EP0099400A4

EP0099400A4 - Asphalt pavement mix containing diatomite filler and a method of mixing same.

Info

Publication number: EP0099400A4
Application number: EP19830900762
Authority: EP
Inventors: John Howard Kietzman; Mario Peter Tocci
Original assignee: Manville Service Corp
Current assignee: Johns Manville
Priority date: 1982-01-27
Filing date: 1983-01-17
Publication date: 1984-07-06
Also published as: IT1164601B; PT76154A; FI830260A0; EP0099400A1; WO1983002619A1; ES8403179A1; NO833251L; FI830260L; DK439483A; IT8347622A0; ES519271A0; DK439483D0; ZA83548B

Abstract

A high asphalt content paving material containing diatomite. The diatomite stabilizes the additional asphalt preventing bleeding, rutting and shoving. The diatomite must be present in sufficient quantity to adsorb at least the excess asphalt and, preferably, all of the asphalt present in the mix. A method of calculating this optimum diatomite content is presented. Further, a method of adding the diatomite to the mix in order to minimize damage to the diatoms is disclosed.

Description

ASPHALT PAVEMENT MIX CONTAINING DIATOMITE FILLER AND A METHOD OF MIXIN3 SAME

Background and Summary of the Present Invention Development work on asphalt paving materials has been conducted for many years by many people. During these investigations, just about every substance known to man, including diatomaceous earth (diatomite) , has been tried as an additive to pavement mixes. However, some efforts to use diatomite as a filler in standard asphalt mixes produced mixes which were too "soupy". Such results would suggest that use of diatomite in high-asphalt content pavements (i.e., pavements with asphalt contents 10-65% above normal concentrations) would be inappropriate. Further, published results of lab tests conducted by the Apshalt Institute in the 1960's indicated that diatomite-containing mixes tended to be water susceptible (i.e., they adsorbed water reducing the cohesive strength of the mix, thereby leading to pavement failure) . In a pavement mixture, asphalt plus the fines (material passing 200 mesh) form the mastic. The asphalt mastic, in conjunction with the fine aggregate (those passing No. 8 or No. 10 rcesh) , functions primarily as what may termed, the mortar, holding the larger aggregate together. For a standard mastic, the thinner the film coating, the stronger the bond and the better the stability of the pavement. However, thin film coatings of asphalt dry out or harden more quickly. Once the asphalt hardens and loses its pliability, its resistance to traffic loading is decreased causing raveling and cracking so that the pavement must be replaced. Therefore, to date, pavers have had to trade off cohesive strength for pavement life. Further, most aggregates and fillers are incapable of preventing the flow of asphalt, so that hiφ-asphalt-containing pavements cannot normally be used without bleeding, rutting or shovin ? or some combination thereof.

The present invention makes such a trade-off unnecessary. It has been found that by adding diatomite to high-asphalt-containing asphalt and, preferably, sufficient to theoretically adsorb all the asphalt, a mix having optimum cohesive strength and extended pavement life can be achieved. This is due to the fact that the adsorption of the asphalt maintains the film thickness of the asphalt mastic at its optimum minimum thickness and, at the same time, forms a thicker mortar layer that shields the mastic bond interface between aggregate particles from the atmosphere and from water, thereby reducing the amount of age (oxidation) hardening and the water susceptibility of the pavement. Further, diatomite increases resistance to asphalt flow to such an extent that bleeding, rutting and shoving of the pavement are effectively minimized. By limiting the quantity of diatomite in the mix to an amount not substantially greater than the amount capable of adsorbing all of the asphalt in these high-asphalt-content mixes, water susceptibility can be avoided. An additional benefit of this diatomite-containing mix is its retention of skid-resistance. One of the factors contributing to loss of skid-resistance in a pavement is its compaction under traffic loading which reduces the void content and, hence, reduces the water permeability of the pavement surface. With a less permeable surface, water drainage is decreased which increases the dangers of hydroplaning and skidding. Many diatomite-containing mixes resist over-compaction and maintain a sufficient quantity of voids under traffic loading to insure permeability. These voids provide a place for the water trapped between tire and pavement to go, thereby reducing the likelihood of hydroplaning and skidding. Further, the diatomite provides the mortar portion of any pavement mix with abrasion-resistance which greatly reduces the rate at which the asphalt and sand is eroded from the pavement and, in turn, prevents exposure of the larger aggregate, thereby protecting it from polishing. Exposure and polishing of certain types of aggregate make even a well-drained pavement slippery.

The present invention also solves the previous problems related to diatomite mix consistency. Conventional practice in pavement batch mixing plants involves dry mixing the hot aggregate and any mineral filler prior to the addition of liquid asphalt. This was believed to be necessary to achieve proper dispersion of the filler in the mix and to avoid clumping. Further, it was believed an extended mixing time after the asphalt was added was necessary to insure complete coating of the mineral filler in order to avoid water susceptibilit .

In preparing the mixes, Applicants discovered that premixing the diatomite with the aggregate resulted in crushing of the diatoms making the resultant mix 'soupy' . The process of the present invention involves premixing the components of the aggregate (usually sand and stone) , adding the asphalt, mixing for a conventional period of time which thoroughly coat the aggregate with asphalt and then adding the diatomite to the mix. Surprisingly, the diatomite is able to achieve satisfactory dispersion in the mix even though added so late in the mixing process. Further, the diatoms do not suffer the crushing effects noticed in the conventional procedure; the asphalt apparently acts as a cushioning agent or lubricant for the diatoms protecting them from the abrasive action of the aggregate. In addition, Applicants find it unnecessary to continue the mixing for a long period af er the diatomite is added. Satisfactory coating and dispersion can be achieved in a minute or less. In fact, it is preferred that each of the mixing steps be accomplished in h minute or less.

Other features, characteristics and advantages of the present invention will become apparent after a reading of the following specification.

Brief Description of the Drawings FIG. 1 is an enlarged view at 250X magnification of one grade of diatomite that may be used in the present invention; FIG. 2A is a schematic depiction of two particles of prior high-asphalt-content mixes;

FIG. 2B is a schematic depiction of two particles of the present diatomite-containing asphalt;

FIG. 3 is a graphic representation of the improvement in durability that can be expected from the diatomite-modified, high- asphalt content mix;

FIG. 4 is a graph illustrating the effects of water on various mixes that do and do not contain diatomite;

FIG. 5 A-D are photographs of core samples of actual pavements following 11 months of high traffic usage. r_e.ta-.1ed Description of the Preferred Embodiments

FIG. 1 is a 250X magnification of one type of diatomite which has been found suitable for use in the pavement mix of the present invention. This grade of diatomite is sold under the trademark "CELITE 292" and is available from MANVTT,T,F, PRODUCTS CORPORATION. One of the primary functions of a filler material in a high-asphalt-content paving mix is to thicken the mastic and inhibit flow of the asphalt. Ωie capability of the filler to inhibit flow will be a primary factor in determining how much additional asphalt can be added without the asphalt seeping or bleeding out of the pavement during placement or under subsequent heavy traffic. There is a direct correlation between the amount of additional asphalt and the amount of increase in pavement life (cost factors being a countervailing consideration) . As a means of comparison, limestone, which is a conventional filler material, has a particle surface area of .05 square meters per gram of iraterial. The above-mentioned "CELITE 292" has a particle surface area of 12 square meters per gram of material. That is to say, this particular grade of diatomite has 240 tiroes the surface area per unit weight that limestone has. Since resistance to flow is directly proportional to surface friction (and, therefore, to surface area) , this diatomite is 240 times more effective at stabilizing high-asphalt-content pavement mixes than is limestone. Further, the internal structure of the diatoms increases this filler's ability to hold the asphalt once it has absorbed the asphalt. The combination of high surface area and internal structure produce a "filtration barrier" for the viscous asphalt. This barrier would require a pressure several times greater than that experienced under heavy truck traffic to overcome its resistance to flow.

In spite of diatomite's ability to stabilize high-asphalt- content paving mixes, it has never been utilized for such mixes. Standard asphalt content mixes with diatomite were indicated by laboratory tests conducted in the 1960's fcy the Asphalt Institute to be susceptible to water degradation unless precautionary procedures were used. The precautionary procedures believed necessary included premixing the diatomite with the asphalt prior to the addition of the aggregate to achieve adequate coating of the diatomite in order to limit its ability to absorb water. Such a precaution limited the amount of diatomite that could be used in the mix to less than 1% by

- weight because of the resulting stiffness and foaming. This procedure also required significantly greater mixing time. This limitation on the amount of diatomite used, limited the potential benefits of adding diatomite to the mix. Further, mixing diatomite in the pavement in the conventional manner, i.e., dry mixing the aggregate and diatomite to achieve proper dispersion prior to adding the asphalt, resulted in abrading of the diatoms fcy the aggregate which reduced them to powder making the mix 'soupy'. Further, a high percentage of the crushed diatomite became airborne and was removed by the dust collector. Due to the soupy consistency of these mixes, in at least one instance of which Applicants are aware, the entire prepared batch had to be scrapped and efforts to use diatomite^"as a filler were abandoned.

The present invention contemplates a procedure which, although contrary to proven techniques of mixing pavement materials, has been shown to be effective. This process involves mixing and heating the aggregate components (generally sand and stone) in a pugmill or the like, adding and mixing the asphalt with the aggregate and, lastly, adding the diatomite to the mixture with a rather short mixing period being necessary to achieve dispersion of the diatomite. As mentioned, this is contrary to established techniques which dictate that mineral fillers be added to the aggregate early in the mixing process to achieve proper dispersion. This was particularly true with asbestos, the only filler prior to this invention which has been shown effective at stabilizing high-asphalt-containing pavements. Significant amounts of aggregate premixing and long periods of mixing after the asphalt was added, were required with asbestos in order to obtain the desired dispersion, prevent clumping, and achieve adequate asphalt coating of the fibers. In contrast, each of the mixing steps of the present invention can be completed in one minute or less and, preferably, in h minute or less. The asphalt apparently coats the aggregate cushioning and lubricating the diatomite against the abrasive effects of the aggregate. Further, the exposure time resulting from mixing has been reduced, further protecting the diatoms.

In preparing three different sample mixes of the present invention, the aggregate which is typically 75-90% of the mix by weight and, which was in this case, 35% coarse stone (median size of hⁿ diameter) , 30% medium stone (k" diameter or less) and 35% sand (below no. 10 mesh) , was pre ixed for 15 seconds in a pugmill before the asphalt (55-100 penetration grade) was added and 15 seconds more before the diatomite was added. Because the diatomite had to be dumped from the bags by hand, the time needed for its addition to the pugmill varied from 5 to 20 seconds, depending on the amount of diatomite that was added. Following the addition of the diatcmite, the mixture was blended for 10 seconds (total mix time to obtain an average of 4055 lbs. of asphalt mix, varied from 45 to 60 seconds) . In the future, the diatomite will be packaged in plastic bags made of, for example, 'TΪVEK' plastic available from Dupont, which will melt without a trace at 290°F, below the temperature of the asphalt mix. This will permit preroeasured quantities of diatomite to be added, bag and all, in a shorter, simplified procedure. ^"

The asphalt mastic (asphalt, 200 mesh fines and, in this case, filler) in conjunction with the fine aggregate, perform the function of a mortar holding the coarse aggregate together. The tensile strength of a mastic is inversely proportional to its film thickness. The effects of increasing asphalt content in prior mixes can be seen in FIG. 2A. Two stones 10, depicted as spherical (the worst case for surface contact) are coated with asphalt mastic film 12. Increased asphalt content means increased film thickness and, hence, a reduction in the tensile strength of the mastic (a reduction in the cohesive strength of the mix) . The limestone filler merely increased the density of the mastic doing little to prevent loss of ' cohesive strength. Further, limestone is not entirely effective at preventing asphalt flow, which generally leads to bleeding of the asphalt from the pavement.

Contrast the mix of the present invention as depicted in FIG. 2B. The diatomite adsorbs the extra asphalt, thereby keeping the thickness of asphalt mastic film 12 at its optimum minimum thickness for the greatest cohesive strength. Hie "reservoir" 14 of asphalt, which is held fcy the diatomite, surrounds and protects the adhering interface from oxidation giving the pavement longer life due to a reduced rate of age hardening. FIG. 3 is a graphic representation of the anticipated benefits in pavement life expectancy, ϊhe graph indicates at least a doubling of pavement life for typical standard dense-graded pavements. -7- These data are taken from a conventional asphalt concrete pavement (life expectancy: 7-15 years) shown by curve A, and a high-asphalt content pavement containing asbestos fiber which has been in place for 18 years (life expectancy: 30-40 years) shown by curve B. It is reasonable to expect similar (or better) results from the mix of the present invention due to diatomite's ability to permit corresponding increases in asphalt content without bleeding and to its superior resistance to compaction. The measurements shown along the vertical axis were taken in accordance with a conventional technique (ASTM D-5) which measures hardness by determining the depth of penetration in tenths of a millimeter for a 100 gram weight during a 5 second time period at 25°C. The critical period for pavements occurs between a penetration of 30 when raveling starts and 20 when severe cracking begins. Resurfacing becomes necessary when the asphalt in the pavement reaches this range of hardnesses.

While FIG. 3 graphically depicts the benefits of adding diatomite to standard asphalt concrete pavements, it should be noted that similar benefits are anticipated with other dense-graded pavements such as sand asphalts (those lacking coarse stones) and stone-filled sheet asphalts. . The following chart depicts some examples of the maximum asphalt contents for each of these three pavement types with and without diatomite added.

TABLE I

EENgrG-CEDB__BBSD5 _*SHΪ__TQlflΞtøS % T3IALWEIGHT

SADCCKEEND SEISE CENIENr

% CFA33$_. % OF-__3E.

BASINS 8 IΘAINEDCN 8 __KD_MEIEr- %

C 10 M CR10 M _____££_____. M33I-TED

1. AsfhalL G_r__rete 35 to 60 65 to 40 4.5 to 6.5 7.5 to 9.0 35 to 65

2. Sb3B-f_lled 60 to 85 40 to 15 6.5 to 8.5 8 to 10 25 to 30 a_eεt Aεξhalt

3. Sard Λεξ-teL * £5 to 100 15 tD O 8.5 to 11.0 12-5 to 18.0 45-to 50 QC Sre=t Aεriϊ≡αt

* Intended only for use as impermeable, crack resistant interlayer, e.g. , as bridge deck membrane. (Its fine surface texture and impermeability make it unsuitable as a permanent wearing course.)

OMP Some states and municipalities use 8 mesh as the dividing line between sand and stone while others use 10 mesh. Hence, these values are used alternatively in TABLE I and the actual cut-off will depend on the place of application. Ωie asphalt content is varied in the manufacture of pavement mixes as a function of the gradation (sand content) of the aggregate.

Care should be taken in adding the appropriate amount of diatomite to achieve the particular result desired. In certain instances, the paver may wish to add sufficient diatomite to adsorb only the asphalt which exceeds normal^" concentrations. Ωiis will maintain the mastic film thickness at the level it would have had, had not additional asphalt been added. However, the preferred embodiment has sufficient diatomite present to theoretically adsorb all the asphalt present in the mix. Due to the recommended mixing procedure in which the aggregate is precoated with asphalt and to physical constraints of the diatoms, the diatomite will not adsorb all of the asphalt that it is theoretically capable of adsorbing. Tests have shown providing sufficient diatomite to theoretically adsorb all the asphalt, produces optimum results. Ωiis optimum amount of diatomite to be added to the pavement mix can be calculated for any particular grade of diatomite using the wet density of the diatomite (computed by using Lompoc SΩϊ 234) , in the following mariner. 'CELITE 292' has a wet density of 16.1 pcf and an apparent density of a 'solid' diatom structure of 1.8 gtη/cc or 112.3 pcf. As a convenience, we calculate the volume V^ occupied by 16.1 lbs. of 'solid' diatom structure.

V_χ = wt./density = 16.1/112.3 = 0.143 cu. ft. This same 16.1 lbs. of actual (rather than solid) diatomite when soaked will occupy a volume V₂ V₂ = wt./wet density = 16.1 lbs./16.1 pcf = 1.0 cu. ft. The volume available for liquid adsorption is

V₂ - V_j^ = 1.0 - 0.143 = 0.857 cu. ft. per 16.1 pounds of diatomite. Converting this to a more useful value of adsorptive volume V₃ per 100 lbs. yields 100 lb./16.1 lb. = V₃/0.857 cu. ft., V₃ = 5.323 cu. ft.

100 pounds of the asphalt used in this mix will occupy a volume of

V₄ = wt/density = wt/SpG X density of water = 100/1.04 X 62.4 = 1.54 ft? Accordingly, for this diatomite and this asphalt, the amount of diatomite needed to theoretically adsorb all the asphalt is

V. ₃ = 1.54/5.323 = 28.9% wt. of asphalt.

That is, the weight of diatomite needed to theoretically adsorb all of the asphalt in the mix is 28.9% of the weight of that asphalt. Since

'CELITE 292' is the least dense and most adsorptive grade of diatomite that is ∞irmercially available, the amount needed for other grades will generally be greater, perhaps as high as 33%. Note, if the specific gravity of the asphalt used were higher, the volume a given weight would occupy and, hence, the .percentage of diatomite needed to adsorb it, would be less, perhaps as low as 26%. While this method has been termed a means of calculating the optimum diatomite content for a particular mix, it could obviously be used to calculate the amount of diatomite necessary to adsorb only the excess asphalt. Note also, that the Optimum* diatomite content is also the threshold value for creating additional voids as discussed hereafter.

Two characteristics of a pavement surface influence skid- resistance: macroscopic surface texture - the coarseness of the surface texture; and microscopic structure - the quantity of voids in the ^' pavement. (Of course, such factors as tire tread wear and inflation pressure are important factors influencing skidding on a given pavement, but emphasis is given to pavement structure here.) Choice of an aggregate which resists wear and/or which resists becoming polished by traffic, can be beneficial in preserving the macroscopic surface texture of the pavement. However, the open spaces on the pavement surface between stones provided by a coarse aggregate can accommodate only so much water drainage. It is actually the microscopic structure (the void content) of the pavement surface which is the most significant factor in determining the skid-resistance of pavement.

In standard wearing course pavements, the percent of voids is primarily a function of the amount of asphalt content which,, in turn, partially determines hew much compaction a pavement undergoes during placement. The quantity of voids will continue to decrease during the life of the pavement due to co paction from traffic-loading. Tests, in which compaction of pavements is mechanically accelerated, have shown that the diatomite-modified pavement of the present invention resists

SUB OMPI over-compaction, thereby maintaining the void content at or near its original level at the time of placement. These voids provide a means of escape for water trapped between a tire and pavement significantly reducing the hazards of skidding and hydroplaning. Apparently, the diatoms and sand interact to form an open-lattice structure in the interstices between the larger aggregate which is pliably held in place by the asphalt. This lattice structure, oddly enough, does not compact under repeated loading but, rather, maintains the void-content level of this pavement. In addition, the diatomite provides wear-resistance and strength to the mastic portion of the mix thereby reducing the rate at which the mastic erodes from the pavement. This also has the effect of permitting the pavement surface to wear at a more even rate, reducing the exposure of stones that may be susceptible to polishing. Polished stone can make even a pavement with good drainage slippery. Of course, the addition of diatomite to paving materials which contain no large aggregate, e.g., sand asphalt, cannot "maintain" voids in these intentionally-designed, impermeable pavements.

While quantities of diatomite of up to 26-33% of the asphalt have been shown to have significant beneficial results, amounts significantly exceeding those percentages can have detrimental effects on asphalt hardening and water susceptibility. Small amounts above the optimum or threshold value can increase the void content above normal levels thereby increasing drainage and skid-resistance. This would be beneficial in geographic areas where significant rainfall increases skidding hazards. This increase in voids may occur without substantial detrimental effect on the mix. (The cohesive strength of the mix will probably be less than for optimum diatomite levels but still greater than standard pavements.) Caution should be used, however, because increasing the amount of diatomite in the mix above the optimum calculated level fcy even 10% of the weiφt of asphalt can (a) cause the mix to be dry and grainy; and, (b) increase the water susceptibility of the mix. Both of these problems can be attributed to the presence of excess absorptive capacity of the diatomite. The excess diatomite will have insufficient asphalt to fully coat it leaving a dry, grainy mix. With excess absorptive capacity, the diatomite will, absorb asphalt needed to coat the sand or, absorb any water it may come in contact with, reducing the cohesive strength of the mix. In the former case. -li¬ the affinity for water displayed by the sand will produce a similar loss of strength.

FIG. 4 graphically depicts the effects of water on standard pavement (6.0% asphalt) and the three sample mixes previously mentioned. Mix I had 6.6% asphalt (10% above normal) and .6% diatomite. Mix II had 7.4% asphalt and 1.8% diatomite. The diatomite in mix II is 24.3% of the asphalt by weight, slightly below the calculated optimum of 28.9%. Mix III contained 7.3% asphalt and 2.4% diatomite by weight, or diatomite-content representing 32.9% of the asphalt (exceeding optimum by 4% of asphalt weight) . Flexural strength of each pavement was tested initially and then following 24 hours of vacuum saturation in water. Die addition of diatomite to mix I prevented the added asphalt from decreasing cohesive strength. Cohesive strength of mix I would probably have been higher, had not the aggregate used been highly porous. Ihe porous aggregate adsorbed significant amounts of asphalt, adversely affecting the cohesive strength of this mix. The 1.8% diatomite mix (mix II) showed 78% retention of strength and a flexural strength exceeding 100 pounds. The 2.4% diatomite mix (mix III) showed only a 40% strength retention as compared with 49% for the standard mix. However, it should be noted that the final strength is more significant than the strength retention of the mix and the 2.4% diatomite mix was still three times as strong as the standard mix following saturation. Ωiese data do tend to indicate that excessive diatomite (more than 10% above the optimum percentage of asphalt weiφt) , could negate entirely the cohesive strength benefits of adding diatomite.

It is interesting to note that the slope of the standard asphalt and the mix containing 1.8% diatomite are generally the same. This indicates that the addition of diatomite (which permits increase of asphalt content) , optimizes the cohesive strength of the mastic. Tests indicate that increasing asphalt content in mixes with limestone filler decreases cohesive strength. This data supports the earlier discussion relative to FIGS. 2A and 2B and the influence of adding diatomite. Therefore, even if water degradation were to continue at the same rate in both the standard and optimum diatomite mixes to a point where no cohesion remained in the standard mix, cohesive strength

OMPI of the diatomite-modified mix would still exceed the original strength of the standard mix.

This capability to optimize the cohesive strength of the mastic, creates several interesting possibilities for diatcπiite- modified mixes. Ωie signficance of aggregate interlock (i.e., the affinity of the aggregate for bonding by asphalt) to the strength of the mix would be reduced. The cohesive strength provided fcy the addition of diatomite would more than compensate for any interlock the aggregate could bring (or fail to bring) to the mix. Accordingly, lower quality, less bondable (or water susceptible) aggregate could be used at a significant cost savings. This optimizing of mastic strength will also overcome such factors as incomplete coating of the aggregate (too little mixing), and incomplete drying of the aggregate.

The three sample mixes and the standard mix tested for ETG.4 whose compositions were discussed earlier, were prepared in truckloads on the order of 17 tons each (eiφt 2+ ton batches). These mixes were placed on a highly-trafficked street in downtown Houston. After 11 months of traffic and weather exposure, three core samples of each mix were taken. Photographs of these samples are shown in FIGS.5A-D. The mortar of the standard mix (FIG. 5 ) , has eroded significantly and the tops of the exposed coarse stones in the aggregrate have already become beveled and polished. Mix I, containing only 0.6% diatomite (FIG. 5B) , shows the best abrasion-resistance of all samples. Ωie abrasionr-resistance of mixes II and III (FIGS.5C and 5D) are sliφtly less than mix I but, still significantly better than the standard mix. The fact that mix III is still in good shape after almost one year exposure to the heavy Houston rainfalls, indicates that the additional diatomite has not made this pavement water susceptible.

. While the test data were primarily obtained from dense-graded hot mixes, the results indicate potential benefits applicable in other pavements, as well. For example, diatomite is potentially useful for addition to cold mixes and to the newly developed open-graded friction courses (OFCs) designed for hiφ-speed hiφway uses. The OPC pavement is a thin overlay (generally % to 3/4") which is intentionally quite porous so that the voids provide channels or water to penetrate the wearing course. Beneath the OPC pavement is a water-impermeable, usually sloped or crowned layer. Water drains through the OFC layer to the sloped impermeable layer and drains laterally to the edge of the roadway. Problems with this new skid-resistant surface include (1) bleeding of asphalt, (2) compaction under traffic reducing permeability, (3) shortness of useful life. Die nature of diatomite indicates its addition will readily cope with (1) and (2) and since diatomite enables the addition of more asphalt to an amount of 5% or more by weiφt, item (3) can also be solved.

Althouφ the basic advantages of diatomite, its ability to stabilize the hiφ-asphalt content of pavements, increase cohesive strength, decrease mastic erosion and preserve skid resistance, have been generally discussed together, it will be understood that diatomite can be used to achieve some of these benefits at the expense of the others. One such example, where the diatomite content exceeds optimum in order to increase void content at minor expense to cohesive strength, has already been given. FIGS. 4 and 5B indicate diatomite can decrease mastic erosion with little or no increase in cohesive strength. Adding diatomite to stabilize the asphalt and increase cohesive strength in mixes where asphalt is added to increase density, is exemplary of employing diatomite to obtain these advantages without influencing skid-resistance. Such usage would primarily occur in creating impermeable layers beneath the actual wearing courses, by varying the usual aggregate composition, in addition to increasing asphalt content.

Various changes, modifications, and alternatives will become apparent following a reading of the foregoing specification. For example, certain naturally occurring deposits of diato aceous earth, contain bitumen and other hydocarbons. Efforts are underway to remove some portion of the hydrocarbons for use as fuel without destroying the diatomite. Obviously such diatomaceous earth, either with or without hydrocarbon removal, could be used in formulating the pavement mix of the present invention and the asphalt that is added separately, decreased by an amount which corresponds to the level of hydrocarbons already present in the diatomite. Accordingly, it is intended that all such changes, modifications, and alternatives as come within the scope of the appended claims, be considered part of the present invention.

Claims

WE CLAIM:

1. A paving material comprising asphalt in an amount which is 10-65% above normal concentrations in paving materials; an aggregate which constitutes at least 75% of the mixture fcy weiφt; and a filler material composed of diatomaceous earth, the concentration of said filler being sufficient to substantially adsorb all of the asphalt above normal concentrations.

2. Die paving material of Claim 1 wherein the concentration of said filler material is sufficient to adsorb substantially all of said asphalt.

3. The paving material of Claim 1 wherein the concentration of diatomaceous earth is no greater than the amount necessary to adsorb all the asphalt.

4. The paving material of Claim 1 wherein the asphalt concentration is between 5.0 and 18.0% of the paving material fcy weiφt.

5. The paving material of any of Claims 1 through 4 wherein the diatomaceous earth present in the material is between 26 and 33% fcy weiφt of the amount of asphalt present. 6. The paving material of Claim 1 wherein the aggregate has a plurality of degrees of coarseness.

7. The method of preparing a paving material comprising the steps of mixing components of an aggregate, adding asphalt to the aggregate and mixing it therewith to form a coating thereon, adding a filler in the form of diatomaceous earth to the coated aggregate and mixing to achieve substantially uniform dispersion of all constituents added throughout the paving material.

8. The method of Claim 7 wherein each of the mixing steps is performed in one minute or less. 9. The method of Claim 7 wherein each of the mixing steps is

- performed in h minute or less.