Measurement of Oxidation Stability of
Road Asphalts A. P. ANDERSON, F. H. STROSS,
AND
A. ELLINGS, Shell Development Company, Emeryville, Calif.
Two test methods are described for determining the oxidation stability of asphalts, to enable forecast of their road behavior. The first method is rapid and can be used for routine testing of asphalt shipments. It uses data obtained by means of the standard loss on heating test. In the second, more accurate method, an asphalt solution in benzene is subjected to the action of oxygen at an elevated pressure but at a temperature within the range of thoee found in service. No volatilization of asphalt constituents takes place. This method is designed for testing unknown asphalt. A number of representative asphalts have been classified by these methods in comparison to their road performance.
T
HE tests at present commonly used in specifications for
road asphalts fall into two classes: (1) those which show the physical character of the asphalt, such as specific gravity, flash point, penetration, softening point, ductility, and solubility in various solvents, and (2) tests intended to indicate the service behavior of the asphalt, such as the Oliensis test, and those determining penetration of residue after heating, fluidity factor, susceptibility factor, etc. It is well recognized that the tests of the second group are more nearly identification tests for crude source or processing methods than measures of the quality of the bitumen (6, 7) ; thus the need for a test which measures the durability of asphalts directly is apparent. I n this paper two test methods are described-the first, termed resistance to hardening, can be used as a simple routine test, while the second, leading to what is termed the deterioration index, gives more accurate results but takes someaThat longer to carry out.
Resistance to Hardening Road failures, other than those caused by improper original construction, appear to be largely due to hardening of the asphalts. A number of investigators have established the fact that when the penetration of an asphalt drops below a certain point, the road in which the asplialt is incorporated will fail because of brittleness (4, 9, 10, 11, 13, 19). The obvious conclusion, which also has been generally accepted in practice, is that a softer asphalt has the advan-
tage over a harder asphalt of similar hardening characteristics, since the former will take longer to reach a critical penetration (1, 7, 9, I S ? 16). While a number of proposed test methods, such as the Shattuck test (14), the Raschig and Doyle blowing methods (I%'), and the Lewis oven aging test (6) recognize the influence of hardening on the durability of an asphalt, they do not take into account the advantage of the softer material. (These tests have other failings, which are discussed below.) This is because the hardening is expressed as a percentage of the original value, which is fundamentally inadequate, as shown in Figure 1, in which the per cent drop in penetration in the standard loss on heating test (A. S. T. M. Designation D6-39T, Note 2) for some asphalts is plotted against their service rating.
TABLEI. SERVICE RATIKGS O F SEVERAL Aspmuma --
asphalt
l'enetrattutr
---Ohserver.4
R
C
ll
171
?;
B t >yopetrolatum R 10% petrolatunr C
+
172 "10
175 84
u
F G H
.5 P
88 81
T
3
-
Identification and analytical d a t a for these asphalts are given in Table 11.
The service rating is a measure of the performance of an asphalt, and is derived from experimental paving sections in which the asphalts were used under mixing and laying conditions which ryere as well standardized as possible. These sections were strips of pavement laid in the San Francisco Bay Area which were uniformly subjected to light traffic for the period of one year. The ratings were made visually according to the amount of raveling exhibited by the pavementh and cannot lay claini to a precision greater than one unit. Table I gives the ratings made by four observers in order to show the degree of reproducibility to be expected. Grades 1 and 2 signify a very good performance, 4, 5, and 6 poor to very poor performance. The borderline cases are rated as 3. c Figure 1 shows that a n asphalt which hardens as little ab w 12 per cent has a service rating of 4, while another with a zt c penetration drop of 20 per cent performs excellently. The alternative Sicholson method (8) corrects the defect 11 C 3 0 - C under discussion, but suffers in that high-temperature blow1 I I I I ing, as used in this test, is too far removed from the condi10 20 30 40 50 tions on the road. The latter defect is inherent also in the % DROP IN PENETRATION (LOSS ON HEATING) method proposed by Steinbaugh and Brown (17). This point FIGURE1. CORRELATION as well as the reasoning which led to the development of the proposed methods are discussed under Deterioration Index OF Loss ON HEATING TEST from Oxygen Bomb Test. WITH SERVICE
$1
INDUSTRIAL AND ENGINEERING CHEMISTRY
46
TABLE11. ANALYTICALDATA Designation
PenetrationQ
Asphalt
and Ring Ball SqftenIng Point
54 a..7 95 102 84 99 187 92 54
104.6 117.5 128 116.5 102 5 106 109 122 131 110 110.5 119 115 114 103 117 125
Loss on Heating % Weight 0.16 0,051 0.04 0.01 0 11 0.137 0.29 0.316 0.3 0.02 0.069 0.06 -0.01 0.0 0.17 0.081 0.079
87 115 68 38 91
112 110.5 116.5 125 112.5
0.46 0.6 0.44 0.26 0.48
F.
+ +
C C
C
6
E F G
H
I I
I
J
I< K K
L
Mid-continent paving asphalt Mid-continent paving asphalt Mid-continent paving asphalt California, straight run B 5% petrolatum B 10% petrolatum Louisiana paving asphalt Louisiana paving asphalt Louisiana Davine asohalt California 'paving as'phalt A California paving asphalt B Mexican, str,aight run West Texas, straight run West Texas, straight run blown California experimental blend California exDerimental blend California experimental blend West Texas cracked, experimental Californiacracked, experimental California cracked, experimental California cracked ,experimental Californiacracked,exDerimental
190 92 60 68 182 206 177 84
Penetratioq of Residue
O F .
155 83 57 63 154 176 140 69.5 44 88 87 90 83 93 149 81 47.5 ~~
65 71 43 27 59
Penetrations were determined on samples used for loss on heating and, being some cases differ slightly irom those determined on oxygen bomb test samples. b Cleveland open cup.
However, data obtained by means of the loss on heating test, the Lewis test, or other test methods (8, 12, 14) can be used in a manner to take into account the initial viscosity of the asphalt. The method may be called resistance to hardening test, because the test results are a rough measure of this property. These results can be plotted against the service ratings of the asphalts (Figure 2), and it is apparent that a critical resistance value of approximately 55 exists, above which no asphalt with a poor service record (4to 6) is found. This limit effectively excludes poor asphalts. Below 55 there are no asphalts with the good performance record of 1 or 2, so that no good asphalt is unfairly excluded. The procedure of evaluating the results follows: The asphalt specimen is heated, in the conventional manner, to 163" C. for 5 hours in a standard oven after which the penetration of the sample is taken; this process is then repeated. The logarithms of the penetrations after 5 and after 10 hours are plotted against time in hours. A straight line is drawn through the two points and extended t o 10 penetration, which is used as the z axis. The intercept on the z axis gives some indication of the time required by the asphalt to drop t o this penetration and is termed the resistance to hardening (Figure 3). Alternatively, this value can be computed analytically by means of the equation: Resistance to hardening
=
5
e -UJ
(Yi
Soluble OhFlash 86' Be. ensis Pointb Sulfur Naphtha Spot 625 610 610 600 530 515 540 560
a75 E80 510 500 675 625 480 490 500 440 420 445 455 440
%
%
2.4 2.4 2.5 1.0 1.0 0.9 4.6 4.8 4~. 9 1.0
89.9 82.7 79.3 91.7 91.3 91.5 77.1 74.3 72.9 91.9 89.7 72.9 87.5 80.8 77.1 71.8 71.2
2 5 2.95 2.7 5.2 5,s 6.2
3.G
1.2 1.1 1.3 1.29
73.3 73.5 73.3 71.5 70.6
---
-
+ $ + +++ +
Vol. 14, No. 1
Actually the time to reach 10 penetration is somewhat greater than would be indicated by the simple extrapolation based on the rate of change in Denetration in a n earlystage- of the reaction. However, as may be seen from Table 111, the extrapolation gives a fair correlation with service ratings according t o road performance. The method of determining resistance to hardening gives but a rough indication of the service to be expected from a n asphalt. The reasons for this become evident from a more detailed analysis
which forms the basis development of the precise method for mining the durability asphalt given in the following section.
Deterioration Index from Oxygen Bomb Test Oxidation and evaporation are generally considered the most important factors that produce the changes known to take place in paving asphalts under the influence of n-eathering in the road. Attempts in the past to formulate tests for predicting these changes, and hence the service behavior of asphalts, have made no distinction between these two factors (4, 8, 10, 11, 16, 14, 15). Moreover, the temperatures used in these tests are high and simulate or exaggerate the conditions of mixing the asphalt with the aggregate rather than those prevailing on the road. Xow the hardening during mixing is not negligible but is a constant i n the life of any one asphalt, and is generally predictable. On the other hand, it is weathering on the road that governs the change of properties
TABLE 111. RESISTANCE TO HARDENING Resistance to Hardening 160 120
Original Penetration A 190 92 60 B petrolatum 182 B lOy0 petrolatum 206 177 C 84 54 E 95 I 187 92 54 Ii 115 68 38 a kveraged from d a t a of Table I. .isphalt
y1 = log penetration after 5 hours' heating y2 = log penetration after 10 hours' heating
I
! l 3
Service Rating5
ion
+ +
+
o f the more deterof an
75 70
!an "-
60 65
S5
85 55 50 30 25
20
1
1
2 2 2-3 2-3 3 4 4 6 6
I
I
3
0
I
I
100
3
0
3
TARLE IV. RATEOF HARDENING AT DIFFERENT TEMPERATCRES
I
0
I
80 RESISTANCE
I
I
I
60 40 riAPDENIYG VALUES
Ll
20
FIGURE2. CORRELATIOX OF RESISTANCETO HARDENING TESTWITH SERVICE
Ifid-continent asphalt Mexican (slightly oxidized) asphalt Western cracked asphalt 5 From oven aging tests.
Time to Drop from 80 t o 70 Penetrationa .4t 160' C. At 500 c Hours Hours 1.02 257 1.29 0.34
112 214
January 15, 1942
ANALYTICAL EDITION
47
asphalt, grading of the aggregate, and bther factors. As indicated, the time necessary for an asphalt t o drop to the critical penetration under standardized oxidizing conditions should be a measure of the durability of a n asphalt. Since the attainment of this penetration would in most cases take too long at the low test temperatures stipulated, i t is necessary to develop a method of predicting this time from the behavior at shorter times. Direct extrapolation proved impossible, for it was found experimentally that the penetrationisnotasimple function of time. The solution found was to combine the oxygen-absorption behavior of the asphalt, in amount and rate at a fixed time, with the hardening caused by a given oxygen absorption.
FIGURE 3. RESISTANCE TO HARDENING
of an asphalt. The rate of change is very different from that experienced in t h e mixer, Since the temperature, the Elative influence of oxidation and evaporation, and the surface of asphalt exposed are very different. An increase in temperature changes not only the rate, but very frequently also the character of the reaction (Table Iv). At 160' c. the cracked asphalt hardens three times as fast, and the Mexican asphalt approximately two-thirds as fast as the midcontinent material. The cracked material at 50' C. hardens only slightly faster but the 48 Mexican asphalt hardens over twice as fast as ul the mid-continent material. Thus, in the case a of the Mexican material, the relative rate of 40 0 hardening has actually reversed itself. 0 For the reasons mentioned i t is desirable Lz % p " 32 t o measure the stability of the asphalt at 68 temperatures within the range of those enE Z countered on the road, and, in the interest of z k 24 obtaining a more basic knowledge of the re$3 E: actions, i t is desirable to separate the in16 5 fluences of oxidation and evaporation. Since experience and preliminary experiments in:2: 8 dicated t h a t oxidation is generally by far the g more important factor in the weathering of well-refined asphalt, an effort TWS made to 0 devise a n oxidation test in which volatilization cannot occur, and in which essential conditions, including temperature, approach those prevailing on the road. If we assume optimum road construction methods, oxidation and evaporation can be considered the principal fundamental causes of eventual failure. The immediate causes are changes in physical properties brought about by these factors, such as the increase in viscosity Tvvhich eventually results in brittleness, the change in cohesion, the in adhesion of the asphalt to the mineral aggregate, the Change in PlasticitY, and perhaps others; of these, the increase in viscosity (as measured by drop in penetrationl seems to be by far the most important. Based O n the in the literature (4, 9,la, 11, 15, 19) and the authors' experience, the critical penetration below which a n asphalt tends to become brittle was taken to be 20. This represents a n average only, for use in calculaThe fluctuate between l o and depending on the climate, elasticity and plasticity of the
The oxidation is carried out in a stainless steel bomb similar to that described by Egloff et d.(%') and modified by Yabroff and Waiters (20). The bomb is immersed as directed by Yabroff and Walter8 (10)in an oil bath which can be regulated to 0.1' C. at the working temperature. For observation of the pressure, an automatic recorder, or for more accurate 'TOrk, a Bourdon-type ga e which can be estimated to 0.2 pound per square inch can be usef. One hundred grams of the warm asphalt are poured into a 240-ml. (8-ounce) oil sample bottle containing 67 grams of c. P .
'
30q
I
4
8
12
I6
l
l
I
I
20 24 T I M E , HOURS
l
28
l
1
32
1
1
36
1
I
40
FIGURE 4. PRESSURE DROPWITH TIME 1 2A 2B 2C 3 4 6 6
K I I I E
F
C A
118 penetration 187 penetration 89 penetration 63 penetration 97 penetration 81 penetration 84 penetration 90 penetration
benzene. After n-eighing, the bottle is shaken until the solution is homogeneous; when it has reached room temperature it is placed in the bomb, n-hich is at 25" C. Next, the bomb is sealed and flushed uith oxygen by charging it to 100 pounds per square inch gage (70,265 kg. per square meter, 6.8 atmospheres above atmoqpheric pressure) and releasing it again. The flushing is repeated and the bomb is then charged 1%-ithoxygen a t shghtly more than 100 poullds per square inch gage; after a short period (15 minutes) to test for possible leaks, the pressure is adjusted to 100 pounds per square inch, and the temperature is raised to 50" C. The pressure is recorded from now on, either continuously by automatic recorder or manuallr, as required for the plot described below. For each asphalt a long run of 40 hours (or longer, for greater accurac?), a short run in which the pressure drop should
INDUSTRIAL A N D ENGINEERING CHEMISTRY
48
0 A T
Kext, the logarithm of the penetration after oxidation is plotted against the pressure drop for the three (or more) runs. The amount of oxygen needed to reduce the penetration to the value of 20 is obtained by extrapolation, and is called the hardening rating and expressed in pounds per square inch (Figure 5). The deterioration index indicative of the service behavior of an asphalt is obtainedfrom the two quantities as follows:
K 118 PEN, I 187 PEN 1 89 PEN.
n 1 53 PEN
E 97 PEN
x
+ F 81 PEN 6 C 84 PEN ,d A 90 PEN
I
I
I00
go'+
-k 70
-
60
-
50
-
Y
t +
ga
40-
n
30
-
20
I
1
I
I
I
I
I
I
I
40 50 60 70 80 PRESSURE DROF: POUNDS PER SQUARE INCH PER 100 GRAMS
10
20
30
YO
FIGURE 6. HARDENING RATINGCURVEFOR ASPHALTE
be at least 4 ounds per square inch, (0.272atmosphere), and an intermediate cieck run is made. At the end of each run, the oxygen is released, and the asphalt is recovered as soon as possible to prevent its hardening in the solution. The equipment serving to recover the asphalt from the benzene solution for the penetration measurements is that described by Ford and Arabian (3). Better agitation of the hot asphalt is provided by substituting for the single capillary described there a curved tube having several outlets for the gas directed toward the bottom of the flask. The penetration of the asphalt is then measured (A. S. T. M. Designation D5-25). The data so obtained are used in the following manner: The pressure drop is found by subtracting the observed gage pressure from the pressure of the original system at 50" C. This datum pressure, calculated from the gas law and corrected for the solubility of the oxygen in the solution, the increase in vapor pressure of the benzene, and expansion of the materials, is approximately 108 pounds per square inch gage (7.35 atmospheres) or the system under consideration. This value was corroborated by experiments on inert materials and consequently used in all the calculations. The pressure drop is plotted against time, for the longest run; the other runs should check this plot within the limits given below. From this graph the pressure drop a t 40 hours and the slope of the tangent a t this point (in ounds per square inch per hour) are evaluated. The product o r t h e pressure drop and the slope is called the oxidation rating. Several of the asphalts tested are thus plotted in Figure 4.
Vol. 14, No. 1
Deterioration index =
Runs should be made in duplicate or triplicate, and should check within 1 pound per square inch of their average; larger differences indicate leaks or other disturbances. The largest errors are introduced in the evaluation of the rate of oxidation and in the extrapolation. The deterioration indices of the duplicate samples should b e within 10 per cent of each other. Other types of bombs could be used; however, for expression of the results in terms of deterioration index as here defined these would need t o be standardized by means of asphalts that have been rated in the specified apparatus. The oxidation rating relates the oxygen consumption of the asphalt to time by multiplying the oxygen pressure drop a t 40 hours by the rate of oxygen consumption at that time. This procedure permits a n over-all characterization of the oxygen-absorption behavior of the asphalt, since i t in effect projects the oxidation beyond the limited periods of time permissible in laboratorv tests. The hardening rat:ng was derived from the linear relationship which was found to exist between the logarithm of penetration and the oxygen consumption (except for a short initial period in which the-asphalt haidens more-for a given oxygen consumption, which prohibits the use of the original penetration as a point, on the curve). d typical plot is shown in Figure 6. The ratio of the two ratings gives the desired relation of critical oxygen consumption to time and, as is evident from Equation 1, is expressed directly in pounds per square inch per hour; i t therefore has the dimensions of a rate, and represents the rate at which the asphalts deteriorate.
$ 1
i
ai
i i
i i 20
1
40
60
80
I
I
IO0
120
DETERIORATION INDEX
FIGURE 7. CORRELATIOK OF DETERIOR.ATION INDEX WITH SERVICE
ANALYTICAL EDITION
January 15, 1942
49
Literature Cited DATAAND INDICES AND SERVICE RATINGS TABLEV. DETERIORATION
(1) Buchanan, J. E., Asphalt
Forum, 11, 56 (1938). (2) Egloff, G., Morrell, J. C., Lowry, C. D., and Dryer, C. G., IND.ENQ. CHEM., Lb./sq. in. L b . / s q . i n . / h r . ( L b . / s q . in.)l/hr. Lb./sq. in. 24, 1376 (1932). ' 1 171 16.5 0.18 3 200 1 (3) Ford, T. F., and Arabian, 90 15.6 0.20 3.1 134 1 57 17 0.19 3.2 115 1 K. G., ISD. EXQ.CHEM., B 68 30.6 0.27 5.3 66 None ANAL.ED., 13, 140 (1941). laid (4) Hubbard, P., and Gollomb, B 5% petrolaturn 172 30 0.21 6.3 107 5.9 1 M., Assoc. Asphalt PavB 10% petrolaing Tech., Proc. Tech. turn 210 29.5 0.24 7.1 146 4.9 1 Sessions., p. 165, Decem1.9 98 1.9 1 20.5 0.09 C 175 2.9 94 3.1 2 ber, 1937. 84 21.5 0.13 52 22.8 0.19 4.3 90 4.8 2 (5) Lang, F. C., and Thomas, D 88 33.6 0.23 7.7 89 8.7 2 T. W-., Univ. Minn. Eng. E 9i 28 0.40 11.2 86 13 2-3 F 81 24.6 0.23 5.5 56 9.9 2 Expt. Station, Bull. 15, G 84 21.5 0.22 4.7 59 8.1 1 (November, 1939). H 99 21.2 0.26 5.4 71 7.6 2 (6) Lewis, R. H., and Welborn, 11 92 11.9 2-3 36.5 0.3 I 187 11.9 67 17.7 3 J. Y., Assoc. Asphalt Pav89 37 0.32 38.5 0.32 53 12.3 48 25.6 4 ing Tech., Proc. Tech. J 87 33.7 0.28 9.3 57 16.3 3 Sessions, p. 14 (January, K 118 49.5 0.36 17.8 67.5 26.4 4 1941). 60 61 0.33 16.8 45 38 6 36 50.5 0.36 18.2 15 126 6 (7) Lewis, R. H., and Welborn, 1. 91 45.6 0.42 19.2 59 33 6 J. Y., Ibid., p. 86 (January, 1940). (8) Nicholson, V., Ibid., p. 208 (December, 1937). (9) Powers, J. W., Montana Natl. Bitum. Conference, p. 344, 1937. Figure 7 shows the correlation found between service rating (10) Raschig, F. L., and Doyle, p. c., Assoc. Asphalt P Q V ~ WTech.. and deterioration index. It is evident that while a deterioraProc. Tech. Sessions, p. 228 (January, 1937). tion index lower than 15 indicat,es fully satisfactory perform(11) Ibid., p. 200 (December, 1937). Ibid., p. 215. (12) ance, poor durability can definitely be expected with a n index (13) Schaub, J. G., and Parr, W. K., Montana Natl. Bitum. Conferabove 20. No exceptions have been found to occur for any ence, p. 157, 1939. of the wide variety of types tested. (14) Shattuck, C. L., Assoc. Asphalt Paving Tech., Proc. Tech. SesThis method is designed for the testing of new asphalts, the sions, p. 186 (January, 1940). (15) Skidmore, H. W., Zbid., preprint (December, 1941). stability of which is unknown; further samples of a n asphalt (16) Skidmore, H. W., and Abson, G., Ibid., p. 195 (December, which has been accepted on the basis of this test can be 1937). routine tests such the resistance to checked by (17) Steinbaugh, V. B., and Brown, J. D., Ibid., (December, 1937), hardening method described a t the beginning of this paper. D. 138. (18) Thurston, R. R., and Knowles, E. C., IRD. ESG. CHEM.,33, 320 Acknowledgment (1941). esDeciallv D. 323. (19) Vokac,'R.,*Monta/a Natl. Bitum. Conference, p. 226, 1939. The authors wish to the assistance of L, (20) Yabroff, D. L., and Walters, E. L.. IND.E s o . CHEM.,32, 83 Griffin in much of the experimental work. (1940). Asphalt
Penetration
Pressure Drop a t 40 Hours
dP/dt at 40 Hours
Oxidation Rating
+ +
Hardening Rating
Deterioration Index (Lb./sq. in./hrJ X 100 1.5 2.3 2.8 12.7
Service Rating
acknowledge
New Indicators for Iodate-Iodine Monochloride Andrews Analytical Procedures G. FREDERICK SMITH AND C. S. WILCOX, University of Illinois, Urbana, Ill.
T
HE use of iodate as oxidant in volumet,ric analysis at high concentrations of hydrochloric acid was originally developed by Andrews (1) and further popularized by Jamieson (4). The Andrews iodate-iodine monochloride reactions, while precise and versatile, require a high hydrochloric acid concentration (2.5 to 6 M ) , and the use of carbon tetrachloride or chloroform as immiscible solvent to indicate the completion of the reaction, by the presence or absence of a violet coloration. The present paper describes conditions under which the Andrews reactions can be applied using dyestuff internal indicators, the color destruction of which corresponds with the completion of the oxidations involved. The iodate-iodine monochloride reactions are simplified and made more rapid and convenient, without appreciable loss of accuracy or versatility. The Andrews (1) procedures as extended by Jamieson (4) have been reviewed by Lang (2). I n order to apply the Andrews reactions t o solutions a t much lower concentra-
tions of hydrochloric acid, Lang used (2j potassium cyanide to cause the formation of iodine cyanide in place of iodine monochloride. By this change starch may be employed as indicator, but this development is burdened by the presence of hydrogen cyanide. The same advantages are attained by tlie Berg modification, which employs acetone to form iodoacetone in place of iodine monochloride or iodine cyanide (2). The applications in this case are limited in number. The determination of iron by the Andrems procedure in the presence of organic matter was described by Heisig ( 3 ) . The dyestuff indicators, some of which are used in the present work, were studied by Smith and Bliss (6).
Requirements of Internal Indicators I n the iodate-iodine monochloride reactions using carbon tetrachloride or chloroform as an extraction solvent indicator, the equivalence point is determined by the disappearance of the iodine color on the addition of the first slight excess of iodate. A dye suitable for use as an internal indicator for such
