A study of the ageing of asphaltic bitumen

A STUDY OF THE AGEING OF

ASPHALTIC BITUMEN

P R O E F S C H R I F T

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS DR O. BOTTEMA, HOOGLERAAR IN DE AF-DELING DER ALGEMENE WETENSCHAPPEN, VOOR EEN

COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 26 MEI 1954

DES NAMIDDAGS TE 2 UUR DOOR

WILHELMUS PETRUS VAN OORT

NATUURKUNDIG INGENIEUR GEBOREN TE UTRECHT

1954

DRUKKERIJ BOEIJINGA - APELDOORN

DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROFESSOR DR M. J. DRUYVESTEYN

The work presented in this thesis was carried out at The Royal Dutch/Shell Laboratories. Amsterdam. My sincere thanks are due to the management of these laboratories for their consent to publish this work.

Aan mijn Moeder. Aan Marijke.

XI.

L I S T O F S Y M B O L S

a = layer thickness (m) b =^ width of strip (m)

/kg^

c = concentration of free gas

_\-v

/ k g \

CQ = solubility Ij^sl /m^\

D = diffusion coefficient \sec) /m^\

Do = diffusion coefficient of fresh material \sec) (N\

E = dynamic elasticity modulus \m^j (N\

El =^ dynamic elasticity modulus of base \m^/ (N\

E2 ^= dynamic elasticity modulus of layer(s) applied . . . • Ims) e = base of natural log

F ^ surface area (m^) fK = resonance frequency (sec~^)

/ k g \

G =: concentration of oxygen to be bound I m ' ) Go = maximum concentration of bound gas i m ' )

/ k g \

g = concentration of bound gas \m^)

h = height (m) hi = thickness of base (m)

h^ = thickness of layer applied (m) Ix = gas stream Lg^. I

k = constant (0.006, 1 , 2 , . . .) / m« \

k = reaction constant , \kg sec/

/ := length of strip (m) / k g \

M = total quantity gas absorbed by whole layer \m^/ m = total quantity gas absorbed at x in layer dx . . . • I j ^ I

n =: integer between 0 and u~>

P = total shearing force (N) s = rate of solution Isec)

t = time (sec) / m . \

V := velocity of displacement isec)

x = coordinate (m) a = constant j8 = constant c ^ — 7 '^o (N sec\ t] = coefficient of viscosity I m^ ) C = tangent of the angle of shear

^ = (" + Vs) ^

Qi = density of base (m^l /kg

^2 ^ density of layer appUed I s T = fcGflf T = shearing force i/kG ym" kGo 0

b. Ageing phenomena of asphaltic bitumen 1 0 C H A P T E R II.

Exposition of the problem 15 CHAPTER III.

Experimental methods 1 / a. Measurement of oxygen absorption 1 /

b. Preparation of asphaltic bitumen/sand mixtures 2 2

c. The microviscometer 2 3 d. Measurement of stiffness of small samples at short loading time 2 5

e. Exposure of thin layers of asphaltic bitumen, especially for the purpose of

determining their viscosity 3 1 C H A P T E R IV.

Measuring results 3 3 a. Oxygen absorption 3 3 b. Change in mechanical properties 3 6

C H A P T E R V .

Theoretical description of the ageing process 4 1

a. Literature survey 4 1 b. Differential equation for the ageing process 4 5

' C H A P T E R VI. - « ^

Solution of the differential equation 4 9 a. Solution with a constant diffusion coefficient 4 9 b. Solution with a decreasing diffusion coefficient 5 3

C H A P T E R VII. p-fl«

Verification of the theory 6 0 C H A P T E R VIII.

Application of the theory 6 9 C H A P T E R IX.

Development of an accelerated ageing test 8 2

Appendix 88 1. Check measurements in the microviscometer 8 8

a. Variation of rate of shear 8 8 b. Comparison of viscosity as measured by microviscometer and by other methods 8 9

c. Influence of treating solutions of asphaltic bitumen and exposure of coats of

asphaltic bitumen on their viscosity 8 9 d. Effect of evaporation of asphaltic bitumen components during exposure on

glass plates 9 1 e. Viscosity of asphaltic bitumen recovered from asphaltic bitumen/sand mixtures 9 2

2. Apparatus for the measurement of the dynamic modulus of elasticity of small

samples of visco-elastic materials 9 3 3. Solution of the differential equation 9 8

a. Solution with a constant coefficient of diffusion 9 8 b. Solution with a decreasing coefficient of diffusion 1 0 1 4. Procedure for adapting the theoretical time-absorption curve to the

experi-mental curve 1 0 3 Summary 105 Samenvatting 107

XV.

LIST O F L I T E R A T U R E

A 1 H. Abraham. Asphalt and allied substances. N e w York (1945). A 2 H. Abraham. Ibid. p . 1867.

A 3 G. J. van Amerongen. Thesis Delft (1943).

A 4 A. P. Anderson and K. A. Wright. Ind. Eng. Chem. 33, 991 (1941).

L.A 5 A. P. Anderson, F . H. Stross and A. Filings. Ind. Eng. Chem. (Anal. Ed.) 14, 45 (1942).

A 6 T h e Asphalt Institute. Asphalt Handbook. New York (1947). B 1 J. D. Babbit. Can. J. Phys. 29, 437 (1951).

B 2 J. H. Bateman and H. L. Lehmann. Proc. A. S. T . M. 29, 943 (1929). L B 3 H. C. Bennett and D. W . Parks. J. Soc. Chem. Ind. 58, 565 (1939). l.B 4 J. R. Benson. Proc. Highway Res. Board 17, 368 (1937).

B 5 S. Brunauer, P. H. Emmett and E. Teller. J. Am. Chem. Soc. 60, 309 (1938). C 1 M. Couette. Ann. Chim. et Phys. V I 21, 433 (1890).

C 2 J. Crank and G. S. Park. T r a n s . Far. Soc, 45, 240 (1949). C 3 J. Crank and M. E. Henry. Trans. Far. Soc. 45, 636 (1949). C 4 J. Crank and M. E. Henry. T r a n s . Far. Soc. 45, 1119 (1949). C 5 J. Crank. T r a n s . Far. Soc. 47, 450 (1951).

C 6 J. Crank and G. S. Park. T r a n s . Far. Soc. 47, 1072 (1951). D 1 P. V. Danckwerts. T r a n s . Far, Soc. 46, 300 (1950). D 2 P. V. Danckwerts, Trans, Far, Soc, 46, 701 (1950).

]_E 1 A. R, Ebberts. Ind. Eng, Chem, 34, 1048 (1942),

E 2 V. A, Endersby, F , H. Stross and T . K, Miles, Proc, Assoc. Asph, Pav. Techn, ''" 13. 282 (1942).

E 3 A. Evans. J, Inst, Petr. 18, 957 (1932).

F 1 R, J. Forbes. Bitumen and Petroleum in antiquity. Leyden (1936). I F 2 G. H. Fuidge and J, G, Mitchell. J, Soc. Chem. Ind. 61, 133 (1942).

H 1 J. P. den Hartog, Mechanical Vibrations, New York (1947).

I H 2 Highway Res, Board. Bibliography on resistance of bituminous materials to deter-"" ioration caused by physical and chemical changes. Bibliography no. 9,

Washing-ton (1951).

H 3 Hiroshi Fujita, Textile Res. J. 22, 281 (1952).

\H 4 A. J. Hoiberg. Proc. Assoc. Asph. Pav. Techn, 19, 325 (1950).

H 5 P. Hubbard and C, S. Reeve, Ind, Eng, Chem. 5, 15 (1913),

H 6 P. Hubbard and H, Gollomb, Proc. Assoc, Asph. Pav. Techn, 9, 165 (1937), r H 7 F, N, Hveem. Proc, Assoc. Asph. Pav. Techn. 15, 111 (1943).

] 1 W , Jost. Diffusion in solids, liquids, gases. New York (1952).

K 1 A, S. Kuzminskii, L, L. Shanin and N . N . Lezhner. Dokl. Akad, Nauk, S.S,S.R. 79, 467 (1951). Translation in Rubber Chem. and Techn, 25, 230 (1952). L I J. W . A. Labout. T o be published.

LL 2 R. H. Lewis and W . O'B. Hillman. Publ. Roads 18, 85 (1937). [ L 3 R. H. Lewis and W , J, Halstead. Publ, Roads 24, 121 (1946).

M 2 J. L. Meyering and M, J, Druyvesteyn, Philips Res, Rep, 2, 81 (1947). M 3 M, Mooney and R. H. Ewart, Physics 5, 350 (1934).

., N 1 S, L. Neppe, T r a n s . South-African Inst, Civ. Eng. 1, 195-223 (1951); 2, 103-134 (1952),

N 2 L. W , Nijboer, Plasticity as a factor in the design of dense bituminous road carpets, Amsterdam (1948).

O 1 W . P. van Oort. T r a n s . Instrum. and Meas. Conf. Stockholm (1952). Microtecnic 7, 246 (1953).

O 2 H. Oberst and K. Frankengeld. Acustica 1, AB 181 (1952). P I G. S. Park. Trans, Far, Soc. 46, 684 (1950),

P 2 G. S, Park. T r a n s . Far, Soc, 47, 1007 (1951), P 3 G. S. Park. T r a n s . Far. Soc, 48, 11 (1952),

P 4 J. T , Pauls and J. York Welburn, Publ, Roads 27, 187 (1953),

P 5 J. Ph. Pfeiffer. The properties of asphaltic bitumen. Amsterdam (1950). (In collaboration with H, Filers, J, W . A, Labout, R, N . J. Saal, M, C. Siegmann and H, W . Slotboom).

P 6 J. Ph. Pfeiffer. Ibid, p, 65,

P 7 J. Ph. Pfeiffer. Ibid. p . 114, 116, 263. P 8 J, Ph. Pfeiffer, Ibid, p . 266,

— L P 9 C, V, d. Poel, Proc. Sec, Int. Congr. on Rheology. Oxford (1953), P 10 C. V, d. Poel. T o be published.

P 11 S. Prager, J. Chem. Phys. 19, 537 (1951).

P 12 S, Prager and F. A, Long. J. Am. Chem. Soc, 73, 4072 (1951). R 1 L. F. Rader, Proc, Assoc, Asph. P a v . Techn. 7, 29 (1936).

R 2 The Radio Amateurs Handbook, Concord, New Hampshire (1951), p. 476, S I R. N, J. Saal and G, Koens. J. Inst, Petr. 19, 176 (1933).

8 2 R. N, J. Saal, P, W , Baas and W . Heukelom. J. chim. phys. 43, 235 (1946), S 3 C, L, Shattock, Proc. Assoc. Asph, Pav, Techn, 11, 186 (1940).

S 4 H. W . Skidmore and G. Abson. Proc. Assoc, Asph, Pav, Techn. 9, 195 (1937). S 5 H. W . Skidmore. Proc. Assoc. Asph. Pav. Techn, 12, 69 (1940),

T 1 A, M. Thomas. J, AppL Chem. 1, 141 (1951),

T 2 R. R. Thurston and E, C. Knowles. Ind. Eng. Chem. 28, 88 (1936). T 3 R. N, Traxler and H. E. Schweyer. Proc. A. S, T , M. 36, 544 (1936). T 4 R. N, Traxler and E. C, Coombs, Proc. A,S.T,M. 37, 549 (1937), W 1 C. Wagner, J, Metals 4, 91 (1952).

W 2 A. A. W h i t e . Proc, Highway Res. Board 27, 197 (1947). L W 3 D. M. Wilson. J, Soc. Chem. Ind, 5 1 , 61 (1932).

A.

CHAPTER I

I N T R O D U C T I O N

a. The nature and properties of asphaltic bitumen ^^) ^ ^)

Asphaltic bitumen is a dark substance mainly of a hydrocarbon nature, produced from natural hydrocarbons; it is completely soluble in carbon disulphide and at room temperature its consistency may range from highly viscous to almost solid. It should not be confused with coal tar pitch, a product resulting from the destructive distillation of carbonaceous materials, from which it differs in chemical composition.

The principal source of asphaltic bitumen is petroleum, many types of which provide asphaltic bitumen as the residue after their more volatile constituents have been removed by distillation. Asphaltic bitumen can also be obtained from crude mineral oil by precipitation with the aid of the appropriate agents. Deposits of asphaltic bitumen are occasionally to be found on the earth's surface or just under it (Trinidad, Neuchatel). These deposits generally contain an admixture of inorganic substances. The easy accessibility of such deposits, which are also found near the Euphrates, is the cause of asphaltic bitumens being one of the earliest known building materials. As long ago as B.C. 3000 the production in Mesopotamia amounted to hundreds of tons per year. There are still constructions, like roads, floors, wharves, water mains, that were made thousands of years ago with the aid of natural asphaltic bitumen as the binder holding the stones together. Obviously, these constructions satisfy very high requirements of resistance to weathering.

Also in modern applications the good binding properties and protective effect of asphaltic bitumen are utilized to a very large extent. The annual world consumption at present amounts to many millions of tons. In road construction and hydraulic engineering, which account for some 70 % of the total production, it is particularly the binding capacity of asphaltic bitumen that is important. Mineral particles of widely varying size, depending on the construction, are held together by relatively small quantities of bitumen. As a protective agent bitumen alone can be used; it is also often applied in the form of impregnated paper or felt to

improve its mechanical strength, or to simplify processes of application. T h e substance here referred to asphaltic bitumen or simply bitumen is called asphalt in the United States. Asphaltic bitumen consists of a highly complicated mixture of compounds, mostly of a hydrocarbon nature in the molecular weight range from a few hundreds to some thousands. Of these the following groups may be distinguished: saturated aliphatic compounds, naphtenic compounds, aromatic compounds and compounds with olefinic double bonds. These groups are frequently present in combinations, as, for instance, a ring system with paraffinic chains, the ring system then possibly being a combination of aromatic and naphtenic structures.

In addition to pure hydrocarbons, heterocompounds, containing sul-phur, oxygen or nitrogen occur.

Depending on method of manufacture and origin, the chemical com-position differs. Thus olefinic double bonds do not occur in products normally obtained as the residue of distillation, but they do when the crude has been subjected to cracking.

T h e knowledge of the variety and quantity of the individual compounds making up asphaltic bitumen does not yet go much beyond this very rough picture. Their number must be estimated at a high figure and it is questionable whether complete analysis will ever be possible. It is even extremely difficult to analyze a light gasoline to its extreme constituents.

W i t h the object of to some extent grouping the multitudinous molecules occurring in bitumen, at least within very wide limits, many methods of separation, such as precipitation, chromatography, extraction and molec-ular distillation, have been applied. Owing to its simple procedure precipitation in particular is an important aid. Separation of this sort is still highly arbitrary, as the quantity and nature of the fraction also depend on the degree of dilution and temperature applied during sepa-ration. The following fractions are distinguished:

asphaltenes: insoluble in low-boiling saturated hydrocarbons, soluble in carbon tetrachloride;

maltenes: soluble in low-boiling saturated hydrocarbons.

From the definition of asphaltenes it is evident that several saturated hydrocarbons can be used for precipitating them from bitumen. T h e amount of precipitate depends a good deal on the precipitating agent used. Thus, the percentages by weight of asphaltenes from one bitumen may vary from 33 when pentane is used to 15 with dimethyl cyclo-pentane. It is therefore necessary to specify what precipitant has been made use of when mentioning asphaltenes: for instance, pentane

asphal-3

tenes, 60/80 gasoline asphaltenes (gasoline with a 60—80 °C boiling range).

T h e separation into maltenes and asphaltenes is mainly one according to molecular weight, the asphaltenes being the high molecular compounds, In addition, the aromatic nature of the material also plays a part in this separation.

A typical example of the chemical composition of a normal type of bitumen according to elementary analysis proves to be the following:

maltenes 60/80 gasoline asphaltenes O/o by wt of bitumen 71,7 28.3 O/o by wt of fraction C 84,8 83,1 H 10,6 7.6 S 3.5 4.6 N 0,4 0,4 O 0.7 1.8 Ash 0 2.5 C/H ratio 0,68 0.91

T h e greatest difference between maltenes and asphaltenes is to be found, according to the elementary analysis, in the C/H ratio. This shows that asphaltenes have a strongly aromatic nature. The mean molecular weight of maltenes, as recorded in the table, is about 500, the minimum molecular weight of asphaltenes is certainly several thousand.

As regards chemical reactivity it may be observed that asphaltic bitumen is on the whole very stable, especially at atmospheric temperature, Hence its suitability as a protection against corrosion, etc. At higher temperature (100—300 °C) it reacts with oxygen, sulphur, chlorine, sulphuric acid, nitro compounds and similar strong reagents. Its reaction with oxygen, in particular, is of great technical importance, as this is responsible for special changes in rheological behaviour connected with the formation of asphaltenes by oxidation. This widens the field of application of bitumen. The technical realisation of oxidation at high temperature is the blowing process. During the blowing of a bitumen part of the reacted oxygen is dissipated as H2O and CO2; coupling mechanisms cause asphaltenes to be formed.

Oxidation at atmospheric temperature is an exceedingly slow process, that can therefore only be observed with special measuring techniques. Low-temperature oxidation eventually has perceptible physical conse-quences. Light has an accelerating effect on this oxidation. The study of a number of aspects of this oxidation process constitutes the subject of the present thesis.

non-aqueous colloidal system of high viscosity. Sometimes it is a sol, some-times a gel, according to the content and nature of asphaltenes and to the aromaticity of the maltenes. The asphaltenes are associated with high-molecular aromatic components from maltenes and form micelles, which are the disperse phase of the bitumen. The continuous or inter-micellar phase consists of compounds of lower molecular weight. The asphaltenes will be completely peptized if the asphaltene content is not too high and sufficient aromatic constituents are present; the micelles can then still move independently, the system is a sol. W h e n the asphaltene content is higher there may not be sufficient aromatic con-stituents to keep every micell completely peptized. The micelles then incompletely peptized may attract each other and thus an irregular structure originates in the bitumen; the spaces in the skeleton are filled with intermicellar liquid. A system of this kind is a gel. W h e n the rheological properties of bitumen are discussed it will become apparent what differences are thus caused in its mechanical behaviour.

T h e equilibrium between micelles and intermicellar phase is affected by temperature. As the temperature is higher the intermicellar phase increases at the expense of the micelles. The gel properties, if originally present, being lost. A time effect in mechanical properties of an asphaltic bitumen can be observed when it has been melted and again brought to the original temperature; the bitumen hardens at a rate decreasing with time. This effect is known as physical hardening and it indicates that the attainment of a fresh equilibrium after a temporal change in temperature is rather a slow process.

The addition of a solvent in which the intermicellar phase and the absorbed constituents of the micelles are soluble, moves the equilibrium towards liquidity. This may result in the nuclei of the micelles becoming insoluble, having lost their peptizing skin, so that they are precipitated, It is this mechanism which is assumed to be responsible for the precipita-tion of asphaltenes — which are the nuclei of the micelles — by low-boiling saturated hydrocarbons. Nature and quantity of the precipitant, as well as temperature, may greatly affect the quantity of asphaltenes that are eventually obtained from one bitumen as will be clear from the described mechanism.

The intermicellar phase can be split off as such from the bitumen by covering the surface with some porous material, such as limestone powder; only the fairly mobile, small molecules are then absorbed by the powder (exudation). The liquid recovered from the powder — the inter-micellar phase —• has lower viscosity than the maltenes have. Its quantity, too, is smaller than that of the maltenes. Only bitumens with a gel structure display exudation.

^111 I P P W T » ^ ' ™ » ^ ' " ' ^ ' ""W-i^w^iiippiilflIIIW^w"**'"»! I - ^^- " '" '. . • , ^ p w ï » i i

5

T h e aids at our disposal to differentiate the various kinds of asphaltic bitumen consist mainly of rheological tests that have been standardized. Usually, these tests are the logical outcome of practical requirements. In their application rheological properties play the leading part and chemical properties are only of incidental consequence. This is due to the fact that commercial grades of asphaltic bitumens vary greatly in mechanical properties, whereas the chemical properties of all kinds exhibit far fewer differences. The principal routine methods of testing bitumen are briefly described below. These must satisfy requirements of sim-plicity, reproducibility and clarity.

T h e hardness of asphaltic bitumen is determined by the penetrometer. A needle of specified dimensions is loaded with a total weight of 100 g. T h e distance the needle penetrates into the bitumen during five seconds' loading is measured. T h e temperature is kept at 25 °C by placing the cup containing 60 g of bitumen in water of that temperature. The number of tenths of millimetres that the needle penetrates into the bitumen, under these conditions, is called the penetration. A product with a penetration of about 200 is a soft grade, of about 50 a medium and below about 20 a hard grade,

Bitumen has no melting point, but a softening range. In order to find the temperature at which a bitumen has a certain liquidity, the term Ring and Ball softening point has been introduced. This refers to a test in which a brass ring of standard dimensions is filled with bitumen. The bitumen is loaded with a steel ball weighing 3,5 g and is heated in a water bath at a rate of 5 °C rise in temperature per minute. The tem-perature at which the ball is seen to have dropped one inch is defined as the Ring and Ball softening point. For various bitumens this varies between 35 and 125 °C. The pretreatment that the bitumen has under-gone affects the result of the determination of both the penetration and the R and B softening point. Specifications for the pretreatment have therefore been laid down.

The sensitivity of the hardness of a bitumen to temperature fluctuations can be determined by measuring the penetration at different temperatures. W h e n the values obtained for several bitumens are plotted as log pen. against temperature, almost straight lines are obtained, therefore log pen.

= A T + K, A and K being constants for a given bitumen.

A knowledge of temperature susceptibility of resistance to deformation is of great importance in actual service and it is therefore necessary to record this property, too, in a figure. This can be done by determining factor A, since A = -^^—'-. Considerations of a practical nature have

d I

20 PI 1

formula A = - ^ • -—• It is found that the PI may vary from —2.5 to + 8 . A low PI value implies high temperature susceptibility of the penetration and a high one the reverse. The theoretical limits of the PI numbers are—10 and + 20.

T o find the PI it is not necessary to carry out penetration tests at several temperatures. For, if the penetration of several bitumens is determined at the R and B temperature of each, a figure in the vicinity of 800 is invariably found. The constant A can therefore be calculated from one penetration and the R and B softening point,

T o characterize the various types of bitumen, both the penetration at 25 °C and the softening point R and B are given. It is usual to devide the asphaltic bitumens into three groups according to PI value. The first group has a PI smaller than — 2 , the second group from —2 to + 2 , the third group more than + 2 . Bitumens of the first two groups are obtained by distillation or by distillation followed by partially blowing. Bitumens of the third group are obtained by blowing soft residues. The PI value is closely related to the characterization of asphal-tic bitumen as ascertained rheologically, which will be discussed later on.

Another important property is the viscosity, particularly at high temperatures. Its importance derives from the requirements that the product must meet during application. T o measure the viscosity at high temperatures, at which bitumen is a pure Newtonian liquid, no arbitrary quantity need be defined, as was necessary in standardizing the pene-tration and R and B softening point. T o determine the viscosity of bitumen at high temperature (100—250 °C) the capillary viscometer is employed. By using capillaries of various diameters it is possible to cover a sufficiently wide range of viscosities (10^ N sec/m2 to 10-^ N sec/m^. *)

Determination of the viscosity of asphaltic bitumen in the temperature range below 100 °C, in which its viscosity may vary from 10^ to 10^ N sec/m2 is done with the aid of viscometers of the type of concentric rotation viscometers. In this test the bitumen is between a fixed wall and a rotating spindle, and the viscosity is found by measuring the stress and the deformation. The same viscosity range is covered by the micro-viscometer devised by Labout, which is described in this thesis.

For bitumens having purely viscous properties it has been empirically found that viscosity is related to penetration as follows:

1.58 X 10^ j.r I ^ss ri = ^TTi— J^ sec/m''

' pen.^" '

7

The brittleness of asphaltic bitumen is an important property of the product in its practical application. Besides temperature, which greatly affects brittleness, rate of deformation also plays a part. Understandably, the test applied for determining brittleness is accurately specified; nor-mally the Fraass breaking point determination is applied. A layer of bitumen 0.5 mm thick is spread on a sheet of steel of specified dimensions. Under standardized conditions the steel is bent at decreasing temperature. The temperature at which the bitumen gives way is called the Fraass breaking point. The prescribed conditions ensure that the layer of bitumen is slightly stretched at a low rate of deformation. Fraass breaking points vary from + 15 to —40 °C.

Besides these routine tests there are methods for ascertaining other properties of asphaltic bitumen. Some of these are: determination of thermal stability, of resistance to vibration, and others for purposes of identifying the bitumen and for directly testing its suitability for some application.

Extensive investigations have been carried out to study the rheological properties of asphaltic bitumen. Rheologically, bitumens may exhibit all the characteristics from purely viscous to ideally elastic. In general bitumens are visco-elastic materials. The rheological behaviour of a visco-elastic

stress

material can be described by aje = -r-^ as a function of time. deformation

The ratio ajs is called stiffness.

An ideally elastic substance is characterized by aje = constant, regard-ness of the extent of deformation and duration of loading. In a graph in which aje is plotted against time the resultant line is horizontal. In this case aje is known as the elasticity modulus or Young's modulus.

A purely viscous substance (Newtonian liquid) is characterized by a straight line in a graph giving log aje against log t at an angle of 45°. Deformation is then proportional to time at constant loading and constant temperature.

At sufficiently short loading times all bitumens behave as ideally elastic substances. At sufficiently long loading times bitumens exhibit purely viscous behaviour. There is a continuous transition in the aje-time graph from the elastic region to the viscous region. If this transition range is short, bitumens are concerned with a PI of —2 or less. These bitumens have a low content of asphaltenes. ^ ' ) ^ *°)

A slow transition corresponds to higher PI values. If the PI is between —2 and + 2 the bitumen has a sol character (completely peptized asphaltenes). In addition of elastic properties viscous properties are also of importance, leading to a retarded elastic effect. The deformation is dependent on the load and on loading time.

If the PI is more than + 2 the bitumen has a gel character. The structure due to the partial peptization of the asphaltenes is the cause of 0/e still being dependent on the extent of deformation. After a given deformation has been exceeded the structure breaks down, owing to which deformation becomes more than linear with time. At the beginning of deformation the substance exhibits elastic behaviour; it is thixotropic and may have a yield value.

T h e maximum value for aje to be found for asphaltic bitumens amounts to 3 X 10^ iV/m2. Any lower value can be found (softer grades, higher temperatures, long times of loading).

Fig. 1. Relation between — and t at two temperatures.

T h e temperature susceptibility of the aje value of an asphaltic bitumen can be characterized by determining how much the curves shift in the graph log aje versus log time as the temperature changes (fig. 1). It is important in this connection that the shape of the curve for a given bitumen is independent of temperature, the rheological type determines its shape. Bitumens belonging to the same rheological type but of dif-ferent hardness, therefore show curves of the same shape but shifted in horizontal direction.

If the shift of the curve in a horizontal direction is measured, it is found to be almost the same for all bitumens for a given temperature change in a wide range of loading times. In other words, the distances marked b in the figure I are equal when the temperature changes from T i to T2.

If, however, the distance in a vertical direction is measured, so at constant loading time, it is obvious that, the slope of the curves being different, the distances differ too; so a is greater than c. The change in aje caused by temperature change at constant loading time can therefore be used as a measure of the temperature susceptibility of aje. It has been

dlog pen. mentioned that for practical purposes

9

Since the penetration is determined at a fixed deformation time (5 sec.) the rheological meaning of the PI is clear.

Pressure affects the rheological properties of asphaltic bitumen. This is connected with the presence of free volume between the molecules. Increasing molecular weight means decreasing free volume, so that pressure greatly affects viscosity. A rise in temperature enlarges the free volume, diminishing the influence of pressure on viscosity. It is the inter-micellar phase, in particular, that is responsible for these phenomena. A low asphaltene content generally means that the viscosity of the intermicellar phase is high, so that again pressure greatly affects viscosity, The effects may be considerable: application of 500 atm. pressure may result in a viscosity increase up to 600 times the original value. But values of 20 times the viscosity also occur under this pressure.

Every application makes its own particular demands on asphaltic bitumen. It is the object of applied research to find the most suitable combinations, for which a large range of bitumens with different proper-ties is available. This research deals mainly with three groups; problems concerning the technique of applying the bitumen to any construction in which is is used, the function that the construction must fulfil and the durability of the construction.

The processibility of asphaltic bitumen always requires low viscosity. This can be achieved by heating it, so that, for instance, it can be mixed with mineral aggregate in a mixer. It is important to keep oxidation at the mixing temperature (often 170 °C) as low as possible.

Reduction of viscosity can also be obtained by dissolving the bitumen in volatile liquids. Many and various economical and technical factors have to be taken into consideration in deciding what process to use.

T o ensure low viscosity during the entire process of application use can also be made of emulsions of asphaltic bitumen in water. These are emulsions in which the bitumen is the dispersed phase. T h e particle size is about 3 /i; concentrations up to 60 % bitumen in water are used. A bitumen in water emulsion generally behaves like a Newtonian liquid. High concentrations of bitumen may cause gelation. During storage and transit an emulsion must be stable, but after application it must coagulate rapidly.

T h e research work necessary to find what bitumen has the best properties for a given construction is particularly concerned with mecha-nical properties. Thus, protective coatings must not flow under stress of the own weight, they must be able to follow the deformations of the object to which they are applied, mechanical properties of road surface mixtures must be exactly known at extreme temperatures in order to become informed when brittleness or high deformations occur,

In road making and hydraulic works adherence is an important factor. As long as the surfaces of the mineral particles are dry and clean, adherence is no problem. W e t surfaces require special precautions. Sometimes surface-active agents are utilized to obtain a smaller angle of contact, so that the bitumen flows more readily over the surface, Emulsions are suitable for application on wet surfaces.

The durability of a structure is dependent to a great extent on the durability of the bituminous binder or protective agent. Hardening and cracking are flaws that may occur in the long run. Generally speaking it may be said that structures in which asphaltic bitumen has been used are of great permanence. Thus, road carpets stand up to many decades of constant service.

b. Ageing phenomena of asphaltic bitumen.

Literature survey.

One of the important factors determining the lifetime of constructions in which asphaltic bitumen has been used is the influence exerted on the asphaltic bitumen by the weather. The entire complex of changes in the properties of asphaltic bitumen by atmospheric influences, to the detriment of the construction concerned, is called ageing. The degree in which asphaltic bitumen resists these influences is called its durability. There is extensive literature on the ageing phenomena of asphaltic bitumen. Not one of the studies published, however, does more than describe the phenomena observed. Frequently correlations between ageing phenomena and properties of the asphaltic bitumen are sought by purely empirical methods and many methods are described that aim at obtaining direct information on behaviour after long exposure by short-time tests.

There is not much point in giving here a full Hst of all the publications that have appeared on the ageing of asphaltic bitumen; surveys of this kind have recently been published by Neppe ^ ' ) and the Highway Research Board " 2), together comprising some three hundred publica-tions. This large number is certainly evidence of the importance of the problem of ageing in the practical application of asphaltic bitumen. Short summaries on the subject are given by Abraham ^ 2) and Pfeiffer P ' ' ) .

The articles bearing on the investigation to be described in this thesis

are referred to briefly.

T h a t ageing makes itself manifest in hardening of asphaltic bitumen was noted by Hubbard and Reeve ^^) as early as 1913. They observed a decline in penetration from 228 to 55 after exposure of a thin layer of asphaltic bitumen to the atmosphere and light for one year.

11

That the observed hardening upon exposure under atmospheric in-fluences is a low temperature oxidation is demonstrated by Neppe ' ^ ' ) , Hoiberg ^*), Ebberts ^ ^) and others.

More observations on hardening of asphaltic bitumen in road carpets are described by Hubbard and Gollomb ^ ^ ) . T h e retarding effect of transfer of oxygen through the narrow pores in the road surface is observed and they base on their observations various specifications for constructing road carpets of better durability. W i l s o n ^ * ) deals with similar problems. W e leave out of consideration here to what extent these specifications are to be reconciled with requirements of mechanical strength when the road has to stand up to a load of traffic, etc.

Pfeiffer''*) also records results of hardening measurements on sam-ples of asphaltic bitumen from various road carpets at the end of fifteen years' service.

Several authors ^2) Ri) S3) S4) W2) point out that already during the laying of road mixes at the required high mixing temperature of about 170 °C hardening is caused by oxidation. This must naturally be taken into account in judging asphaltic bitumen taken from road carpets that have been in service.

An effect for which allowance must also be made in determining the hardness of asphaltic bitumen is what is known as its physical harden-ing, This is in a certain sense also a symptom of ageing as it makes itself felt in an increase of resistance to deformation upon storage at room temperature for may weeks ^ 3) T4) "Phg increase in resistance to defor-mation may sometimes amount to a factor 3. T h e effect disappears, how-ever, when the asphaltic bitumen is heated beyond its R Ö B softening point. This hardening has nothing to do with oxidation, but must be attributed to a gradual approach to stabilization of the structure of the asphaltic bitumen P *).

Many publications have appeared on testing methods that aim at recording the nature of asphaltic bitumen ageing in a measure, preferably by a test not taking too much time and yet providing information on its behaviour in the long run.

As long as the precise nature of the ageing process is not known, how-ever, it is very difficult to find an appropriate method. Moreover, it is always difficult to tell what combination of light, temperature, layer thickness, water and air must be selected. Many authors do agree that it is best to work on thin films, because then the condition in which the asphaltic bitumen exists in road carpets is the most closely approached. It is then necessary to use films at most some twenty to thirty microns thick, but this entails having too little material available for analyses. The only evaluation that can then be made is by the appearance of the

layer. Investigating ageing after exposure of thick layers of asphaltic bitumen in bulk, Neppe ^ i ) rightly asks how far the results may be correlated with the behaviour of thin films.

The most effective test is still the exposure of films to the atmosphere, Lewis and Hillman ^ 2) and Benson ^ 4j record results of such tests, Benson describes interesting microscopic observations on coagulation with transmitted light through thin films of aged asphaltic bitumen on object glasses. The films he uses are 25 j^ thick.

It is clear that outdoor exposure is never suitable for predicting durabi-lity within a reasonably short space of time. Besides, comparisons are often badly interfered with by changing conditions outside human control,

Oxidations of asphaltic bitumen in the dark at temperatures between 150 ° C and 350 ° C is often suggested as a means of achieving the desired acceleration of ageing for purposes of investigation. Lewis and Halstead L3) and W^ilson^3)^ for instance, carried out investigations of this kind between 160 and 200 °C. These tests are, however, more valuable for ascertaining the hardening that occurs during the prepa-ration of asphaltic bitumen aggregate mixtures than for studying ageing. Some authors — Endersby, Stross and Miles ^ 2) and Anderson, Stross and Filings-^5) — observed whether oxidation tests at high temperature may produce results that correlate with ageing in actual service. Their findings are negative, though Neppe ^ i ) does not always entirely agree with them. His criticism is directed towards the way in which these authors work out their experimental data on tests on oxidized asphaltic bitumens, however,

Rather singular blowing tests at 350 ° C as suggested by Skidmore ^5) and McKesson'^i) as a means of assessing ageing are rejected by Neppe ^ ' ) . He rightly points out that the chemistry of this oxidation is quite different from that, of oxidation, say, at 50 °C. Some of the work described by Thurston and Knowles ^ 2) seems to be much on the same lines as the accelerated ageing test, because thin films of asphaltic bitumen and components of it are oxidized at 200 °C on grains of sand (thick-ness of film about 15/^). Essentially, this investigation is a blowing in-vestigation; nothing is said on correlations with ageing.

Oxidation at low temperature is suggested by Anderson, Stross and Ellings ^5) as a means of ascertaining the durability of asphaltic bitumen. They oxidize the asphaltic bitumen in solution in benzene at 50 °C in an autoclave under oxygen pressure of about 8 atm. The result is eva-luated by the amount of oxygen absorbed during a given time and by the change in penetration of the asphaltic bitumen recovered, these two quantities being combined in a certain way to a "deterioration index".

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" ^

13

Though the temperature is appropriate this method has the drawback that the physical constitution of the pure asphaltic bitumen, which is certainly a factor in the absorption of oxygen in actual constructions, can play no part in the test. Only the chemical nature of oxidation at low temperature is taken into consideration. Hoiberg ^4) and Neppe ^ i) both point out this drawback.

An interesting method is described by Ebberts ^ i ) . Asphaltic bitumen is poured into an Erlenmeyer flask so as to form a layer of a few microns thickness on the bottom. An alkaline solution of K M n 0 4 is put into the flask and this is allowed to stand at 50 °C for a given time, generally not longer than 90 min. The amount of K M n 0 4 consumed is taken as a measure of the quality of the asphaltic bitumen in regard to ageing; so in this test it is not any mechanical change in the asphaltic bitumen itself that is considered. T o get a true picture it would be necessary to esta-blish the relation between oxygen consumption and mechanical change H 7). It would also be desirable to ascertain whether the highly reactive permanganate causes no other oxidation processes in the asphal-tic bitumen than does oxidation under the same conditions with gaseous oxygen.

Bennet and Parks ^3) describe a methode in which the asphaltic bitumen, in a layer of 0,13 mm on a strip of rubber, is exposed to the atmosphere for three days. The strip is elongated at a standard rate and the temperature is lowered until the bitumen cracks. This is cer-tainly an ingenious method, but its drawback is that its interpretation is unknown.

The Beckton durability test ^'^) makes use of a method that is elegant but difficult to check, to determine brittleness after ageing. A 3 /< film of asphaltic bitumen on grains of sand is oxidized at 45 ° C for 72 h. The coated sand is then subjected to abrasion by a jet of air. The amount of asphaltic bitumen worn off is a measure of its durability. This method, again, cannot be understood in detail,

Many investigators have made use of accelerated ageing tests in which radiation with ultra violet light is the accelerating factor. The most well-known instrument for this purpose is the weatherometer, in which radiation, artificial rain and heat or cold can be applied for any required period. T h e effect of this ageing is judged in various ways, the appear-ance of the panel tested generally playing an important part. For applications of asphaltic bitumen in which light exerts influence this is a useful method, though the wave length range of the light emitted must be carefully selected. Under radiation a much accelerated process of ageing is invariably observed.

investigations to be described in the present thesis, we may be permitted to refer to the surveys given by Neppe ^ i ) and the Highway Research Board'^ 2) _ where ageing under the influence of light is dealt with ex-tensively.

T o sum up what has been pubhshed in the many papers on ageing, it may be said that:

a. oxidation is the principle factor in ageing.

b. increasing the temperature causes acceleration of ageing; at too high temperatures (e.g. beyond 70 °C) other reactions set in.

c. light has a highly accelerating effect on ageing. d. ageing phenomena should be studied in thin films.

e. the mechanical results of ageing are increased hardness and brittleness. f. no fundamental theory is available.

g. a multitude of tests is described whose results generally do not meet the requirement of providing a criterion for normal ageing.

4S.

CfiAPTER n

E X P O S I T I O N O F T H E P R O B L E M

The phenomenon of ageing of asphaltic bitumen in practice makes itself felt as a change in the mechanical properties of constructions in which it has been applied.

In mixtures containing mineral aggregate, asphaltic bitumen serves as a binder between the particles of mineral mutually and with the sub-stratum. T h e durabihty of this bond determines the durabihty of the entire construction. The strength of the bond depends on the adhesion of asphaltic bitumen to mineral particles and on the cohesion of the asphaltic bitumen itself. Which of the two bonds first gives way when cracks make their appearance is not always clear. There is little funda-mental information on this point.

In actual practice it is of even greater importance to gain an under-standing of the factors determining the rate of ageing. From trials it is known when a construction must be expected to give way. Brittleness has much to do with it. Rate and extent of deformation, viscosity and rheological type of the asphaltic bitumen are essential quantities.

T h e same factors are considered in judging when a bituminous facing loses its protective effect through crack formation.

It is apparent from the hterature that the action of oxygen is one of the principle factors responsible for the occurrence of ageing pheno-mena. Physical hardening, which can also be called a form of ageing because of its dependency on time, will be left out of consideration here. By establishing the conditions under which mechanical and other measure-ments are carried out on aged and unaged asphaltic bitumen, this effect, whose effect is anyway limited, can be ruled out in comparing results.

Whatever may be the combination of forces causing a construction incorporating asphaltic bitumen eventually to give way, the slow oxida-tion of the asphaltic bitumen is the process that is largely responsible for the time this takes.

Any fundamental theory of this slow oxidation process is lacking and the object of the investigation described in the present thesis is to

con-tribute to the knowledge of the process. An investigation of this type provides a means of connecting up the findings and this in its turn may lead to improving the product so as to increase its durability.

In every investigation of ageing there is always the problem of ob-taining information quickly on the behaviour of the product in the long run. A fundamental knowledge of the process may point the way to an accelerated ageing test and its interpretation.

In order not to make a fundamental study of the ageing of asphaltic bitumen too complicated, ageing in the absence of light was investigated. From a practical point of view such an investigation is very important as most asphaltic bitumen is applied in road carpets and similar con-structions, where the greater part of the asphaltic bitumen incorporated in the mixture is subject to slow oxidation in the dark owing to the usually porous structure of the mixture.

T h e rate of ageing of asphaltic bitumen is controlled not only by its chemical nature, but also by the transfer of oxygen from the surround-ings to and into the asphaltic bitumen. It is therefore also a physical problem, being one of diffusion in particular.

There are therefore three aspects to the study of this slow oxidation process: that of the transfer of oxygen, of the mechanical consequences of oxidation and of the resultant chemical changes in the material.

In this thesis a study is made of the time-absorption curve for oxygen under conditions defined as closely as possible. This comprises both experimental and theoretical investigations and its object is primarily to acquire understanding of phenomena involved by the transfer of oxygen and of related factors determining the velocity of the entire process.

By way of following up this theoretical investigation and with the object of collecting practical data, an investigation into the change in mechanical properties as a result of ageing is also dealt with. As a con-sequence of the visco-elastic character of most of the asphaltic bitumens this investigation ought to comprise measurements of the stiffness both in the short loading-time region as well in the long loading-time region. Owing to the limited amount of aged material available special attention has to be paid to the development and to the application of micro-tech-niques. The development of a micro elasticity method in the frequency region of 60 c/s has been undertaken and use has been made of a micro viscometer already available.

CHAPTER in

EXPERIMENTAL M E T H O D S 1) a. Measurement of oxygen absorption.

The conditions under which the absorption of oxygen was measured were chosen so as to accord the most closely with conditions of actual service. This involves measurements on asphaltic bitumens in thin films at temperatures between 20 and 70 °C.

Some tentative measurements had shown that the order of magnitude of oxygen absorption is 3 cm^ ( S T P ) 2) per g of asphaltic bitumen per 100 h at 50 °C when the thickness of the film is 7 ^ , T o ensure reason-able accuracy of the absorption measurements it was necessary to use several grammes of asphaltic bitumen. T o spread this amount out in a

Fig. 2. Absorption apparatus.

^) The measurements have been performed by messrs. J. P. Spaanderman, A. D. Langeveld and Th. W . Niesman.

film of, say 7/(, within the smallest possible volume, grains of sand as nearly as possible of the same size were coated with asphaltic bitumen

(see III, b ) .

T h e actual absorption apparatus operated by the conventional volu-metric method (fig. 2). It consists of a gas burette having a content of 100 cm3 and graduated into 0.2 cm^, a mercury pressure gauge up to 800 mm Hg and a 0.5 dm^ flask, which contains the asphaltic bitumen as a coating round grains of sand. T h e entirely apparatus is welded glass to glass, with connecting tubes having an internal diameter of 2 mm, fitted with good cocks, the flask being connected via a ground joint. T h e apparatus is assembled against a wall. Altogether, four such appa-ratuses were constructed, so as to be able to carry out several determi-nations simultaneously (fig. 3). The cocks were greased with Apiezon M grease.

Fig. 3. Absorption apparatus.

A pump line connected to a normal mechanical oil-pump enables the apparatus to be evacuated to a pressure of 0.1 mm Hg.

T h e flasks are placed two by two in a water thermostat that can be adjusted to any temperature between 20 and 70 °C with an accuracy of 0.05 °C by means of a mercury contact thermometer, electronic relay, immersion heater and agitator. The flasks are painted black on the out-side to keep out the light. T h e water level in the thermostat baths is

19

kept steady with the aid of a Mariotte bottle. T h e measuring burette is surrounded by a glass jacket to prevent rapid temperature changes from having any effect on the volume of oxygen in the burette.

The measurement proceeds as follows,

The asphaltic bitumen/sand mixture is weighed into the flask and the latter connected to the apparatus. Then, for one hour, the entire appara-tus is evacuated by the oil-pump, the flask being in the water thermo-stat bath, which has been adjusted to the appropriate temperature. The cocks to the pump line are then turned off and the apparatus checked for leakage. Pressure in the apparatus may not increase then. Oxygen from a cylinder (98.7 % Og, 0.6 % Ng, 0.4 % A, 0.3 % CO2) is admit-ted via a special line up to the required pressure. A pressure of 734 mm Hg is chosen, i.e. always lower than barometric pressure. If leakage were to occur during the test, which takes several days, this would at once be apparent by the apparatus filling, so erroneous results may be detected and ruled out.

After the cocks have been turned off an hour is allowed to pass before the first reading is taken in order to establish temperature equilibrium, so as to make this initial error negligible.

Every subsequent reading, generally twice a day, is taken by forcing up the mercury in the burette with the aid of a levelling vessel until the pressure is again 734 mm Hg. As a rule it is not necessary to adjust the pressure several times a day. Of course pressure decreases on account of absorption, but this seldom amounts to more than 1 or 2 mm Hg, so that experimental pressure fluctuates round the required pressure by this variation, which can have little effect on the measurements. Every volume of gas recorded is reduced to 0 °C and 760 mm Hg ( S T P ) , the length of the column of mercury in the pressure gauge also being adjusted so that the pressure is always 734 mm Hg of 0 °C. T o derive this value correctly it is necessary to know the gas volumes at both temperatures (of water bath and room temperature). These are known for each appa-ratus by cahbrations.

The difference between the initial reading and each subsequent read-ing gives the oxygen absorption as a function of time.

The accuracy of reading is 0.05 cm^ on the burette; pressure can be adjusted accurate to within 0,2 mm Hg, At a total reading of 10 cm3 the relative error is 2.5 %. The absolute error is always at most 0,25 cm^. For a number of measurements it was required that the accuracy should be increased to within 0,05 cm^; for this purpose a special apparatus was built, also operating volumetrically (fig. 4 ) . The burette has a content of 25 cm^, graduated into 0.1 cm^. Accuracy is further increased by placing the burette and pressure gauge in an air thermostat bath of

28 °C, accurate to within 0.01 °C, the thermostat being entirely insulated with felt and asbestos, A fan ensures good air circulation; the tempera-ture is again regulated by a mercury contact thermometer, electronic relay and heating element. A water bath of the same kind as described for the other four apparatuses is placed close to the air thermostat and it contains the flask with the preparation to be tested.

/ INLET ! BURETTE 3 MANOMETER 4 DIFFERENTIAL MANOMETER 5 WATER THERMOSTAT 6 SAMPLE CONTAINER 7 LEVEL CONTROLLER a AIR THERMOSTAT 9 INSULATION

10 DEVICE TO ADJUST MERCURY LEVEL 11 TO VACUUM PUMP

Fig. 4. Accurate absorption apparatus.

T o get the required degree of accuracy it was also necessary to adjust measuring pressure with a precision greater than 0.2 mm Hg. W i t h the aid of a differential pressure gauge filled with dibutyl phthalate (very low vapour tension at reasonably low viscosity), pressure can be adjusted with no greater difference than 0.01 mm Hg in comparison with the pressure in the buffer vessel. T h e buffer vessel is previously adjusted to the required pressure (e.g. 734 mm Hg again). The vessel stands in the air thermostat. Variation in pressure in this vessel resulting from temperature fluctuations of 0.01 °C of the air thermostat amounts to about 0.03 mm Hg. The total error in pressure adjustment therefore comes to 0.04 mm Hg, the absolute error in absorption being 0.05 cm^.

Care must also be taken to ensure that the dibutyl phthalate in the differential manometer at the prevailing temperature and pressure of 28 °C and 734 mm Hg, respectively, is saturated with the measuring gas, it being essential that the buffer vessel is also filled with this gas.

T h e above two measuring systems are used especially for measuring oxygen absorption at 50 °C.

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21

T o get a more complete picture of how oxygen absorption proceeds it was necessary to carry out a series of measurements at 20 °C. It was to be expected that at this temperature the process would be much slower, making it necessary to follow the oxygen absorption for several weeks. A simple arrangement (fig. 5) made it possible to carry out the measurements for a long time at room temperature without overcharging the apparatus. niTUMEN SAND MIXTURE -0UAKT2 WOOL BAROMETRIC PRESSURE

Fig. 5. Simple absorption apparatus.

A glass bulb having a content of 100 cm^ is welded to a 50 cm^ burette graduated into 0.2 cm^. The burette is open at the bottom end. Diametrically opposite the burette there is a charging opening with a ground-in stopper. A wad of quartz wool is placed in the passage from the bulb to the burette. The asphaltic bitumen/sand mixture is put in the bulb and oxygen passed into the bulb from the top until the entire apparatus is filled with oxygen. The burette is then placed in a mercury vessel, 35 cm long and 3 cm in diameter, and the charging opening closed. By varying the level of the apparatus in relation to the mercury vessel, the mercury inside and outside the burette can be adjusted to the same height, so that the pressure in the apparatus is equal to barometric pressure. W h e n the necessary corrections have been made the absorp-tion of oxygen can thus be measured as a funcabsorp-tion of time. The tempe-rature at which absorption is measured is not quite constant, varying with the temperature of the room, but this variation was not more than 4 ° C .

T h e outside of the bulb is painted black to exclude light. Altogether, ten such apparatuses were used.

T h e required gas volume corrections for small fluctuations in tempera-ture (20 °C ± 2 °C) and pressure (barometric) can be very simply made by placing among the group of apparatuses a reference apparatus of the same dimensions, but filled with clean sand. The variations in the volume readings of this burette at once give the factor by which the readings of the others have to be corrected. It proves possible to achieve an absolute accuracy of ± 0 . 1 cm^.

b. Preparation of asphaltic bitumen/sand mixtures.

T h e carrier material chosen for the films of asphaltic bitumen was river sand. This has grains without sharp angles or edges and each grain is approximately spherical. W h e n dry, the sand is screened to get the fraction passing mesh 20 and retained by 30 ( A S T M code). Micro-scopically, the mean diameter of the grains was found to be 0.8 mm. If the grains are taken to be spherical 357 g of sand has a surface of 1 m2. If the asphaltic bitumen is evenly spread over all the grains a mix-ture, containing 2 % by wt of asphaltic bitumen on sand has a film thick-ness of 7 microns.

About 1 kg of sand of the above fraction is put into a mixer having a capacity of about 2 dm^. The mixer can be closed, so that it is possible to force the air from it by inert gas with which the asphaltic bitumen cannot react. CO2 was chosen for its great density. W i t h the sand in it and while CO2 is passed in, the mixer is heated to about 100 °C above the R 6 B softening point of the asphaltic bitumen tested. The mixing vanes are revolved from time to time. W^hen the right temperature has been reached and there is no more air in the mixer, the weighed out asphaltic bitumen is introduced through a small trapdoor in the lid. It is mixed for 15 minutes and then the mixer is cooled down as quickly as possible. T h e content is quickly transferred to a brown bottle, this being filled with CO2 and well sealed.

T o see whether the required amount of asphaltic bitumen is present on the sand, it can be dissolved in benzene, filtered and recovered by evaporation. It was found that 95—98 % of the asphaltic bitumen put in the mixer is coated on the sand.

A mixture of 2 % asphaltic bitumen with 0.8 mm grains of sand is still a typically "lean" mix, the grains hardly sticking together. The asphaltic bitumen on the sand is a more or less transparent, brown skin. Any original differences in colour between the grains is still perceptible.

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23

Mixtures containing more than 3 % of asphaltic bitumen begin to get sticky, the thickness of the film no longer being so clearly defined.

W h e n the asphaltic bitumen/sand mixture is introduced into the ab-sorption apparatus, a few grammes of solid K O H in the form of pellets are put in with it. The object of this is chemically to bind any traces of CO2 that might be liberated from the asphaltic bitumen after it has been pumped off, so as to prevent them from interfering with the deter-mination of the gas volume. Any water formed during the oxidation reaction can also be bound by this K O H .

c. The microviscometer.

The change in mechanical properties due to ageing may result in a shift in horizontal direction of the a/e-t curve (fig. 1), but also in a simul-taneous change in the type of the curve. In general, it is therefore neces-sary to measure at least at two suitable points of the time scale of the a/«-timc curve.

Fig, 6. Microviscometer,

The determination of the viscosity gives more or less unequivocal results of bitumens with a PI up to about + 2.

Depending on how the asphaltic bitumen has been prepared, the vis-cosity of freshly-made asphaltic bitumen may vary between 103 and 108 N sec/m2. An instrument frequently used for determining the

visco-sity of asphaltic bitumen is the rotation viscometer, which requires 25 g of asphaltic bitumen for a determination, however, c i ) s i ) M 3 )

A special microviscometer for determining the viscosity of aged asphaltic bitumen, only requiring about 50 mg of material, has been designed by Labout ^ ' ) . T h e principle on which this instrument is based is the linear shear of a layer of asphaltic bitumen bounded by two paral-lel surfaces, under the influence of a constant shearing stress.

Fig, 7, Shearing of a bitumen layer.

A film of asphaltic bitumen 20 to 50 microns thick — depending on its viscosity — is applied between two similar, polished plates of glass, each of 30 X 20 X 7 mm (fig. 6). The lower plate is held in a vide, the upper is fitted with a frame on which there is a 2-cm scale graduated into 1/50 mm. T h e frame is connected.to a weight by a thin, flexible cord passing over a pulley running on bearings. T h e plates must be in an exactly horizontal position. T h e movement of the upper glass is read off from the scale via a mirror and with the aid of a microscope magni-fying 45 times.

T h e apparatus stands in an air thermostat, whose temperature can be adjusted between room temperature and 40 ° C to an accuracy within 0.1 °C. Viscosity can be calculated directly with the aid of the definition

formula for the viscosity of a Newtonian liquid: (fig. 7) d^

'^'^-dt ( I I I - l )

where t = shearing stress N m

fl =^ coefficient of viscosity N sec m ' f = tangent of the angle of shear

t = time (sec.)

At a constant displacement velocity the expression becomes:

V = ^ ( " 1 - 2 ) ( F is the surface area of one glass plate).

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25

The asphaltic bitumen must be applied between the plates in such a way as to ensure good adhesion at every point of the surface and constant thickness of the layer. This may be done quite simply by allowing an appropriate quantity of asphaltic bitumen to spread over a plate after the plate has been heated to about 50 ° C above the R 6 B softening point of the asphaltic bitumen. Then the second plate, also heated to that temperature, is carefully placed in position on the former. Gentle pressure on it ensures good adhesion. Uniformity of the layer can be assessed by observing the colour of the asphaltic bitumen in transmitted light and the clarity of the transmitted light. These must be the same at every point. After the edges have been cleaned the thickness of the layer can be found by weighing. Owing to having been melted the asphaltic bitumen in the microviscometer is invariably isotropic.

Horizontal movement of the plates in relation to each other during measurement increases shearing stress. T h e movement is, however, at most 5 % of the length of the plate, and in consequence, the rate of shear of Newtonian liquids remains constant within accuracy of measure-ment.

The microviscometer can cover the range between 103 and 10^ N sec/m2.

If an asphaltic bitumen can be rheologically classified among New-tonian liquids (penetration index at most — 2 ) , its viscous nature predo-minating, the microviscometer produces unequivocal results. This is also the case when the asphaltic bitumen is of a sol type. For bitumens with a gel character, (penetration index more thans + 2) the viscometer readings are dependent on the shearing stress applied. On the whole this effect does not interfere much with interpretation, as the change in viscosity is the magnitude to be determined.

The accuracy of the instrument proves to be such as to enable the viscosity to be determined accurately to within 10 %. Appendix la-b mentions a number of control methods.

d. Measurement of stiffness of small samples at short loading time. In general, accurate measurements of this property can be made on a transversal vibrating bar. In the case of asphaltic bitumen usually 30—50 grams of material is required P ' ) . Since it is the basis of the new method for measuring much smaller quantities of material, a very brief description will here be given of the method with the homogeneous transversely vibrating, freely supported bar.

A bar of rectangular cross section is placed with its nodal lines on two slack strings. By means of a small electromagnet (fig. 8) fed by an

A.C. source of variable frequency, the bar is set into transverse vibra-tion, via a thin iron plate attached to it. T h e amplitude should remain so small that the stress remains proportional to the strain. A second iron plate attached to the other end will now vibrate with the bar. A wire coil with a magnetic iron core is placed opposite this plate, which plate induces in the coil an e.m.f. of the same frequency as that of the gene-rator. After amplification this voltage can be measured with a valve voltmeter. T h e frequency of the generator is now varied until the deflection of the voltmeter reaches its maximum. Then resonance has

been reached and the stiffness can be calculated from ^')

.Cdyn f^ I' Q 48

h' {k + V2)* ^' (111^3)

By varying the length and/or the thickness of the bar, a fairly wide frequency range can be covered.

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TRARSOOCFR IPICKUPl OSCILLOSCOPC VALVE VOLFHETEH

Fig. 8, Apparatus for testing bars vibrating freely.

If there is not enough material available it is necessary to support it so that the whole becomes sufficiently rigid to be placed on two strings. A bar thus consisting of two layers, of which one is the material to be examined, is called a bar lamellated on one side. Sometimes it is useful to cover two sides of the supporting strip. Then the bar is lamellated on both sides,

It is possible to calculate the stiffness (and also the damping) of the material applied on the rigid base (e.g. a thin steel strip) from the difference in resonance frequency between the lamellated bar and the uncovered base, again using the transverse vibration of the freely sup-ported bar 01) 0 2 ) .

examina-27

tion are, unfortunately, not simple. T h e comparatively simplest result is obtained for the bar lamellated on both sides if the layers applied to either side of the base are equally thick. This is due to the symmetry of the system.

The calculations are based on the homogeneous properties of the base and of the layers applied. The results of the calculations are

Bar, lamellated on one side:

£ , : = - ;i-3 (A - M ^ + B) . ( I I I - 4 )

" 2

where

A = 6 C + £ i /ii (2 /ii- + 3 fti /zs + 2 /i^^) B = - i 5 i / i i V ( 1 2 C - f £ i V )

C = J^ij^ihi Qi + h Q2) ^ = 0.006, 1,2 Bar, lamellated on both sides

' ^ (k + V^)*^' ' O Q • 12 where

P =2hsQ^ + hiQi 7 h ^ h ^ h

Q==hih,'+^ + ' ^ ^ fc = 0.006, 1, 2, . . . A calculation of the difference in resonance frequency to be expected when using as the base a thin steel strip 10 X 1 X 0.01 cm as a function of the stiffness and of the thickness of the layers shows the limitations of this measuring system (fig. 9 and 10). A complicating circumstance is that there are two factors influencing the magnitude of the shift in resonance frequency due to the coverage of the steel strip. The reso-nance frequency of the strip will increase as a result of the stiffness of the applied layer (or layers), but it will decrease owing to the influence of the mass of the layer(s). Especially in the region of low values of the stiffness of the applied material the mass effect dominates, resulting in a lower value of the resonance frequency of the lamellated strip as compared with the uncovered strip. From fig. 9 and 10 it can be seen that measurement on materials having a stiffness of, say 10' Nlm2 (a bitumen with a penetration of 55 at 25 °C, PI zero, 50 c/s) can hardly (111-5)

be performed in layer thicknesses below 1000//. Then the system has become insensitive to variations in stiffness of the applied layer(s). Harder materials (stiffness above 10^ N/m2) can be measured in thinner layers.

o

3.10'N

LAYER THICKNESS IjU I

Fig, 9. Strip lamellated on one side. Frequency difference as a function of layer thickness for different values of the stiffness of the applied layer.

W i t h the apparatus shown in fig. 8 the accuracy of the frequency difference measurement amounts to 0.5 c/s, so only the hardest types of bitumen can be measured in layers below 1 0 0 ^ thickness. In order to have a means of carrying out measurements on softer asphaltic bitu-mens after ageing (and also with a view to having available a more accurate apparatus which might be of value for measurements on paint films and on polymer materials) a new apparatus based on the principle of a differential measurement has been developed. In this apparatus

simul-29

taneously set into transversal vibration. T h e steel strips are as nearly similar as possible. The power supply for each strip is obtained by means of a feed-back system consisting of a linear amplifier, the input

£fl9s)

iv-Layer thickness {JJ) Fig. 10. Strip lamellated on both sides. Frequency difference as a function of layer

thickness for different values of the stiffness of the applied layers.

voltage for which is supplied by the strip itself. The output voltages of both amplifiers, each having the frequency of the strip concerned and vibrating in resonance (ca 55 c/s), are mixed electronically and after amplification made visible on an oscilloscope. The beat frequency of these