US3276890A - Asphalt paving composition - Google Patents

Description

United States Patent 3,276,890 ASPHALT PAVING COMPOSITION Jack M. Reese, West Point, Miss., assignor to Renrock, Inc., West Point, Miss., a corporation of Mississippi No Drawing. Filed Apr. 15, 1963, Ser. No. 272,878

' 6 Claims. (Cl. 106283) This invention relates to asphalt concrete, particularly for paving.

Reference should be made to the Asphalt Handbook (1960 edition) published by the Asphalt Institute of College Park, Maryland, from which the terminology used herein is taken, with minor exceptions.

Broadly, asphalt concretes are made up of an aggregate of a size range and distribution suited to the service intended, bound by an asphaltic cement or binder. The particular aggregate used in this invention is not a matter of significance since this invention is generally applicable to all aggregates. If an aggregate can be used in an asphaltic concrete of another kind, it can be used with my invention. The selection of aggregates is an operation more involved than is apparent to the average observer and is the subject of much technical literature. Extensive information on the selection of aggregates may be found in the Handbook on Bituminous Construction published by the Barber-Greene Company Handbook indicated above. I

In determining the amount of asphalt required for binding the aggregate, it should be remembered that the asphalt should adequately bind and stabilize the aggregate. Additionally, it must seal the pavement against the destructive action of air and water. On the other hand, an excessive amount of asphalt will impair the stability of the concrete body and lead to a slick surface. The amount of asphalt in the surface course is particularly critical. When the surface course is porous for any reason, including the use of insuflicient binder, air will penetrate into the structure and oxidize the binder. Oxidation results in hardening of the asphalt, and when its viscosity reaches a value somewhat above 1x10 poises at 77 F. the asphalt will no longer serve as a satisfactory binder at low atmospheric temperatures. When the binder becomes too hard, the pavement surface erodes under the action of traific. An inadequacy of 0.5% asphalt in a mixture may result in a road that requires excessive maintenance. From this, it will be recognized that it is desirable to employ asphalts of lesser viscosity and superior oxidation resistance for best results.

The foregoing criteria for the selection of asphalts are well known in the art. It is also known that the lower viscosity asphalts do not adequately bind the aggregate by reason of their fluent nature. Accordingly, it is customary to increase the viscosity of the asphalt by adding thereto a quantity of finely divided mineral matter, known as a filler or stabilizing agent. It has been observed that the same result is obtained by using a harder asphalt so far as the immediate result is concerned but such harder asphalts may be expected to have a shorter life. The mineral filler is frequently obtained by screening the aggregate to be employed to obtain a fraction passing the 200 mesh screen. Among the minerals that have been recognized as acceptable as fillers are: limestone, silica, Portland cement, clay, slate, talc, mica, diatomaceous earth and asbestos. It has also been recognized that there and the Asphalt is a great variation in the effectiveness of fillers; the minerals named are in an ascending order of effectiveness with asbestos recognized as having more than four times the value of limestone on a volume basis.

Not only is it important to test mineral aggregate and asphalt separately, but tests should also be made on the combinations of these materials to establish the proper proportions and characteristics for asphalt concrete mixtures. The Asphalt Handbook at pages 36 through 45 and 67 lists five recognized testing methods. Although no two methods can be related with mathematical precision, the results are generally comparable. The product of this invention has been tested by the Marshall method; this procedure is the official one used by the United States Corps of Engineers for designing air field paving.

Due to the necessity for procuring the aggregate at a point relatively close to the point of consumption, because of the great weight and bulk of material handled, a substantial degree of latitude must be employed in the selection of the optimum blends of ingredients for use at any specific location. The choice of suitable mixtures satisfying acceptable requirements is referred to as the;

design of a paving mixture.

This invention is based upon the discovery that asphalt concretes of unusually high stability value (as determined by the Marshall method) are produced by the use of a mineral filler consisting of Selma chalk limestone. Selma chalk limestone is the name of a geological stratum found over a wide area of the southern United States and particularly in the vicinity of Selma, Alabama, from which it takes its name. As is also indicated by the other portion of its name, it is primarily calcium carbonate of a soft, chalky nature. A typical analysis is:

A similar stratum, probably of the same geologic origin extends from southwestern Arkansas westward to the vicinity of Sherman, Texas, and thence in a generally southerly direction through eastern Texas.

The material is widely sold in finely ground condition under the name of agricultural limestone. It is usually ground to pass substantially through a No. 4 sieve and more than half will pass through a No. 40 sieve. Since asphalt concretes are normally intended to bear comparatively heavy loads, it is axiomatic that mineral aggregates should have very substantial crushing strength, far in excess of that possessed by Selma chalk limestone. Also,

since conventional limestone has the lowest satisfactory ternal grinding takes place during the batching and mix- Patented Oct. 4, 1966' ing operations employed during the production of asphalt concrete asit is usually made, due to the softness of the Selma limestone in relation to the hard aggregates needed for load bearing applications of the concrete. Consequently, it is assumed that a larger amount of the agricultural limestone is found ultimately in the filler than might be assumed from the initial sieve analysis of the agricultural limestone.

In a typical application, Where a range of 3 to 8% mineral filler Would be used, it has been found that agricultural limestone is sufiicient. Although the 10% used exceeds the 8% normal range, only of this or 2% is available in the filler range, yet the results are superior to those obtained when dust from a hard aggregate is used as the filler.

Although asphalt may be admixed with aggregate and mineral filler in a number of ways as by hot mixing, as cut-back, that is, diluted with a solvent, or as an emulsion. Further, the emulsion may make use of either an anionic or cationic emulsifying agent or the emulsion may be inverted, with the water as the internal phase. In thepractice of this invention, very gratifying results have been obtained in the use of an anionic emulsion, particularly of the slow setting type. Heretofore, asphalt concretes made by the hot mix method have been considered so superior for general paving that many State highway departments specify their use to the exclusion of other types. Following the teachings of this invention and using an anionic asphalt emulsion, asphalt concretes meeting the highest duty requirements can be produced without difliculty. Since the mixing apparatus needed for use with an emulsified asphalt is much less complex and less expensive, it is evident that great savings can be made.

In the tables that follow, there is demonstrated the manner ,in'which suitable levelling course and surface course paving materials are designed.

The first step is designing the asphalt concrete is to obtain a sieve analysis of the solid material to be employed. Table I shows this. Table II shows the specification to which the asphalt emulsion is produced; this is the type identified as SS- l.

TABLE I.SIEVE ANALYSIS OF MATERIAL Material Sieves Gravel Sand Agriculture Limestone The amounts listed under the respective material columns are the percentage amount passing each sieve.

TABLE IL-SPECIFICATIQNS ON EMULSION ASTM Furol viscosity in seconds 77 F., 40-400.

4 Percent soluble in carbon disuliide 97.0% min. Percent ash Max. 2%. Ductility, 77 F 40 centimeters min. Specific gravity, 77 F 1 min.

Next, tentative proportions of the solid ingredients are selected and made up and a new sieve analysis made of the mixed ingredients as shown in Tables III-A and III-B.

TABLE IIIA.PROPORTIONS OF MATERIALS AND COMPOSITE GRADATION Design Leveling Course Surface TAB LE III-B Sieves Leveling Course Surface The amounts listed under the respective material colratios in what are deemed to be in the acceptable range.

These are tested according to ASTM Standard D1559.

TABLE IVA.PROPORTIONS OF MATERIALS AND'PHYSL CAL PROPERTIES OF MIXES; LEVELING COURSE A B O Sp. Gr. of

Material 46. 0 45. 0 36. 8 36. 4 9. 2 9. 1 8. 0 9.0 Theo. Sp. Gr. of Mix 2.308 2. 276 Sp. Gr. 01 Sample 2.227 2.219 Percent Density 96.5 97. 5 Stabi1ity 948 898 Flow Value 7 9 Percent Moisture 4. 2 5. 2

TABLE IVB.PROPORTIONS OF MATERIALS AND PHYSI CAL PROPERTIES OF MIXES, SURFACE COURSE The foregoing tests and calculations were carried out under the supervision of Bruce G. Marshall, originator ofthe standard test. On the basis of the experimental results, he suggested that 8.5% of emulsion be used in the leveling course, to produce a Stability Rating of about 925 and a Flow Value of 8. For the surface course, the. recommended amount of emulsion-was 9.0. However, because the Stability Rating was greater. than required, it was recommended that the limestone be decreased from 25% to 15% with corresponding increase in the amount of sand. .As will be noted from Table IV-9 of the Asphalt Handbook, for heavy tnafiic,v a minimum Stability Value of 750 is desired along with :1 Flow Value of 8. Since he asp alt concrete of this particular design was intended for paving city streets, the minimum acceptable flow value was sought. For lighter traflic a Stability Value of 500 is recognized as acceptable 'but with the trend toward increasingly heavy trafiic, the higher Stability Values are favored. As shown by the results of Tables 'IV-A and B, the Stability Values obtained by the use of Selma chalk limestone ground to pass a No. 4 sieve tend to exceed the expectations of an especially skilled designer of asphaltic compositions.

Utilizing recognized standards for the selection of the kind, amount and size distribution of aggregate, recognized standards for the kind and amount of asphalt and established performance standards for asphalt concretes this invention resides in the use of Selma chalk limestone as the mineral filler for the asphalt. Desirably, it contemplates the inclusion of to of such limestone in the fine aggregate as well. Also, although it has been recognized that the hot-mix process of formulating asphalt concretes is most desirable, the use of this limestone yields excellent results with anionic emulsions thereby reducing the cost of producing acceptable paving mix.

I claim:

1. In an asphalt concrete containing an aggregate and asphalt, the improvement consisting of Selma chalk limestone as the asphalt filler.

2. An asphalt concrete containing as essential ingredients: an aggregate of size distribution suitable to the use intended for said concretes, the asphaltic residue of an anionic asphalt emulsion and finely ground Selma chalk limestone in an amount sufiicient to form the filler of the concrete.

3. An asphalt concrete containing as essential ingredients: an aggregate of size distribution suitable to the use intended for said concretes, asphalt and finely ground Selma chalk limestone in an amount suificient to form the filler of the concrete.

4. An asphalt concrete containing as essential ingredients: an aggregate of size distribution suitable to the use intended for said concretes, asphalt and finely ground Sehna chalk limestone, a portion of said being of fine aggregate dimension and a further portion being finer than No. 200 mesh to act as filler for said asphalt.

5. An asphalt concrete having a Stability Value in excess of 500 as determined by the Marshall method wherein substantially all of the mineral filler under N0. 200 mesh and a minor but significant amount of the fine aggregate is made up of Selma chalk limestone.

6. A composition as in claim 5 wherein the asphalt is the asp'haltic residue of an anionic emulsion.

References Cited by the Examiner UNITED STATES PATENTS 2,086,581 7/ 1937 Smith 106-28 1 X'R 2,773,777 12/ 19-56 Alexander et a1. 25 2-3115 3,046,149 7/ 1962 Moore et a1. 106-273 3,052,639 9/ 1962 Wright et a1 106-96 ALEXANDER H. BRODME/RKEL, Primary Examiner.

MORRIS LIEBMAN, Examiner.

J. B. EVANS, Assistant Examiner.

Claims (1)

11. IN AN ASPHALT CONCENTRATE CONTAINING AN AGGREGATE AND ASPHALT, THE IMPROVEMENT CONSISTING OF SELMA CHALK LIMESTONE OF THE ASPHALT FILLER.