US2049985A - Method of preparing a paving composition - Google Patents

Description

Patented Aug. 4, 1936 PATENT OFFICE METHOD OF PREPARING, A PAVING COMPOSITION Bertrand H. Wait, New'Rochelle, N. Y., and Ralph T. Haller, Plainfield, N. J.

No Drawing. Application March 8, 1934,

' Serial No. 714,630

2 Claims.

Our invention relates to paving,and more particularly to the method of preparing a mixture of a bituminous cement and an aggregate which may be laid either hot or cold and in which will satisfactory under heavy trafiic wherever it has been convenient to lay the asphalt directly from the hot mixing plant. Such a hot mix pavement is tougher than the cold mixes hereinafter re- 2 especially when laid with a tight and waterproof mixture, of presenting a slippery surface due to the fact that the excess of hot asphalt in the mix is brought to the surface of the pavement during rolling. It'is' necessary that such hot mixtures be laid within a short time after mixing as they have no workability and cannot be laid after they have cooled. Hence, a mixingplant 30 necessary.

Hot mixes of the foregoing character have a further undesirable feature in that a large percentage of their compression is secured under the roller and they are not substantially further compressed under traflic. It is a well known fact that the roller will not give the maximum pressure desiredand that a hot mix pavement .has a greater tendency to shove and rut than 40 would be the case if this same type of pavement could remain in a friable condition for a short period after laying to permit traffic further to consolidate it. If the hot mixes are laid with an asphalt of'suflicientl'y low penetration to 'elimi- 5 nate shoving and-rutting, they have'a tendency to crack due to temperature changes.

Later, other bituminous paving mixtures were developed covering for the most part cold mixes,

that is to say, mixes which could be laid cold. In'

making some of these so-called cold mixes, the aggregate is first treated with a volatile liquefier such as naphtha or kerosene and then coated with hot bitumen. Cold mix pavements have a large percentage of voids and their stability depends entirely upon the interlocking of the aggregate,

ferred to, but has the disadvantage generally, and

in'close proximity to the paving job is absolutely The voids permit air and water to penetrate the pavement and oxidize and disintegrate the bitumen. Furthermora'the bond between the bitumen and aggregate is not as good as in the hot mixes.

In accordance with another cold mix method, the aggregate is first treated with a flux oil, and powdered asphalt is then mixed therewith. In this case the production of an asphaltic cement and the proper mixing of the materials is wholly dependent upon the proper fiuxing of the solid asphalt, and the desired stability and durability are often delayed for considerable periods until such fiuxing takes place. i

In accordance with our invention there is duced an asphalt or tar composition for use in the construction and repair of. roads and pavements which approximates in its toughness and durability the best hot mixes above referred to and at the same time approximates the best cold mixes in its workability, ease of handling and long storage characteristics. According to our invention, we first, take a portion only of the total aggregate whether it be all fine or partly coarse and partly fine, and where the aggregate is both coarse and fine, preferably, but not necessarily, all of the coarse aggregate and a portion of the fine, and thoroughly dry and heat the same. While it is hot, we add thereto a hot bituminous cement in a quantity sufiicient to coat thoroughly. and uni 0 formly the particles of the aggregate and form a thin film of cement thereon. We find that hot bituminous cement having a penetration between and 140 degrees American Society of Testing Materials, serial designation D5-25 is satisfactory. A thorough and lasting contact is thereby produced between that portion of the aggregate which has been used and the hot bituminous binder which will assist in producing in the pave- 0 ment the same desirable toughness, accelerated stability and durability as is obtained with hot asphaltic mixes. 7

We then add to the cement coated aggregate a portion or all of the remaining aggregate which V is, of course, cold. The cold aggregate is thoroughly mixed with the batch of hot aggregate and hot bituminous cement already in the mixer and the mixing is carried on for a sufiicient time to distribute the cold aggregate thoroughly through the mix. During this process, the temperature of the entire mass is materially lowered below. that of the first mixture and the cold, fine aggregate coats and separates the particles 55 of cement coated aggregate, thus breaking up the adhesion which existed between them.

In case all of the remaining aggregate, to complete the mixture, was not added during the above process, it may be now added, or it may be added with the flux oil or pulverized asphalt which is used to complete the mix as set forth below.

Any oil such as a suitable crude petroleum or any other agent which is preferably liquid at temperatures of 40 degrees F. and over and which will flux and blend with the asphalts in the mix is then added in a sufiicient quantity not only to coat the dry particles in the mix but also to cut back somewhat the bituminous cement with which the hot aggregate has been treated. The time of mixing for this operation shall not be sufiiciently long to remove the hot asphalt from the aggregate.

After the materials have been mixed and coated with the fluxing oil, we then add pulverized asphalt to the mixture in such quantities and of such consistency that it will combine and thoroughly blend with the flux and the hot asphalt already in the mix to provide the desired hardness of blended asphalt in the finished pavement as soon as suflicient pressure is applied to bring the particles into close contact and permit them to cement together.

The composition made in accordance with our invention, due to the use of hot bituminous cement and the hot aggregate, has a degree of toughness comparable to that which is obtained in hot asphalt pavements and, due partially to the fact that the cement coated particles are more or less separated, and partially to the presence of the pulverized asphalt and the flux oil, the material is as easily handled and readily worked as those mixtures which are made wholly from fiux oil and pulverized asphalt, or from asphaltjwith a large percentage of liquefier. In other words, the composition embodying our invention combines the desirable characteristics of the hot mix and those of the cold mix, the hot asphalt treatment providing toughness, and the flux and pulverized asphalt treatment providing workability and non-skid properties on the surface of the course, vary with the location, sub-base, traffic and the tightness of the surface desired. Such requirements will determine the quality and stiff-' ness of the bituminous cement and to some extent the gradation of the mixture. Except for special cases, our method contemplates the use of an aggregate of which at least 25% thereof, for the coarser mixtures, will pass a screen having a A; inch square opening, and for the finer mixtures where surfaces similar in appearance to sheet asphalt are desired, of which may pass such a screen.

The percentage of cold aggregate used depends somewhat upon the time which will elapse between the mixing and the laying, but in all cases it should be sufficient to break up the adhesion between the hot coated particles. Where the mixture is to be stored a comparatively short time before laying, a major portion, say approximately 75% of aggregate, coarse and fine, may be uniformly good results, it is to be understood that in some cases, depending upon the type and gradation of the mixture being manufactured, it may be desirable to heat more or all of the fine aggregate and intermix therewith a portion or all of the coarse aggregate in a cold state. Again, it is not necessary that the fluxing material be incorporated before adding the pulverized asphalt. In some cases it may be desirable to add the asphalt first and the fiux afterwards, or first to add a portion only of the flux, then the pulverized asphalt followed by the remainder of the flux. In other words, we'desired to make it perfectly clear that following the separation of the hot cement coated particles by the addition of the cold aggregate, our procedure is not limited to the particular sequence of steps as set forth above. The steps ow adding the flux and the pulverized asphalt are of course essential,'but the order of their per- 20 formance is more or less immaterial.

.As an example of a pavement successfully laid by our method, we. coated 55% of=the aggregate hot with 3% of 30 penetration hot asphalt, added 3 /z% of suitable flux and 2%'of hard pulverized asphalt with the added cold aggregate and. obtained a pavement that had immediate stability when laid without sacrificing the storage characteristics claimed.

This pavement under continued observation shows resistance to displacement under heavy traffic, while at the same time it possesses a high ductility which prevents cracking at low temperatures.

The ratio of hot asphalt, flux and pulverized asphalt used will depend on the character of each component and upon the character of the final cement desired tomeet existing road conditions.

It is therefore obvious that in designing a pavement by this method a careful study should be made of the materials selected for use and thereby determine the proper ratio of the different asphalts for use in the pavement.

What we claim is:

1'. The-method of preparing a paving composition including a mineral aggregate which comprises thoroughly drying a portion of the aggregate by heating the same to a substantial degree, mixing the heated aggregate while in a hot, dry condition with hot bituminous cement until the particlesthereof are thoroughly and uniformly coated with a thin film of said cement, mixing with thehot, bitumen-coated aggregate a cold aggregate in a clean, uncoated condition and in a quantity sufficient to break up the adhesion between the particles of the hot coated aggregate and substantially to fill the voids therein, and

thereafter intermixing fluxing oil and powdered asphalt therewith; the quantity of fluxing oil being sufiicient to coat the particles of aggregate which were added cold, and the quantity of pulverized asphalt being suflicient to combine and blend with the fluxing oil and the first added bituminous material to form a friable mixture.

2. The method of preparing a paving composition including fine and coarse aggregates, which comprises thoroughly drying the coarse aggregate by heating the same to a substantial degree, mixing the coarse aggregate while in a hot, dry condition with hot bituminous cement until the particles thereof are thoroughly and uniformly coated with a thin film of said cement, mixing with the hot bitumen-coated aggregate fine aggregate in a clean, cold, uncoated .condition and in a quantity suflicient to break up the adhesion between the particles of the hot coated aggregate and substantially to fill the voids therein, and thereafter intermixing fluxing oil and powdered asphalt therewith; the quantity of fluxing oil being sufficient to coat the particles of fine aggregate, and the quantity of pulverized asphalt being sufiicient to combine and blend with the fluxing oil and the first added bituminous material to form a friable mixture.

BERTRAND H. WAIT. RALPH T. HALLER.