supply hoppers, 10, 11 and 12 are filled from conveyor 13 and elevator 14 by charging the various grades of stone into hopper 15 located at or below ground level. Elevator 14 is suitably a bucket elevator of common design and conveyor 13 can be either belt or helix type. Hydraulically operated clapper valves 16, 17, and 18 regulate the amount of each material discharged into weigh hopper 19 equipped with hydraulically operated discharge valve 20.
Asphalt supplied in 50 to 400 lb. blocks is powdered in mill 21, suitably a hammer mill such as the well known Williams or Raymond mill. It is collected in hopper 22 equipped with hydraulically operated discharge valve 23 which: controls the charge delivered into weigh hopper 24 where the desired amount is accurately measured for each batch. When the correct weight is indicated by movement of beam 26, an electronic signal closes the discharge valve-23.
Flux oil from tank 27 flows by line 28 to metering cylinder 29 provided with piston 30 operated by hydraulic cylinder 31 having an adjustable stroke controlled by screw 32, operated either manually or automatically to deliver any desired exact volume of oil thru discharge line 33. Check valve 34 prevents oil returning to tank 27 while spring loaded discharge valve 35 prevents discharge of oil except when ressure is applied by piston 30. It is desirable to locate valve 35 at the extreme end of the discharge line 33 to prevent drip when not operating between mixing cycles.
Turbine mixer 36 is preferably of the Smith Turbine type made by T. L. Smith Company of Milwaukee, Wisconsin, wherein plows travel in a circular path, weaving the aggregate rapidly in and out while travelling at a speed of about to feet per second. The charge of aggregate is usually between 1,000 and 4,000 pounds and mixing is completely uniform within 60 seconds or less. The cutaway view of the mixer is shown at 37 with plow 38 car ried on arm 39 attached to rotating hub 40.
Hydraulically operated discharge plate 41 closes the bottom opening, permitting the mixer to be emptied within less than 5 seconds with the mixer running. The finished mix falls through chute 42 into waiting truck suggested at 43. It is preferred to operate all controls automatically by means of a timer which operates the valves in sequence in a manner well known in the art. The mixer 36 may run continuously when the plant is in operation. Following is an example: a truck-trailer rig having a capacity of tons arrives at the plant to receive a load of mix. A two ton batch of stone, previously weighed into hopper 19 is dropped into the mixer 36 followed immediately by 50 lbs. powdered hard asphalt from 24. After seconds mixing, the stone has a uniform gray color showing the asphalt particles dispersed evenly throughout. On the instant, 12 gallons of flux oil is added from line 33 and mixing continued 55 seconds. Discharge plate 41 then opens for a period of 5 seconds after which the cycle is ilepeated. Time required for loading the truck with 20 tons is thus 15 minutes.
The following data illustrate the uniformity of mixing obtained with powdered asphalt and stone in a Smith Turbine mixer of 2,000 lbs. capacity equipped with six plows travelling at 9 feet per second. The charge of stone was made up of 800 lbs. No. 4 inch mesh) and 1,200 lbs. of rock dust.
Screen analysis of the stone is as follows: Percent Passing /i mesh 100 Passing 4 mesh 71 Passing 8 mesh 51 Passing 30 mesh 41 Passing 200 mesh (with washing) 7 /2 Screen analysis of the asphalt follows: Percent Passing 50 mesh 90 Passing 100 mesh 55 Passing 200 mesh 20 Second Mix, gm.
60 Second Mix, gm.
Sample 1 Sample 2 These results show asphalt content of 1.625% of the mix in which 1.725% asphalt was incorporated. Surpris ingly, they show perfect mixing in 40 seconds, the asphalt concentration being exactly the same in remote sections of the mixer.
Flux oil in the amount of 100 lbs. was then added and mixing continued for 60 seconds. The oil was No. 250 slow curing (8.0.) to which was added 5% by volume of an aromatic solvent corresponding to xylene, for the purpose of reducing viscosity to aid rapid mixing and speed hardening of the concrete on the roadway. When the dough-like product is compacted by rolling, it can be used for light trafiic immediately and will support heavy road traffic within three to five days. The xylene aids hardening on evaporation and by increasing the rate of solution of the asphalt particles in the flux oil. Test strips were made by spreading a layer 3 /2 inches thick and rolling to a thickness of 2 /2 inches.
Following are typical analyses of slow and rapid curing liquid asphalts (flux oils) suitable for our process:
The hard asphalt employed in our process will have a specific gravity of about 1 and softening point of about 250300 F. ball and ring method. Penetration-ASTM at 77 F. may range from 0 to 5. It is very brittle and easily reduced to powder in an impact mill, care being taken to prevent the mill from heating to a degree where the asphalt particles become tacky. Powdered asphalt is conveniently transferred in an air stream from mill to hopper. The amount of asphalt powder employed in our process ranges from about 10 to 35 lbs. per ton of aggregate with fiux oil proportions of about 75 to 125 lbs. per ton, usually about lbs. or 12 gallons. The flux oil dosage varies somewhat with the type of oil used, generally more oil is used with the slow curing type. Addition to the oil of about 2 to 10% of a volatile aromatic solvent such as xylene aids in the curing of the pavement.
Following are examples of paving mixtures which have been found satisfactory. In each example, one ton of aggregate of the composition described herein above, was used.
EXAMPLE 1 Lbs.
Rock 2000 Powdered asphalt 12 Flux oilR.C. 250 108 On rolling the pavement, a firm surface resulted which was not affected by rain.
EXAMPLE 2 Lbs. Rock 2000 Powdered asphalt 22 Flux oilblend of R.C. 250 with 25% by vol. of
SC. 250 and 5.5% petroleum xylene solvent 108 Rolling twice gave good compaction, cured rapidly.
EXAMPLE 3 Lbs. Rock 2000 Powdered asphalt 25 No. 2400 oil 60 No. 800' R.C. oil 40 Three hours after laying pavement, it was hard enough for service. Next day it was very hard and appeared fully cured.
Although we have described our process with respect to certain limited applications by Way of example, We intend its scope be determined only by the claims which follow. Uniform dispersion referred to in the claims means that the asphalt content of the mix is the same throughout the batch, regardless of what area of the batch is analyzed. By rock dust is meant that part of the stone aggregate passing a standard 200 mesh sieve. The term plastic used herein with reference to the paving mixture is meant to describe the dough-like consistency thereof.
Having thus described our invention, what we claim is:
1. The method of paving with an asphalt and stone mixture without heating which comprises thoroughly mixing a stone aggregate with comminuted hard asphalt to form a uniform dispersion of asphalt particles on the stone surfaces, then adding a fluxing oil and continuing mixing until uniformly dispersed in the aggregate and thereafter applying the plastic mixture to a surface and compacting it thereon to form the desired pavement before solution of asphalt particles in said fiuxing oil is complete, and said mixture becomes rigid.
2. The method of claim 1 wherein said stone aggregate contains about 2 to percent by weight of rock dust which forms a uniform mixture with said powdered asphalt during the said initial mixing operation, thereby assisting coating the stone when subsequently contacted with oil.
3. The method of claim 1 wherein the amount of powdered asphalt employed is about 10 to 35 lbs. per ton of stone aggregate.
4. The method of claim 1 wherein the amount of fluxing oil employed is about 75 to 125 lbs. per ton of stone aggregate.
5. The mehod of claim 1 wherein the said fluxiug oil contains from 2 to 10% of a volatile aromatic solvent to aid the solution of asphalt particles in said oil.
6. The method of claim 1 wherein said stone aggregate is charged to a mixing zone simultaneously with said asphalt powder and the rate of mixing is sufficiently rapid to effect a uniform mixture within about seconds and thereafter the fluxing oil is mixed at a rate sufficiently rapid to effect a uniform mixture in less than about seconds.
References Cited UNITED STATES PATENTS 2,128,291 8/1938 Finley 9423 X 2,229,872 1/1941 Pullar 9423 2,349,445 5/1944 McGrane 9423 2,572,068 10/1951 Sommer 9423 2,876,686 3/1959 Birney 9423 2,978,351 4/1961 Pullar 9423 X 3,074,807 l/1963 Dorius 9423 X NILE C. BYERS, JR., Primary Examiner.