US3201269A - Asphaltic concrete pavement and method of preparing same - Google Patents

Description

United States Patent 3,201,269 ASPHALTEC CONCRETE PAVEMENT AND METHGD 0F PREPARENG SAME Philip C. Doyle, Rocky River, Ohio, assignor to The Standard Gil Company, Cleveland, Ghio, a corporation of Ohio N0 Drawing. Filed Mar. 12, W53, Ser. No. 264,762 1 Claim. (Cl. 106-28tl) This application is a continuation-in-part of application Serial No. 26,970, filed May 5, 1960, and now abandoned.

This invention relates to an improved asphaltic cement for paving purposes. More particularly, this invention relates to a method of improving an asphalt cement by retarding the loss of ductile characteristics which normally occurs during high temperature mixing with aggregate so as to improve the service behavior of an asphaltic pavement in which said asphalt cement is incorporated.

Good ductility is recognized by those skilled in the art as an important and desirable property for anasphalt cement intended for paving purposes. It is believed that the ductility of an asphalt cement is related to the actual service behavior of an asphaltic cement in which it is incorporated so that as a qualitative matter the better the ductile characteristics of the asphalt cement present in the asphaltic pavement, the better Will be the service behavior of the pavement.

In my paper entitled Cracking Characteristics of Asphalt Cement, which appears in volume 27 of the Pro ceedings of the Association of Asphalt Paving Technologists, at page 581, I propose that the ductile characteristics of asphalt cement are meaningful with respect to the tendency of an asphaltic pavement in which it is incorporated to fracture or crack in service, particularly if the ductile characteristics of the asphalt are measured on a ductilometer at lower temperatures and speeds than those commonly employed. In this paper tests are reported on core samples which were drilled from pavements actually in service, showing both good and bad resistance to surface cracking. It was found from these various tests that when the ductile characteristics of the asphalt cement recovered from these core samples were run at 55 F., and 1 cm./min..speeds on ductilometer the ductility values of the asphalt cement could be closely correlated with the service behavior of the pavement. By comparison, it was found that when the ductility of the recovered asphalt was determined at temperatures of 77 F. and at speeds of 5 cm./min. the ductility values obtained were not always consistent with field results with respect to the cracking tendency of the pavement. Besides showing that ductility measurements of the asphalt cement at 55 F. and 1 cm./ min. speeds on the ductilometer offer a convenient means for predicting the service behavior of an asphaltic pavement, the results reported in my paper for these low temperatnure ductility measurements stress the importance of preserving good ductile properties in the asphalt in order to insure that the asphaltic pavement will provide good service behavior.

It has been found that asphalt cements generally have initially good ductile characteristics so that if this high degree of ductility were possessed by the asphalt after being laid into a pavement, good service behavior of the pavement could be expected. However, it is well known that the ductility of the asphalt cement is materially lost during the high temperature mixing of the asphalt cement with aggregate so that the ductile characteristics of the asphalt cement in the pavement is usually of a relatively low value. For example, a typical asphalt for paving having a penetration value of 7085 measured at 77 F. obtained from a Mississippi-Arkansas-Illinois crude had a ductility of 150+ cms. when measured on a ductilometer at 55 F., 1 cm. speed. This asphalt, however, after Patented Aug. 17, 1965 ICC being mixed with gravel aggregate in a standard mix mill at 310 F. in accordancewith conventional trade practices showed a ductility value of only 10 cm. when measured at 5 F. and 1 cm. speed.

This, of course, demonstrates the drastic eflfect the high temperature mixing has on the ductile characteristics of an asphalt cement. It is believed that the loss in ductile characteristics which occurs during the mixing operation is due primarily to an oxidation mechanism induced by the high temperatures that must be employed. It is to be understood, however, that the present invention is not at all dependent upon what mechanism is responsible for this loss in ductile value since the fact remains that such a loss does occur. There is, of course, some progressive hardening and loss of ductility in the laying of the asphalt and in its exposure to the elements while present in the pavement, but this loss is negligible in relation to the loss which occurs during the mixing operation. Consequently, if some way could be found to inhibit the loss occurring during the mixing operation, the ductile characterisitcs of the asphalt could be largely preserved and the pavement in which it is incorporated would have greater resistance to cracking.

Generally, the degree of ductility loss in the mixing operation increases with the increase of temperature and/ or time employed in mixing and the time the finished mixture is held in a truck at the mixing temperature prior to its actual laying. According to good trade practices, the temperature at which a mix mill is operated is generally selected as the temperature which will give seconds viscosity on a Saybolt-Furol viscometer for the particular asphalt that is used. It is believed that this temperature provides the best film thickness of asphalt on the aggregate for paving purposes. However, since few mix mills are equipped with thermocouples, the operating temperature in actual practice of these mills has a tendency to vary over wide limits and the problem of the loss of ductile characteristics in the asphalt cement becomes particularly aggravated. Also, since the locations of these hot mix paving plants are frequently distant from the paving job, it often happens during cold climatic conditions that the asphalt must be mixed at much higher temperatures than the proper mixing temperature in order that the paving mix will arrive at the job at a sufiicient temperature for paving. Moreover, frequently the time of mixing is not carefully or uniformly controlled at the mix mill with commensurate depreciation of the ductile temperatures or at proper mixing temperatures but for excessive mixing periods.

The primary object of the present invention, therefore, is to minimizethe loss of ductile characteristics in an asphalt cement which occurs during the high temperature mixing with aggregate so as to improve the asphalts resistance to cracking in the pavement. It is a further purpose of this invention to permit much greater latitude in the time and temperature of mixing of asphalt with aggregate. A still further object of this invention is to permit asphalt cements which are originally borderline with respect to ductile characteristics to be used in paving operations where they formerly could not be employed.

These objects and other objects which. will become apparent from the following discussion may be accomspouses 6 plished in accordance with the present invention by adding to the asphalt cement before the mixing operation a phosphorus compound selected from the following general formula:

wherein R is a phenyl radical or an alkyl substituted phenyl radical containing up to a total of 12 carbon atoms in the alkyl substituent or substituents, and preferably not more than 4. R may be, for example, phenyl, tolyl, xylyl, butyl phenyl, hexyl phenyl, 2-ethyl hexyl phenyl, dodecyl phenyl, butyl octyl phenyl, or butyl hexyl phenyl. The phosphorus compounds of the invntion are well known to the art and are commercially available. Such materials are readily prepared by reacting phenols or alkylphenols with phosphoric acid according to known methods or by any other suitable means forming no part of the present invention.

In order to gain any significant effect, at least 0.01% by Weight based on the weight of the asphalt cement of one or a mixture of phosphate compounds from the foregoing general formula must be used. Actually, there is no upper limit but amounts over 2% by weight usually cannot be justified economically. A preferred range is generally from 0.5% to 1% by weight.

The asphalt ingredient of this invention may be any natural or manufactured bituminous material which may be mixed with any of the common aggregates, such as crushed limestone, slag, crushed rock, sand, gravel, etc. to form an asphaltic concrete for paving. The desirable properties for such an asphalt cement may be found in highway specification manuals or in Abrahams text Asphalt and Allied Substances. In general, asphalt cement forpaving purposes is required to meet penetration specifications and the preferred penetration will usually be from 50-200 at 77 F.

, In preparing an asphaltic concrete for paving, the phosphate compound is introduced into the asphalt cement at some convenient point before the cement is charged to the mix mill, employing any suitable mixing means which will insure that the additive is evenly distributed throughout the asphalt cement. The cement is then mixed with aggregate in the proportions of from 4 to 8 parts by weight of asphalt cement to 96 to 92 parts by weight of aggregate, with heating at an elevated temperature in the range of from 280 F. to 350 F. As indicated hereinabove, the preferred temperature vithin this range is the one which gives a viscosity of 120 seconds on the Saybolt- Furol viscometer, for the particular asphalt cement used.

The preparation of an asphaltic concrete pavement in cludes the additional steps of transporting the mixture, While still at an elevated temperature, from the site of mixing to the'place where the mixture is to be laid as a pavement, and laying the mixture as an asphaltic concrete pavement. In order to allow for proper handling the temperature of the mixture at the place it is to be laid as a pavement should not be below 250 F.

For best results, the pavement should be built up from one or more layers of asphaltic concrete each of which has been compacted to a thickness of 1 to 3 inches, and contains not more than about voids. The phosphate additive functions during the mixing and transporting of the asphaltic concrete at an elevated temperature to inhibit a ductility loss in the asphalt in the finished pavement.

A better understanding of the present invention Will be gained from the following discussion of Oven Weather Tests comparing the loss of ductile characteristics during mixing with aggregate of an untreated asphalt cement with the same asphalt treated with a phosphorus corn; pound of the invention.

The Oven Weather Test simulates the high temperature mixing in a conventional mill and correlates therewith. Results from this test have indicated that ductile characteristics measured on a sample of asphalt cement recovered from the hot paving mix prepared and heated in accordance with this test at 55 F, 1 cm. speed, parallel the ductile characteristics When measured at 55 F, 1 cm. speed, for a sample of the same asphalt cement recovered from a hot paving mix just before it is laid as a pavement following mixing in a conventional mix mill at the proper temperature to give 120 seconds viscosity (Saybolt-Furol) for the asphalt cement employed and including an average of approximately one hour transportation of the hot mixture to the job site.

In executing this oven test, 3000 grams of standard Ottawa sand is mixed with 120 grams of asphalt cement for two minutes at the temperature which gives 120 seconds viscosity as determined by means of a Saybolt-Furol viscometer for the asphalt cement of the test. Immediately after this mixing, the mixture is spread to a uniform depth in shallow aluminum pans measuring approximately 5 /2" in diameter and /8" in depth and allowed to cool to room temperature for at least one hour. The asphaltic mixture is scraped from the pans and the asphalt is recovered by extraction with benzene in accordance with Absons Method and Test which is recorded in detail beginning at page 48 of The Association of Asphalt Paving Technologists Proceedings for 1952, vol. 21. The original asphalt and the recovered asphalt, on evaporation of the benzene, were tested for ductility according to ASTM designation Dll3-44 at 55i0.9 F. and at a rate of speed of 1 cm./min.

In the table below, the oven weathering of an asphalt cement having a penetration of -100 at 77 F., obtained from a Mississippi-Arkansas-lllinois crude is compared with the same asphalt after treating with tri-tolyl phosphate and tri-phenyl phosphate. In each case the phosphorus compound was added to the asphalt cement While it was maintained in a molten and mobile condition. A mechanical stirrer was provided to insure that the phosphorus compound was evenly dispersed throughout the asphalt.

it is obvious from these data that the phosphorus compounds of the invention markedly reduce the loss in ductile characteristics during high temperature mixing with aggregate.

it is to be understood that various modifications of the foregoing invention will occur to those skilled in the art upon a reading of the description. All such modifications are intended to be included as may be reasonably covered by the appended claim.

I claim:

A method for preparing an asphaltic concrete pavement exhibiting improved retained ductility comprising the steps of adding to an asphalt cement having a penetration of from 50-200 at 77 F, in an amount within the range of 0.0l to 2 wt. percent based on the Weight of asphalt cement, a phosphorus compound of the general formula:

6 wherein R is selected from the group consisting of phenyl, References Cited by the Examiner and alkyl substituted phenyls in which the alkyl sub- UNITED STATES PATENTS stituent contains a total of up to 12 carbon atoms, mixing from 4 to 8 parts by Weight of said asphalt cement con- 2693425 11/54 Hardman 106 '273 talning said phosphorus compound With from 96 to 92 5 OTHER R CE Parts y Weight of aggregate at an elevated tempefatllfe Page 528 and chapter XXVIII, volume one, fifth ediof from 280 F. to 350 F., and While still at said eletion, Abraham, Asphalts and Allied Substances, New vated temperature, transporting the mixture from the site York, D. Van Nostrand Company, Inc.

of mixing to the place the mixture is to be laid as an asphaltic concrete pavement, laying the mixture as an 10 ALEXANDER BRODMERKEL Primary Examine"- asphaltic concrete pavement. MORRIS LIEBMAN, Examiner.