SUGGESTED PROCEDURE IN DESIGN OF
Sheet Asphalt I
I. BENCOWITZ
percentage of asphalt. The resulting mixture is consequently too rich. It is also known that films of asphalt are thinner than films of air and liquids generally used to determine the percentage of voids (12). Thus the percentage of voids determined by such methods yields values which are too high in reference to asphalt. The desirability of some voids in the compressed mixture may be of a more fundamental nature. Pavements which gave perfect service for several years, were known to fail under service suddenly for no readily recognized cause. This is accounted for only by the assumption that continuous compression under traffic reduced the percentage of voids below the safe minimum (19). The likelihood of this assumption is supported by the fact that the strength of an asphalt pavement is due largely to the friction and interlacing of the irregular particles of the aggregate (8). Under traffic, owing to internal wear, the particles are crushed, the irregularities are reduced, and the interlocking effect is destroyed. It is interesting to speculate as to whether a superior paving could not be designed with round particles, provided a sufficiently stable asphalt could be found. At any rate, the initial percentage of voids in the aggregate cannot serve as a measure of the amount of asphalt to be added. The ultimate percentage of voids desirable in the finished pavement will depend on the asphalt, the kind of aggregate, and the shape of ita particles. Since 1924 several methods have been proposed for measuring the resistance to deformation of compressed test specimens. These stability-determining tests have become the most important tools in the design of paving mixtures. Of the numerous methods suggested, those of Hubbard-Field (I I ) and Skiddmore are the best known and most widely used. The underlying principles of both methods are identical. The former is suitable for a specimen made of fine aggregate; the latter is adaptable to mixtures made with coarse aggregate. Both determine the total force required to shear completely parallel faces of the test piece. The chief limitations of these devices are described by Hubbard (IO): “We do not know yet what values to assign to the stability test; i. e., it may be best to work for a range that will be considerably below the maximum. However, we now have a yardstick for measuring stability, and the most satisfactory range is obtained by correlation of test results with the service behavior of pavements which have been subjected to the test.” The Hubbard-Field stability test employs a static force until a cylindrical specimen, 2 inches (5 cm.) in diameter and 1 inch (2.5 om.) in depth, is completely forced through a ring is/., inches (4.4 cm.) in diameter. On the other hand, the forces under actual traffic conditions are impact forces which are more severe than static forces of the same magnitude. To simulate actual service conditions more closely, Tarwater developed a rolling stability testing machine (19). This consists of a revolving bank carrying eleven rollers. During the test, while the bank revolves, its entire weight (450 pounds or 204 kg.) rests upon the test specimen, which is held in a collapsible mold open on one end near the surface to allow displacement of the material. The number of revolutions required to produce a displacement of 0.3 inch (7.6 mm.) is considered to be an index of stability.
Texas Gulf Sulphur Company, Gulf, Texas
The procedure described consists of determining the brittleness of briquets by tumbling them in a revolving drum. When the percentage loss after an hour is plotted against the percentage asphalt, a characteristic curve is obtained which shows that the safe range of asphalt is that which corresponds to a brittleness range between 0 and 2. Briquets within this range of brittleness are then subjected to a stability test which consists of determining the load necessary to force a sphere, a/4 inch (1.9 cm.) in diameter, into the specimen to a depth of */8 inch (9.5 rnm.). This test shows that the stability decreases when the proportion of filler increases beyond a certain value, depending upon the quality of the sand. The combination of these two tests determines specifically the best proportion of asphalt and filler to be used with a given sand.
T
WO of the most important variables to be determined in the design of sheet asphalt axe the proper proportion -- of asphalt and fine aggregate (iller) to sand. The requirements of a good mixture are well known though not specifically defined. It frequently suffices to establish the best of several possible combinations. The methods of testing, therefore, are relative rather than absolute. Until recently the ‘(pat” test was generally used to determine the proper proportion of asphalt (20). It is still used extensively as a field test, and experienced technologists believe it to be reliable. The character of a stain made by a test specimen on paper indicates whether the mixture is too rich or too poor in asphalt. The simplicity of this method argues in its favor. Its disadvantages, on the other hand, are obvious. It is highly empirical. Years of experience are necessary before the appearance of the stain can be correctly and reliably interpreted in terms of percentage of asphalt. A number of investigators held that the percentage of voids in the aggregate is an indication of the amount of asphalt to be used. This, to a large extent, is true. On the other hand, it is well known from service records that the best paving mixture is not that containing the minimum percentage of voids (4, 19). This can be explained by the fact that, since asphalt does not “wet” the aggregate perfectly, some air films and air pockets are unavoidable (3). The absolute volume occupied by air cannot be reduced, and only the apparent percentage of voids is reduced by increasing the 98
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JANUARY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
99
alone, whereas, when the asphalt proportion is below the required minimum, the brittleness will increase sharply. Upon the assumption of this principle the following brittleness apparatus was built.
80
Apparatus and Experimental Procedure
FIGURE 1. BRITTLENESS NUMBERus. PER CENTASPHALTBY WEIGHT
A reliable interpretation of the results of all these stability tests is impossible unless a service record of similar mixtures is available. This is a severe handicap not only to the research man, who works with new materials which have never been tried in service, but also to the highway engineer, inasmuch as the performance of a paving mixture in service depends upon so many variables that it is frequently well nigh impossible to correlate failure to any one factor. It is desirable, therefore, to develop tests, the results of which could be readily interpreted without the need of a vast experience or a priori knowledge of the service records. These should enable the research man to determine the best composition and study the variables essential to obtain such mixture; and the highway engineer should be able to decide whether a newly proposed material shows sufficient promise to justify large-scale trials, which in the end must be the final test. The tests are to establish the highest resistance to fracture and deterioration under impact and the greatest resistance to deformation under load a t the highest temperature encountered in service-i. e., 60" C. (6). It is assumed that a mixture which answers these qualifications is of sufficient promise to justify large-scale trials. The procedure described, developed, and used in this laboratory since 1933, seems to satisfy these requirements. I n their present form the testing devices are not free from objections, but the procedure has possibilities and it is thought worth while to bring it t o the attention of others.
The brittleness was determined in a machine which consists of a cylindrical drum, 24 inches (60.96 cm.) in diameter and 24 inches in length, mounted horizontally on shafts welded into t h e ends of the drum. The shaft does not go through the drum. On the inside of the drum is welded a shaft, 4 inches (10.2 em.) wide, from end to end parallel to the axis of rotation and bent at an angle to the shell so that at each revolution the specimen is picked up once, carried to the maximum height, and allowed to fall freely. It rotates 27 r. p. m. The shell is perforated with six rows of a/,-inch (9.5-mm.) holes so that all fines are allowed to sieve through, thus avoiding any cushioning effect and abrasion. The specimen is introduced through a small door a t one end. The test briquets were made in collapsible molds in the following manner. The aggregate, kept in a steam-jacketed pot at 150-160" C. for not less than 24 hours, was weighed and added tlb a previously weighed and melted amount of asphalt in a steamheated pot. Mixing was done by hand until it appeared to be homogeneous to visual inspection. The collapsible mold, .previously preheated and provided with a hopper, was filled with a generous excess of the hot mixture, tamped by hand, and then compressed under 2500 pounds (1134 kg.). The pressure was allowed to remain for 5 minutes. The pressure was then released, the hopper removed, and the excess mixture struck off to the proper level by means of a hot knife. The briquets remained in the molds not less than an hour and were allowed t o age at least 24 hours. Six briquets were made from each batch. All tests were made on specimens from the same batch. Each briquet weighed from 700 to 750 grams and was 6 X 2 1 / 2 X l'/a inches (15.24 X 6.35 X 3.81 em.) in size. The briquet was weighed, allowed to remain in the refrigerator for at least 24 hours, and then rotated in the tumbler for an hour at room temperature and weighed again. The percentage loss in weight after an hour is defined as the brittleness number.
Materials Although several different asphalts and fillers were used in this study, mixtures with only one asphalt and one filler are considered sufficient to demonstrate the procedure of testing employed. The materials are described in Table I. Sand A is the type used by local railroad companies for engine traction. Sand B is a quality available in this locality and generally used in making Portland cement concrete. Sand C was prepared in the laboratory by crushing stone and grading it by hand. The grading adopted is similar to that used by the U. S. Bureau of Public Roads (15). The particles of sands A and B are round, those of sand C are flat and angular. TABLEI.
-GradingPassing Retained on sieve No. sieve No. 8 10 20
30 40 50 80 100 200
Brittleness Number
It is generally recognized that all stability tests should be accompanied by some brittleness or fracture test, for it is possible to design a mixture which will possess a high resistance to deformation and yet fracture and deteriorate under a frequent repetition of small impact forces. On the other hand, an intuitive analysis suggests that the brittleness of a specimen is likely to be a function of the proportion of asphalt present. As the proportion is increased to a large excess, the brittleness of the mixture will approach that of asphalt
CHARACTERISTICS OF MATERIALS USED IN THIS INVESTIGATION
a
10 20
30 40 50 80 100 200
...
Sand A
.... 2.5
55.5 38.5
3.0
Send B Sand C -Per ' cent by weight16.0 26.7 25.2 25.2
....
3.9 6.4
10.3 14.2
....
3.7 1.0 0.6
30.9
....
0.4
16.7
0.3
0.5
....
Round Round Shape of particle 2.605 2.586 Specific gravity 32.6 37.7 Per cent of voidsa Limestone : Specific gravity 2.585 Per cent retained on No. 200 sieve 20.0 Per cent of voids'" 36.5 Asphalt penetration in 5 set, at 77' F. (25' C.)under load of 100 grams Determined by the Cone method (8,p. 206).
16.8
0.8
Flat and angular 2.608 42.89
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Discussion of Results The experimental data representing averages of three tests are given in column 4, Table 11. Columns 2 and 3 show the percentages by weight of asphalt and aggregate, respectively. Column 5 gives the corresponding percentage of voids in the compressed mixtures. The percentage of voids was calculated by the following formula : v = 100 - D M aA )
(s.+
where V D M A S
Q
I,
= = = = = =
per cent voids in compressed mixture apparent density of compressed specimen per cent of weight of aggregate per cent of weight of asphalt true density of aggregate density of asphalt (1.0)
The density of the sample was determined by weighing the dry specimen, immersing it in 50 per cent alcohol for an hour, and measuring its volume by displacement in 50 per cent alcohol. In Figure 1 the brittleness numbers are plotted against the percentage weight of asphalt. Each curve has a well defined “knee,” which extends over a range of 2 or 3 per cent asphalt. The branch to the left is very abrupt. Bearing in mind the almost unavoidable difference in proportioning, which exists between laboratory specifications and field plants, these curves, without any need of other tests, determine the safe minimum proportion of asphalt to be used-vie., that which corresponds to a brittleness number no greater than 2. For no matter how desirable other properties of mixtures corresponding to the upper part of the knee may be, no highway engineer will consider it safe to design a mixture so dangerously near the sharp rise of the curve. It so happens, as will be shown in the next section, that the maximum stability also corresponds to the lower portion of the knee in the brittleness curve. These curves also explain the generally known fact that the range of asphalt variation is dangerously narrow, when the percentage of filler is high. Figure 6 gives curves representing the effect of increasing proportions of filler upon the brittleness number. The curves for 10 and 15 per cent filler almost coincide. In the case of 20 per cent limestone dust the range of asphalt variation is narrower. Mixtures containing 20 per cent filler, on the other hand, show a more abrupt change in the brittleness curve and at higher asphalt content. The actual rise of the brittleness curve is much sharper and the knee is shorter than those shown in these curves when effects of temperature and loss in weight are considered. This behavior will be discussed in subsequent paragraphs. This test, therefore, seems to be a valuable yardstick for the determination of a safe minimum percentage of asphalt to be used with a given combination of aggregates. On the other hand, it is not sufficiently sensitive to differentiate between different kinds of aggregates and proportion of filler. I n a complete procedure of design it should be supplemented by stability test. Any stability test will serve the purpose. However, the test described in the next section seems to have pronounced advantages. The limitations of the brittleness test are as follows: Although this test is only remotely similar to the Deval abrasion test for bricks ( I ) , the characteristics of both are to some extent analogous. It is known that abrasion is not proportional to the weight, and it was shown that the volume and shape factors are important and that the edge effect is considerable ( I S ) . This is true in the case of the brittleness test. The weight of the briquet, on the other hand, plays a much more important role in the brittleness test than it does in the Deval abrasion test. The amount ground away per revolution is proportional to the force of impact, and this is
VOL. 29, NO. 1
a function of the weight of the briquet, everything else being constant. With each revolution this weight is gradually decreased, and the amount ground away, calculated on the basis of the original weight, is less and less as the test proceeds. Thus the percentage lost per hour (the brittleness number) will depend upon the length of the test, and the error for briquets of high brittleness will be greater than for those of low brittleness. This would have been a serious objection to this method if it were not for the fact that we are interested only in that part of the curve which represents brittleness numbers less than 2. That the error involved within this range is negligible is shown by the fact that the brittleness numbers calculated from the results of 1-hour and 3-hour tests check closely. TABLE11. COMPOSITION BRITTLENESS NUMBER, AND PERCENTAGE VOIDOF ~ I X T U R E SPLOTTED IN FIGURE 1 No. of Mixture
Asphalt Aggregate Per oenl by weight
22.53 20.29 15.39 12.70 9.83 6.78 5.17 3.51 2.66 36 37 38 39 40 41
15.39 12.70 9.84 6.78 5.17 3.50
63 64 65 66 67 68 69 70 71 72
20.00 17.90 15.39 14.73 12.70 11.27 9.84 8.33 6.78 3.52
93 94 95 96 97 98
8.22 6.13 5.53 4.70 3.90 2.12
Sand A 77.47 79.71 84.61 87.30 90.17 93.22 94.83 96.49 97.34 Sand B 84.61 87.30 90.16 93.22 94.83 96.50 Sand C 80.00 82.10 84.61 85.27 87.30 88.71 90.18 91.67 93.22 96.48 Gravel 91.78 93.87 94.47 95.30 96.10 97.88
Brittleness No.
Voids in Compressed Sample
% 0.0 0.0 0.0 0.7 4.5 13.5 33.2 84.8 93.0
2.3 3.7 6.5 12.5 19.5 26.4 29.7 32.4 33.6
0.0 1.0 2.5 9.8 22.6 57.0
1.3 7.6 12.5 20.3 22.6 26.2
0.0
0.0 0.0 0.0
0.0 2.5 8.1 34.8 69.1 100.0 0.0
2.4 7.0 20.3 47.0 60.0
3.8 5.3
....
14.8 21.5 23.4 27.8 28.9 32.5
....
.... .... .... .. .. .. .. ....
However, the sharply rising part of the brittleness curves in reality rises considerably more abruptly. In other words, the brittleness numbers corresponding to the rising part of the curve are too low and the deviations from correct values are greater, the higher the brittleness. Another important factor is the temperature. Just as the stability is determined at temperatures at which it is the lowest, so should the brittleness be determined at the lowest temperature which can be conveniently maintained in the laboratory. We have at present a small unit built to fit a 6 cubic foot (0.17 cubic meter) household refrigerator. However, since few laboratories are equipped with large refrigerators, the next best thing is to carry out the test at room temperature as low as possible and constant within 5’ C. This requirement is obviously important if the results of different laboratories are to be compared. For each individual laboratory, however, the temperature is not as serious a factor as it seems, especially if the effect of the temperature is borne in mind; viz., at lower temperatures the abrupt change in brittleness occurs at somewhat higher asphalt content. Essentially, the effect of lower temperatures is to give a higher brittleness number of briquets which are appreciably brittle, even at room temperature-i. e., a brittleness above 2 per cent.
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101
This fact is illustrated in the following table, where brittleness numbers of several mixtures obtained under different temperature conditions are given. I n line one am given the brittleness numbers obtained with briquets kept and tested a t room temperature (1&20° C.). I n the second line are figures obtained a t 30-33" C. I n the third line are given brittleness numbers of similar briquets which were kept in the refrigerator for a t least 24 hours and tumbled a t 30-33" C. The effect is too small to change the nature of the brittleness curve essentially: Aged and tested at 18-20° C. Aged and tested at 30-33" C. Aged in ice box and tested a t 30-33" C.
1.6 0.7 1.2
1.2 0.9 0.9
20.6 20.0 25.5
66.0 51.7 62.1
Certain steam-refined asphalts, however, are extremely brittle a t low temperatures, and prechilling in the ice box yields appreciably higher brittleness numbers.
Stability Test
-
The Hubbard-Field stability test and some of the others measure the sum total of all forces which contribute to the strength of an asphalt mixture. These forces include those of plastic or viscous flow, the adhesive forces of the asphalt towards the aggregate, the tensile strength of the asphalt, the friction of the particles, and the mutual attraction of the particles of the mineral aggregates. That friction and interlacing of the aggregate are important factors contributing towards the stability is not difficult to conceive. Green and Haslam (7) showed that the yield value of pigment-vehicle mixtures involves the interfacial tension and the friction between the particles; Horsefield (8) supports the view that friction and interlacing of the particles of the aggregate determine the stability of asphaltic mixtures. That there is a force of mutual attraction of the solid particles acting through the film of asphalt is not improbable. That such molecular forces exert "a powerful influence on the molecules of the extremely thin liquid glue film" between surfaces of quartz and steel is postulated by Lee (1.6). That such molecular attraction exists is also supported by evidence obtained with joints between steel-steel and steel-copper surfaces. It is not unlikely that similar forces exist between the particles of an asphalt mixture. Of all the factors contributing to the strength of a paving mixture, those most responsible for failure are the continuous flow and mobility (viscous or plastic) of the asphalt and the lowering of the frictional and interlocking resistance of the aggregate. It is desirable, therefore, to design a method which will measure the stability of a mixture involving these two factors only. The specimen should deform because of continuous flow or mobility, without rupturing the bond between the asphalt and the aggregate. Such a test can be made with a plunger which covers only a fraction of the surface of the specimen, so that there is compression underneath and flow immediately adjacent to and around it (18). That such a plunger should have the geometry of a hyperbola of revolution, such that the rate of increase of the radius is proportional to the depth of penetration, is obvious. Unfortunately, hyperbolas of revolution are diacult to machine. As a practical alternative, spheres suggest themselves.
Design and Procedure How (9) was probably the first to use a ball penetration test, similar to the Brinell hardness test, for the determination of the stability of asphalt paving mixtures. Subsequently others used similar methods (16). In the design of a sphere penetration method as a stability test, numerous details of construction are possible, but they are of little importance. The chief problem is the choice
FIGURE 2.
STABILITY TESTING DEVICE
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Mirror Thermoregulator Asbestos plate Eleotric heaters Pilot light lieht .4sbest& fiber ins,ulation, insulati 4 inohes thick, around thermostat .4sbestos Vertical shaft, 1 inch in diameter Concentric shaft used to raise shaft 12 and thus remove load from speeimrn when tesc is completed cc 14. Gear Gear, serving t o lift shaft 13, operated by: 15. Handwheel Handwheel 16. Counterweight 17. Brass pulleys mounted on ball bearing, two on each side of shaft 18. Ball bearines 19. Micrometer 20. 200-pound (90.7-kg.) spring balance 21. Handwheel for applying load 22. Sorsw onnratnd hv 21 23. ii-~sgilh'lbh&Tw~vIre 24. Silk strin connecting micrometer to shaft 12 25. Key whict keeps screw 22 from turning when load is applled 26. Standard 1-inch aluminum pipe
of the most suitable variables: (1) choice of the independent variable, time or load; (2) radius of the sphere to be used; (3) ultimate depth of penetration; (4) use of a confining mold or not; ( 5 ) dimensions of the specimen. Any combination of these variables should yield interesting
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INDUSTRIAL AND ENGINEERING CHEMISTRY
results, some more so than others. The availability of so many combinations is an attractive feature for the research man. The procedure adopted in the test to be described consists in increasing the load uniformly and intermittently at a rate of 3 pound (226.8 pounds (1361 grams) per minute in steps of grams) every 10 seconds until a ball a/a inch (1.9 om.) in diameter is depressed s/8 inch (9.5 mm.) under the increasing load. The s ecimen, resting on a level plate, is not confined by a mold so t t a t the pressure exerted by the sphere in radial directions is resisted by the material itself and not by a confinin mold. The specimen is of the same dimensions as those used in the brittleness measurements4 X 21/a X 11/2 inches (15.2 X 6.4 X 3.8 cm.). Preliminary tests indicated that specimens larger than these did not change the results appreciably. The ultimate load (in pounds) required for a depression of a/8 inch is considered an index of the stability. However, weight is given to the performance of the specimen under the testwhether it cracked (C), broke (B), or yielded a perfect impression of the sphere without any visual signs of rupture. This is the simplest representation suitable for control laboratories. For the research man, however, this test reveals a closer insight. Several different representations are possible; for example, the relation of the initial and final rates of deformation is of interest. A diagram of the apparatus is outlined in Figure 2. The air thermostat shown was subsequently replaced by a water bath, the temperature
4.5
1
LOADI N KILOGRAMS 9.1 13.6 18.1 22.7
I
I
27.2
3L8
I
of which was kept constant a t 60’ C. bath closer checks are possible,
* 0.2’.
With a water
A specimen, which was allowed to reach temperature equilibrium for at least an hour, was placed under plunger 4, and sphere 5 was inserted in the space between. The plunger can be adjusted to accommodatespecimens of different thicknesses and spheres of several different diameters. The plunger is lowered by means of handwheel 21, which operates spring balance 20, until micrometer 19 indicates that no downward movement results, while the weight on the spring balance increaaes to pound. Vertical shaft 12 communicatingthe load to the sphere is counterbalanced by weight 16 so that a little less than pound on the spring balance is needed to move it downward. The operation with an air thermostat is considerably simpler; contact of plunger and sphere is then indicated by mirror 6. After contact is made, the load is increased in ste s every 10 seconds and the reading of the micrometer recordef at regular intervals. When the micrometer indicates that the plunger has moved downward inch, the test is stopped and the plunger is lifted by means of concentric shaft 13. The final reading on the balance is recorded as the stability. The specimen is then removed and its condition inspected. This visual inspection, however, is unnecessary; the operator can tell at any time during the test whether the specimen failed or remained sound throughout the test. If the readings of the micrometer are plotted against the corresponding loads, a permanent record is obtained which yields a correct re resentation of the performance of the specimen during the test. guch records can be made by a simple automatic recorder attached t o the apparatus.
A few characteristic curves are shown in Figure 3, where the depth of penetration is plotted against the load. The curves are similar to those obtained by Emmons and Anderton (6) with their method of stability measurement which consists of compressing a specimen 8 X 6 X 2 I/* inches (20.3 X 15.2 X 5.7 cm.) in a mold containing three extrusion openings. The load is applied by means of a 20,000-pound (9,072-kg.) compression machine through a plate with a clearance of 1/16 inch (1.6 mm.) around the sides of the mold. Loads corresponding to a displacement of 0.5 mm. are taken to represent the stability. The similarity of the curves is of interest only in so far as the objective which they wished to attain with their device is identical with the objective of this investigation-vis., ‘(to cause an internal movement and rearrangement of the particles without leaving them free t o dissociate themselves completely during the progress of the test.” The objections to their method of testing are the bulkiness of the specimen (19) and the likelihood that slight variations in the mold dimensions will make large differences in the stability, as has proved to be the case in the HubbardHeld test (17).
Results of Tests
FIGURE 3. VARIATIONOF DEPTHOF PENETRATION WITH LOAD The number rtbove each ourve designates mixtures given in Table 111.
The data are given in Table 111. Mixtures made with sands A, B, and C, containing variable quantities of limestone dust, and asphalt, are included. The stabilities (average of three tests) corresponding to pounds required to move the sphere downward 3 / / ~ inch are given in column 5. Column 6 shows the corresponding percentage of voids in the compressed specimen, computed as described in the preceding section. I n Figures 4, 5, and 6 are plotted the brittleness numbers, the stability, and the percentage of voids against the percentage of asphalt by weight. The most interesting feature of the stability curves is that a definite maximum is obtained. Also, this maximum corresponds with the lower portion of the rising part in the brittleness curve. When the proportion of asphalt is excessive or deficient, the stability is practically independent of the amount of filler, and the mixtures corresponding to the upper part of the knee in the brittleness curve, crack (C) or break ( B ) before the stability test is completed, or while it is in progress.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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P
s
t
109
graphically where a combination of the two tests described here yields an insight into the usefulness of a mixture which other methods fail to reveal. The question may arise as to whether this method of stability determinations is sufficiently precise t o indicate the optimum percentage filler to be used with a given sand. As shown in Figure 4 (sand A), the maximum stability is obtained a t asphalt contents below that necessary to avoid excessive brittleness. Furthermore, the irregularities of the stability curves seem t o indicate an experimental error of TABLE111. COMPOSITION, BRITTLENESS NTJMBER, AND PERCENTAGE VOIDSIN COMPRESSED BRIQUETS BrittleMixture
No.
FIQIJRE4. EFFECTOF FILLERON CORRELATED PROPERTIES IN COMPACTED SPECIMEN OF SANDA
Highway engineers have recognized that, although the stability of mixtures carrying as much as 30 per cent of filler is high as determined by previous methods, the stability is greatly affected by slight variations in bitumen content, and that a mixture containing a proportion of filler somewhere between 10 and 20 per cent is most practical (18). The curves in Figure 6 prove this fact. The stability, as determined by the sphere penetration method, yields a lower value for 20 per cent filler content than for mixtures containing 15 per cent filler. Furthermore, if the maximum stabilities are plotted against the per cent filler, a break in the curve is noted at about 15per cent filler. None of the other methods of measuring stability (as far as the writer knows) indicates these facts, although the practical highway engineer has learned to beware of high-filler, high-stability mixtures. Figure 4 shows that a high-filler content may yield a sharply rising stability curve. The sphere penetration method indicates, however, that such stabilities are due t o rigidity, since they are accompanied by cracking (C) before the test is completed-i. e., before the sphere has penetrated to a depth of half its diameter. Presumably, the value of an asphalt pavement is enhanced by its pliability. Rigidity is not desired even if the increased stability is advantageous. Furthermore, since such high-stability mixtures invariably lie within the danger zone of the brittleness curve, they cannot be considered as useful. This case demonstrates
ness
Aggregate No. -StabilityPer cent buweiaht Pounds KO. S a n d A,, 6.3% Limestone Dust 10 13.63 86.37 0.0 25.0 11.3 11 88.37 11.3 0.0 26.0 11.63 12 90.48 12.0 1.4 26.6 9.52 2.0 27.0 13 8.44 91.66 12.2 4.1 ZS.O(C). 11.3 14 7.32 92.68 93.82 7.2 26.0(C) 11.3 16 6.18 95.00 42.3 22 0 C ) 10.0 6.00 16 96.20 63.3 16:5[C) 7.0 17 3.80 Sand A, 10.0% Limeatone Dust 88.12 0.0 13.0 18 14.88 5.9 14.3 86.95 0.5 31.6 13.05 19 16.9 88.89 0.9 36.0 11.11 20 90.91 1.8 39.0 17.7 21 9.09 91.96 8.06 2.9 62.0(C) 23.6 22 95.24 26.6 31 6 C) 14.3 23 4.76 96.38 100.0 13:OlB) 6.9 3.62 24 Sand A, 14.3% Limestone Dust 86.71 0.0 16.0 7.3 25 14.29 87.60 40.0 18.1 0.0 26 12.60 90.32 41.6 18.8 0.7 27 9.68 91.30 18.8 3.0 41.5 28 8.70 93.33 41.0(C) 18.6 18.1 29 6.67 95.46 36.O(B) 16.3 38.6 4.46 30 Sand A, 18.2% Limestone Dust 86.27 0.0 21.0 9.6 31 13.73 11.8 88.00 0.0 26.0 12.00 32 12.5 90.00 0.4 27.6 33 10.00 91.67 5.6 85.0(C) 38.6 34 8.33 36.6 47.0(8) 21.3 93.62 35 6.38 Sand B, 7.0% Limestone Dust 83.10 0.0 8.0 3.6 42 16.90 85.60 0.4 17.0 7.7 43 14.60 16.3 88.06 0.8 36.0 11.94 44 12.7 89.39 1.0 28.0 10.61 46 12.2 27.0 90.77 1.4 9.23 46 93.71 9.4 22.O(C) 10.0 6.29 47 95.61 44.1 29.0(8) 13.1 3.39 48 Sand B, 12.7% Limestone Dust 84.00 0.0 17.0 7.7 49 16.00 86.30 0.0 22.5 10.2 13.70 60 88.74 0.0 44.5 20.2 11.26 61 19.1 90.00 0.7 42.0 10.00 62 91.30 1.0 40.6 18.4 53 8.70 94.00 13.8 35.0(C) 15.9 6.00 64 95.46 69.6 29.O(B) 13.1 4.66 65 Sand B, lS.O% Limestone Dust 84.81 0.0 14.0 6.3 16.9 66 87.01 0.8 18.6 8.4 12.99 67 88.16 0.9 36.0 15.9 68 11.84 16.3 90.53 1.0 36.0 9.47 69 91.83 1.8 31.6 14.3 8.17 60 94.36 35.00 36.0(C) 16.9 6.64 61 96.00 100.00 18.0(8) 8.1 4.00 62 Sand C, lO.O’% Limestone Dust 80.00 0.0 15.0 6.8 73 20.00 83.33 0.3 46.0 20.4 16.67 74 31.7 86.90 0.5 70.0 13.10 76 24.9 88.89 0.8 65.0 11.11 76 90.91 1.0 39.0(C) 17.7 77 9.09 93.02 2.4 33.0(C) 16.0 6.98 78 95.23 84.9 19.O(B) 8.6 4.77 79 Sand C, 15.0% Limestone Dust 0.0 20.00 10.0 4.6 80 14.6 0.5 6.6 16.67 81 13.10 83.0 0.6 37.6 82 67.0 30.4 0.9 11.11 83 1.2 48.0(C) 21.8 9.09 84 2.6 30.O(B) 13.6 6.98 85 19.O(B) 8.6 91.9 86 4.77 Sand C, 20.0% Limestone Dust 80.00 0.0 15.0 6.8 20.00 87 7.3 83.33 0 7 16.0 16.67 88 33.1 86.90 0.8 73.0 13.10 89 30.4 11.11 88.89 0.9 67.0 90 90.91 1.6 64.O(B) 24.5 9.09 91 93.02 78.3 26.O(B) 16.3 6.98 92 5 C = crack; B = break. Asphalt
~I
Voids
% 3.66 7.43
1ii:io
19:36 24:11 I
.
.
6.26
...
14:64 21.32 23.24 4.46 6.80 10.80 12.70 14.60 17.00
0.10 3.72
...
8.50 13.OS
...
0.43. 3.76 7.87 10.33 15.44 18.22
...
0.34 2.36. 3.12 6.64 12.17 16.66
...
0.15 0.50 0.72 2.81 9.27
... ... 1.74
7.26 13.20. 18.6@ 23.21 26.13.
...
2.62 3.32 10.98. 16.701 21.43. 26.242.862.84 2.94 6.62‘ 13.04 19.13.
104
INDUSTRIAL AND ENGINEERING CHEMISTRY
magnitude sometimes as great as the difference caused by variations of 5 per cent filler. The author feels justified in answering this question affirmatively. In the first place, the sphere penetration stability method can be improved to yield considerably more precise data than the author was able to attain with his homemade apparatus. Again, the irregularities of the curves do not necessarily imply corresponding experimental errors. I n comparing properties of mixtures, it is essential to evaluate what are corresponding compositions. There is no correspondence between mixtures containing different proportions of filler but the same percentage of asphalt, when the relative proportions are expressed in weight relations. A much closer approximation to equivalence could be attained in terms of available percentage voids and total surface of the aggregate. A lengthy discussion on the relation between the percentage voids and the grading of the aggregate and proportion of filler is beyond the scope of this paper. It is easy to visualize, however, that the amount of asphalt needed to fill the voids and cover the surfaces of the aggregate at first decreases as more and more filler is added and finally begins to increase with excessive m e r . Mixtures containing more asphalt than is needed to obtain this "saturation" point will exhibit approximately the same stability, regardless of the amount of filler. On the other hand, the proportion of filler determines the amount of asphalt necessary to attain this asphalt saturation point. I n view of this qualitative consideration it is obvious that no parallelism among the stability curves can be expected. Furthermore, since the stability is a measure of two forces (resistance to vertical compression and lateral displacement), certain irregularities of the curves should be expected.
OF FILLER ON CORRELATED PROPERTIES IN FIGURE 5. EFFECT COMPACTED SPECIMEN OF SANDB
VOL. 29, NO. 1
The only points of the curves which are strictly comparable are the values of the maximum stabilities. In the case of sand A (Figure 4) the stability curves are very close together, This is probably due to grading of the sand rather than to lack of sensitiveness in the stability measurement. The transition between the comparatively coarse particles (minus 50 mesh) and the particles of the filler is too abrupt to form a compact mix. The influence of small variations of filler, therefore, on the stability cannot be well defined. If these considerations are sound, this stability method is sufficiently precise to differentiate among gradings of the sand as well as variations in the proportions of filler.
Effect of Temperature upon Stability When the stability obtained a t different temperatures between 35" and 60" C. is plotted against the temperature, straight lines are obtained. These results check those obtained by Hubbard and Field. However, the stability at 0" C. is decidedly out of proportion. The data are as follows: Mixture No. 10 11 17 20 31 35 49
50 51
a
, 60' C. 241/n
28 lS'/a(C) 37l/a 21 47 2 1 '/a 22'/2 441/a
Stability at: 48' C. 370 79 73
c.
$:);
65
... ... 69 86
0-30
c.0
...
