US 3044373 A
Description (OCR text may contain errors)
July 17, 1962 A. SOMMER 3,044,373
BITUMINOUS SURFACES Filed Feb. 20, 1958 INVENTOR: ASommer 3,044,373 Patented July 17, 1962 land Filed Feb. 20, 1958, Ser. No. 716,370 1 Claim. ((11. 94 23 This invention relates to bituminous surfaces compacted from above, such as asphalt pavements, especially for landing strips for aircraft. The invention is particularly concerned with a process for increasing the stability and pressure-resistance of asphalt surfaces and rendering them resistant against the effects of heat and oil.
Exhaust produced byjet propulsion systems contains considerable quantities of free fuel, such as kerosene and other petroleum fractions in hot condition, and the discharged gases are often accompanied by flames.
It is obvious that great heat and free solvents spilled over the surface of landing strips have a detrimental effect on bituminous structures, since the bituminous binder is not only dissolved but frequently burnt up by the exhaust of jet aircraft. For this reason, bituminous structures which are generally highly suitable for landing strips owing to their resiliency, are inadequate for jet airplanes and the like.
Attempts have been made to replace the asphalt bitumen by coal tar pitch which is somewhat more resistant against solvents, and another suggested expedient has been the surfacing of bituminous layers with certain oil-resistant plastics, such as polyvinyls. Such treatment, however, is not only expensive, but also, like coal tar pitch, offers no protection against excessive heat or flames. Furthermore, such treatment does not add more rigidity or load resistance to the pavement.
An object of the present invention is to change the character of the asphalt bitumen in order to render it oil and heat resistant and more rigid.
Studies which were carried out in solving the objects of the present invention disclose that in asphalt which is currently used for structural purposes, the predominant component consists of asphaltenes, and such bitumen is completely soluble in the usual solvents, as well as being also almost completely soluble in petroleum fractions. Asphalt bitumen is also fusible at varying temperatures according to its hardness. There are, however, asphaltic bitumens which are less soluble and fusible, which types are generally called asphaltites (Grahamite and the like). Finally there is a category calledthepyrobitumens, which are neither fusible nor soluble, such as impsonite, and which have a higher percentage of carbon in their molecules.
It is known that asphalt bitumen can be hardenedeither by evaporation or by treatment with oxygen at comparatively high temperatures. It may be assumed that the above named stages of asphalt have been gradually formed in nature by a process of hardening within low limits of temperatures, by aging. Inasmuch as hardening by evaporation or destructive treatment is out of the question in the forming of structures, any such processes can be considered only if they take place without deformation of the structure and without destruction of the binder, namely, without evaporation or decomposition.
The present invention is based in part on my discovery that if in mixtures of mineral aggregate and asphaltic bitumen, the latter is present in thin uniform films of less than 0.01 millimeter thickness surrounding each individual stone element of the aggregate, and if such mixtures after compacting contain a certain percentage of voids or hollow spaces, namely, from to 25%, the bituminous films when exposed to a temperature of about 200 C.,
undergo a gradual molecular change and hardening. The bituminous component which is initially in the form of asphaltene (which is fusible and soluble), changes into asphaltite and finally into pyrobitumen. These various stages although of different physical character, all represent bituminous substances,'which While not of the conventional plastic consistency, exert a very strong binding power on the structure with which they are most intimately fused.
In other words, it is possible by such a process of hardening at temperatures below the point of decomposition of the bitumen, to convert the bitumen into a nonfusible and non-soluble hydrocarbon and thus make the structure resistant against oil and heat; furthermore, a higher degree of rigidity can be attained. The structure while still bituminous, behaves much more like a mineral structure, such as stone, or cement concrete. Since it is now possible to apply a rigid surface to a conventional bituminous base, such a surface requires only a minor thickness, namely, or thereabout; it adheres firmly to the bituminous base and, while resistant to action of heat or solvent, it can be replaced, if necessary, at comparatively low cost.
While the described process for hardening compacted bituminous compounds, when applied to molded or pressed shapes, such as blocks or briquettes, should take place in a closed heating chamber, it is obvious that upon a finished pavement heat can be applied upon its surface from above the surface only. This can be carried out, for example, by a movable heating apparatus containing infrared light sources so adjusted that the radiation is directed onto the surface to be treated.
For further illustration of this invention, an example of carrying out the process is now described with reference to the accompanying drawing, which forms part of this specification and which is to be understood as being explicative of the invention and not limitative of its scope. Other embodiments incorporating the broad principles underlying my invention are possible without departing from the spirit and scope of the appended claims.
The sole FIGURE of the drawing shows a transverse section through a bituminous pavement and a diagrammatic representation of a surface heating apparatus for carrying out the surface hardening process according to the present invention.
In the drawing, the reference numerals 1 and 2 desig nate the base of stone and cement concrete, respectively. This base supports a layer 3 of bituminous concrete, for example of the Topeka type having a thickness from one to two inches. The top surface layer 4 is applied according to the present invention and preferably comprises an aggregate-not coarser than sand and in many cases an aggregate of filler gradation (stone dust). The mix for this top layer 4 is preferably produced by a process wherein each individual sand or dust particle is enveloped in a thin bituminous film. Particularly, the method of preparing a bituminous paving mix described in my Patent No. 2,572,068 can be used. According to this method the mix is produced, in contrast to the conventional pressure of kneading mix, by whirling the aggregate up in a pug mill device into a floating Zone above the mixing blades, and atomizing the liquid bitumen into that floating Zone, so that both ingredients penetrate each other in a suspended condition. By this procedure bitumen can be so dispersed in the loose aggregate that each particle, independent of its size, is coated with a uniform, thin and controllable film of bitumen.
A compound thus produced, containing anywhere from 6 to 8% of bitumen of comparatively soft character penetration) can be spread and compacted by rollers to a thickness of about of one inch. Compacting will be carried out to-such a degree that the layer will have a V by means of the illustrated apparatus.
void content of hollow cells of from 10 to 25% of its volume. I
The hardening process of the layer can now be efiected This apparatus hasthe shape of a carriage, comprising a chassis or framework movable on wheels 6. ,The chassis carries a motor 7 which drives a dynamo-electric machine 8 generating electric current to supply :a plurality of sources of infrared radiation 9 disposed a short distance above the level of the bituminous surface 4' to be: hardened. Each infra-red radiation source is provided with a parabolic reflector 10 which directs the entire radiant heat developed by the sources 9 onto the bituminous layer 4. Thus this layer 4 beneath the chassis Sis heated to a temperature of about 200 C.
The chassis 5 may have a certain width and the infrared sources 9 may be in the form of tubes of a length corresponding to the width of the chassis, so that a substantial surface area of the layer may be heat-treated at one time. The chassis Scan be attached to a truck, or .it maybe self-propelled by the motor 7, so that it can be moved slowly over the entire surface of the pavement .to be hardened, such as an aircraft landing s i 7 It .was found that the time of exposureof abituminous layer to a temperature of about 200 C. should be for about one'half hour to one hour and a half in accordance the desired degree of hardness, since the hardness increases with the length of time of the exposure to heat.
Specimens made of bituminous layers treated in accordglnce with the described process are infusible and insoluble in such solvents as kerosene or other fuels. 7
.It is apparent that the process herein described is capable of many variations and modifications within the scope of the present invention.
What is claimed is:
In a process for making bituminous surfaces from a composition in which solid mineral particles are coated with an asphaltic bitumen binder, wherein said mineral particles are whirled up and are sprayed with liquified bituminous binder While suspended in air, for coating each individual solid particle with a thin film of binder notexceeding a thickness of 0.01 millimeter, thesteps of spreading said mixture to form the top layer of a pavement of a thickness of %to 1 inch, compacting said layer to such a degree thata void content of 1010 25% will remain in the compacted mass with air ,in the voids, and then hardening the bituminous component of the layer by heating the compacted bituminous layer to a temperature of about 200 C. to cause the heated air to harden said film. A
References Cited in the file of this patent UNITED STATES PATENTS 917,004 Clifitdrd Apr. 6, 1909 1,677,600 Schutte July 17, 1928 2,572,068 Sommer Oct. 23, 1951 FOREIGN PATENTS 12,798 Australia 1928 217,084 Australia 1956 766,133 Great Britain u--. Jan. 16, 1957 OTHER REFERENCES Asphalts and Allied Substances, by Abraham, 1945, 5th edition, vol. 1, pp. 79, and '101, pub. D Van Nostrand Co., N.Y.
Journal of the American Concrete Institute, p. 179, August 1957.