US1946517A - Sand rock asphalt pavement - Google Patents

Description

Patented Feb. 13, 1934 UNITED STATES PATENT OFFICE Uvalde Rock Asphalt Tex., a corporation of Company, San Antonio, Texas No Drawing. Application February 4, 1932 Serial No. 590,989

4 Claims.

The invention relates to an improved paving material and more particularly to an asphalt pavement having wear and weather resisting qualities and which will be relatively non-skid.

Among the objects of the invention is to provide a pavement having the desirable features of limestone rock asphalt and sheet asphalt. The former type of pavement is generally recognized for its wear and weather resistance, although it is admitted to be deficient in flexibility and as having the objectionable feature of becoming slippery when wet. Since the-latter type is made of well graded sand filler and asphalt it is considered as one of the best of asphalt pavements although not up to the standard set by limestone rock asphalt for wear resistance.

Another object is to produce a less costly limestone rock asphalt pavement which can more nearly compete in price with artificial types of asphalt paving such as asphaltic concrete sheet asphalt, etc. Since rock asphalt is expensive to mine or quarry and since it is only found in certain sections and as the localities where it is found are usually some distance from the place where pavements are to be laid, the cost of the rock asphalt delivered on most paving jobs is much higher than the cost of the materials for other asphalt pavements, the bulk of which latter materials can usually be secured locally.

Another object is to produce a sheet asphalt type of pavement that can be laid directly on any firm or unyielding foundation without the necessity of using a binder course. At present asphalt pavements can not be successfully laid less than several inches thick because it has to be laid in two courses, the lower layer being a binder made of coarse aggregate so as to provide a key or bound for the upper layer of regular sheet asphalt mixture consisting of graded sand, filler and asphalt. The sheet asphalt mixture or wearing course can not be successfully laid directly on the foundation because the push or pressure of the roller will cause the same to spread while being laid.

Another object is to make a sheet asphalt type of pavement from a poorly graded sand which ordinarily would not be suitable for such use. Sheet asphalt requires that the sand be graded uniformly from the largest to the finest particles so as to produce a minimum of voids and also requires that the sand be made up of sharp angular grains rather than rounded grains. Sands or combinations of sands suitable for sheet asphalt are usually relatively expensive, whereas there are many other sands that are relatively cheap.

To accomplish the above objects the invention contemplates pulverizing the rock asphalt in what is known as a hammer mill. This pulverizer or hammer mill has a revolving shaft provided with a plurality of hammers, which upon rotation of the shaft operate to crush the stone between the hammers and a breaker plate, whereupon the pulverized stone leaves the crusher through a set of grates installed below the hammers. This type of crusher or pulverizer is preferred because when used with limestone rock asphalt it produces a product that is well graded from the largest to the smallest particles, thus containing a relatively low percentage of voids. Regulation of the hammer mill is secured so that it will pulverize the rock asphalt as fine as economically practical, preferably to the extent where the product will be of such fineness that seventy-five per cent will pass through a ten mesh screen.

After pulverization of the rock asphalt the next step is the addition thereto of approximately an equal amount of sand which is poorly graded as to size, there being a preponderance of particles of approximately one size to produce a maximum rather than a minimum of voids.

The rock asphalt and sand are then mixed and heated and any type of asphalt paving plant that is being generally used for making asphalt paving mixtures may be utilized for carrying out this step in the process. When the mixture has been heated to the proper temperature, namely three hundred degrees Fahrenheit, there is added sufiicient asphaltic flux to coat the sand particles and to soften the native bitumen in the rock asphalt so as to produce a fiuxed asphalt with a penetration of twenty to sixty at seventy-seven degrees Fahrenheit, the exact penetration being determined by local conditions. The invention does not contemplate the addition of suflicient flux to fill all the voids as determined by an analysis with a void meter of the compacted materials before being heated and compressed, there being added only sufiicient flux to substantially fill the voids after the mixture has been heated and finally compressed into a pavement.

To determine the proper amount of flux to be added it is desirable to make use of a trial and error method as follows. A laboratory sample containing approximately eight per cent total bitumen, that is, native asphalt plus flux, is molded under a pressure of about six thousand pounds per square inch, a slightly greater pressure than would be obtained with a roller, while the pavement is being laid. The bitumen is then extracted and the voidage of the aggregate is determined by the usual methods. From the voidage the per cent bitumen required to exactly fill the voids may be calculated and knowing the per cent of the bitumen contained in the rock asphalt it is then an easy matter to calculate the amount of flux necessary to add in order to have sufficient bitumen that is, native asphalt plus flux, to fill the voids.

Limestone rock asphalt is a finely porous rock the pores of which are filled with hard asphalt. When the asphalt is extracted, the porous limestone is very fragile and easily crushed. In a similar way, when rock asphalt is heated to a point where the asphalt becomes fluid, say, from three hundred degrees Fahrenheit to three hundred and seventy-five degrees Fahrenheit, it is easily broken, or crushed under pressure inasmuch as the heated asphalt being fluid offers but little resistance to said crushing. Therefore, when a heated mixture of rock asphalt and sand containing an excess of voids are compressed under the pressure or" a roller, the rock asphalt, being relatively fragile, is broken down into finer particles and many of the voids are filled by these fine particles, consequently such a mixture will contain considerably less voids after being compressed than before being compressed. For this reason the amount of bitumen in the finished pavement will have to be less than would be indicated by the voidage as determined by the usual methods.

Having a preponderance of larger size particles, an unbalanced mix with excess voids, the

paving material made in a manner as above described can be laid on any solid foundation without the use of a binder, for it does not push or shove to any great extent under the roller while being laid. Instead there results a mechanical bonding action between the larger size particles as they tend to key together when pressure is applied.

As has been explained previously, under this invention almost any kind of sand, no matter how poorly graded as to size is suited. The cheapest sand that is obtainable locally near the job therefore can be used with the result that the cost of the pavement will be materially reduced.

It is to be understood that I do not wish to be limited by the exact embodiment of the device shown, which is merely by way of illustration and not limitation, as various and other forms of the device will of course be apparent to those skilled in the art without departing from the spirit of the invention or the scope of the claims.

I claim:

1. The process for the manufacture of a rock asphalt paving mixture which comprises pulverizing limestone rock asphalt to a fineness where seventy-five per cent will pass a ten mesh screen, adding thereto sand poorly graded as to size to produce a maximum of voids, mixing the ingredients, heating the mixture to a temperature around 300 Fahrenheit, and finally adding an asphaltic fiux sufiicient to coat the sand particles and soften the native bitumen in the rock asphalt.

2. The process for the manufacture of a rock asphalt paving mixture which comprises pulverizing limestone rock asphalt to a fineness where seventy-five per cent will pass a ten mesh screen, adding thereto sand poorly graded as to size to produce a maximum of voids, mixing and heating the ingredients and finally adding to the mixture an asphaltic flux in sufiicient amount to produce a fluxed asphalt having a penetration of twenty to sixty at Fahrenheit.

3. The process for the manufacture of a rock asphalt paving mixture which comprises pulverizing limestone rock asphalt, adding thereto sand poorly graded as to size producing a maximum of voids, mixing and heating the ingredients, and finally adding to the mixture an asphaltic flux in amount sufficient to substantially fill the voids after the mixture has been compressed into a pavement.

4. The process for the manufacture of a rock asphalt paving mixture which comprises pulverizing limestone rock asphalt, adding thereto sand poorly graded as to size producing a maximum of voids, mixing and heating the ingredients, and finally adding to the mixture an asphaltic flux in amount sufficient to coat the sand particles and to substantially fill the voids as determined after the mixture has been heated and compressed into a pavement.

GLENN H. ALVEY.