The aging of asphaltic bitumen

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THE AGING

Of

ASPHALTIC BITUMEN

by

JR. A. J. P. VAN DER BURGH, J. P. BOUWMAN

and

G. M. A. STEFFELAAR

(Netherlands State Road Laboratory)

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Please address correspondence to

RIJKSWATERSTAAT

KONINGSKADE 25 - THE HAGUE - NETHERLANDS

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November, 1960.

The

Aging

of

Asphaltic

Bitumen

by Jr. A. J. P. van der Burgh, J. P. Bouwman and G. M. A. Steffelaar, Netherlands State Road Laboratory

Summary

The aging in the course of the years of asphaltic bitumen used in bituminous constructions has various causes. One of these, the action of oxygen, is the subject of this publication, although in particular the investigation relates to the rate and intensity of aging caused by the action of oxygen under pressure and without the presence of light. The chemical reactions involved, however, are not dealt with.

The investigations showed that of the various factors which could serve as the most suitable basis for a quantitative examination of the phenomena of aging, the penetration was preferabie to the ring and ba1l softening point, the ether asphaltenes content, or the increase of weight. In evaluating the general results, attention is paid to the shape of the aging curves, and an attempt is also made to explain what happens when the aging procedure is interrupted

and then resumed after remixing the batch.

From the study it has been inferred that asphalts may be prepared which are much less liable to aging than normal ones of the same grade.

The probability of parallelism between natural and artificial aging has also been shown by the results of some of the experiments.

The experimental part of the investigation consisted of keeping thin layers of various types of asphaltic bitumen in oxygen at 60°C. and 20 atm. for a varying number of days, and then measuring the changes in the penetration figure, the softening point and other similar characteristics. In addition, several practical experiments relating to the influence of the atmosphere were carried out (natural aging). The materials used were asphaltic bitumens, either as such or mixed with filler, oil or rubber.

The results have shown that the characteristics measured, such as the penetration figure and the softening point, altered most quickly at the beginning of the experiments, but that those changes, and therefore the aging rates of the asphaltic bitumens, subsequently became slower and slower. Although the number of available data relating to experiments with natural aging is fairly smalI, it can be assumed that natural and artificial aging proceed in similar ways. Further investigations into the re1ationship between the characteristics measured and the length of time lead to the hypothesis that aging proceeds differently during all. initial period of about 7 days from the way it does subsequently, when it has the nature of a unimolecular reaction. The question of whether it is possibie to manufacture asphaltic bitumen of a given hardness which is less than norma1ly sensitive to the aging influence of oxygen can, on the evidence of experiments with "stepped" or interrupted aging, be answered in the affirmative: if a soft asphaltic bitumen is artificially aged until the desired hardness is reached, the result is a product of which further aging under the influence of oxygen is less rapid than that of untreated asphaltic bitumen of the same hardness.

The influence of temperature and pressure, as well as that of the thickness of the layer, in the accelerated aging experiments has also been investigated. The trials dealing with the layer thickness, indeed, produced remarkable results which can perhaps be explained by the occurrence of the phenomena of de-mixing and convection in thick layers of asphaltic bitumen.

Fina1ly, several other experiments on the artificial aging of asphaltic bitumen mixed with macadam are discussed, showing that in such cases aging does not proceed in exactly similair ways for different types of asphaltic bitumen. Tt was also found that a mixture of asphaltic bitumen and tar ages more quickly than pure asphaltic bitumen does, particularly in the initial period.

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THE AGING OF ASPHALTIC BITUMEN

1.

Introduetion

1. 1. In the course of time the properties of asphaltic bitumen undergo modifications, the causes of which can be divided into three groups:

a. alterations in the physico-chemical structure without the absorption or release of matter and without the action of chemical agents;

b. release of matter, such as volatile components or the products of decomposition, and c. chemical action from outside.

The speed at which these modifications occur may be affected by temperature and pressure and, so far as chemical reactions are concerned, by the concentrations of the reacting materials, by catalysts and by actinic radiation (light).

The causes mentioned under a. generally bring about minor modifications which can be removed almost completely by heating the bitumen (thixotropy); in certain cases, however, fairly considerable alterations have been observed in the properties of asphaltic biturnens and bituminous mixtures which have been kept away from light and air for some years (see 4.8.b).

1. 2. The purpose of the experiments was to study the above-mentioned modifications, particularly in sofar as they are caused by the chemical influence of the atmosphere when light is excluded.

In fact, in practice asphaltic bitumen undergoes modifications in bituminous con-structions as a result of the influence of light, air, water, fluctuations in temperature, etc., which are known under the name "aging". Some of the aspects of these aging phenomena form the principal item in the subject examined in this paper, i.e., the influence of oxygen and the part played by pressure and temperature.

2. Method

2. 1. There are various ways of carrying out experiments on the aging of asphaltic bitumen. For example, the modifications which asphaltic bitumen ondergoes in practice may be directly observed, or the bitumen may be placed under experimental conditions which, apart from a few simplifications, closely approach reality. Experiments may, however, also be carried out which, although further removed from reality, have the advantage of taking only a short time, compared with natural (prolonged) aging.

A description of the conditions under which the artificially or otherwise induced aging of various asphaltic biturnens and bituminous mixtures has occurred is given below. 2. 2. a. Accelerated aging experiments were carried out as follows:

About 5 gr. of asphaltic bitumen was poured into six round stainless steel dishes with flat bottoms and a vertical edge (thickness of wall 1 mm, internal diameter 90 mm, internal height 6 mm) to form a layer about 1 mm thick. The dishes were placed horizontally one above the other about 30 mm apart on a rack made of copper wire in a steel cylinder having an internal diameter of 100 mm and a height of 198 mm (capacity 1.55 litres), which was sealed by means of a lid with a flange, rubber packing,

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THE AGING OF AS PH ALTIC BITUMEN

and six bolts. A bronze tap and a manometer were fitted in the lid. The material to be tested could be examined in various ways by means of this apparatus:

1st. Keeping it in an atmosphere composed of oxygen under increased pressure and temperature. The tap in the apparatus was connected to an oxygen cylinder and, without removing the air, oxygen was blown in until a pressure of 18 atm. was obtained. The apparatus was then placed in a heater at a temperature of 60°C. so that within a short time the pressure reached 20 atm. as a result of the increase in temperature. After a fixed number of days the apparatus was removed from the heater, the tap was opened, a day's cooling was allowed, and then the contents of the dishes were collected.

2nd. Keeping it in oxygen atmosphere under increased pressure and at room temperature. The operations given under 1st. were repeated, except that the pressure was brought immediately up to 20 atm. and the apparatus was not placed in the heater.

3rd. Keeping it in an oxygen atmosphere at ordinary pressure and increased tem-perature. Before the lid was put on the apparatus, oxygen was fed into the cylinder via a tube to drive out the air. The tube was removed then, the lid put on and the apparatus placed in the heater at a temperature of 60°C.

4th, 5th, 6th. The same experiments were carried out in the same way using nitrogen instead of oxygen.

2. 2. b. To find out how deep any aging caused by the influence of oxygen penetrates into the bitumen, cups made of various materials (cardboard, plastic, tin) and glass beakers were filled with asphaltic bitumen to a height of 6 cms and then kept for 3 days at 20 atm. and 60°C. in oxygen. Then samples were taken from the top, middle and bottom parts of the bitumen, and in one case in particular also from the centre and the outer parts of each layer.

As a control test a beaker was filled with asphaltic bitumen; after cooling, three samples were taken from it at different heights and examined.

In addition, a quantity of asphaltic bitumen was kept for 3 days at 60°C. in a sealed tube. Before sealing, the air in the tube was not evacuated.

2. 2. c. The modifications in asphaltic bitumen taken from hydraulic mastic asphalt which had been under water for several months were examined.

2. 2. d. Several asphaltic biturnens and bituminous mixtures were poured out and spread on eternit sheets measuring 1 sq.m. which had been previously heated with a burner, thus forming layers about 1 mm thick. The sheets were then exposed to the influence of the atmosphere for four years on the roof of the laboratory.

Portions of the materials being examined were also kept in sealed bottles and tins in a dark cupboard.

2. 2. e. Several bituminous mixtures containing rubber were applied to concrete tiles in layers 0.5 mm thick, and the tiles left lying on the roof of the laboratory for several years. 2. 3. To find out how far the properties of asphaltic bitumen or a bituminous mixture are altered by the natural or artificial aging described above, the following characteristics were as a TUle measured before and after treatment:

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a. the penetration figure at 250

C.; b. the softening point (ring and bali);

c. the penetration index, and

d. the percentage of ether asphaitenes.

In some cases the following were also measured:

e. the percentage increase in weight;

f.

the external appearance;

g. the brittle point (Fraass); h. the UbbeIohde drop-point, and

i. the percentage of components insoluble in carbon tetrachloride.

2. 4. Materials examined (several tests were carried out with some, only one test with others):

A. Asphaltic bitumens and mixtures of asphaltic bitumens with filter:

1. Kuwait asphaltic bitumens:

a. 280/320; b. 80/100;

c. same, unblown (Ghent); d. 60/70, and

e. 50/60.

2. F 50/60 Shelfalt:

a. unmixed;

b. with Enci-filier 1 : 1, and

c. with marbie powder 1 : 1. 3. Mexican asphaltic bitumen:

a. 80/100;

b. same, from Germany, and c. 50/60.

4. Venezuelan asphaltic bitumen:

a. 80/100 from Belgium, and

b. 50/60 (bIown).

5. Asphaltic bitumen from emuision. 6. 150/200 cut-back bitumen. 7. Trinidad épuré.

B. Mixtures of asphaltic bitumens:

280/320 Kuwait

+

Trinidad épuré 1 : 1. C. Mixtures of asphaltic bitumens with rubber:

1. 80/1 00 Kuwait.

a. with 2,9% Hycar OR 15;

b. with 3 % Pulvatex, and c. with 5 % Pulvatex.

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THE AGiNG OF ASPHALTIC BITUMEN 2. 150/200 cut-back bitumen:

a. with 5% Pulvatex, and

b. with 5% Mealorub.

D. Mixtures of asphaltic bitumen with oil:

1. üld 1928 asphaltic bitumen and 8.3% anthracene oil. 2. üld 1929 asphaltic bitumen:

a. with 8.6% anthracene oil;

b. with 4.4% creosote oil without tar acids, and

c. with 4.52% creosote oil without tar acids.

3. 20/30 asphaltic bitumen:

a. with 20.2% residue X 100, normally distilled, and b. with 20.1% residue X 100, vacuum distilled.

4. 50/60 asphaltic bitumen with 17% creosote oil without tar acids. 5. Fluxed Boeton asphalt.

6. Fluxed Trinidad asphalt.

E. Mixtures of asphaltic bitumens, oil and rubber:

50/60 asphaltic bitumen

+

17% creosote oil without tar acids:

a. with 6% Pulvatex, and

b. with 6% Mealorub.

3. Results

3. 1. Table I shows the results of all artificial aging experiments with layers of asphaltic bitumen of about 1 mm thickness at 20 atm. and 60°C. in oxygen, as described in 2. 2. a. and 2. 2. a. 1st.

Figure 17 shows the results of the experiments with 80/100 Kuwait asphaltic

bitumen in oxygen and in nitrogen at 20 atm. or normal pressure, and 60°C. or room

temperature (see 2. 2. a.).

pT increased pressure and temperature; p = increased pressure; normal temperature; T = normal pressure; increased temperature.

The results of the experiments reported under 2. 2. b. involving a 6 cm thick layer of 80/100 Kuwait asphaltic bitumen for 3 days at 20 atm. and 600

e.

in oxygen are given in Table 11, together with the control test and the test with the sealed tube.

The results of the experiments described under 2. 2. c. involving asphaltic bitumen (probably 50/60 and 60/70 Kuwait) taken from hydraulic mastic asphalt are given in

Table lIl.

THE AGiNG Of ASPHALTIC BITUMEN

Table I. Summary of the results of artificial aging tests on layers of asphaltic bitumen 1 mm thick at 60°C. and 20 atm. in oxygen

Double columns: the tests were done twice with the same material, but not as duplicates. d Pen R+B P.1. Fr EA IW Material 280/320 Kuwait 80/100 Kuwait Kuwait, unblown 80/100 (Ghent) 50/60 Kuwait 80/100 Mexican 80/100 Mexican from Germany 80/100 Mexican from Germany 80/100 Venezue1an from Belgium

number of days in the test; 0 days means original material. penetration figure at 25°e.

softening point (ring and bali) in oe. penetration index.

brittle point (Fraass) in oe. percentage of ether asphaltenes. percentage increase in weight.

I d Pen R+B I _P.1. _Fr _EA _IW

i

I _I I 0 303 36.0 I +0.3 -21 9.4 I 3 121 42.3 -1.1 -22 11.8 0.6 I I ₇ ₁₀₅ _44.0 I -1.0 -19 _13.0 I 0.8 114 96 45.1 I_I -0.9 -21 14.3 1.0 0 89 83 _{46.41 47.8 1-0.7 -0.5} - 2 0 _{13.91 :6.6} 1 47 54.5 -0.3 17.9 0.5 2 42 55.7 -0.3 19.5 0.6 3 40 37 54.0 57.4 -0.7 -0.2 -10 16.9 19.1 0.6 0.9 7 31 32 58.2 60.6 -0.4 + 0.1 7 18.4 22.3 0.7 1.1 14 26 25 61.2 65.0 -0.2 + 0.4 - 9 20.8 22.2 0.9 1.4 28 17 73.3 + 1.0 25.2 2.0 0 79 47.0 -0.9 13.3 1 79 54.2 -0.5 14.9 0.5 2 40 54.9 -0.6 15.1 0.6 3 38 55.6 -0.5 15.5 0.7 7 32 58.0 -0.4 16.4 0.9 14 29 60.9 0.0 16.6 1.2 28 26 63.3 +0.2 19.0 1.5 0 52 53.6 -0.2 -13 17.1 3 23 63.7 0.0 - 7 20.6 0.8 0 77 50.0 -0.1 -19 20.7 3 36 61.5 +0.5 -JO 24.4 0.7 7 29 64.0 +0.5 -13 25.9 0.8 14 21 68.3 +0.6 - 6 27.3 1.2 0 76 51.2 +0.2 20.7 1 45 58.9 +0.6 23.7 0.5 2 39 60.4 +0.5 25.0 0.6 3 38 60.4 +0.5 24.7 0.7 7 32 64.2 +0.8 26.9 0.9 14 28 67.6 +1.1 27.8 1.2 28 17 76.7 + 1.5 31.3 2.0 0 79 49.1 -0.3 18.0 1 41 55.4 -0.4 20.1 0.6 2 35 57.2 -0.4 20.1 0.8 3 32 58.9 -0.2 22.2 0.9 7 27 60.9 -0.2 22.7 1.1 14 18 68.3 +0.3 25.0 1.6 28 9 93.0 +2.3 32.0 3.3

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THE AGING OF ASPHALTIC BITUMEN -2

+

v w

o

w 2

+

n

o

M<:terial d Pen

i

R+B P.1. Fr EA

i

ith tar acids 7 34 61.8 + 0.5 -12 28.0 0.5

" 14 28 65.4

+0.7 -13 29.7 0.9

Id 1929 asphaltic bitumen 0 93 49.1 +0.2 -22 23.7

-+ 4,52% creosote oil 3 34 62.0 +0.9 -12 27.1 I 0.4

ithout tar acids 7 37 63.3 +0.9 -12 27.9 1

1 _0.6 0 202 38.8 -0.5 <-24 14.2 I 0/30 asphaltic bitumen I -20,2 % res. X 100 3 61 53.3 +0.1 -20 16.9 I 0.9 ormally distilled 7 52 57.1 +0.5 -19 18.0 1.1 " 14 44 60.3 +0.8 -18 19.3 1.3 0/30 asphaltic bitumen 0 134 43.6 -0.3 <-24 13.3 -20,1 % res. X 100 3 47 58.8 +0.6 -22 16.7 0.8 acuum distilled 7 39 61.6 +0.7 -20 18.4 1.1 14 36 65.2

+

1.2 I -21 19.1 1.3 " 1I_, I

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-Table 11. Aging tests on thick layers of asphaltic bitumen

No. Pen _RIB

I

_P.1. EA Packing lW after

--~---~----~

-Table 111. Natural aging of hydraulic mastic asphalt

Material Pen R+B P.I. EA

original asphaltic bitumen (50/60)

" " " " " " " same recovered after 10 months in water

" " " " same recovered after 27 days in water

" " " "

same recovered trom freshly-prepared mastic asphalt

same recovered from freshly-prepared mastic asphalt

original asphaltic bitumen (60/70)

14.8 16.0 18.7 19.1 16.2 16.3 17.9 17.9 17.0 17.7 19.2 16.2 -0.7 -0.7 -0.7 -0.5 -0.7 -0.6 -0.6 -0.8 -0.4 -0.4 -0.3 -0.7 47.2 52.2 50.6 53.8 44.4 50.0 48.5 50.8 50.3 53.4 49.2 49.5 82 50 57 46 111 64 74 55 67 49 77 64 "

"

" 21

Table IV. Natural aging througb influence of the atmosphere on tbin layers of asphaltic bitumen and bituminous mixtures

Fraass Ubbelohde: !nsoluble

nu: AGiNG OF ASPHALTIC BITUMEN

Figures 18, 19 and 20 show the results of the tests in which diverse materials spread into thin layers on eternite sheets were exposed to the air, while other batches of those materials were kept in closed vessels in the dark (see 2. 2. d).

Table IV gives a summary of the results of the tests mentioned under 2. 2. e., in which 150/200 cut-back bitumen, 50/60 asphaltic bitumen and mixtures of them with 5% or 6% of Pulvatex, Mealorub and 17% of creosote oil without tar acids in layers 0.5 mm thick, were kept for several years on concrete tiles on the roof and subsequently scraped off and tested.

Brittleness: determined at scraping off in February 1954; low figure - very brittIe; high figure - slightly brittle.

brittie point in

oe.

drop-point in

°c.

percentage of components insoluble in carbon tetrachloride.

4. Analysis of thc rcsults

4. 1. When the figures in Table I, which relate to a score of asphaltic bitumens and bituminous mixtures which have undergone the same test, are made into graphs, it is noticeable that series of points are obtained for the penetration figure and the softening point (and, less clearly, also for the ether asphaltene content and the increase in weight) which suggest similarly shaped curves. That is to say, all the examined materials appear, in this test, to behave in a similar way (see the graphs in Figures 1 - 4).

The series of points for each separate material naturally show random irregularities, which prevent any attempt to find an expression for the aging curve. Yet because of the relatively large number of these series of points, it is possible to establish an average series of points in which accidental deviations are likely to play a much smaller part. In determining the average itmust be remembered that the values for penetration, softening point, ether asphaltene content and increase in weight of the various materials have been measured after an aging of 0, 1, 2, 3, 7, 14, and 28 days, although the series is not complete for each material as in many cases the values for 1, 2 and 28 days are missing. Since the series of points lie at different levels, according to whether the materials are harder or softer, it is not possible to make up the average series of points solely from the averages found for each period of time. However, on the basis of the presumed similarity between the aging curves, the following method can be used: each series to be multiplied by a factor in such a way that for zero time the same value for the quantity concerned is obtained.

Ifthe average of the various series calculated in this way is determined, the series thus obtained appears to be almost the same as the average of groups taken from the total number of series of points (see Table V). This supports the supposition that the aging curves are basically similar.

0) = 16.0. On the graphs g 20 is shown by a thick line and small circles. The series g 5 and g 13 were also multiplied (after obtaining the averages) by factors in th is way.

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THE AGING OF ASPHALTIC BITUMEN ---;-- ---;-- ---;-- ---;-- ---;-- ---;-- ---;-- ---;-- - -Pen. 10 OH>--n~~~-~\~--~o---'---~-'-"-9Ol---J\...-.j..\\.~-++I\-1-~J.___--_-I---1 801-- \ 0123 7 14 28 d

Figure 1. Variation of the penetration figure for various asphaltic biturnens during artificial aging, with d = number of days. The thick line with small circ1es shows the average.

R+Bl00 90 80 70 60 50 40 30 20 10 01 23 7

-[

-,

I

1-14 28 - - d

Figure 2. Variation of the softening point for various asphaltic biturnens during artificial aging, with d = number of days. The thick line with small circ1es shows the average.

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THE AGiNG OF ASI'HALnc BITUMEN

-I

28 -_ d EA 40 ---_._,--- -35 30 25 20 15 10 5 0 7

Figure 3. Variation of the percentage of ether asphaltenes for various asphaltic biturnens during artificial aging, with d

=

number of days. The thick line with small circ1es shows the average.

_ J

28 - d

Figure 4. Increase in weight in percentage for various asphaltic biturnens during artificial aging, with d = number of days. The thick line with small circ1es shows the average.

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THE AGiNG OF ASPHALTIC BITUMEN

_ . _ - - - . _ . _ - - . - - - -4. 2. It, as a result of the foregoing, the series of points marked by g 20 are taken as representative for the aging process, the question arises whether a simple mathematical expression can be found for the aging curves which can be drawn through these series of points.

For physico-chemical reasons it is to be expected that the ratio between the characteristics of asphaltic bitumen and the duration of aging wil! be of a logarithmic or exponential type. It it is assumed that aging occurs mainly by means of chemical reactions (decomposition and oxidation) and that the characteristic quantities, measured at a given moment, are proportional to the extent to which these reactions have occurred up to that moment, and thus to the number of molecules which have reacted, and if it is remembered that decomposition reactions are unimolecular and that oxidations can also be considered to be unimolecular reactions if the concentration of oxygen is taken as constant, then the formula f(V) = ekt _{applies, in which V represents the aging} (measured by penetration figure, etc.), f a linear function, e the base of the natural logarithms, t tbe time and k a constant.

It a graph of such a function is drawn on half-logarithmic paper, with the t-axis on a linear scale, a straight line is formed.

It has appeared from the investigation of the g 20 series for penetration figure and softening point that the values for 7, 14 and 28 days always lie on a straight !ine on the half-logarithmic graph, although this is not the case with the value for 0, 1, 2, 3 and 7 days. However, the last-mentioned values certainly lie on a straight line if the t-axis is also made logarithmic and two-sided logarithmic paper is used (see Graph No. 5)

R +B Pen EA IW)( 10 4

,

10 8 6

L __

~-~+--1>--=---1R+B EA

L__

----~--<>---~--O':..,Pen IW xlQ 4 '4 28 - d

Figure 5. Rectilinear results on logarithmic scale of the vertical axis anct of the horizontal axis for ct = 1 to ct = 7, but on linear scale of the horizontal axis for ct

=

7 to ct = 28. On the basis of these fa cts, the aging curves can be described mathematically as follows:

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THE AGING Of' A~PHALTIC BITUMEN

- - -

in which a, b, c and d are positive or negative constants. Y is the quantity in which the aging is measured (penetration figure, softening point, etc.), X is the time.

These equations each represent an approximate expression of part of the aging curve, and may be considered solutions of the differential equations:

dY Y dY cX

dX-

a·

_X-

and dX - log e

which show the aging per unit of time, respectively during the first 7 days and subsequently.

4. 3. If the processes described in paragraphs 1 and 2 are applied to the values obtained for the penetration figure, the softening point, the percentage of ether asphaltenes and the percentage increase in weight, the curves shown in Graphs 6, 7, 8 and 9 are obtained.

The indices 1 and 2 relate to the following equations (d is number of days):

log Pen₁ - 0.1976 log d

+

1.756

}

penetration figure log Pen₂ - 0.01157 d --I- 1.670

4. 4. The two quantities from Table I which have not yet been mentioned, viz., the brittle point (Fraass) and the penetration index, must be discussed separately, because they cannot be treated in the same way as the other data.

4. 4. a. The brittle point (Fraass)

The number of values relating to this is too small for mathematical treatment as described in the foregoing paragraphs.

But it is reasonable to assume that the modification in the brittle point during the aging process is similar to that of the other quantities, so that it is permissible to

determine an average g 15 series from the 15 available series in the way already described. This series reads as follows, if the first value is taken as - 20:

d : Fr:

o

-20.0 7 -15.2 14 -13.9 Values for 1, 2 and 28 days were not determined.

The series of points does not seem to contradiet the assumption that the brittle point behaves in a similar way to the other quantities (see Graph 10).

Pen 100 90 80 70 60 50 40 30 20 10 Penl _I

LOG. Pen 1 =1.7560.1976LOG d -LOG. Pen 2= 1.6 70-0.01157 d

\

P~~

_~~

----

... Pen!

--

Pe;2 0 1 2 3 7 14 28 - - - - d

Figure 6. Mathematical approximation of the mean curve for the penetration figure. Penl is valid for d

<

7, and Pen₂ for d

>

7.

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Ttffi AOfNO OF ASPHALTIC 81TUMEN 28 _ d 14 R +B)2 - 1 :..--::~:::f:=--~---li(R~fB)1 I

1-I

LOG. CR

+

B) 1 = 0,0 5 0 9 LOG d

+

175 5 lLOG.(R+B)2=o.0037Id+i,772- -7 50

i

1

i

-I I I I 40 (R+B) 1 01 2 3 R +. BI 76[!•.I !. IT.I ! 70 ---: . I 1

I

60(R+B)2

Figure 7. Mathematica! approximation of the mean curve for the softening point. (R

+

B)l

is valid for d

<

7 and (R

+

B)2for d

>

7.

LOG. EA 1=0,0758LOG.d +1,258 LOG.EA 2=0.00386d +1,295_ I EA 30

I

26 I ! 22 18 14 +It~ 10

+t+

6

Itt

2 EA 1 01 23 7 14 EA 28 - - - - _ d

Figure 8. Mathematical approximation of the mean curve for the percentage of ether asphaltenes. EA_I is va!id for ct

<

7, and EA₂ for d

>

7.

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~ ~~~~~~~T_H_E~A_G_I_N_G OF ASPHALTIC BITUMEN IW 1,7

-!

'lW 2 I

- I

l,S -1 1,3 - -

--

I

IWl 1,1

-I

0,5 -t

-I

0,3 LOGIW 1=0,349LOGd-0,387_{L OG.IW 2=0,014 4d - 0,19 2}

6--1

0,1 - _ ..

-7 14 28

d

Figure 9. Mathematica! approximation of the mean curve for the increase in weight. IW₁is va!id for d

<

7 and IW₂ for d

>

7.

Fr 20 19 18 17 16 15 14

1\

\

'\

\

r---...-

---13 12 0 ₃ ₇ 14 d

Figure]O, Average modification of the britt!e point (Fraass).

4. 4. b. The penetration index

As the penetration index is not a directly measured but a derived quantity, It IS evident that it behaves according to the same general laws as the penetration figure and the softening point.

If an attempt is now made to determine an average series from the calculated P.1. in order to draw a graph, difficulties arise as the result of the rather complicated

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relationship which exists between the penetration figure and the softening point on the one hand and the penetration index on the other.

If Pfeiffer's and Van Doormaal's equation (Pfeiffer, The properties of asphaltic bitumen, 1950, page 166): d log Pen (T) dT 1 50 20 - P.l. 10

+

P.l.

is used as a starting point, the following is obtained by integration:

log Pen (T) = T 50

+

B; and

d. calculation of the P.l. with the aid of formulae derived from the expressions given in paragraph 3 for the Pen, tand the R

+

B, t curves.

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THE AGING OF ASPHALTIC BITUMEN -[ I I,B

J----,-~.--r--~-~--~---~~7

1,6

1--+--+--++-_-+----+----"L.._f---_ _

~--=----=----1 1,4~~I----+---+---+---::;;~---I 1.2

H-+-++---+--...

+...--...--...---:::Tf"'=---1 1.0H--+-+-I-~----+...c="~pI 1 - + - - - 1 QB l.? 0.6 P~~-f-t----t---+---::;;j2e - - - j

~

Q41---+J+-4--+-~-f-V-;;;>""''''t---+-e---I ~ 1 0.2Hf-+--H~--!.--b-~-!---I . / ...~ OHl++-+...---".-"'~--f__-__+---"-'---1 2B ---~._d 14 10 7 -O'2I----1l;;O;!~~/I;.'--j---+--f---+---I 01 23

Figure 11. Modification of the penetration index (P.I.) in aging. For the signs see 4.4.b. The small circles show the points mentioned under 3. in 4.4.b.

The curves certainly lie at different levels, but, with the exception of P.Ll' are approximately straight lines lying at roughly the same angle to the time axis. Even the P.L

2 is very approximately a straight line, which can be simply represented by:

P.L2 = 0.0378d

+

0.683

Although the P.L_I curve has been derived from the Penl and (R

+

B) curves which

meet the experimental data very weIl, it seems to be of little value in describing the size of the P.J. in the period of 0 - 7 days. Itis more appropiate to say that the P.l. rises from the beginning broadly proportionately to the time. The P.L increases every day in the aging test by an absolute amount of about 0.038.

4. 5. How can the shape of the aging curve as described in the previous paragraphs now be explained physico-chemicaIly?

Itmust first bepointed out that the aging behaves after about 7 days as an unimole-cular reaction and in doing so bears out the hypothesis referred to in par. 2. It is, however, more difficult to explain the behaviour in the starting period. The equation

obtained,

dV

V

- - - = a _. (see par. 2)

dt t

does not correspond to areaction the rate of which is determined solely by the concentra-tion of the reacting substances. Because the pressure and temperature are kept constant, the only remaining influences on the reaction rate are: (i) catalytic or actinic agents (the

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THE AGlNG OF ASPHALTIC BITUMEN

.~----latter can in this case be excluded), and(ii)the addition or removal of one or more of the substances involved in the reactions in accordance with laws which have little or no relation to these reaetions. As an example of the latter, the interchange of oxygen and reaction products between the gaseous and liquid (bituminous) phases may be mentioned, an interchange which is governed by the laws of solubility, absorption and diffusion, complicated by the fact that the liquid phase and the dividing surface undergo modifi-cations.

It must therefore be assumed that after a while these phenomena no longer have any infiuence on the modifications in the reaction rate, since they have become stationary.

However, bearing this in mind it is difficult to derive theoretically an equation far the reaction rate in the starting period. Consequently an attempt was made, by means of the following experiments, to look further into the possibilities indicated above.

80/100 Kuwait asphaltic bitumen was subjected to the normal aging test for 14 days. The material was then collected, weil mixed, and then again aged for 1, 2, 3, 5, 7 and 14 days. Using the same method, 280/320 asphaltic bitumen was re-aged for

1 and 2 days after having been aged for 16 days.

In these experiments the following items were measured regularly: penetration figure, softening point, penetration index, percentage of ether asphaltenes and percentage increase in weight.

Ifthe values obtained trom this "stepped aging" are drawn on graphs and thus farm

curves which correspond to the "starting curve" of normal aging, this would indicate that the form of this starting curve is indeed determined by diffusion and absorption phenomena. It, on the other hand, curves are formed which correspond to the part of the normal aging curve after 14 or 16 days, which would mean that the aging continues just as if the test had not been interrupted, the explanation of the form of thc starting curve would have to be sought in the ariginal chemical composition of the asphaltic bitumen. In this latter case, it would be possible, by artificial aging, to manufacture an asphaltic bitumen of fixed hardness which would be less sensitive to (further) aging than an asphaltic bitumen of the same hardness which had not been previously treated in this way.

Graphs 12 to 16 show the values found, together with comparable curves obtained by converting them and several mathematical approximation curves.

The results show that the aging of the "pre-aged" bitumens takes place more quickly than is the case after 14 or 16 days without interrupting the experiment, but more slowly than that of fresh asphaltic bitumens. But liWe can be said with certainty about the form of the curves of the "stepped aging" in view of the small number of values, although the Pen-curve makes it seem particularly likely that this form corresponds to the "starting curve" of norm al aging.

An attempt was therefore made to find empirical equations giving the approximate average of the stepped aging that can be derived from the values obtained (marked with the indices 3 and 4 in three of the graphs).

Curves 1 and 2 are the normal aging curves for d

<

7 and d

>

7 respectively. The curves with the index 5 show the course that the stepped aging would take if the interruption of the experiment and the fresh mixing of the material had no influence on it. They have been drawn by taking the value for d = 14 on the curves with the index 2 after adequate multiplication as starting point on d

=

O.

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It can be conc1uded from the foregoing that both diffusion and absorption pheno-men a and the chemical composition of the fresh asphaltic bitupheno-men have an influence on the farm of the aging curve and that therefore, it must certain1y be possib1e to manufacture by artificial aging a bitumen which is 1ess sensitive to further aging than a fresh bitumen of the same hardness.

Itmust also be pointed out that in "The properties of Asphaltic Bitumen", p. 111, Pfeiffer gives an empirical equation for the quantity of oxygen which has reacted as function of the time in an aging test on Mexican asphaltic bitumen at 100°C. and

normal pressure.

This equation reads: G = c.ta

G

a . - or, since G t

in which G = quantity of oxygen used, t corresponds to

dG dt

= time, a and care constants. This equation

V: dV dt V a. -t

In other words, Pfeiffer applies the starting curve to the total aging, although he gives na explanation of the formu1a.

Figure 12. Aging of pre-aged asphaltic bitumens ("stepped" aging). 0

norma! aging;

D

= 80/100, stepped aging; X = 280/320, norma! aging;

+

stepped aging.

80/100, 280;320,

Indices of the curves: 1 and 2: mathematica! approximation of the average curves for norma! aging (for d

<

7 and d

>

7 respective!y); 3 and 4: mathematica! approximation of the curves for stepped aging; 5: theoretica! shape of the curves for stepped aging, assuming that the interruption of the test has na influence.

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THE AGlNG Of ASl'HALTIC BITUMEN R.B

.

80 70 Figure 13. 27 26 25 24 23 22 21 Figure 14. 5 7 8 I i 2345 78 p',~... -...-;

r'

,I (g.B) j,., ...---- : .... --'- ...t-' ... 14 15 16 17 18 19 21 14 15 16 17 18 19 21 .0 28 -d 28 - • d

Figures 13 10 16. Aging of pre-aged asphaltic biturnens ("stepped" aging). 0 = 80/100, normal aging;

LJ

= 80j100, stepped aging; X = 280/320, normaI aging;

+

= 280/320, stepped aging.

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THE AGING OF ASPHALTIC BITUMEN IW

.

o

I 234 Figure 15. +If-l--++--H

o

-II-t--+-+-Figure 16. 28 -.d 28 .d

Indices of the curves of the figures 13 to 16: 1 and 2: mathematicaI approximation of the average curves for normal aging (for d

<

7 and d

>

7 respectively); 3 and 4: mathematical approximation of the curves for stepped aging; 5: theoretical shape of the curves for stepped aging, assuming that the interruption of the test has no influence.

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THE AGlNG OF ASPHALTIC BITUMEN

- - - " - - - " -

-4. 6. The accelerated aging test, which supplied the figures in Table I, was performed by subjecting the asphaltic bitumen to the influence of oxygen for a fixed period of time at increased temperature and pressure. The influence of the time in this test has been explained in detail above; but what is the influence of pressure, temperature and oxygen (compared with an inert gas)? This can be seen from Graph No. 17.

4. 6. a. Raising the pressure to 20 atm. O₂ without raising the temperature results in slower aging than raising the temperature to 60°C. without increasing the pressure. A combination of increased pressure and temperature has, with regard to the penetration figure, a roughly additive effect; for the other quantities, the action is greater than the sum of increased pressure and temperature separately. Abnormality occurs only in the case of the ether asphaltenes; reduction of the ether asphaltene content in the test involving increase in temperature only, which is, perhaps, an accidental deviation.

The phenomena mentioned can be explained as follows: Raising the pressure to 20 atm. increases the concentration of oxygen twenty times. Insofar as the aging consists of oxidation, this also applies to the reaction rate, at least if diffusion in the bitumen also increases by the same factor. The viscosity of asphaltic bitumen is also increased by raising the pressure; the increase can be estimated in this case as at least 10 to 20% (see Pfeiffer, pp. 74-76). Consequently, the aging rate will increase by a factor smaller than 20, e.g., 16, as a result of raising the pressure to 20 atm.

Raising the temperature generally speeds up chemical reactions by a factor of 2 to 4 per 10°C., or an average of about 3. This means that the 40° rise from room temperature (20°) to 60° speeds up the reactions from 16 to 256 times, with an average of 81 times. In addition, the viscosity of asphaltic bi tu mens the softening point of which lies between 20° and 60°C. decreases by a factor of 102 _{to 10}3 _{when the temperature is}

raised to 60°C., which increases the diffusion rate of the oxygen in the material by a factor of the same size (see Van Dort, "A Study of the Aging of Asphaltic Bitumen", 1954, p. 70, and Pfeiffer, p. 70.). It can thus be said that the aging rate becomes hundreds of times as fast as a result of the said ir'Jcrease in temperature.

That the combination of pressure and temperature increases has an accelerating effect on aging which is often greater than the sum of the effects of the said measures separately can perhaps be explained by the fact that raising the pressure at a higher temperature causes a smaller percentage increase in the viscosity of asphaltic bitumen than at a lower temperature (see Pfeiffer, p. 75.).

4. 6. b. When the tests are carried out with nitrogen instead of oxygen, the relationship between the effects of increasing the pressure and raising the temperature is roughly the same, although in the absolute sence these effects are much smaller. Since the oxygen was not completely removed in the tests to which Figure 17 refers, some oxidation of course occurred.

According to Van Oort ("A Study of the Aging of Asphaltic bitumen", 1954, p. 17) the oxygen absorption of 1 gr of asphaltic bitumen in a layer 7 microns thick at

SO°c. and norm al pressure during 100 hours is about 3 mI.Itfollows that 1 gr of bitumen at 60°C. during 3 days absorbs about 7 mI of oxygen. The cylinder contains 5 gr of bitu-men, which would therefore absorb 35 mI of oxygen. If the air is not removed before-hand, 1.5Iof air remain in the cylinder; they contain 300 mi of oxygen, a great excess therefore in respect of the asphaltic bitumen, the more so since the latter is present in a layer much thicker than 7 microns.

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THE AGING OF ASPHALTIC BITUMEN - - - _.._ -17 9 0 !,-P,:e,--n_. ~-1 8 EA---,- - p T 02 I

_---1

1 3 [±==±==-:.:~

o

3 60 R

+

B.~__._~ 58H--"----"-r 56 54 52

Figure 17. Jnfluence of temperature, pressure and oxygen in aging. T ture, p = increased pressure, 02 = oxygen, N2 = nitrogen.

increased tempera.

In the tests with increased pressure and temperature, therefore, failure to remove the air from the cylinders results in the oxygen concentration being and remaining roughly equal to that in air at normal pressure. The aging effect will then roughly correspond to that which occurs in air at norm al pressure at 60°C. The latter combina-tion has not been reproduced in experiment, but the effect is certainly somewhat smaller

than that obtained in the test with oxygen at 1 atm. and 60°C.

The deviation in the ether asphaltenes mentioned under a. did not occur in the tests with nitrogen.

4. 7. As in the foregoing, only asphaltic bitumen in thin layers (about 1 mm) was under consideration, it may be asked how deep the aging proces actually penetrates into the asphaltic bitumen. Most books on the subject deal almost exclusively with aging tests on thin layers (Abraham, "Asphalts and Allied Substances", 5th edition, 1945, pp. 1476-1491; Van Dort, "A Study of the Aging of Asphaltic Bitumen", 1954, pp. 10-14), because in the bituminous constructions used in practice the asphaltic bitumen usually occurs in layers ranging in thickness from a few microns to one or two millimetres.

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THE AGING OF ASl'HALTlC BITUMEN

Van Oort states, moreover, that the aging penetrates only very slowly and is in practice noticeable only in the surface layer (pp. 72, 108).

The figures given in Table 11 refer, however, to pieces of asphaltic bitumen 6 cm thick. Ifthe increase in weight after 3 days (average 0.12%)is compared with that of the layers I mm thick from Table I, it is clear that the absorption of oxygen cannot be limited to the surface layer. This appears to be contrary to the view of Van Oort, but it must be remembered that in the case of fairly Iiquid material at the test temperature (60oe.) and in a layer 6 cm thick, convection probably plays a large part, which will not be the case with layers only 1 mm thick.

Further examination of Table 11 shows that there are no observable differences caused by using various sorts of cups (cardboard, tin, plastic) and glass beakers. The averages of the results of the first seven tests can, therefore, be taken. The decrease or increase in the penetration figure, the softening point and the percentage of ether asphalt-enes for each zone (upper, middle and lower) can be calculated from these averages as percentages of the original values. The same can be done for the data relating to the outer edge and centre of each layer, but here only one observation is involved.

The figures thus obtained can be arranged as in Table VI (the figures for the contral test and the test with the sealed tube have been dealt with in the same way).

Table VI.

₊ 3.2 + 7.7

The following can be deduced from this Tabie:

The figures relating to all the top, middle and bottom layers which, as the averages of seven tests, are fairly reliable, show that aging is strongest in the upper layer (as could be expected) and weakest in the middle layer.

The aging in the lower layer is midway between that in the other two layers. This is difficult to explain except by convection and demixing. The (1ess reliable) figures for the outer edge and centre show strongest aging in the outer edge of the topmost layer and the centre of the lowest. The least aging apparently occurred, according to these figures, in the centre of the middle layer, i.e., the middle of the whole mass. All this is true not only for cardboard and plastic cups, for which permeability cannot be entirely

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excluded, but also for glass! Some aging also appears to have occurred in the sealed tube; the tube contained roughly the same amount by volume of air as asphaltic bitumen i.e. about one-fifth of the quantity of oxygen which the asphaltic bitumen would absorb if the oxygen pressure were constant at 1 atm.

4. 8. Tables 111 and IV and Figures 18, 19 and 20 give the results of the natural aging tests, but as the available figures are far less numerous than those for the artificial aging

tests, only relatively superficial conclusions can be drawn.

4. 8. a. The tests with mastic asphalt for hydraulic construction (Tabie 111) stand rather apart from all the other tests because no influence from oxygen in the gaseous state has occurred and the continuous contact with water has perhaps played a part in the aging process. The fairiy considerable aging which has been found after the recovery of asphaltic bitumen from fresh mastic asphalt is striking. This may be a case of "aging during mixing" (see Van Oort, p. 11.)

ON THE Pen. ROOF

6 ° - - i1

- j " l

: :

i

+

J

30 - ---, IN THE 60Pen._1~-1DARK _-l 50r+~~~~~ 4 0 ~~.+----l-3 0~....'>.,-l-"'::--'~=-+

Figure 18. Natural aging; modification of the penetration figure, y circles show the average. On the roof; in containers in the dark.

years. The small

Asphaltic bitumen taken from the samples which have been under water shows, after 27 days and after 10 months, a drop in the penetration figure and a rise in the softening point, which is norm al in aging; but after 21 months the penetration figure seems to have risen again and the softening point to have fallen. The percentage of ether asphaltenes follows an irregular path; nor is there much to report about the penetration index.

4. 8. b. An examination of Figures 18, 19 and 20 (roof tests on eternit sheets next to keeping in containers) gives the impression that in this case the aging curves have

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THE AGING OF ASPHALTIC BITUMEN ---~-- - - ----~---ON THE 9

°

R t.-"S'-r-,-,R...=0-r,O,-,-F-,-_ I 8 6 + -82 78 74 70 6 2 58 54H---+--t

°

90

Figure 19. Natural aging; modification of the softening point, y

show the average. On the roof; in containers in the dark. years. The small circles

IN THE 32EA DARK 28 28 24 24 -T· 20 20

----1

16 16 12 12 8 8 4 4 Y

°

2 3

°

2 3 4

Figure 20. Natural aging; modification of the percentage of ether asphaltenes, y The small circles show the average. On the roof; in containers in the dark.

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roughly the same shape as above with artificial aging. In addition, the curves are probably

of the same type as those for the tests in cylinders. At a rough estimate, a given number of years in the roof tests corresponds to the same number of days in the artificial aging; this accords with the overall calculation of the relative aging rate in par. 4. 6. a, making

allowance for the fact that in the roof tests sunlight also has an acce1erating effect. The fairly large variations which seem to occur in the mixtures kept in closed containers in the dark are curious. Here, too, in some cases (although not as an average) the properties appear to alter in the reverse direction after some years; the mixtures again become softer. This phenomenon, which has also been observed after keeping under water (see a), has not yet been explained.

4. 8. c. The usual characteristic quantities (penetration figure, etc.) of the materials which have been aged on concrete tiles (Tabie IV) have not been determined. On the basis of a visual appreciation (exterior and brittleness), it can be said that the rubber-containing mixtures appear to have most advantage. The addition of creosote oil seems to have no favourable effect.

5.

Conclusions

5. 1. Among the characteristic quantItIes of asphaltic bitumen measured in these experiments the penetration figure is the most suitab1e for a quantitative investigation of the phenomena of aging. However, whether the penetration figure is also a means of measuring the practical consequences of aging, e.g., in bituminous constructions, is a question which is not answered in this study.

5. 2. It seems that all asphaltic bitumens and bituminous mixtures age in roughly the same way and according to the same rules, although there may be differences in the rate of aging.

5. 3. A satisfactory quantitative, physico-chemical explanation of the rate of aging, especially at the start of the aging process, has not yet been made.

5. 4. There are too few figures available concerning natural aging to make it possible to say definitely that natural and artificial aging occur in a completely analogous way; however, there are no indications to the contrary.

5. 5. The retrogression of aging phenomena found in certain cases with asphaltic bitumens in closed containers has not yet been explained. The same applies to the phenomena occurring in aging tests with asphaltic bitumens in thick layers.

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In the series of Rijkswaterstaat Communications the following numbers have been published before:

Nr. 1. J. J. Dronkers and J. C. Schöfeld: Tidal computations in shallow water;

A. Waalewijn: Report on hydrostatic levelling across the Westerschelde.

Nr. 2. Ir. H. Ph. van der Schaaf and P. Vetterli, Ing. Dipl. E.T.H.:

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'"

...