I
CHARLES MACK Research Department, Imperial Oil, Ltd., Sarnia, Ontario, Canada
Physical Properties of Asphalts in Thin Films
b Film strength of thin films of asphalt, used as binder in bituminous pavements, varies linearly with film thickness t o a maximum and then decreases, and increases with surface tension. At film thicknesses between 0.02 and 0.007 cm. asphalts behave like solids.
A S P H A L T Sserve
as binders for mineral aggregates in the construction of bituminous pavements. It is usual to correlate the usefulness of an asphalt as a binder with its properties in mass, such as viscosity or penetration and ductility. However. asphalt is present in pairements in the form of thin films. Experimental evidence indicates that liquids in thin films have properties different from those in mass ( 4 ) . Molecules in the interior of a liquid are subjected to intermolecular attractions which contribute mainly to the cohesive forces. Molecules a t the surface of a wettable solid have a free surface and are exposed to a stronger field of forces, which is responsible for adhesion. The effective distance over which these forces act is different for polar and nonpolar liquids. I n nonpolar liquids this surface zone is probably never more than a few molecules thick. I n polar liquids, to which asphalts belong, it is often great enough to be observed by macroscopic methods. The principal effects are a considerably increased viscosity and an abnormally great elastic strength of the liquid near the surface of the solid. The suggested explanation is that the surface molecules
422
orient theinselves in the direction of their polarization, and that this causes a similar alignment of the molecules icithin the liquid, which often extends to a distance of thousands of molecules from the surface. X-ray diffraction patterns of such surface zones tend to confirm this theory. The viscosity of paving grade asphalts is of the order of 106 poisrs a t 77' F., lchereas heavy lubricating oil fractions of similar molar volume have viscosities of the order of 20 poises a t the same temperature. The high viscosity of asphalts is a manifestation of strong attractive forces bet\veen the individual molecules. Based on the above concept, asphalts in thin films on solid surfaces should approach the properties of solids. and the force necessary to break such a film is comparable to the tensile strength of solids.
Experimental
In road construction, asphalt is mixed with mineral aggregate a t elevated temperature. The coefficient of cubical expansion is of the order of 3.5 X 10-6 per ' C. for the mineral aggregate and 6 X per ' C. for asphalts. If the transition from the liquid state to a solid state takes place in the asphalt above the ambient temperature, the difference in the coefficients of expansion causes internal stresses in the asphalt film. These stresses keep the film expanded and have the effect of a compressive stress acting normal to the surface of the film. If the internal stresses are large enough. they
INDUSTRIAL AND ENGINEERING CHEMISTRY
causr rupture of the film or lormation of caviiics. T o stud!. this phase, films of naphtha, lubricating oils, and yrc:asrs contained bettvern microscope slidrs M jected to pressure. Thesc materials expanded \rhcn compressed. After release o f t h t pressure, there \vas a rapid rlastic recovery. folloivcd by floiv of the material. This Ho\v \vas not unifurm. but started at niiinero~~spoints and follo\vrd an irregular pattc'rn, beyond {chich there leas no lateral fjo\c, \+i'll-1 ordinary liquids. the pattern leas not sufficiently permanent for photogra1)hic reproduction. Hoivever. greases retaincd the pattern permanently. Figure 1 sho\vs thr Hofc pattern of a soda soap qrcasr which had been compressed undrr a pressure of 1 kg. per sq. cm. After release o f the prcssurr. the area of the expandrd film rrmainrd essentially unchanged, icith flow occurring Leithin the film. Figure 2 shoivs a similar cavitation pattcrn for a \'en+ zuelan asphalt bvhich was compressrd a t elevated temperature to a film thickness of 1.07 X 10+ cm. and was then cooled in the absence of pressure. This observation was described as early as 1911 by Budgett ( 2 ) and morc recently by Kumler (5). A probable explanation for this behavior is as follows: A liquid, placed on a solid surface, spreads until the forces associated with the work of spreading are in equilibrium with the forces of adhesion. When subjected to a compressive load, the film is in a state of stress and expands. O n release of the pressure, the internal stress relaxes through do\\-. Under these circumstances, a
film of a nonpolar liquid eventually flows back to its equilibrium thickness. However, if the liquid is polar and the forces of attraction between the molecules of the solid surface and those of the film are great, the expanded film should retain its position because of strong orientation effects. If flow takes place, it can occur only a t points of weakness in the film and results in cavitation. These points represent loci of stress concentration in the film, or weaker forces of attraction between solid surface and film, or a combination of both. Although the phenomenon of cavitation, described above, was obtained under exaggerated conditions, a sufficiently large difference between the coefficients of expansion of the adhesive and adherent will set up internal stresses. This can cause the formation of microcavities in a uniform-appearing film. The presence of cavities tends to weaken the forces of attraction in the film or the film strength. As a measure of the film strength, the force per unit area was determined which is necessary to separate two steel plates cemented by a thin film of asphalt. The steel plates were chosen, because they had approximately the same coefficient of expansion as mineral aggregates. They had a maximum surface irregularity of 1.8 X 10-6 cm. They were further polished with naphthenic acids, and cleaned with naphtha to ensure zero contact angle between asphalt and steel. The plates were always freshly prepared and kept free from foreign surface contamination. The films were prepared by placing a weighed amount of asphalt on a plate kept in a n oven a t elevated temperature. After liquefaction, the other plate heated a t the same temperature, was placed on the asphalt. The plates were marked so that the same surface irregularities were always opposite each other. The film thickness was varied by the amount of asphalt used, by the time of residence in the oven, and by the temperature which was maintained a t 250" or 300' F. The plates were cooled in air for 3 hours, then placed in an insulated box and exposed to an air stream of 77' F. for 2 hours. The plates were finally attached to a Baldwin testing machine by means of universal joints, and the film strength was determined under a constant rate of loading of 5 pounds per second at 77" F. After rupture the area of both film surfaces was measured with a planimeter. This datum together with the amount of asphalt used served for the calculation of the film thickness. Only those results were taken into consideration where rupture occurred in the film. At thicknesses below 2 X cm. the asphalt film was not uniform in appearan'ce after rupture and showed a cavitation pattern similar to that shown
in Figure 2. As a result of this, the film strength measurements were erratic. However, a t thicknesses in excess of 2 X 10" cm., the surfaces of the ruptured films were uniform. After every test, the surface of the ruptured film was examined under a stereoscopic microscope a t 45-fold magnification. No flow occurred in the asphalt film during rupture within the range of film thicknesses studied, nor was cavitation visible. Flow took place only a t thicknesses of the order of lo+ cm. and greater, depending upon the type of asphalt. I n several cases the asphalt films were subjected to shear under a constant load for 30 minutes without any indication of Aow. T h e shearing stress varied with the area of the film. For the asphalts tested (described in Table I), the conditions of no flow were:
Conditions of No Flow Film
Shearing ThickStress, ness, Kg./Sq. Cm. X Asphalt California red Western Canadian red Venezuelan red
Cm. 1.23 1.47 0.97 1.33
108 1.98 2.36 2.52 3.05
Figure 1. Flow pattern of compressed material after release of pressure. Soda soap grease, X4
I t would appear, therefore, that asphalts in thin films, prepared by the method described above, have properties approaching those of solids. The materials used consisted of a series of 85 to 90 penetration asphalts of varying flow properties, a coal tar pitch, and an asphalt containing 5% of GR-S rubber. Inspections on the asphalts (Table I) include data for surface tension and viscosity a t 77" F. The surface tension was measured by the pendant drop method, using the tables computed by Fordham (3) for the calculation of the surface tension. The viscosity measurements were carried out in a rotating cylinder viscometer with stationary cup. Asphalts with anomalous flow properties show the following relationship between stress and shear rate. Shearing stress = B (shear rate)b where shear B With b = 1, the B becomes the smaller than 1,
(1 )
and b are constants. flow is Newtonian and viscosity. When b is the viscosity decreases
Figure 2. Flow pattern of compressed material after release of pressure. Venezuelan asphalt, X4 VOL. 49, NO. 3
MARCH 1957
423
with increasing shearing stress. For asphalts with such flow properties, the viscosities given in Table I represent the equilibrium values a t very loiv stresses where the internal structure remains unchanged (7). Figures 3 and 4 show the relationship between the logarithms of film strength and film thickness in decreasing order of steepness of the curves. The strength increased in all cases Ivith the film thickness and reached a maximum a t an optimum film thickness. Beyond this point. the strength again decreased. The maximum strength is approximately 2017 pounds per square inch for the asphalts and 276 for the coal tar pitch. The optimum film thickness, hoxvever, varies with the sourcc and production method.
0 WESTERN CANADIAN CALIFORNIAN
REDUCED
REDUCED
A WESTERN CANADIAN t GR-S RUBBER
..- . v)
a Y
'
2.3
F (3
z w
a I-
2.2
v)
5LL
" s
2.1
2.0 0.3
0.4
0.5
0.6
COAL TAR P I T C H
2.5
Film Strength as Function of
WESTERN
,
0.5
0.4
0.3
CANADIAN
07
0.6
BLENDED
1
Attractive Forces and Cavities
As rupture occurred in the film. tilm strength is partly a function of the secondary valence forces betJveen adjacent asphalt molecules. LVith asphalts containing polar and nonpolar cornpounds, the attractive forces are mainly due to induction and dispersion forces. The energies associated Lvith these forccs both decrease with thc sixth power of the distance bet\veen the molecules. and may, therefore, be expressed by the Lennard-Jones equation ( 6 ) .
'
2.3
3
IL
"0
2.2
0.3
0.4
LOG
0.5
0.6
Figure 3.
where ~ ( r is ) the potential energy between tivo adjacent atoms separated by a distance I: and .4, R, tn, and n arc constants. The value of IZ is 6 and IIZ is usually 12 and greatrr. The first term on the right-hand side of the equation represents the potrntial enerqy due to repulsive forces, and the other th(. potential due to attractive forces. I n terms of the minimum potential energy, TO)? Equation 2 may bc written as folloivs:
Differentiating with respect to T gives the secondary valence force betiveen two adjacent atoms. di
7;,
Introducing this term into Equation 4 gives the following expression for the masimum force due to the potential of d secondary bond.
424
0.5
0.4
LOG
0.6
0.7
FILM THICKNESS (cm. x I O 3
Film strength as function o f film thickness
Table I.
Inspection o f Asphali
Softening Point
Californian reduced Western Canadian reduced Western Canadian blendedd Western Canadian reduced S% GR-S Western Canadian oxidized Venezuelan reduced Venezulan oxidized Coal tar pitch
+
This force is a maximum a t r = hence d f m r d r , = 0 and
0.3
F I L M THICKNESS (cm. x io3 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
5
270
lOOf
1.00
0.86
36.0
264
100-C
1.00
0.89
34.1
86
260
100+
0.90
1.76
33.2
80
224
lOOf
0.90
2.50
(34.1)''
110'/2
87
111
86'
111" 125
'2
113'/?
224
loo+
0.76
6.62
32.0
1151,'~
223
lOOt
0.80
4.22
33.0
118
220
100;
0.71
12.10
32.1
llbl/y
205
100'-
0.85
4.66
45.9
Reduced western Canadian crude bleiided with heavy lulic di.>till:tte. S u h c e tension is a - s u m d t o he >:imp ns that of w e l t r r n (':iti:irIi;in wit.
VENEZUELAN
strains are very small; therefore, exp (e) can be replaced by e 1, and
REDUCED
+
€
-.v)
p.
=
(L4/u)l'a
(9)
where a = 3.7 - cy. The film strength varies with the film thickness, and the ratio of two film strengths may be expressed by WESTERN
OX I DI ZED
CANAD IAN
u m / u = (€/Em).
2.3 I
I
in
I
I
where e, is the strain corresponding to the maximum film strength, urn. The strains are small and can be expressed as change in height over original height of the film, e = Ah/h. Since rupture occurs, when the molecules are separated by a certain distance, it can be assumed that Ah is constant for a given asphalt, and
1
I
I
5
J LL
VENEZUELAN
OX I D IZED
um/u = (hm/h)a
2.1
A I .o
0.5
0.4
0.6
0.7
0.8
0.9
LOG F I L M THICKNESS (crn. x io3I Figure 4.
Film strength as function of film thickness
tion, Equation 6 may be written as follows : This is also the force necessary to break the bond. The force per unit area, jm/cm2 = urn, is obtained by summing u p the forces over the individual centers of attraction per unit area. This summation cannot be carried out for asphalts, as their chemical structure and degree of orientation of the molecules in a film are not known. Following a suggestion by de Boer (7), the summation may be replaced by an integration. Integrating over the surface of a molecule, number of molecules in a row of 1 cm., and number of rows per square centimeter, we obtain (6)
um = C/r,3
where C is a constant combining the constants of Equation 5, the constants of integration, and sometimes also a n anisotropy factor due to imperfect orientation. O n account of the presence of cavities, the value of u, is lower than the theoretical value and varies with the number of cavities present. To take the latter into consideration, the plane may be divided into a fraction x occupied by x, molecules, and a fraction, y = 1 occupied by the cavities or by loci subject to cavitation. For this condi-
-
xu = xC/ra (7) As there was no flow in the asphalt film, the cavities may be associated with a n increase in volume under a tensile stress according to
(8)
y o = yDe0 = yDec
where D and c are constants and e is the volume strain. With the volume increasing only in the direction of tension, the volume strain is equal to the tensile strain, e = e. Differentiating the logarithmic forms of Equation 7 and 8 to eliminate constants C and D,we obtain on rearranging d ( x u ) = udx - 3xudr/r = udx d(yu) = udy +ycude/c
-
3xude
The term dr/r is the differential strain, de, adding and keeping in mind that dy = -dx, gives du = -3xude
4-ycudc/e
and
Integration between E and o keeping x and y constant, leads to exp
(e)
-1 =
(10)
&82(A/u)1/a2
where A is a n integration constant. The
(11)
This equation describes the results obtained, As the maximum film strength, u,, is always larger than u, a is positive when the optimum film thickness, h,, is larger than h, and negative for h > h,. With a = 3x - cy, a is positive when 3 x > cy-i.e., in this region the secondary bond forces are strong and probably cause orientation of the asphalt molecules. I n the region where a is negative, cy > 3 x , and the secondary bond forces diminish with increasing film thickness. In view of the dependence of a on x , a variation of a with film thickness should be expected. The fact that a was practically constant for a given asphalt seems to indicate that, for the test conditions, the distribution of the centers of attraction was approximately the same. The constancy of a may be also explained by a linear dependence of c on x. As the plates are pulled apart, the asphalt film is under strain. A knowledge of the magnitude of this strain is of importance, in that small strains are associated with brittle films. I t is only with difficulty that the observed very small deformations can be separated into those of the metal parts of the testing equipment, and of the asphalt film proper. .4n estimate of the maximum strain a t the point of rupture may be obtained from the following consideration. Eliminating the repulsive forces, because they fall off very rapidly with increasing distance between the molecules, the attractive forces per unit area as a function of the distance can be expressed in a manner similar to Equation 6. Combining with the cavitation effect, in the same way as above, gives the strain as a function of stress similar to that of Equation 9, after modification for the proper boundary conditions. Considering only stresses normal to the plane, the asphalt film a t rest is under a compressive strain due to the load of the upper plate. During the VOL. 49, NO. 3 e
MARCH 1957
425
Table (I. Maximum Film Strength and Optimum Film Thickness o f Asphalts Opt
rliln
hfnx;.
Tliirk-
I'lllll
St reti et 11, ne+-. Lh. dq Cin. X -1sphnlt
Coal tar pitch California reduced W. Canadian reduced W. Canadian reduced 5 % GR-S W. Canadian blended Venezuelan reduced W. Canadian oxidized Venezuelan oxidized
+
Stniiii at
'
2 Slh,,, I)yne-/Sq.
IIlPl1
103
276.1 211.6 206.4
3.33 2.58 2.96
1 484 1.400 1.220
1.918 1.907 1.980
10-1 6.0 X lO--d 2.0 x 10-4
27,500 27,900 23,000
207.0 205.1 200.0 188.4 193.5
4.00 3.38 4.03 4.47 5.14
1.210 0.633 0.392 0.333 0.253
0.650 0.837 0.454 0.370 0.250
2.3 x 4.9 x 9.2 x 2.4 X 2.3 x
17,000 19,700 16,400 14,800 12,500
U
I1
I
Illl~,tllre
7.7
x
10~~4 lo-'* 10-13 10-1'
io-."
(3111.
mum film rhickncsscs, the values o f tlw strain obtained at the point of rupture a s calculated from Equation 13: and the values of a and (I'for the ascendins and descending parts of the film strenqth film thickness curves. r , T h e coal tar pitch has the greatest film strength of 276 pounds per square inch whereas the film strengths of thr Differentiating and dividing by Equaasphalts are approximately the same, tion 12 gives varying. bmveen 188 and 211. Since, ivithin the range of film thicknesses d€ 1 du - - - ____~~_._~ used, rupture occurred in thc. asphalt e a u [ i u / u ,I' - 1] film, these film strengths are, therefore. Integration between c,,,, tlie strain a t a measure of thr cohesion o f asphalts film strength rt,z. and 0. u~,,. and uo gives a t the optimum film thicknrss. The values of n and a' drcrease in the same order as the film strciigtti.;. The parameter, a. is 3 u - cy according from Ivhich follo\\,s to Equation 9? \vliere factor 3 refers to the sccondary bond forces, c is a constant associated \rirh the cavities. and s and y = 1 - Y arc: the fractional areas Discussion of Results occupied by inolccules and cavities. I t follows from the definition of a, that Table I1 gives the values of the maxix = (0 r) '(3 c), indicating that m u m film strength in decreasing order T increases Lvith increasing. i i . for the asphalts used. The data include . i s values o f the ctrain a t the point f J f the values of the coi,responding optiexperiment the film is under tension, and this strain becomes zero, \\hen the tensile stress is uo and equal to the load of the upper plate per unit area. I'or this condition, the strain as a function of stress becomes I
-
$
'1
+
+
72-
70
-
6 8
-
>
t v)
0
6 6 -
Figure 5. Relation of viscosity of asphalt t o optimum film thickness
V
-
v)
>
6 4
W
-
0
highl!. dependent on t h e valrir of (I according to Equation 1 i, this strain also decreases Luith drcrc.asing values of a. The data sho\v t h a t t h e coal tar pitch. reduced (MTornian asphalt, and reduced \vestern Canadian asphalt without and \vith GR-S rubber had thr larqcst strains. CJf the o ~ l e ro f In-'. 'I'hr hlonded I-duccsd \vrs[erii Caiiadiaii asphalt had a strain of 4.0 X l(J--*.whrrcas those for t h c r c d u w d and oxidized \'enrzuclan and osidi;.cd \vestern Canadian asphalrs w c i ~ ( priir~ tically zero. \\'it11 a tensilr loading (if -5 pounds Iit'r secund. the time of load ai)plication a t thc optimurn iilrn stretigth varied i v i t h thr film area and \vas of tlic order of 2.50 to 400 srconds. Kastsd on the calculated strains. the strain ratrs ~verc'vcry small and \cere of the order o f IiJ we: and less. It can: therc~fore, Ix asstin~t:dthat a variation in the rate of loadinq does not affect the valum of he film strcan,Tth,as long iis the rninirniiin rime of load application is 100 seconds. 'I'hr addition of .576 GR-S rubbt.r to tlie \\-('stern Canadian asphalt h a r d l y atfwts the iiroperties of the asphalt filin, vsce1)t that ttic optimum film ttiicknrss is increased. Ir ajipcars rliat ~iaraniercrii a n d ir. drpcndcncc upon + art' a iiirasurc o f tlic dcgrrr of packin? oi' tlic: inolec~ules i n a thin film. Break ocriirs dtrririq srretchinq of a film when thc distance of the inolrc~trles has rcactictl ii certain valur.. ' r l i f , potrn'ial rnerq?- incre;iscs pvitti increasing distance of the mol(,rules. 'l'hc increase in distance. a t hrrak. o r thr. srrain, drpt,iids crlion the original d r q r r ~of packin? of rhc. rnolectilrs and their potrritial rncrgy. It is grcatcst. \ \ h t 9 r i tlir inolcciil~:sare closely Ilackcd o r are in pcxitions of low potential cnergy. and decreases ivith lo pickinq a n d grrater p o t t . T l i r i s t h r ordc-r in lvhich t h c qiven i n Tahle II also rciircsents tlir ordc.1. 01' incrcasinq potcntial c'nergy in [he original film 'Thcz first four asphalts t v i t l i tlic qreati,sr strains are highly aromatic. I t apprdrs! therrfore, that high aromaticity in asphalts is inducivc to close packing i n thin filins. . i s the film strength test is accoinjxinicd by tlir formation of t\vo nrlv surfaces. thr rict tvork esprndcd i i i ('II'ating thcse surfaces can be c.spcctcd to he equal to the frce surface energy g a i t i d . The forct, per unit area nrcwsary t o do this ~vorliis //asea
J 6.2
-
60
I
426
1
I
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
=
2 S,ih
(14)
rchere .r' is the free stirfacc rncrgy in ergs per sq. cin. or the surface tension in dynes per cni., and h is the film thickness. These data are also given in Table I1 for the optimum film thickness. A comparison shoivs that tlir thcrmodynamic concept of film strength docs not apply, as the values of the lattrr arr of the order of l o 4 dynes per s q . cin.,
whereas the values of the maximum film strength are of the order of lo7 dynes per sq. cm. There exists, however, a qualitative relationship between maximum film strength and surface tension, in that asphalts of similar surface tension have similar film strength. It is generally agreed that the viscosity of a liquid is a function of the size of the molecules, which may be units in either a chemical or physical sense. Since the optimum film thickness approaches the equilibrium thickness, it can be assumed that the latter is determined by the size of the units and is, therefore, a function of the viscosity. For the majority of the asphalts used, the relationship between log viscosity a n d log optimum film thickness is linear (Figure 5). The exceptions are the coal tar pitch and the asphalt containing rubber. Their abnormal behavior may be attributed to the fact that they are suspensions. The coal tar pitch contained 18.6y0 of free carbon. Microscopic examination of the mixture of asphalt and rubber revealed the presence of swollen rubber particles. Relation of Film Strength to Mechanical Properties of Pavements
Mineral aggregate, when dry, forms , a loose mass with no coherence of the particles. After addition of asphalt and compaction, a certain force must be applied to separate the particles. This force is related to the film strength. Although the film strength is affected by the properties of the aggregate surface, the results obtained on steel surfaces can be expected to bear a qualitative relationship to the mechanical properties of bituminous pavements. A compacted bituminous paving mixture hardens under prolonged loading. T h e compressive stress, a t which maximum hardening is obtained, represents the bearing strength. Ax this stress the pavement has also its greatest density. At greater stresses the density decreases because of separation of the mineral particles and finally the pavement fails (8). The bearing strength is, therefore, a measure of the cohesion between the mineral particles imparted by a film of asphalt. Hence, the relationship between bearing strength and asphalt content should be similar to that between film strength and film thickness. Table 111 gives the bearing strength of sand-asphalt mixtures a t 77" F. as a function of asphalt content, using reduced Californian and Venezuelan asphalts. Both asphalts have a penetration of 90 a t 77" F., and are obtained from the same crude oil as the ones previously described. The mixtures consisted of 80 parts by weight of beach sand passing 40 retained on 50 mesh sieve, and 10 parts of filler passing the 200-mesh sieve, to which various
Table 111. Bearing Strength as Function of Asphalt Content
Bearing Strength, Lb./Sq. Inch
Asphalt Content, Parts/QOParts Mineral Aggregate
Californian Reduced 33.5 50.6 72.2 53.8 41.5
Venezuelan Reduced 46.0 53.5 60.5 67.0 61 .O 55.7
7 8 9 10 11 12
quantities of asphalt were added. The bearing strength was measured (8) on cylinders 1 inch in height and 2 inches in diameter, which had been compressed twice under a load of 3000 pounds per square inch by the double plunger method. The data indicate that bearing strength increases with the asphalt content to a maximum, in agreement with experiments conducted with thin asphalt films between steel surfaces, and is proportional to the asphalt content raised to the power of a. The values of a are 1.9 for the California and 1.0 for the Venezuelan asphalt. The corresponding values from the film strength measurements are lower (1.4 and 0.392, respectively). The ratio of maximum film strength to maximum bearing strength is 2.93 for the Californian and 2.99 for the Venezuelan asphalt and is practically constant. Calculation of the thickness of the asphalt film carried by the total aggregate gives values which are considerably smaller than those observed for the film strength measurements. Omitting the filler and considering the sand (density of 2.5) to be perfect spheres 0.042 cm. in diameter, the optimum thickness of the asphalt film carried by a grain is 0.00124 cm. for the Californian and 0.00199 cm. for the Venezuelan asphalt. The film thickness a t the point of contact between two grains is twice as high0.00248 cm. for the Californian and 0.00398 cm. for the Venezuelan asphalt. The corresponding optimum film thicknesses obtained from the film strength measurements are practically the same (0.00258 and 0.00403 cm., respectively). Although this result may be somewhat fortuitous, it seems to indicate that the filler becomes part of the film. Based on this concept, the optimum film thicknesses of the point of contact between two sand grains are 0.0039 cm. for the Californian and 0.00532 cm. for the Venezuelan asphalt. The filler, being
.
part of the asphalt film, partly replaces the cavities formed in the asphalt film. This concept explains why the values of a obtained from the bearing strength measurements (1.? and 1.O) are greater than those from the film strength measurements (1.4 and 0.392). Aromatic asphalts form the densest films. I t follows that the diffusior: of water into films should be very small in this case. This is of importance with regard to the mechanical stability of bituminous pavements, in that water c a n reduce their bearing strength by weakening the film, through reductio:) of the void content, or by a combination of both. The sand-asphalt mixtures were immersed in water for 96 hours a t 77' F. under a pressure of 6 inches of mercury. Their bearing strengths were as follows :
Pounds/Sq. Inch Bearing Strength Californian Venezuelan Before water immersion After water immereion
72.2
67.0
72.5
59.6
Hence, water immersion did not affect the bearing strength of the mixture containing the aromatic Californian asphalt and reduced the strength of the mixture prepared with the lesp. aromatic Venezuelan asphalt by 11%. Another item of importance is the finding that the optimum film thickness increases with the viscosity of the asphalt. As the viscosity varies with temperature, the optimum film thickness must also depend on temperature. If the same reasoning can be applied to the optimum asphalt content, it must, by consequence, affect the design of bituminous pavements. The design is often based on data obtained at 140' F., and such a pavement would be deficient in asphalt content a t lower temperatures. This phase requires further investigation. literature Cited
Boer, J. H. de, Trans. Faraday SOC. 32,lO (1936). Budgett, H. M., Proc. Roy. SOG. (London) A86,25 (1911). Fordham, S.,Zbid.,Al94, l(1948). Henniker, J . C., Rev. Mod. Phys. 21, 322 (1949). Kumler. W. D.. J . Phvs. Chem. 44. 612 (1940j. Lennard-Jones, J. E., Proc. Phys. SOC. 43,461 (1931). Mack, Charles, J . Polymer Sci. 13, 279 (1954). (8) Mack, Charles, Proc. Highway Research Board 33, 138 (1954). RECEIVED for review April 11, 1956 ACCEPTEDSeptember 21, 1956 Division of Gas and Fuel Chemistry, Symposium on Bituminous Materials, 129th Meeting, ACS, Dallas, Tex., April 1956. VOL. 49, NO. 3
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M A R C H 1957
427
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