NO843623L - DIATOMITE MODIFIED ROADS - Google Patents

Description

Background of the invention and summary of the invention

The present invention relates to hot-mix asphalt mixtures for road surfaces. The invention relates more particularly to a diatomite-modified hot asphalt pavement mixture which has an asphalt content within the normal range, but which has improved aggregate interlocking and superior compactibility.

In a road surface mix, the asphalt plus the fines (material that passes through a 200 mesh sieve) make up the mastic. The asphalt mastic functions in connection

etc. with the finely divided aggregate (such as passes through a sieve no. 8 or sieve no. 10) as what can be described as the mortar and holds the coarse aggregate together. The adhesion/cohesion strength of the mastic is inversely proportional to its thickness, i.e. the thinner the mastic coating on the aggregate, the stronger the bond and the more stable the road surface. However, thin film coatings oxidize more quickly, so that the asphalt loses its flexibility and crumbles. The resistance to traffic load decreases drastically and causes unraveling and cracking. Similarly, thin coatings are more susceptible to water penetration. For these reasons, a number of road surface manufacturers increase the asphalt content in an attempt to extend the life of the road surface. In reality, the best road surface construction conditions require that the asphalt content is as high as possible without this leading to over-compression of the road surface due to traffic. Mixtures with a high asphalt content have thicker films, fewer voids and lower permeability and are therefore less susceptible to oxidation and degradation due to water. However, simply increasing the asphalt content above normal will adversely affect the stability of the road surface for most road surfaces. The increase in the thickness of the mastic coating on the aggregate particles leads to over-compression and allows bleeding, rutting and sliding. Moreover, asphalt is generally the most expensive road surface component, so that it would be an expensive precaution to increase the content of this in the mixture.

There are basically two forces that serve to

holding the road surface mixture together: adhesion/cohesion strength and aggregate interlocking (frictional forces between aggregate

the particles that resist displacement).

The road surface's cohesive strength (i.e. the mastic's cohesive strength) is mainly dependent on the quality of the asphalt, which will affect such factors as the interfacial tension between asphalt and aggregate and the asphalt's degree of moisture and adhesion to the aggregate. (It should be noted that the properties of the aggregate and the fines also exert some influence on these factors which affect the cohesive strength). The second force, the aggregate interlocking, is a function of the size grading of the aggregate in the mixture, the shape (angularity) and surface structure of the aggregate and the density of the aggregate packing. It should be noted that as used herein, the term "aggregate interlocking" includes the frictional forces of all particles in the mixture regardless of size (ie, including fines). When asphalt pavement materials are exposed to temperatures above 32°C, the asphalt begins to soften (ie lose cohesive strength), and aggregate interlocking plays the main role in maintaining road surface stability.

Mineral fillers are sometimes used in a trial

on increasing the viscosity and density of pavements in the hope of increasing aggregate interlocking and of volumetrically diluting the asphalt in pavements with normal asphalt content in order to

increase compressibility. Finally, ground rock dust, such as limestone dust, will volumetrically dilute the asphalt. The volumetric dilution does not have the desired effect in terms of reducing the void content in the road surface. However, rather than increasing aggregate interlocking, the inclusion of such a filler may actually reduce aggregate interlocking. This is due to the fact that the volumetric dilution of the asphalt is permanent and serves to increase the thickness of the mastic film. A thicker mastic film increases the separation between the aggregate particles and affects particle friction (ie aggregate interlocking). Depending on the filler and its diluting ability, the mastic's increased thickness can actually overshadow any advantages of increased density. In addition, a permanent volumetric dilution of the asphalt leads to an unstable road surface (rutting and

pushing in the road surface). The use of such a mineral filler in an attempt to increase the density and compressibility in order to thereby extend the service life of the road surface by reducing voids and thus oxidation rate and sensitivity over-

for water will therefore generally prove ineffective and may well have the opposite effect.

A variety of fillers (including rock dust) can

increase the viscosity as much as three times and make laying difficult or impossible. Even minor increases in viscosity can prevent correct compaction with the result that the laid road surface has an excessively high void content, which accelerates oxidation hardening and water permeability. In reality, care must be taken in determining the correct amount of diatomite to be added in order to

avoid too strong an increase in viscosity. In a similar way, adding the filler, if this is highly absorbent, will not dilute the asphalt and facilitate compaction. The filler will rather adsorb (absorb and hold)

the asphalt and make correct compaction impossible.

It has been shown that diatomite, when added in small amounts (0.5-2%), can function as a particularly suitable temporary asphalt extension agent in the hot mix. Its h<ave ov<p>r surface area per unit of weight and the diatoms' open microstructure inhibit asphalt flow and prevent bleeding of asphalt. This same open microstructure together with the other components in the mixture prevents too strong compression by forming a lattice-like structure. This lattice structure serves to maintain a correct dispersion of the fine material in the mastic and prevent it from separating. It is surprising that diatomite's high surface area and open microstructure do not also increase viscosity too much, which would have made laying and compaction difficult. It is even more surprising that the highly absorbent diatomite does not immediately absorb the asphalt, which would have robbed the road surface of its high asphalt content properties. The pressure of the outgassed water vapor contained in the diatoms, and/or the extremely small tertiary openings (0.03yum) in the diatoms apparently inhibit asphalt adsorption until after the road surface has cooled below 93°C, i.e. until after the road surface has been laid . The diatomite then begins to adsorb the asphalt.

In addition, diatomite's distinctive structure makes it particularly suitable for reducing the void content and increasing the density. The different shapes and sizes of the diatoms and diatom fragments, as well as the ability of the diatom fragments to break down further, enable the diatomite particles to fill almost all "interstices" in a road surface aggregate. Diatomite modification of road surface therefore provides a material that exhibits increased density and viscosity while achieving compressibility and increased life of a high asphalt pavement and also improved shear strength resulting from improved aggregate interlocking, and better erosion resistance of the mastic, without any increase in bleeding, rutting or sliding characteristic of high asphalt and other diluted asphalt mixtures. the adsorption that takes place after the road surface has been laid actually significantly reduces the chances of bleeding, rutting and sliding. Diatomite's ability to both dilute and absorb asphalt makes it a distinctive additive that "covers up" defects in b the landing recipe. Moreover, the increase in the shear strength of the mixture and the abrasion resistance of the mastic suggests that the thickness of the overlay can be reduced without reducing the wear life, with a corresponding reduction in laying costs.

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

Brief description of the drawings

Figs. 1A-1E are photographs of core samples taken from

five different road surface mixtures after eleven months of wear in Houston and show respectively: Fig. IA are samples of a standard (5.5% asphalt) road surface that does not contain diatomite,

Fig. IB are samples of a mixture II containing

7.4% asphalt and 1.8% diatomite,

Fig. 1C are samples of a mixture I containing

6.6% asphalt and 0.6% diatomite,

Fig. ID are samples of a mixture III containing 7.3% asphalt and 2.4% diatomite,

Fig. 1E are samples of a mixture IV containing

6.0% asphalt (normal concentration) and 0.6% diatomite,

Figs. 2A-2D are photographs of core samples taken from four different pavement mixes laid in Calgary, Alberta, the photographs showing respectively: Fig. 2A is a sample of a pavement mix containing 7.8% asphalt and 2% asbestos fibers, Fig. 2B is a sample of a road surface mixture containing 7.8% asphalt and 2% diatomite, Fig. 2C is a sample of an asphalt mixture containing 6.5% asphalt and 1% asbestos fibers, Fig. 2D is a sample of a road surface mixture containing 6.5% asphalt and 1% diatomite, Fig. 3 are curves and projections for two-way pavements, including a standard pavement containing 6% asphalt (curve "A") and an asbestos-modified pavement containing 7.5% asphalt and 2% asbestos fibers (curve "B") , from pavement mixes used to build the Trans-Canada Highway.

For comparison, four data points from samples taken from four pavement mixes laid in June (mix "C") and October (mix "B") of 1981 in Alberta, Canada, with cores taken in October 1982, are superimposed on these curves. The respective mixtures for these data points are indicated in the "key",

and

Fig. 4 is a diagram showing how stability meter readings vary with asphalt content in common pavement mixes and shows the benefits of adding small amounts of diatomite to the standard Houston plant mix shown in Fig. 1A.

Detailed description of the preferred embodiments

As described more fully in US Patent Application 06/343075, filed January 27, 1982, three different test mixtures of diatomite modified pavements were prepared using a natural (or uncalcined) grade of diatomite sold by MANVILLE CORPORATION under the trademark "Celite 292", and various increased asphalt contents. These test mixes of dense quality pavement were

placed on a heavily trafficked downtown Houston street along with a regular mix that did not contain diatomite. Although the main purpose of these experimental layings was to determine whether diatomite, in suitable quantities, could stabilize mixtures with a high asphalt content, a fourth mixture was also laid. This mixture contained no addition of asphalt (ie a standard 6% by weight) and 0.6% diatomite. After eleven months of downtown car and truck traffic and under Houston weather conditions (intense heat and heavy rainfall, both of which have potentially adverse effects on the road surface), three core samples were taken from the wheel treads for each of the five compounds. These core samples are shown in Figs. 1A-1E.

Figs 1A-1D relate to the use of large amounts of diatomite to stabilize increased asphalt content. These figures are presented here as a means of comparison with Fig. 1E which shows the core sample of mix IV containing a normal amount of asphalt (6%) and 0.5% diatomite. It was this trial run, originally intended as another blank (along with the standard mix) for Mixes I, II and III, that led to the discovery of the benefits of small amounts of diatomite as a filler/stretcher.

As a result of this initial trial and subsequent plant and laboratory sampling, it has been determined that additions of small amounts (0.5-2% by weight) of diatomite to pavement mixes containing normal concentrations (5-12% by weight, depending on the mix) of asphalt serves to temporarily dilute the asphalt while increasing the mixture's density and viscosity and enables improved compressibility during laying, so that the void content in the road surface can be reduced, thereby reducing the road surface's susceptibility to oxidation hardening and degradation due to water. This diluting ability is possible despite diatomite's natural tendency to adsorb (absorb and hold onto) the asphalt. The moisture in the diatoms (typically 3-6% by weight) is presumably evaporated by the addition of the diatomite to the asphalt-coated hot aggregate mixture (generally heated to approx. 177°C). This water vapor is slowly outgassed over a period of hours by the diatoms (due to Van der Waals'ke forces) until the mixture cools below 93°C (ie until after initial compression), and this outgassing delays adsorption of the asphalt . The length of time during which absorption is delayed can of course be varied by increasing or decreasing the moisture content of the diatomite. In addition to its ability as a tensioning agent, the diatomite improves aggregate interlocking in the road surface, and this manifests itself in increased strength during shear tests.

Although diatomite is intended to be added to asphalt mixtures with normal asphalt quantities to act as an extension agent (ie, instead of adding additional asphalt, it will be understood that what may be considered a "normal" amount of asphalt by some paving authorities may in reality be an insufficient amount to correctly coat the aggregate. It is therefore within the scope of the invention to use diatomite together with mixtures in which the asphalt is increased by 0.5-1% to counteract such a deficiency. It will also be apparent to the person skilled in the art that because diatomite temporarily dilutes the asphalt, it may actually be possible to reduce the amount of asphalt to below "normal" concentrations. Although this is a real possibility, it is highly recommended

that the diatomite used in such mixtures is pre-treated with a surface-active silicon or petroleum material or by another suitable treatment which is intended to make the diatomite hydrophobic. This will remove the risk of making the mixture susceptible to degradation due to water as a result of the reduced asphalt content (ie unused absorption capacity of the diatomite).

A comparison between Fig. 1B-1E and the standard asphalt mixture according to Fig. IA shows the effectiveness of the diatomite in terms of improving the abrasion resistance of the asphalt mortar. The core samples according to Fig. IA show a typical mortar abrasion rate of approx. 0.64 cm per year. Further abrasion of the mortar in the standard mix will allow the surface aggregate to begin to unravel. All four of the diatomite mixtures, on the other hand, show mortar with superior abrasion resistance. In addition, the mortar is effectively a wear-resistant surface that shares the load with the aggregate. The wear advantages of diatomite-modified road surfaces are therefore twofold: 1) the abrasion-resistant mortar stays in place and holds the aggregate firmly against unraveling, and 2) the mortar is able to receive wear which distributes the wear load more evenly over the road surface and makes the road surface -erosion slower. These results suggest that a thinner than normal overlay of diatomite-modified road surface can be used without adverse effects on the wear life. However, it should be noted that a dilution of the overlay

will reinforce the need for a proper foundation (ie that care must be taken when repairing existing roads if the overlay is applied directly over old asphalt).

Table I shows the core analysis for the five samples shown

on Fig. 1.

As shown in table I, the mixture IV shows, in the same way as the other diatomite-modified road surface mixtures, two advantages of the diatomite addition: 1) a significant reduction in void content and 2) a reduced asphalt hardening rate. These advantages are interrelated with each other, as the first contributes significantly to the second. The reduction in void percentage reduces the internal surface area exposed to oxidation hardening and degradation due to water. The diatomite-modified samples were in fact practically impermeable to water (when tested using a 30.5 cm hydraulic pressure). These factors clearly contributed to the reduced asphalt hardness levels evident in the penetration data shown in Table I. Figs. 2A-2D are photographs of cores from test road pavements laid in Calgary, Alberta, Canada. The purpose of these layings was to compare the diatomite-modified road surfaces with road surfaces reinforced with asbestos fibers that have been widely used in Canada for the past 20 years. Fig. 2A is a core sample of a road surface mixture containing 7.8% asphalt and 2% asbestos fibers. The same mixture

where the fibers are replaced with 2% diatomite is shown in Fig. 2B.

Fig. 2C is a core sample of a road surface mixture containing 6.5% asphalt and 1% asbestos fibers. Fig. 2D is the same mixture where the fibers have been replaced with 1% diatomite. Visual inspection of the cores shows that the mastic in the diatomite-modified samples has superior abrasion resistance compared to the mastic in the asbestos fiber cores. Table II shows the core analysis for the four samples shown in Fig. 2.

The data in Table II suggest that although both the asbestos and diatomite samples generally have a correspondingly high density and low void percentage, the diatomite cores are significantly less permeable to air (and therefore less prone to oxidation). Similarly, the diatomite samples should be less permeable to water because there is a certain tendency for the asbestos fibers to cause a softening effect which is not present in diatomite due to the diatoms' more varied non-linear structure.

The data in Table II suggest that the diatomite samples should oxidize (age harden) more slowly than their asbestos counterparts. To substantiate this, asphalt penetration tests were carried out with core samples taken 12 to 16 months after the original laying. The results of these tests are reproduced in Fig. 3 and have been plotted with two expected lifespan curves A and B for road surfaces used in the construction of the Trans-Canada highway in 1967. Curve A represents an expected lifespan projection for a road surface containing 7 .5% asphalt and 2% asbestos fibres. The points on each curve represent real asphalt penetration measurements made from cores taken immediately after laying,

at the age of 2 years and at the age of 6 years. The dashed line at a reading of 30 suggests the generally accepted /. critical minimum value below which the road surface loses sufficient compliance and the road surface begins to crack and unravel.

It appears from Fig. 3 that the two diatomite samples actually outperform their asbestos counterparts and can be expected to last as long or longer than the asbestos-modified road surface according to curve B. This would give these diatomite road surfaces an expected life of from 37 to 45 years. Although such factors as weather and heavy road traffic can make it difficult to achieve such a long lifespan, it can be expected that the diatomite-modified road surface will last up to 60% longer than standard asphalt mixes.

At the time the Calgary trial samples of mixture "C"

was laid, small amounts of both the asbestos-modified and diatomite-modified mixtures were taken from the road surface laying machine. These samples were compressed in the laboratory using rotary shear and subjected to a series of tests for comparison purposes. The results of these tests are reproduced in Table III.

The test results given in Table III show

that the diatomite samples outperform the asbestos samples for almost every test category. These data therefore support the visual abrasion resistance and asphalt penetration test results by confirming that diatomite modified pavement mixes can be expected to last as long as or longer than asbestos modified mixes.

In an attempt to confirm diatomite's behavior with different asphalt grades and aggregate types and grades, additional tests were conducted using two different samples obtained from a Los Angeles road surface paver. The two mixtures had a maximum stone size of 0.95 cm and 1.27 cm, and in each mixture a very hard asphalt grade was used. These samples were returned to the laboratory and split, and various amounts of diatomite and additional asphalt were added to create the desired sample mixes. These mixtures were then compacted using a Marshall compactor before the various analytical tests were carried out. The results of these tests appear in table IV.

The test results for the first sample suggest a significant increase in shear strength with the addition of 0.5% diatomite to the 5.2% and 5.7% asphalt mixes. For the first sample, in reality, 0.5% diatomite appeared to be better than 1.0%. In contrast, 1% diatomite for the second sample has a significantly stronger effect on the shear strength than 0.5%.

This apparent contradiction is believed to be the result

of differences in aggregate interlocking (it should be noted that as the shear tests were performed with samples at 50°C, i.e. above the asphalt's softening temperature, the shear strength measurements mainly reflect the degree of aggregate interlocking).

As previously indicated, the aggregate interlocking is affected by such factors as the size grading of the aggregate, the aggregate's shape (or angularity) and surface structure (degree of grinding) and the density of the aggregate packing. The aggregate interlocking or frictional resistance to displacement is a result of the synergistic mutual effect of all aggregate particles. If there is too much fine matter in a mixture, the spaces between the larger stones will not be filled, and this leads to low aggregate packing and a lower degree of aggregate interlocking in the aggregate.

Diatomite can contribute to aggregate interlocking

due to its angularity and ability to increase the aggregate packing's density. Diatomite is actually remarkably capable of increasing aggregate packing due to its many different particle sizes and shapes and the diatom fragments' ability to fracture and "fill" the spaces between aggregates.

particles. These fractured fragments can be tailored

themselves so that they are adapted to the size and shape of the opening. Diatomite forms a lattice-like structure with the other fines in the mastic and prevents these from separating out.

On the other hand, if the aggregate gradation in the mixture is insufficient in terms of small amounts of fines, additional amounts of diatomite will be necessary to fill the resulting voids. The second sample according to Table IV had a fine matter deficiency which 0.5% diatomite could not completely fill. For this reason, the mixture with 1.0% diatomite had better density and shear strength values, as the additional amount of diatomite replaced the lack of fines. For the first sample, 0.5% diatomite fills the voids sufficiently. The additional diatomite added by the addition of 1% diatomite has no particular positive or negative effect in the mixture with 6.2% asphalt for the first sample, but it appears beneficial in the mixture with 6.7% asphalt in that it overcomes the unfavorable effects of added asphalt.

To substantiate these results, tests with a stability measuring apparatus were carried out with several samples of the mixture from the Houston plant modified with 0.5% diatomite.

(This test with a stability measuring device is called the Hveem stability measuring test after its inventor, and it is considered the best method for determining the degree of aggregate interlocking and has won great confidence in the western states of the United States). The results of these tests are shown by means of a curve in Fig. 4 for a standard asphalt mixture. It appears from the standard curve that the addition of asphalt reduces particle friction (stability measurement value), and this is a result of increased asphalt coating thickness on the aggregate particles. The addition of 0.5% diatomite to the mixture containing 6, 6.5 and 7% asphalt led to better stability measurement values than for the standard mixture. This advantage is particularly significant for the 6.5 and 7% mixtures where the addition of diatomite prevented the stability measurement value from falling below the accepted critical minimum value of 40. It is clear that the increase in aggregate interlocking obtained due to the diatomite and diatomite's

adsorption capacity, counteracts the adverse effects from the addition of asphalt.

As stated above, the diatomite transiently dilutes the asphalt volumetrically. The calculations below show the relative effects of adding diatomite as a stretching agent, in contrast to a mineral filler, such as e.g. limestone. The limestone filler has a specific gravity of 2.5 g/cm 3. If it had been desirable to add 6% filler to one t©n of a mixture with 6% asphalt,

60 kg of limestone would have to be added. A typical specific gravity for asphalt is 1.04 kg/dm 3. The 6% (60 kg) asphalt will occupy a volume of 60/1.04 = 57.7 dm<3>. The 60 kg of limestone filler will occupy a volume of 60/2.5 = 24.0 dm 3. The addition of 6% limestone dust to 1 ton of asphalt mixture will dilute the asphalt volumetrically by 24/57.7 x 100 =?41.6%.

Diatomite (especially the above-mentioned "Celite 292") has

a wet density of 0.258 kg/dm 3 and an apparent bulk density of 1.8 kg/dm 3. The same asphalt dilution of 24 dm3 (or 41.6%) can be achieved with 24x(0.258) = 6.2 kg or 0, 62% diatomite. The real significant difference between the addition of diatomite and limestone dust as a filler is not due to the almost tenfold difference in weight (6.2 kg compared to 60 kg) required to achieve the same volumetric dilution. The most important difference is due to the net final effect in terms of the stability of the mixtures.

As stated above, diatomite has a wet density of 0.258. kg/dm 3 and an apparently massive density of 1.8 kg/dm 3. A calculation of both the volume taken up by 7.3 kg of solid (V^) and of wet (V^) diatomite gives

V = 7.3/1.8 = 4.06 dm<3>

and

V2= 7.3/0.258 = 28.3 dm<3.>

The volume percentage of 28.3 dm 3 diatomite available for absorption is equal

In real practice, not all of the available absorption capacity is utilized. This may be due to retained water vapor or that the tertiary openings in the diatoms are too small for them to be penetrated by the asphalt. In any case, 95% of the absorption capacity appears to be the maximum achievable asphalt absorption. This means that 85.7 x 0.95 or 81.4% of a diatomite sample's volume is made up of open space that can be occupied by asphalt. In reality, diatomite will absorb 0.814:(0.258/1.04) or 3.29 times its own weight of asphalt.

As soon as the outgassing of the water vapor contained in the diatomite ceases (ie as soon as the mixture has cooled below 93°C), only 100-81.4 or 18.6% of the volumetric dilution of 24 dm 3 will remain. This means that the real final volumetric dilution of the asphalt due to the diatomite will amount to exactly 0.186 x 24/57.7 x 100 = 7.7%. As limestone dust has little if any absorption capacity, the originally caused 41.6% volumetric dilution will remain. The mastic coating diluted with limestone on the aggregate particles will be correspondingly thicker and have less adhesion/cohesion strength. In addition, the thicker mastic will adversely affect the friction between the particles and reduce aggregate interlocking.

The permanent volumetric dilution of 41.6% of asphalt achieved with limestone can be expected to affect stability in a similar way as an addition of 0.416 x 6 = 2.5% asphalt. It appears from Fig. 4 that the addition of

an amount equivalent to 2.5% asphalt to even a 5% asphalt mix will have a drastic effect on the stability meter reading (possibly dropping the reading by 25 points or more and causing bleeding, rutting and sliding). On the other hand, 0.62% diatomite would at worst be equivalent to adding 0.077 x 6 = 0.5% asphalt and cause no more than a 4 point drop in the stability meter reading. However, it has been shown that the addition of 0.5% diatomite actually counteracts the adverse effects of such a small permanent volumetric dilution by increasing the aggregate

The addition of small amounts of diatomite is therefore not expected to have any adverse effect on stability.

In the past, road surface installers in the USA have advocated its use

of harder asphalt grades as a means to prevent instability and to increase adhesion/cohesion strength. These asphalts have a long initial penetration reading with a correspondingly shorter service life (ie a shorter time to reach the critical minimum penetration in the asphalt). Experience with diatomite-modified road surfaces in Calgary, where softer asphalts are used, suggests that using diatomite together with the softer asphalts can provide a stable road surface with greatly increased wear life. Diatomite optimizes the adhesion/cohesion forces in a mixture and stabilizes the thinner asphalt as a result of the combination of improved aggregate interlocking and asphalt adsorption.

Normal overlays on road surfaces have a thickness of 3.8-

5.1 cm„ As previously indicated, the wear resistance that the diatomite gives the mastic and the improved shear strength that it gives make it possible to reduce the thickness of the overlay. It is recommended that the thickness of the overlay is two to three times the diameter of the largest stone size in the aggregate. For an asphalt concrete with a maximum stone size of 0.95 cm, the overlay thickness should vary from 1.9 cm to 2.86 cm, and for a 1.27 cm stone from 2.54 to 3.8 cm. For stone-filled sheet asphalt with a maximum stone size of 0.64 cm, the overlay thickness should vary from 1.27 to 1.91 cm. This thickness reduction can contribute to significant reductions in the amount of material used and in the costs per added square meters.

It has been shown that the addition of small amounts of diatomite (0.5-2.0%) can act as a temporary volumetric stretcher for the asphalt in road surface mixtures. The exact amount to be added to a particular mixture to optimize the road surface's properties will depend on such factors as size grading for the aggregate and the asphalt quality to be used. However, it will be understood that the addition of diatomite will make a mixture more tolerant. For example, the incorporation of diatomite can "remedy" the addition of too little asphalt by diluting it or too much by absorbing it. Diatomite can also "remedy" the incorporation of too little finely divided aggregate, as discussed above. The use of diatomite as a filler/stretching agent expands an otherwise narrow percentage range for the various components in the mixture. It is important that the diatomite is added to the mixture after the aggregate has been coated with asphalt, as in the previous application, so that the brittle diatoms will not be exposed to abrasion from the aggregate. Preferably, the different aggregate components will be mixed

in a kneading mill or the like while forming a dry mixture. The mix will then be heated to a temperature above 149°C, and the asphalt will be added to and mixed with the dry mix to coat the aggregate. Finally, the diatomite will be added to the mixture and mixed for a short time (preferably for no more than 30 seconds). The road surface should be laid and compacted before the temperature drops below 93°C, so that the diatomite will not adsorb significant amounts of asphalt before the initial compaction has been completed.

Various changes, modifications and alternatives will become apparent after reading the above description. For example, if the diatomite is used in a mixture that includes recycled road surface, it could act as a carrier for an asphalt renewing agent that is typically used in such mixtures. Secondly, the moisture content of the diatomite can of course be varied to vary the time during which the outgassing takes place, as suggested. Moreover, although only one grade of diatomite has been used for these tests, its use is to be considered as an example, as the use of other grades gives similar results. Finally, although the advantages of diatomite make it particularly suitable for thin overlays, it is not intended that the use of diatomite should be limited to this. Diatomite can be of some benefit for all hot asphalt road surface mixtures, including standard overlays, binder courses etc. It is jfleir-

'for- . meant that all such changes, modifications and alternatives that fall within the scope of the appended patent claims can be considered part of the present invention.

Claims (7)

1. Method for increasing aggregate interlocking and compressibility for a normal hot mix of asphalt road surface material containing between 5 and 12% by weight of asphalt without the need to significantly increase the percentage of asphalt in the mixture, characterized in that 0.5-2-0% by weight of diatomite is mixed into it ordinary asphalt road surface material, since the diatomite increases the density and viscosity of the asphalt mixture and temporarily dilutes the asphalt volumetrically in the mixture, so that the road surface material exhibits increased compressibility until the temperature of the road surface material drops below 9 3°C without increasing the probability of surface bleeding, and since the diatomite adsorbs a significant amount of asphalt after the road surface material has been cooled to below 93°C. 2. Hot mix of asphalt road surface mixture, characterized in that it comprises between 5 and 12% by weight of asphalt, 0.5-2.0% by weight of diatomite, as the diatomite increases the road surface mixture's density, viscosity and aggregate interlocking, where the rest of the mixture consists of aggregate and fillers with different gradations, and since the diatomite serves to temporarily dilute the asphalt volumetrically so that the mixture has improved compressibility until its temperature has dropped below 93°C, and since the diatomite adsorbs a significant amount of asphalt after the road surface has cooled below 93°C. 3. Asphalt road surface mixture according to claim 2, characterized in that it comprises an asphalt concrete with a largest stone size of 0.95 cm. 4. Asphalt road surface mixture according to claim 2, characterized in that it comprises an asphalt concrete with a largest stone size of 1.27 cm. 5. Asphalt road surface mixture according to claim 2, characterized in that it comprises a stone-filled sheet asphalt with a largest stone size of 0.64 cm. 6. Asphalt road surface mixture according to claim 2, characterized in that it is designed as a tread overlay with a thickness of from two to three times the largest dimension of the aggregate in the mixture. 7. Procedure for the production of a road surface, characterized in that several aggregate and filler components with different gradations are mixed together to form a dry mixture, the dry mixture is heated to over 149°C, 5-12% by weight of asphalt is added and mixed with the dry mix to coat the dry mix, 0.5-2.0% by weight of diatomite is added to and mixed with the asphalt coated mix to increase the mix's density, viscosity and aggregate interlocking, the diatomite also serving to temporarily dilute the asphalt volumetrically , the road surface material obtained is laid, and the road surface material is compressed and rolled before it has cooled to below 93°C, so that the laid road surface has fewer voids which leads to slower oxidation hardening and less degradation due to water.