1070-4272/04/7703-0515�2004 MAIK �Nauka/Interperiodica�

Russian Journal of Applied Chemistry, Vol. 77, No. 3, 2004, pp. 515�516. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 3,2004, pp. 521�522.Original Russian Text Copyright � 2004 by Levchenko, Rozental’, Zalishchevskii.

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Asphaltic Dispersion and Its Evolution in ManufacturingAsphaltic Concrete

E. S. Levchenko, D. A. Rozental’, and G. D. Zalishchevskii

St. Petersburg State Technological Institute, St. Petersburg, Russia

Received June 25, 2003

Abstract�Evolution of properties of cold asphalt is investigated. The time required for establishment ofadsorption equilibrium in preparation of asphaltic concrete is determined.

The asphaltic dispersion differs from the classicalcolloidal systems in that its dispersed phase is formedfrom molecules similar to those of the dispersionmedium. The molecules with a polynuclear condensedstructure consisting of 3�4 aromatic and 2�3 naph-thene rings represent planes with alkyl substituentsbound around. Four to six molecules are arranged inparallel to each other to form a quasispherical stackassociate.

Thanks to �-electron systems of the aromatic andheteroaromatic rings, the intermolecular bonds in theassociate appear to be rather strong, and their dissocia-tion starts only above 300�C. It was demonstrated byX-ray diffraction that, in supramolecular structures ofthis kind, the stack thickness is well comparable withthe diameter of the molecules (�0.8�1.7 nm), thatincluding the solvate shell being about 2.0�2.3 nm[1, 2]. These structures are known as asphaltenes,even though it is not absolutely correct.

In practice, by asphaltenes is meant a mixture ofcompounds precipitated from a toluene solution ofasphalt under the action of n-pentane or n-hexane.Along with high-molecular-weight aromatic mole-cules, this mixture can contain low-molecular-weightpolar molecules insoluble in the indicated solvents.Therefore, asphaltenes thus isolated can differ sig-nificantly from those forming the dispersed phase ofasphalt. Furthermore, asphalt contains a great amountof molecules also tending to association, but with alower energy of intermolecular interaction. In allcases, association proceeds by random diffusion ofmolecules in the asphalt bulk, until molecules ofthe appropriate structure come into collision. Thisrequires a time increasing with decreasing tempera-ture, because of increasing viscosity of the system anddecreasing energy of the molecules.

Therefore, it takes a long time to form a thermo-dynamically stable asphalt structure. The lower thetemperature of asphalt, the longer this time. This maybe clearly demonstrated by measuring the penetrationof asphalt without intermediate softening in eight daysunder oxygen-free conditions in the dark. The resultsof such experiments are given in the table.

As seen, the penetration of asphalt decreased withtime, which is due to solely structural transformations.Such an aging effect was attributed to oxidation inthe course of storage. However, in fact, it is a resultof high-temperature oxidation of asphalt. Structuraltransformations accompanying establishment of ther-modynamically stable state are very typical of oxi-dized asphalts. In the course of asphalt oxidation,naphtheno�aromatic oils undergo oxidative dehydro-genation to form asphaltenes. At 220�280�C, this isa kinetically controlled process, while association iscontrolled by diffusion, proceeding at a considerablylower rate.

Another consequence of aging, but of asphalticconcrete pavements, which has not been taken intoconsideration until now, is structural transformation of

Variation in the penetration of asphalt in aging at roomtemperature����������������������������������������

Asphalt

� �25,* 0.1-mm units, in aging for indicated� time, days�������������������������������� 0 � 2 � 4 � 6 � 8

����������������������������������������BND 40/60 � 59 � 57 � 56 � 53 � 50BND 60/90 � 88 � 83 � 77 � 74 � 70BND 90/130 � 120 � 115 � 110 � 102 � 98����������������������������������������* (�25) Penetration of asphalt at 25�C.

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 3 2004

516 LEVCHENKO et al.

asphalt caused by adsorption of certain components onthe mineral ballast. In fabrication of asphaltic concreteat an asphalt plant, the time of contact of asphalt withmineral ballast is 20 min at 160�C. A short time afterasphaltic concrete is laid to the roadbed. Thus, ap-parently, asphaltic concrete is cooled before reachingthe adsorption equilibrium.

It was demonstrated by the hot centrifugationmethod that the adsorption equilibrium of asphalt onboth acidic and basic rocks is established in 3�4 h at160�C and somewhat more rapidly at higher tempera-tures. At room temperature, this takes 6�7 months.Adsorption of asphalt on the mineral ballast surface isa selective process, resulting in extraction of the mostreadily adsorbed components from asphalt, which, inits turn, distorts the thermodynamic equilibrium in theasphaltic dispersion system. As a result, transforma-tion of the asphalt structure changes its characteristics:the penetration considerably decreases and softeningpoint increases.

The effect of heating time � on the performancecharacteristics (penetration �25 and softening pointTs) of centrifuged asphalt is illustrated below.

�, h 0 1/2 1 2 3 4�25, 0.1-mm units 88 71 59 46 39 37Ts, �C 48 50 54 62 66 67

The observed considerable decrease in the penetra-tion and increase in the softening point suggest anincrease in the rigidity of the asphalt structure anddecrease in its volume. Detachment of asphalt from amineral material always proceeds via break of thecohesion bonds, whose strength is lower as comparedto the adhesion bonds. Therefore, in asphalt cooledbefore reaching the adsorption equilibrium, innerstresses are realized in its bulk between the mineralballast grains, which may result in microcracking,penetration of water, and deterioration of the roadpavement. Therefore, the primary cause of deteriora-tion of a road pavement is not an oxidation, but struc-tural transformation of asphalt. Oxidation contributesonly at the next stage, i.e., after microcracks areformed in the asphaltic mass.

As a conclusion, our results show that, to improvethe durability of asphaltic concrete, it is necessary tohold it at high temperature (160�C) for at least 3�4 h,i.e., until reaching the adsorption equilibrium.

REFERENCES

1. Posadov, I.A., Popov, O.G., Proskuryakov, S.V., et al.,Neftekhimiya, 1985, vol. 25, no. 3, pp. 412�416.

2. Bodan, A.N., Khim. Tekhnol. Topl. Masel, 1982, no. 12,pp. 22�24.