CA1113657A - High strength modified asphalt paving composition - Google Patents

Abstract

TO ALL WHOM IT MAY CONCERN:
BE IT KNOWN that WE, LAURENCE (NMI) LATTA, JR., and JOHN B. LEONARD, JR., citizens of the United States of America, residing in the City of Portola Valley, County of San Mateo, State of California, and residing in the City of Hillsborough, County of San Mateo, State of California, respectively, have invented certain new and useful improvements in HIGH STRENGTH MODIFIED ASPHALT
PAVING COMPOSITION
of which the following is the specification.

Abstract of the Disclosure A paving composition formed of aggregate mixed with substantially unblown asphalt modified by dispersing a metal catalyst throughout the asphalt. The catalyst includes one or more of the following metal ions - manganese, copper, or cobalt. The composition is employed for paving roads by conventional techniques.

Description

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sackground of the Invention The present invention relates to an improved asphalt-aggregate paving composition.
Aggregate-con-taining asphalt has been employed as the paving composition ~or roads or the like for many years. The asphalt includes bitumens as a predominant constitutent and is conventionally obtained as a solid residue from the distillation of crude petroleum. The asphalt is conver-ted to a fluid state when paving a road. One fluid form is the suspension or emulsion of the asphalt in water. After spreading and compressing the aggregate-containing asphalt, water evaporates and the asphalt hardens into a continuous mass. Another form of asphalt employed in road construction is a cut-back, i.e., a liquid petroleum product produced by fluxing an asphaltic base with a suitable distillate. A road is formed by layering the cut-back and evaporating the volatile distillate from the mass. The ` advantage of using the above road construction techniques is the avoidance of high tempera-ture application. In an alternative technique, the asphalt and aggregate can be mixed and applied at elevated temperatures at the fluid state of the asphalt to form the road. Then, the asphalt need not be cut-back or emulsified.
It is advantageous to increase the adhesivity of asphalt, especially in the form of a cut-back of emulsion, to be employed with aggregate in road construction. One technique which has been disclosed to increase such adhesivity is set forth in U.S. patent 2,342,861. The examples of that patent illustrate the addition of a lead soap, specifically lead oleate or naphthenate, to asphalt cut-backs or emulsions : , ' . ,:

-to incr~ase th~ir adllesivl-ty for aggregate. Although in all illustrated examples only lead is disclosed as a metal soap to increase adhesivity, -the patent suggests tha-t other heavy metal salts of organic acids could be employed including the following metals: Fe, Al, ~, Zn, Co, Ni, Sn, Ca, Sr, Ba, and ~Ig. The patent discloses a technique of forming the lead soap by heating a lead oxide in the presence of the desired organic acids. Such lead soaps are then added to the desired asphalt.
Heavy metal salts oE high molecular weiyht organic acids, such as naphthenates or linoleates, have been employed to prevent cracking in a blown or oxidized asphalt coatings.
For example, U.S. patent 2,282,703 discloses the use of heavy metals such as cobalt, manganese, iron, lead, vanadium, or zinc dispersed into the blown asphalt for this purpose.
Heavy metal soaps have also been disclosed for use as a dispersant in roofing asphalts to prevent failure of the asphalt due to "alligatoring". U.S. patent 2,928,753 discloses the polyvalent metal salts of copper, cobalt, or manganese in combination with high molecular weight mono-carboxylic acids such as oleic or naphthenic acid. The final disclosed product is an aggregate-free coating of 0.025 inches thick on an aluminum sheet heated so that leveling occurs.
Summary of the Invention and Objects In accordance with the present invention, it has been found that dispersing certain polyvalent heavy metal catalysts in asphalt and then mixing the same with aggregate forms a paving of vastly improved characteristics. In order of effectiveness, manganese, copper, and cobalt in quantities as low as 0.05% to 0.59~ by weight o-f the asphalt procluces an enormous in-crease in comp-ressive, flexural cmcl fatigue strength of the ultimate cured paved road.
~ he asphalt may be fluidized for me-tal catalyst dispersion by con-ventional technlques such as emulsification, cutting back, or by heating the asphalt to a temperature above its mel-ting or softening point. The metal catalyst-containing asphal-t may -then be mixed in this form directly with the aggregate for road construction. It has been found that the modified asphalt may be stored in bulk prior to road formation without hardening.
It is an object of the invention to provide a modified asphalt-aggregate paving composition of exceptional strength and fatigue resistance for use in the formation of roads or the like.
It is a particular object of the invention to provide a modified asphalt which is of suitable viscosity in bulk for paving but which hardens into a thermosetting polymer of exceptional strength after paving.
It is a further object of the invention to provide an asphalt-aggregate paving composition of superior Marshall stability.
It is a further object of the invention to provide a paving compo-sition of the foregoing type which retains a large portion of its strength at elevated temperatures while forming a flexible pavement possessing self-healing properties.
The present invention provides a paving composition comprising a major proportion by weight of aggregate and a minor proportion of substan-tially unblown asphalt which is present as a coating on the aggregate, the asphalt having uniformly dispersed or dissolved therein at least one oil-soluble soap which is a manganese, copper or cobalt salt of a monocarboxylic acid having not more than 30 carbon atoms, or a mixture of two or more thereof, and which is present in an amount such that the composition contains from 0.01 to 0.5%, based on the weight of the asphalt, of manganese, copper or cobalt ions, or a mixture thereof.

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Detailecl Description of the Preferred Embodiments l'he present invention relates to a modified asphalt which, combined with aggregate, forms a paving composition providing significan-tly improved physical properties.
The asphalt is modified by dispersing a metal catalyst throughout the asphalt while the asphalt is in a fluid state. Such fluidity may be accomplished in any of the known conventional techniques such as by heating -the asphalt to ahove its softening point, emulsifying the asphalt, or by forming a cut-back oE the same in a volatile organic solven-t.
The me-tal catalyst of the present inven-tion are formed from a metal salt in which the metal ion is manganese, copper, cobalt or mixtures thereof.
As used herein, the term "asphalt" refers to any of a variety of substantially unblown or unoxidized solid or semi-solid material at room temperature which gradually liquifies when heated. Its predominant constituents are bitumens, which are obtained as the residue of refining processing.
It is important to disperse the metal catalyst of the present invention uniformly throughout the asphalt so that its catalytic effect is imparted to the final product in a consistent manner. The form of metal catalyst is important in obtaining such uniformity of dispersion by thorough penetration of the catalyst throughout the asphalt. For optimum dispersion, the metal catalyst is in the form of a metallic organic salt which is soluble in a significant port-ion of the asphalt. It is known that an excellent anion for the metallic organic salt of this type is derived from a high molecular weight monocarboxylic acid.

~365~7 Such metallic organic salts are oil-soluble soaps of monocarboxylic acicls, preferably having from 6 to about 30 carbon atoms in the chain. Preferable anions for ~his purpose include linoleates, octoates, naphthenates, oleates, stearates, and laurates.
Significant improvemen-ts in the modified asphalt are obtained by adding a relatively small quantity o~ the metal catalyst. Thus, a concentration of metal ion as low as 0.01% by weight of manganese ion based on the asphalt yields a paving composition of improved compressive load strength. It has been found that optimum properties are obtained at a minimum of 0.05 - 0.5% by weight of metal ion.
Levels of me-tal ion above such concentrations, say at 1%, produce only marginal improvements. For economy, the optimum range is from 0.05 to 0.15% by weight of metal based upon the asphalt.
Other forms of the metal catalyst than -the foregoing me-tal soaps may be employed in accordance with the present invention. Thus, for an asphalt containing substantial naphthenic acid, an inorganic compound of the heavy metal such as oxide, sulfate, chloride or hydroxide may be added to asphalt and heated to form a soluble heavy me-tal naphthenate soap in situ. A technique of this type for oxides is described in U.S. patent 2,3~2,861.
It may be possible to employ other dispersable forms of the metal catalysts of the present invention. For example, the metal may be in a chelatable form suitable for reaction with the asphalt. If a material of this type is added in a solid form, it is preferable as a fine powder to assist dispersion in the asphalt.

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One technique for mixing -the Eoregoiny rnetal catalysts with the asphalt of the present lnvention is to heat the asphalt to above its soEtening or melting point until it is sufficiently fluid to thoroughly disperse the metal catalyst. This technique is designa-ted "warm mixing"
herein. For this technique, i-t is preferable to employ the metal catalys-t in the liquid form. For most conventional asphalts, a temperature on -the order of 110 to 120C is ; sufficient for asphalt fluidizing. At such temperatures, the viscosity of the asphalt composition is suEficiently reduced to permit thorough dispersion by manual mixing.
The warm mixing technique may be employed at a facility remote from the road construction site. This is possible because, as set forth hereinafter, the catalyst does not harden the asphalt while it is in bulk form. Thus, the modified asphalt can be stored in bulk until needed. In conventional processing, the modified asphalt is maintained in a fluid state from its time of formation, during normal storage and transport to the road site, during mixing with aggregate and until final paving. In an alternative to remote warm mixing, the metal catalyst could be added to the asphalt at the road construction site just prior to paving.
The modified asphalt is characterized by a viscosity in a fluid state at the elevated temperature of road building comparable to conventional asphalt. However, as set forth below, the cured paved road has vastly superior strength in comparison to one formed with conventional asphalt.
Conventionally, the warm mixed modified asphalt in fluid form is premi.xed with heated aggregate to form a paving composition, suitably in a mixing tank truck. Then, ., ~

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the composition is spread on the road bed and compacted. Af-ter curiny, the road comprlses agyregate bound by a matxix of modifie~ asphalt binder.
It is noted tha-t acceptable curing occurs at ambient temperatures, e.g., 22C. Moderate elevations (e.g., to 50C~
in the temperature of curiny accelerates -the process. However, very high temperatures such as employed to blow asphalt, i.e., on -the order of 230C, would be unacceptable for the present process. As set forth below~ it is is believed -that this is due to the decomposi-tion of peroxides or hydropero~ides required for free radical polymerization to form the superior product of the present invention.
Other conventional techniques for fluidizing asphalt may be employed to permit thorough dispersion of the metal catalyst. For example, the asphalt may be formed into a cut-back by fluxing the asphalt with a suitable volatile solvent or distillate. Thus, the metal catalyst of the present invention could be added directly to the asphalt cut-back. The modified asphalt cut-back could then be directly mixed with aggregate and applied as a paving composition.
Another conventional technique for fluidizing the asphalt to disperse the metal catalyst is to first emulsify the asphalt by known techniques. Like the cut-back technique, an advantage of this mode of fluidizing the asphalt is that dispersion can be accomplished at room temperature. Also, like the cut-back, the metal catalyst-containing asphalt emulsion can be mixed in that form with the aggregate to form the paving composition.
The aggregate of the present invention is suitably -'7 of a type employ~d in the road building industry. It may range from Eine particles such as sand to relatively coarse, ground particles such as crushed stone, gravel or slay.
A major portion by weigh-t of aggregate is mixed with a minor portion by weight of asphalt. The ratio of aggregate to modiEied asphalt is -that typical for road paving applications. Thus, a minimum of about 85% by weight of aggregate and generally abcut 90 to ~6~o by weiyht of the total paving composition is employed in the present invention.
As set forth in the background of the invention, heavy metal soaps have been employed in combination with asphalt for a number of different purposes. For example, ; they have been used as a catalyst for avoiding cracking in blown asphalt, and preventing alligatoring in roofing materials. Also, such metal soaps have been disclosed in aggregate-containing road building compositions to improve adhesivity of the asphalt for the aggregate. For use as a paving composition, there is no disclosure in the prior art that such heavy metals significantly increase the strength of the final product. Furthermore, the prior art teaches the general equivalency of multivalent heavy metal ions for this purpose. For example, in the aforementioned U.S. 2,342,861, experiments were performed employing lead soaps to increase ; adhesivity of asphalt for aggregate. In accordance with the state of the art, the patent asserts that other metals such -as iron, aluminum, manganese, zinc, cobalt, nickel, tin, calcium, strontium, barium, or magnesium could also be employed for the same purpose.
Wlth the above background knowledge, it has unexpectedly found that only selected heavy metals serve as ~.
_g_ 3~5'7 catalysts to enormously increase the strenqth, fatigue resistance and other properties of an agcJregate-containing paving composition while the remainder of the heavy metals have little or no beneficial ef~ect. Thus, it has been found tha-t manganese, copper and cobalt, in that order, are vastly superior to the remainder of the heav~ metals ~or this purpose.
It is believed that extraordinary increases in the flexural strength and fatigue resistance oE the foregoing modified asphalt-aggregate cured composition may be explained in accordance with the following theory. It is believed that the soluble salts of the manganese, copper, or cobalt metal ions dispersed in asphalt causes the asphalt to be vinyl polymerized when mixed with aggregate during curing at lS either ambient or elevated temperatures. This is to be contrasted with the oxidation in conventional asphalt which -~
actually degrades the flexural strength and fatigue properties of the same. There is strong evidence that the double bond content of the various constituents of the present modified asphalt are activated to permit cross-linking and polymerization. A plausible theory as to the effect as to the metal catalysts of the present invention is that they generate peroxides or hydroperoxides which are catalyzed by the metals to yield free radicals which, in turn, are known to catalyze vinyl polymerization. It is totally unexpected that manganese, copper, and cobalt, in that order, would be vastly superior to other heavy metal catalysts for this purpose.
Support for the above theory of vinyl polymerization may be found by the following explanation. It is believed .

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L365'7 that the vi~yl unsaturation oE conventional paving asphalt can be measured by the Woburn Iodine Number Method as set forth in Von Mikusch, J.D., and Fraizer, Charles, "Woburn Iodine Absorption Method", Ind. Eng. Chem. Analytical Edition, 1941, 13, 782. A typical paving asphalt has a Woburn iodine number of about 90. This means -that there is approximately 1 mole of double bond for each 280 grams of asphalt. By the above test, the starting asphal-t yielded a Woburn iodine value of 91. An asphalt including 0.2% cobalt by weight was mixed with aggregate and cured. The asphalt was extracted from this sample and yielded a ~oburn iodine value of 70. The decrease in iodine number indicates that significant vinyl polymerization occurred during curing.
Further support for the polymerization theory of increased strength is found in the ratio of pentane insoluble materials in the final cured modified asphalt product in comparison to the starting material. One such test is ; described in a paper by Kleinschmidt, L.R., entitled "Chromatographic Method for the Fractionation of Asphalt Into Distinctive Groups of Compounds", Journal of Research of the National Bureau of Standards, 1955, 54, 163. The percent of pentane insolubles in the starting aggregate was 12.8% and in the asphalt extracted from aggregate was 26.5%. It is believed that the increase in pentane insolubles is caused by polymerization as set forth above.
It has been found that the modified asphalt of the present invention does not harden or cure when in bulk form. Thus, it remains at a viscosity at elevated temperatures above its melting point comparable to unmodified asphalt.
Thus, the metal only functions as a catalyst for hardening . .

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or curing the asphalt after mixing with acJgreyate. It is believed that this phenomenon may be explained by the requirement tha-t the modified asphalt be in a relatively -thin film, the state in which it exists in combina-tion with aggregate.
Thus, the asphalt tends to Eorm a thin coatiny of, 5 - 10 microns on the surface of sand aggregates which apparently causes catalytic activity in -the metal catalyst. Although -the maximum thickness of the -thin film i-or this effect is not known, it is believed that the film may be as thick as150 rnicrons or more. An explanation for -the requirement of the asphalt to be in a thin film form is -that a certain amount of oxygen is required to penetrate througho~lt the film to permit catalysis to occur. That is, in accordance with the foregoing theory, the metal catalyst of the present invention requires a small quantity of oxygen to form peroxides or hydroperoxides which yield free radicals for vinyl polymerization.
It has been found that increasing the void ratio of the modified asphalt-aggregate composition causes a corresponding increase in the curing rate. This is consistent with the theory that a certain amount of oxygen penetration of the asphalt is required to interact with the metal catalyst to initiate polymerization. Thus, at a 20% void ratio in a sand aggregate composition, significant curing can occur in a week.
; 25 A further disclosure of -the nature of the present invention is provided by the following specific examples of the practice of the present invention. It should be understood that the data disclosed serve only as - examples and are not intended to limit the scope of the invention.

xample 1 Comparative tests were performed with various transition heavy metals as addi-tives -to the asphalt. In each instance, sand of the F6-039 (Shayba) type, classified as a dune sand, was mixed with a modified bitumen asphalt designated AR-8000 by the Sta-te of California in a ratio of 4% by weight of modified asphalt to 96% by weight of sand.
The modified asphalt in each instance included organo-metallic compounds (soaps of napthanates or octoates) in a ratio so that the metal was presen-t at 0.2% by weight of the asphalt. The soaps, in liquid form, were mixed with the fluid asphalt at elevated temperatures (about 110-120C) sufficient to melt the asphalt. Mild manual stirring of the mixture was employed to thoroughly disperse the metal throughout the asphalt.
Short minia~ure cores were molded at 154 - 158C
and cured at 50C for seven days. The cores were tested for compressive strength at both 22C and 50C. The same cores were re-tested under the same conditions after one and two weeks. The cores of the second test were slightly larger in ; diameter so the results should be multiplied by approximately 0.97 for correction. The results of the tests are set forth in the following teble.

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3~ 7 l'he columns designated "enc:Losecl" lndicate that the compositions were cured in containers to eliminate mos-t but not all oxygen contact. The compressive strengths of the enclosed and totally exposed samples are comparable indicating -that there is no significant effect in starving the material of oxygen during curing.
It is apparent from Table I -that manganese is the preferred metal ca-talyst for all tests. However, copper and cobalt also provided extraordinary increases in s-tructural streng-th in comparison -to the remainder of the tested metals.
Example 2 A series of tests were performed on the basic paving composition of Example 1 but using only manganese octoate at 0.2% of the total asphalt. The sand aggregate was loaded at 96~. The results of the above test are summarized in the below table. All samples were cured at 50C for one week exposed to the air. Similar results are obtained at lower ambient -temperatures (e.g., 22C) at increased curing times.

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TABI,~ Il _ tJnmodiEied Modified ~sphalt _ Asphalt - Unconfined Compression Test:
strength at 22C, kg/cm 8.~ 63.2 strength at 22C, kg/cm 0.3 20.3 - Marshall Stabili-ty Test:
stability, lb. 250 1800 flow, 1/100 inch 14 12 - Static Flexural Test:
modulus of elasticity, kg/cm2 780 5175 modulus of rupture, kg/cm 3.4 17.3 ultimate strain (x 10 4) -* 152 ! - Dynamic Fatigue Tests:

modulus of elasticity, kg/cm2 -* 60,000 endurance limit, e x 10 ~ -* 80 ,, .

* These quantities could not be determined for untreated ; sand-asphalt beams.

It was found that full strength at 22C testing temperature was obtained in about four weeks, although a longer period of time was required for full strength at a 50C
test temperature.

The mode of failure was of a plastic nature. The "failed" specimens could be rested several times within relatively short periods of time (e.g., one or two days) and still yield about the same strength. This indicates a combination of plastic flow and thixotropic behavior, hence, a retention of strength near the peak point, and a healing action beyond.

, 3~S'7 The Marshall stability test revealed the stability increased at a decreaslng rate beyond one month.
The static flexure tests were performed by molding beams rneasuring 25 cm in leng-th, 2 cm x 3 cm in cross-section.
The beams were cured at 22C for about one month, and were then tested in static flexure in third-point loadiny, over a clear span of 22 cm. The rate of loading was 0.05 inches per minute, and the tempera-ture at test was 22C. The results are set forth in the above table. The ul-timate flexural strain of the unmodified asphalt-sand beam could not be obtained because i-t deformed continuously for the duration of the test. The modified asphalt beam ultimate strength was approximated by adding the elastic strain near the ultimate load to its plastic strain at rupture.
. 15 Beams of the foregoing type were tested in a dynamic fatigue machine supported over a span of 22 cm loaded repeatedly at their mid-point. A steel leaf was placed crosswise under the beams to push them back up each time the load was removed. The rate of loading was three repetitions per minute, and the temperature at test was 22~C.
In performing the fatigue test, no meaningful results could be obtained for the unmodified asphalt beams as they deformed both vertically and laterally under relatively low levels of load. The results of the fatigue test were plotted on a log-log scale to yield an equation in the standard form Nf = K(e)C wherein Nf is the number of load repetitions to failure, e is the corresponding flexural strain, and K and c are regression constants. K and c were found to be 1.82 x 1012 and 3.29, respectively, where e is expressed in micro units.

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~ i`he clynamic modlllus of elasticity of thc sand-asphalt beams was found to be on the order of 850,000 p51. At this modulus, the modi~ied asphalt-sand produc-t was upgraded to the level of asphaltic concrete in fa-tigue life.
Example 3 Mixes were made of asphalt plus 0.05%, 0.1P6 and 0.2% manganese. Standard Marshall GOreS were molded at 4%
bitumen. Half the cores a-t each level of manyanese were put into a 50C oven. The other half were left on the bench. ,At 7 days of cure time, Marshall Stabili-ty Tests were run. The resul-ts are set forth in Table III below.

TABLE III

% Manganese in Asphalt Cure and Test 0.05 0.10 0.20 :, 22C cure, Marshall Stability (lbs.) 370 1030 1140 - 50C cure, Marshall Stability 1288 2690 3220 CONCLUSION
A graph of this data indicates that the maximum beneficial effect on stability per unit quantity of manganese is between 0.08% and 0.12%. Levels of manganese above this produce only marginal benefits.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A paving composition comprising a major proportion by weight of aggregate and a minor proportion of substantially unblown asphalt which is present as a coating on the aggregate, the asphalt having uniformly dispersed or dissolved therein at least one oil-soluble soap which is a manganese, copper or cobalt salt of a monocarboxylic acid having not more than 30 carbon atoms, or a mixture of two or more thereof, and which is present in an amount such that the composition contains from 0.01 to 0.5%, based on the weight of the asphalt, of manganese, copper or cobalt ions, or a mixture thereof.

2. A composition according to claim 1, in which the salt is a lino-leate, octoate, naphthenate, oleate, stearate or laurate.

3. A composition according to claim 1 in which the salt is a manganese salt.

4. A composition according to claim 1, which contains at least 85%
by weight of the aggregate.

5. A composition according to claim 1, 2 or 3, in which the salt is produced in situ by reaction of said monocarboxylic acid, which is present in the asphalt, with a source of at least one of manganese, copper and cobalt.

6. A composition according to claim 1, 2 or 3, in which the salt is produced prior to addition thereof to the asphalt.

7. A composition according to claim 1, 2 or 3, in which the asphalt coating has a thickness not exceeding 150 microns.

8. A method of preparing a composition according to claim 1, in which the soap is dispersed or dissolved in the asphalt while the latter is in a fluid state, the resulting mixture being mixed with the aggregate.

9. A method according to claim 8, in which the asphalt is maintained in a fluid state by the application of heat.

10. A method according to claim 9, in which the asphalt is maintained in a fluid state by heating to a temperature of 110 to 120°C.

11. A method of producing a paving layer, which comprises applying a composition according to claim 1 to a substrate and curing the asphalt such that the aggregate in the paving layer is bonded together by cured solid asphalt.

12. A method according to claim 11, in which the curing is effected at a temperature not exceeding 100°C.

13. A method according to claim 12, in which the curing is effected in the presence of oxygen.

14. A method according to claim 11, 12 or 13 in which the substrate is the surface of a road.

15. A paved layer which has been produced by a method according to claim 11, 12 or 13.