Adaptive mix proportioning method for use in asphaltic concrete mixing plants

This invention is related to U.S. application Ser. No. 123,206 entitled Asphalt Pavement Mixing Plant With Plural Weight Cells filed Feb. 21, 1980 by the same inventor hereof, now abandoned.
This invention is related to U.S. Pat. No. 4,304,504 entitled Unsegregator Shroud For Hot-Mix Asphalt Lay-Down Machine issued Dec. 8, 1981.
BACKGROUND OF THE INVENTION
This invention concerns asphalt paving mixing plants, and more specifically, to aggregate proportioning measurement and control means for an asphalt mixing plant having plural aggregate storage bins for different size aggregates. It also concerns systematic procedure for mix design, mix analysis, and job control for producing a uniform asphalt pavement of substantial length having uniform physical properties.
Asphalt surfaces generally are used to provide a riding surface for land vehicles or a take-off/landing surface for air vehicles. The primary material constituents of asphalt-hot-mix paving mixtures are mineral aggregates of different sizes and asphalt cement, an oil-base ingredient. The hot mix is prepared at a mixing plant which preferably is located near the site of the pavement to reduce transportation costs.
The aggregates generally are excavated from earth deposits, transported to a crushing plant for crushing and separated according to their size and finally stockpiled for subsequent use. Certain aggregates, such as river sand and hydraulically tumbles stone are not crushed but instead, are stockpiled for use in their present form immediately upon excavation. Evacavation and stockpiling often occur near the mixing plant, but in certain instances, the stockpiled aggregates are again transported to a remotely situated mixing plant and again stored in stockpiles near the mixing plant for use during mixing operations wherein the aggregates are supplied to separate storage bins of the mixing plant. During mixing, the aggregates are proportioned according to a predesignated job mix formula and a predesignated quantity of asphalt cement. The entire combination of aggregates and asphalt cement then is heated to a certain mixing temperature of about 250 degrees to 280 degrees Fahrenheit, and when thoroughly mixed and heated, the mixture is dumped from the bottom of a mixing bin into a dump truck that transports the hot mix to the pavement site. The hot mix then is picked up by laydown equipment and evenly spread on a prepared pavement subgrade or, in the case of an overlay, on top of a preexisting asphalt or concrete surface. Heavy steam rollers then compact the hot mix to produce the pavement. The compacted surface is allowed to cool for about one day prior to being put into service.
During handling of the aggregates as aforestated, a certain amount of segregation and degradation occurs. Efforts are made with difficulty to minimize segregation and degradation. Segregation is the separation of the larger aggregates from smaller aggregates within the blended mixture, while degradation is the degrading of larger sized aggregates into smaller sized aggregates.
Segregation naturally occurs when a blended mixture of aggregates is poured or dumped from the mixing plant into a truck bed, during transportation of the aggregates from the mixing plant to the road site, dumping of the hot mix aggregate blend at the road site, and also during handling by the laydown machine. Heavier aggregates have a natural tendency to be slung and ejected with greater force and therefore take a different path of travel during handling operations such as dumping, scooping and excavating. For example, when a hot mix is dumped from a truck into a pile, the heavier aggregates have a tendency to roll towards the sides and periphery of the pile whereas the finer aggregates tend to fall and collect at a location vertically downward.
Degradation, on the other hand, at worse, occurs within the aggregates prior to being introduced into the mixing bin of the mixing plant. For example, the larger aggregates of a stockpile have a tendency to strike one another and break up into smaller pieces during handling between the stockpile to the mixing plant. Accordingly, the proportioning of aggregates according to size is rather difficult. Degradation for coarse aggregates are known to exceed normally 30% between the stockpile and the output of the mixing plant. Finer aggregates, such as blended sand and crushed fines which pass the number 4 screen size do not degrade during handling.
During the production of asphalt paving mixtures, the paving engineer attempts to maintain the aggregate blend within certain predesignated specifications as previously indicated. A job mix formula is usually designated by the contracting authority as a series of percentages associated with a number of sieves which describe the aggregate blend. A typical job mix formula may be designated as 100% passing in the 3/4" sieve, 80-100% passing in the 1/2" sieve, 70-90% passing in the 3/8" sieve, 55-73% passing in the No. 4 screen, 40-55% passing the No. 8 screen, 20-30% passing the No. 30 screen, 10-18% passing the No. 100 screen, and 4-10% passing the No. 200 screen. Any blend of aggregates within the range designated by the job mix formula specifications generally is acceptable by the contracting authority under present standards. Difficulties are encountered in meeting job mix formula specifications because of segregation and degradation of aggregates as previously indicated, and further, due to lack of adequate feeding controls for the separate storage bins.
At the mixing plant, the separate storage bins of the mixing plant contain an aperture and feed belt of the bottom thereof which regulates the flow of aggregates into the mixing bin. Calibration normally is performed with dry aggregates. A certain rate of flow of dry aggregates is calibrated by adjusting the relative speeds of the individual feed belts associated with each storage bin. However, because stockpiles normally are stored out of doors and are not shielded from climatic conditions, a certain amount of moisture generally is present in the aggregate stockpiles. Because conventional aggregate feeding mechanisms are calibrated to dry aggregate flow rather than wet aggregate flow, certain errors in the blended composition may occur. Hopefully, the aggregates of the stockpile are prepared so that the actual blend will fall within the middle of each percentage specified in the job mix formula, but due to segregation and degradation as previously explained, adequate control of gradation almost is impossible with present systems. It is possible to adjust the feeding controls in accordance with the amount of moisture content in the aggregates, but this procedure is rather complicated in that the moisture content varies on a daily basis and also varies according to the climatic conditions. Calibration procedures to align all of the separate feed systems are complicated and time consuming. Furthermore, the amount of moisture retained in aggregates varies according to the size of the aggregate. Particularly, blend sand and crushed fines will retain more moisture than coarse and intermediate sized rock. Thus, in order to more accurately control the feeding operations from the feeding bins, feeding must be controlled accordingly to the size of aggregates. The prediction of aggregate size in the various stockpiles and the feeding bins is complicated by segregation and degradation problems which were previously set forth.
Using present calibration techniques for proportioning aggregates and by adjusting for moisture content, I have been able to maintain an accuracy such that 50% of my samples taken from the mixing bin were within 1% of tolerance, 90% of the samples taken were within 3% of tolerance, and 10% of the samples taken were within 5% of tolerance. All samples were measured over the no. 4 screen. Some engineers do not even take account of the moisture content of the aggregate stockpiles.
Improper control of aggregate proportioning at the mixing plant causes non-uniformity in the compacted mixture with respect to voidage control, flexibility, and stability. The lift of the pavement is extremely shortened, particularly at certain sections, thereby foreshortening the lift of the entire pavement. The pavement is deteriorated by aeration of the asphalt cement in the compacted mixture which results in oxidation of the asphalt cement. The compacted pavement also is subjected to water seepage which strips the coating of asphalt cement from the aggregate thereby causing the pavement to break up and lessen its flexibility.
In the plant-mix type pavement construction, a road bed and/or subgrade on which an asphaltic concrete mixture is laid generally are prepared by grading, compacting, and leveling. The asphaltic mixture is prepared at a remote plant site and is transported to the prepared road bed. The mixture is then dumped in place, spread by a paving machine, and finally compacted in place by heavy steam rollers.
Plant-mix production consumes large quantities of materials, typically as much as 6000 tons per day, thus possibly depleting any given single aggregate source, such as a rock quarry, sand pit, or other source of mineral deposits. Aggregate materials of pavements of several miles or more generally are gathered from several sources and accordingly nonuniform pavements result from source variations. Furthermore during the initiation of a production run, several miles of pavement typically are laid before the materials engineer has acquired sufficient knowledge of the aggregate qualities to establish consistent physical properties in the pavement. In many cases, consistent qualities may never be achieved either because the production job is completed before the material engineer has had the opportunity to correct for source variations, or the nature and quality of the aggregate changes too rapidly for the materials testing engineer to perform required tests and determine appropriate corrective action at the mixing plant. In many cases, the aggregate source intentionally is changed to shorten the distance between the aggregate source and the mixing plant whereupon attempts are then made to bring the new mixture within job specifications.
At the mixing plant, the amount of asphalt cement injected into the mixing bin can be reasonably accurately controlled as a percentage by weight of the total mix. Effective asphalt cement governs the amount of air voids in the compacted mixture and varies as a function of their shape, absorption characteristics, and sizes. Attempts are made to regulate size by controlling gradation over a multitude of screen sizes, generally 8 to 10, ranging from coarse aggregates passing the 1" screen down to mineral filler passing the no. 200 screen. Gradation however is almost impossible to control the job mix formula at the mixing plant and at the lay-down site due to degradation and segregation of aggregates. Plant control in accordance with changes in absorption characteristics, aggregate shapes, or other natural qualities are at best based upon trial-and-error techniques.
No method previously exists for quickly measuring changes in the natural aggregate qualities at the mixing plant site to effect corrective action at the mixing plant during operation. Further, no method previously exists for quickly and accurately determining the necessary corrective action for effecting such proper control of the production plant at the plant site. Thus uniform asphalt pavements cannot be produced.
Frictional qualities also are difficult to achieve for dense graded mixes. An open graded overlay is now used for friction overlays. Frictional qualities are provided by the sharp edges of 1/2" to 1" coarse aggregate particles protruding upwardly of the riding surface. The asphalt cement in the present open graded frictional overlay is permeable to water and air penetration causing the cement to age prematurely resulting in deterioration of the pavement structure. The underlying surface of the friction overlay also deteriorates for the same reasons. One problem associated with attempts to lay dense graded friction courses is that the coarse rock is pushed down into the underlying asphalt pavement upon load application on the riding surface and thus frictional qualities disappear. Thus to reduce the effects of displacement and reorientation of aggregate particles, the present friction overlay is laid very open or coarse.
The design of optimum asphaltic mixture today is at best a trial-and-error procedure. Good mixes generally result from knowledge of aggregates, experience, and luck. Many limitations in achieving optimum pavement properties exist, even with the expert design engineer using known aggregates. With some aggregates, present techniques cannot be used to determine a mix design that meets job specifications, such as stability, flexibility, density or voidage.
In an effort to produce uniform pavements, firstly mix design is performed by trial-and-error methods to determine what aggregate blendings asphalt cement combination will produce certain predesignated pavement qualitites such as flexibility, stability, and air voidage; and secondly, crushing, mixing, and laydown operations are controlled to produce an asphalt pavement having the desired physical properties. The object is to produce a pavement so that the load is transmitted to the subgrade only through the rock while the mortar of fine aggregate and cement fill the intersticies between the rock. The aggregates are proportioned according to a predesignated job mix formula which comprises prescribed proportions of different sized aggregates generally ranging from a maximum size of one and one-half inch in diameter to a minimum aggregate size that passes the no. 200 sieve.
Two methods now in general use for mix design and quality testing are the Marshall and the Heevm methods. According to these methods, test specimens are prepared by compacting trial mixes within the limitations of predesignated job mix formula tolerances, and then tested in an effort to determine which blend possesses desired physical properties, such as flexibility, flowability, density, stability, etc. This procedure consumes as much time as two to three weeks. It must be repeated when the natural qualitites of the aggregates change during the production operation or when different degrees of segregation and degradation of aggregates occur during stockpiling, transporting and handling.
An asphalt pavement containing a large amount of air voids is an "open graded mix". It is vulnerable to deterioration due to water seepage throughout the pavement. Water weakens the binding effect of the asphalt cement by stripping the cement from the surface of the aggregates. An open graded mix also is subject to aeration which shortens the pavement life due to oxidation of the asphalt cement. Aeration decreases the viscosity of the cement, causing it to become brittle and ultimate breakup of the pavement. Where prolonged road life is not important, an open graded mix, within certain limits, however is used to produce a high friction riding surface. Road pliability having resilient characteristics is sacrificed in open graded mixes.
The nature of the coarse aggregates determine, among other things, the load bearing and riding characteristics of the pavement. Specific qualities of concern include hardness, shape, porosity, and other surface qualities. Coarse aggregate may consist of crushed rock, volcanic rock, hydraulically tumbled stoned, or other large mineral deposits.
A pavement having a small amount of air voids is known as a "dense graded mix". This mixture has a larger quantity of the fine aggregate/cement mortar. Present dense graded mixtures have poor stability and deform under load forces as the coarse aggregates do not properly transmit loads to the subgrade. Where load conditions permit, pavements comprising dense graded mixes are used when high pliability is desired.
A maximum "bulking point" is defined herein as the proprtion (percentage by weight of dry coarse aggregate with respect to total weight of dry aggregate in mixture) at which each coarse aggregate particle touches one another while the mortar of fine aggregate and asphalt cement fill all interstitices between the coarse aggregate. The gradation at which the bulking point occurs essentially depends on the shape of the coarse aggregate. For example, the available volume of space between contiguous hydraulically tumbled stone is different than the available volume between contiguous crushed rock, and thus a mixture including one type of coarse aggregate may have a different bulking point than a mixture of another type of coarse aggregate. A mix at or above the bulking point mixture generally is unacceptable because it lacks, among other things, sufficient flexibility, and resistence to air and water penetration. Some presently specified job mix formulas completely ignore bulking point limitations in the specification.
A "balance point" is defined herein as the gradation at which each coarse aggregate particle are in close proximity to one another while the mortar of fine aggregate, asphalt cement, and desired dispersed air voids fill all interstitices between the coarse aggregate. There should be just enough asphalt cement in the mixture, and just enough air voids to allow for proper expansion and contraction of the pavement under the climatic conditions without penetration of excessive water and air. Thus the air voids are the separating factor between dense and open graded mixes below the bulking point. Excessive asphalt cement reduces stability and too little asphalt cement generally reduces flexibility. The voidage control is difficult to achieve and maintain during the mixing operation because of myriad variables, some of which have been previously indicated, and accordingly, uniform pavements cannot be produced by present techniques.
At least one procedure presently in use for designing a mixture which has the desired quantity of air voids is the manipulation of the job mix formula of discrete sized aggregates over the entire gradation scale. Specifically, Fuller Maximum Density Curves and the Federal Highway Administration 0.45 Power Gradation Chart currently are in use. Their use is described in "Mix Design Methods for Asphaltic Concrete and other Hot-Mix Type" published by The Asphalt Institute, manual series no. 2, fourth edition, March 1974. The basis for the FHA 0.45 power gradation chart is described in detail in volume 31, pages 176 through 207 of the "Proceedings of the Association of Asphalt Paving Technologist", Jan. 29, 1962. The theory of controlling voidage is based on the principle that a gradation deviating from a maximum density curve will contain increased air voids. Thus some obscure relationship between job mix formula and voidage is developed for job control. It is rarely successful particularly in view of segregation and degradation of aggregates while being handled and transported to and from the crushing plant, stock piles, mixing plant, and laydown site. Degradation between the crushing plant and the mixing plant alone may amount to as much as 30%.
Another method for computing voids in the mineral aggregate is disclosed in volume 34, pages 574 through 594 of the "Proceedings of the Association of Asphalt Paving Technologist". The computations are based upon successive correlations of voids in discrete ranges of the gradation spectrum. Page 577 of the treatise illustrates 8 gradation ranges between mineral filler and 3/8 to 3/4 inch aggregate. By summing the voidage contained in each aggregate group, and considering the correlation factors of aggregate voidage, a sum total is obtained which closely approximates the final aggregate voidage. No clue however is given to how one might achieve this ideal combination of aggregate mixture at the mixing plant nor is any consideration given to variation in effective asphalt cement.
In any job, the use of local indigenous materials is desired, if at all possible, because of expensive transportation cost in transporting aggregates across the country. Certain jobs however do not lend themselves to the use of such indigenous materials because of widely varying changes in natural aggregate qualities that render mixing plant control almost impossible. In that case, materials are imported from a distant source.
SUMMARY OF THE INVENTION
In view of the foregoing, it is one objective to provide a uniform asphalt pavement of substantial area which pavement is produced from aggregates of varying natural quality.
It is another objective of this invention to provide an in situ plant control method for controlling gradation of the mixture thereby to produce a uniform asphalt pavement.
It is another objective of the invention to provide a systematic mix design and plant control method for producing a uniform pavement having predetermined physical properties such as flexibility, flowability, stability, workability, voids in mineral aggregates, voids filled, density, and air voids.
In accordance with this invention, a uniform asphalt pavement of substantial area having a substantially constant volume of effective air voids under a standard compaction is produced by a systematic procedure of mix design, mix analysis, and plant control to compensate for variations in specific gravity, particle shape, absorption, size distribution, or consolidation of the aggregates that comprise the pavement. To achieve mix design: firstly, the aggregates are demarcated thereby to define coarse and fine aggregates; secondly, and Index quantity is derived that represents the quantity of fine aggregate particles that is contained in a unit volume of the bulk fine aggregate compacted under the standard compaction, such as the compactive effort defined by the Marshall method; and thridly, a proportion of coarse and fine aggregate is selected wherein the mathematically computed quantity of fine aggregate per unit of the combined volumes V.sub.FA and V.sub.EAC substantially equals the Index quantity according to the volumetric relationship:
V.sub.CA +V.sub.FA +V.sub.EAC +V.sub.EA =1 unit volume
where V.sub.CA is the volume of the coarse aggregate, V.sub.FA is the volume of the fine aggregate, V.sub.EAC is the volume of the effective aspahalt cement, and V.sub.EA is the volume of the desired effective air voids under the standard compaction. Plant control is achieved during the mixing process by periodically selecting a proportion according to the mix design process and adjusting the proportioning controls of the plant to said selected proportion thereby to maintain the desired air void volume V.sub.EA, and thus produce a substantial area of uniform asphalt pavement having a constant or controlled volume of air voids. Mix analysis is performed on production outputs of the mixing plant to assure proper proportioning. Standard compaction can be performed by the Marshall method, or by a similar compacting method that consolidates the fine aggregates to their upper limits of consolidation. Coarse and fine aggregates may be demarcated by the no. 4 or similar size aggregate sieve. A fricition overlay also is produced by this method.
In further aspect of this invention comprises a systematic procedure for determining the flexibility indexes of an asphalt pavement mixture by varying the quantity of asphalt cement and crushed fine aggregates in the mixture.
The advantages of this invention include the ability to produce uniform pavements of substantial area, reduction of testing procedures and time, and reduction of cost of design and construction. More importantly, my asphalt pavements, can have a useful life exceeding 20-25 years, as opposed to six months to five years for present asphalt pavements.
Because an analysis of any mixture of aggregates can be made by this invention, aggregate material from existing pavements already in place can be crushed and recycled to produce new pavements thereby obviating the need to excavate and transport aggregate material from a distant source.
Further, pavements of both high flexibility (12 to 16) and high stability (2000 psi to 3000 psi) not otherwise achievable can now be produced with exceptional frictional qualities. Such asphalt pavements can provide long-lasting safer operation of asphalt concrete airport runways and highways than conventional portland cement runways and highways with friction gratings.
These and other aspects and advantages of the invention will become readily apparent upon review of the succeeding disclosure taken in connection with the accompanying drawings. The invention however is pointed out with particularity in the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts volumetric relations of the aggregate blend as a function of gradation.
FIG. 2 depicts an enlargement of a section of the chart of FIG. 1 that is useful for job control at the mixing plant, mix design, or analysis purposes.
FIG. 3 depicts the density of aggregates of FIG. 2 as a function of gradation.
FIGS. 4 and 5 depict charts useful for mix design to determine structural strength of a mix.
FIGS. 6 and 7 depict charts useful for designing flexibility and stability properties by varying the ratio of blend sand and crushed fine aggregates in the mix.
FIG. 8 shows a conventional hot-mix asphalt paving mixing plant which embodies plural weighing cells.
FIG. 9 shows another embodiment of the weighing cell arrangement.
FIG. 10 shows a modified storage bin for a mixing plant.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
In developing my invention, I made the following observations and discoveries:
The nature and quality of the fine mineral aggregates passing the no. 4 sieve essentially determine all behavioral qualities, e.g. durability, flexibility, compactibility workability, and stability of the blended mixture because a change in its natural qualities affects air voids and effective asphalt cement to a significantly greater extent than the coarse aggregates.
Voidage can be mathematically computed with knowledge of the maximum consolidation of the fine aggregates and asphalt cement which is based upon the specific gravities, particle shape, and absorption characteristics of all aggregates in the mixture.
The composition of fine aggregates supplied to a mixing plant can easily be controlled within close tolerance as its segregation and degradation is neglible. Plant control can be effected by controlling the quantity of fines relative to the quantity of rock.
The nature and quality of the rock within the entire mixture determine only the load bearing strength and rigidity of the pavement. The composition, e.g. blend of different sized rock, does not have any significant effect of other qualities of the mixture.
The degradation and segregation of the rock supplied to a mixing plant can be monitored and controlled within a high degree of accuracy thereby providing effective gradation control at the mixing plant for producing my uniform pavement.
The amount of air voids remaining in a compacted mixture can be controlled by adjusting the proportion of the aggregates over a single screen size comparable to the no. 4 screen, by varying the quantity of asphalt cement.
The flexibility and stability of an aspahlt pavement mixture can be ontrolled by varying the ratio of crushed fine aggregates to blend sand (river sand).
Stability can be increased by including crushed fine aggregates which have been subjected to extended crushing.
The best demarcation between coarse and fine aggregates for defining gradation and controlling the mix proportions at the mixing plant is a sieve size approximating the no. 4 screen (0.187 inches or 4.75 mn.).
The mix proportions can be maintained at the mixing plant by separately controlling and feeding controlled quantities of coarse aggregates and fine aggregates fed to the mixing plant. Blended aggregate samples are withdrawn approximately every two hours, or as needed, and examined for aggregate gradation over a single sieve size, such as the no. 4 screen. Control of the blended mixture of aggregates over this single sieve size is obtained by regulating the feeding controls of the separate aggregate storage bins to control the mixture over all sieve sizes of the job mix formula. A mixing plant having separate feeding controls and separate monitoring means for the coarse and fine aggregates is disclosed in my copending U.S. application Ser. No. 123,206, now abandoned, entitled Asphalt Pavement Mixing Plant Having Plural Weight Cells which was filed of even date herewith. The substance of that application is embodied herein. For support of appended claims, all essential matter disclosed therein is incorporated herein by reference.
Based upon the foregoing observations and discoveries, I have discovered and Index quantity herein called the Riguez Index to which graded blends of aggregates are compared on a volumetric basis thereby to design mixes, analyze mixes, and control the mixing plant. The composition of my uniform pavement comprises aggregate mixtures within the following mix proportions and possessing the natural aggregate qualities as determined by the indicated AASHTO test procedures; but are not limited to these ranges:
______________________________________Fine AggregateBulk Specific Gravity 2.676 (166.98#/CF)Absorption 1.07%Effective Specific Gravity 2.705 (168.79#/CF)Coarse AggregateBulk Specific Gravity 2.708 (168.98#/CF)Absorption 1.84%Effective Specific Gravity 2.758 (172.10#/CF)Asphalt CementSpecific Gravity (AR2000 Asphalt) 1.0146 (63.31#/CF)Job Specifications:Asphalt cement 5% by weightair void volume 5% in compactedmixtureJob Mix Formula:Fine Aggregate Blend (% Passing)no. 4 100%no. 8 70%-85%no. 16 46%-62%no. 40 24%-40% (not more than 7% passing no. 200)Combined Fine and Coarse Aggregate (% Passing)3/4 inch 100%1/2 inch 95%3/8 inch 50%-60%no. 4 44%-54%no. 8 31%-45%no. 16 20%-34%no. 40 12%-22%no. 200 2%-6%______________________________________
Natural variations in absorption and particle shape normally contributes 2% to 5% error in designed air voidage. Natural variations in densities of aggregates normally fluctuate 5% to 15% for aggregates excavated from a single location. Since the quantity of asphalt cement injected in the mixture is based upon total weight of aggregates, and that naturally occurring density variations fluctuate 5% to 15% for aggregates excavated at a single source; the quantity of effective asphalt cement in the mix varies to such a degree causing fluctuations in air voidage of .+-.10% or more, flexibility to fluctuate .+-.5 points or more, and stability to fluctuage by more than 1000 psi or more. These errors are typical for aggregates generally used in asphalt pavement production. Occasionally, they are much worse due to radical changes in the formation of the rock at the site of excavation of the aggregates.
The worst errors though are brought about by the large tolerances allowed in the job mix formula and job specifications that are predesignated by the contracting authority. A typical gradation tolerance allowed between coarse and fine aggregate is 30% to 50% retained on no. 4 screen. It will be seen by the teachings of this invention that variations within that range could produce open graded mixtures having more than 15% to 20% air voids, or a stability of more than 1000 psi under a predesignated Marshall strength, or a flexibility more than 10 points below desired amount.
By using the techniques of my invention, I am able to maintain air voidage, void filled, or voids in mineral aggregate (V.M.A.) constancy within less than 1/2% tolerance.
DERIVING THE RIGUEZ INDEX
The Riguez Index indicates the extent of compactibility of the mixture. It measures only the fines as the coarse rock does not compact. The Index, when periodically determined during production, measures natural variations in absorption, specific gravities, and particle shape of the fines thereby to allow the proper relationship between effective asphalt cement and aggregate to be maintained, and the air voidage to be maintained. When making the volumetric comparison, only the absorption and density of the aggregate are needed.
The Riguez Index is empirically derived by mixing and compacting a mortar comprising a mixture of a representative sample only of the fine aggregate blending to be used in a job and the specified quantity of asphalt cement. Mixing and compacting are performed by the Marshall or similar method which is described in the previously mentioned "Mix Design Methods for Asphalt Concrete and Other Hot Mixes" published by the Asphalt Institute. Prior to compacting, the Marshall method provides for heating and mixing the aggregates at 230 degrees Farenheit, plus or minus 10 degrees Farenheit, to ensure adequate homogenity of the mixture. The compactive effort applied is equivalent to 75 blows with a ten-pound hammer with an eighteen-inch drop on both sides of the hot mix, or an equivalent of 43,000 foot-pounds of pressure. Approximately 1100 grams of the loose mixture is used to produce the specimen of about 21/2 inches in height. After allowing the speciment to cool, the bulk density of this specimen is determined by weighing in an air to determine weight, and measuring its displacement in water to determine its volume. Several specimens are made to assure accuracy. This procedure can be completed within two hours.
The Riguez Index is computed by subtracting the known weight of asphalt cement from the bulk density of the speciment thereby to derive the weight of dry fine mineral aggregate contained in a unit volume. Riguez Values O.D. (oven dried), is defined herein as the Weight of the Dry Fine Mineral Aggregate per Cubic Foot Contained in its own Volume plus the Volume of the Effective Asphalt Cement. A Riguez Index S.S.D. (saturated surface dried) and Riguez Values S.S.D. are used for mix design, mix analysis and job control in my invention. They are determined by adding to the weight of dry aggregate per cubic foot, the weight of asphalt cement absorbed by the dry fine aggregate. The Riguez Index S.S.D. is herein called the Riguez Index.
Different blendings of the fine aggregates or fine aggregates from different sources have different characteristic Riguez Indexes. Thus any physical property of the Riguez specimen can be used as job control parameter to be monitored during production for producing uniform pavements of substantial length from aggregates of differingnatural qualities.
An average Riguez Index for the exemplary aggregates used herein was determined as follows:
Bulk Density of Riguez Index specimens with 5% A.C. 2.47
Riguez Index computations:
2.247.times.62.4=140.2 lbs/CF.times.95%=133.20 lbs./C.F. O.D.
Absorption:
133.2 lbs.times.1.07%=1.42 lbs/166.98 lbs=0.85% in Volume
0.85%.times.63.31 lbs=0.54 lbs A.C.
Riguez Index SSD=133.2 lbs+0.54 lbs=133.74 lbs/C.F.
GENERATING VOLUMETRIC FUNCTIONAL RELATIONSHIPS
FIG. 1 depicts the volumetric behavior of aggregate blendings as a function of proportion for my exemplary aggregates with the indicated job specifications. The curves are mathematically computed using respective density values of rock and fines, respective absorption values of the rock and fines, and the specific gravity of the asphalt cement. Specifically, curve 1 represents the combined volume of the rock V.sub.CA, fines V.sub.FA, and effective asphalt cement V.sub.EAC as a function of gradation. The difference between curve 1 and 100% value represents the effective air voids V.sub.EA that will be contained in a blended mixture as a function of gradation when the blended mixture is compacted with the same compacted effort of the Riguez Index specimen. Curve 1 is derived under the assumption that only the mortar of fines are compacted and that rock is incompressible. Curve 2 represents the combined volume of the rock V.sub.CA and fines V.sub.FA as a function of gradation. Curve 3 represents the volume only of the rock V.sub.CA as a function of gradation. Representations of aggregate behavior also can be made with quantity of asphalt cement, rather than gradation, as the variable without departing from the spirit and scope of my invention.
The Riguez Value Curve depicted in FIG. 1 shows the relationship between densities of the mortar of the fine aggregate portion of the graded mixture as a function of gradation and the density of the mortar of the fines of the Riguez Index specimen. For example, at the balance point (51.71% rock) indicated thereon, a unit volume of the mortar plus asphalt cement plus five percent air contains the same quantity of fines as a unit volume of the Riguez Index specimen. The gradation at which the balance point is reached is a function of the natural qualities of the aggregates. Thus only the fine aggregates of the mixture need be monitored at the mixing point. As soon as the natural qualities change, so does the balance point. Thus the mixing plant can be adjusted if a change in the Riguez specimen is noted during the operation of the mixing plant. If the change is small, only the proportion is altered to compensate for change in voltage. The quantity of asphalt cement also can be altered to correct for voidage changes resulting from changes in the volume of the Riguez Index specimen. If the change is great, a new set of curves are computed to determine the new balance point of the mixture for more accurate compensation of air voidage at the mixing plant. Since the Riguez Index is indicative of maximum consolidation of the fine particles, the Riguez Values show the extent consolidation of particles of the blended mixture including five percent air voids. The Riguez Value curve indicates the percent error as the gradation moves away from the balance point. It is seen that the Riguez Value curve is nearly constant with respect to gradation at the balance point. Thus a range within a few percentage points of the balance point is used for job control at the mixing plant.
It is interesting to note that at some proportion greater the balance point, the Riguez Value curve 4 intersects curve 1. Experimentally conducted tests shows the gradation at which the Riguez Value curve intersects curve 1 in the "bulking point" of the mixture. Thus, in accordance with another aspect of my invention, the bulking point of a blended mixture can be arithmetically calculated. That information is useful for establishing gradation limits or job mix formula limitations for mix design purposes. For the aggregate used herein, the theoretical bulking point was empirically proven by laboratory test to be 85 pounds of rock per cubic foot or 58.8%.
To illustrate the exact procedure for computing the curves 1 through 4 as a function of proportion, I performed step-by-step calculations at three proportion points on the gradation scale, beginning at and descending from the bulking point. As previously indicated, mixtures above the bulking point do not produce good pavements as the air voids becomes uncontrollable. Accordingly, my analysis is not used for aggregate mixtures above the bulking point. These computations can be performed by a digital computer. The graphical presentation resulting these curves is useful for controlling the mixing plant.
Insert Point A, Point B, Point C--Table 1.
__________________________________________________________________________POINT A - CALCULATIONS OF VOLUMETRIC VALUES OF MIX WITH 85# OF__________________________________________________________________________ROCK1. 85#/168.98# CF Dry Rock 50.30%2. Program 5% desired effectiver air 5.00%3. Volume occupied by Rock plus 5% Effective Air 55.30%4. Volume available for Fines plus Asphalt Cement (100-55.30) 44.70%5. Dry Weight of Fines to 1 CF: 133.2# .times. 44.70% 59.54#6. Combined Dry Weight of Aggregates 85# plus 59.54# 144.54#7. Percentage Rock Dry Weight of Aggregate 85#/144.54# 58.80%8. Total Weight of mix without absorption factors 144.54#/95% 152.15#9. Absorption: 85# .times. 1.84% = 1.56#/168.98#CF 0.92% 59.54# .times. 1.07% = 0.64#/166.98#CF 0.38% Absorption on the basis of combination 1.30%10. Asphalt Cement Absorbed 1.30% .times. 63.31# 0.82# 0.82#/152.15# = 0.54% on the basis of Total Mix. 0.54% .times. 95% = 0.51% on the basis of total aggregates. Effective Asphalt Cement 5.00%-0.51% 4.49% Absorption values within a range of the gradation scale will be practically the same. The analysis from this point will be on the mixture with absorption factors: Volume of Rock 85# + 1.56# = 86.56#/172.10# 50.30% Program 5% Effective Air 5.00% Volume occupied by Rock plus 5% Effective Air 55.30% Volume available for Fines plus A.C. (100%-55.30) 44.70% SSD Weight of Fines to 1CF = 133.74# .times. 44.70% 59.78# SSD Weight of Aggregates 86.56# + 59.78# 146.34#20. Total Weight of Mixture 146.34#/100-4.49% 153.22# Weight of Effective Asphalt Cement 153.22# - 146.34# 6.88# Final Volumetric Analysis: Total Weight 153.22# Effective Asphalt Cement 6.88# SSD Weight of Aggregates 146.34# SSD Wt. Rock 86.56#/172.10# = 50.30% Volume of Rock SSD Wt. Fines 59.78#/168.79# = 35.42% Volume of Fines 85.72% Volume of Aggr. 100-85.72% = 14.28% Voids in Mineral Aggregate 6.88#/63.31# = 10.87% Volume of Effective A.C. 3.41% Volume of Effective Voids Percentage Voids Filled 10.87%/14.28% 76.12% Calculated Voidless Mix 153.22#/100-3.41% 158.63#CF Riguez Index 59.78#/35.42% plus 10.87% 129.14#CF Riguez Value of original Index 129.14#/133.74# 96.56%__________________________________________________________________________
__________________________________________________________________________POINT B - CALCULATIONS OF VOLUMETRIC VALUES OF MIX WITH 80# ROCKThese calculations with identical values of Riquez Index,Effective Air and Asphalt Cement, as were used for Point__________________________________________________________________________A.1. 80#/168.98# = CF Dry Rock 47.34%2. Program 5% desired Effective Air 5.00%3. Volume occupied by Rock plus Effective Air 52.34%4. Volume available for Fines plus Asphalt Cement (100 - 52.34) 47.66%5. Dry Weight of Fines to 1 CF 133.2# .times. 47.66% 63.48#6. Combined Dry Weight of Aggregates 80# + 63.48# 143.48#7. Percentage Rock Dry Weight of Aggregate 80#/143.48# 55.76%8. Total Weight of mix without Absorption 143.48#/95% 151.03#9. Absorption: 80# .times. 1.84% = 1.47#/168.98#CF 0.87% 63.48# .times. 1.07% = 0.68#/166.97#CF 0.41% Absorption on the basis of combination 1.28%10. Asphalt Cement Absorbed 1.28% .times. 63.31# 0.81# 0.81#/151.03# = 0.54% on the basis of Total Mix. 0.54% .times. 95% = 0.51% on the basis of total aggregates. Effective Asphalt Cement 5.00% - 0.51% 4.49% Volume of Rock 80# + 1.47# = 81.47#/172.10# 47.34% Program 5% Effective Air 5.00% Volume occupied by Rock plus 5% Effective Air 52.34% Volume available for Fines plus A.C. (100% - 53.34%) 47.66% SSD Weight of Fines 133.74# .times. 47.66% 63.74# SSD Weight of Aggregates 81.47# + 63.74# 145.21#20. Total Weight of Mix 145.21#/100% - 4.49% 152.04# Weight of Effective Asphalt Cement 152.04# - 145.21# 6.83# Final Volumetric Analysis: Total Weight 152.04# Effective Asphalt Cement 6.83# SSD Weight of Aggregates 145.21# SSD Wt. Rock 81.47#/172.10# 47.34% Volume of Rock SSD Wt. Fines 63.74#/168.79# 37.76% Volume of Fines 85.10% Volume of Aggr. 100% - 85.10% = 14.90% Voids in Mineral Aggregates 6.83#/63.31# = 10.79% Volume of Asphalt Cement 4.11% Volume of Effective Voids Percentage Voids Filled 10.79%/14.90% 72.42% Calculated Voidless Mix 152.04#/100% - 4.11% 158.55# Riguez Index 63.74#/37.76% - 10.79% = 131.29# Riguez Value of original Index 131.29#/133.74 98.27%__________________________________________________________________________
__________________________________________________________________________POINT C - CALCULATIONS OF VOLUMETRIC VALUES OF MIX WITH 70# ROCKThese calculations with identical values of Riguez Index,Effective Air and Asphalt as were used for Points A and__________________________________________________________________________B.1. 70#/168.98# = CF Dry Rock 41.43%2. Program 5% Desired Effective Air 5.00%3. Volume occupied by Rock plus 5% Effective Air 46.43%4. Volume available for Fines plus Asphalt Cement (100 - 46.43) 53.57%5. Dry Weight of Fines 133.2# .times. 53.57% 71.36#6. Combined Dry Weight of Aggregates 70# + 71.36# 141.36#7. Percentage Rock Dry Weight of Aggregates 70#/141.36# 49.52%8. Total Weight of mix without Absorption 141.36#/95% 148.80#9. Absorption: 70# .times. 1.84% = 1.29#/168.98#CF 0.76% 71.36# .times. 1.07% = 0.76#/166.98#CF 0.46% Absorption on the basis of combination 1.22%10. Asphalt Cement Absorbed 1.22% .times. 63.31# 0.77# 0.77#/148.80# = 0.52% on the basis of Total Mix. 0.52% .times. 95% = 0.49% on the basis of total aggregates. Effective Asphalt 5.00% - 0.49% 4.51% Volume of Rock with Absorption 70# + 1.29# + 71.29#/172.10# = 41.43% Program 5% Effective Air 5.00% Volume occupied by Rock plus 5% Effective Air 46.43% Volume available for Fines plus A.C. 100% - 46.43%) 53.57% SSD Weight of Fines 133.74# .times. 53.57% 71.64# SSD Weight of Aggregates 71.29# + 71.64# 142.93#20. Total Weight of Mix 142.93#/100% - 4.51% 149.68# Effective Weight of Asphalt Cement 149.68# - 142.93# 6.75# Final Volumetric Analysis: Total Weight 149.68# Weight of Effective Asphalt Cement 6.75# SSD Weight of Aggregates 142.93# SSD Wt. Rock 71.29#/172.10# 41.42% Volume of Rock SSD Wt. Fines 71.64#/168.79# 42.44% Volume of Fines 83.86% Volume of Aggr. 100% - 83.86% = 16.14% Voids in Mineral Aggregates 6.75#/63.31# = 10.66% Volume of Effective A.C. 5.48% Volume of Effective Voids Percentage Voids Filled 10.66%/16.14% 66.05% Calculated Voidless Mix 149.68#/100% - 5.48% 158.35# Riguez Index 71.64#/42.44% plus 10.66% 134.91# Riguez Value of original Index 134.91#/133.74# 100.87%__________________________________________________________________________
Some information about the physical properties of the mixture is readily ascertainable as illustrated in the following Table 1. The balance point information was derived by interpolation means. Total Vol. quantity represents combined volume of aggregates and effective asphalt cement.
TABLE 1______________________________________CALC. BAL-NUM- POINT POINT ANCE POINTBER VALUE A B POINT C______________________________________1 # Rock 85 80 73.48 707 % Rock 58.80 55.76 51.71 49.5113 % Eff. A.C. 4.49 4.49 4.50 4.5120 T.W. #'s 153.22 152.04 150.49 149.6822 Vol % Rock 50.30 47.34 43.49 41.4222 Vol % Fines 35.42 37.76 40.82 42.4422* Vol % Aggr. 85.72 85.10 84.31 83.8622 % VMA (voids) 14.28 14.90 15.69 16.1422* Vol Eff. A.C. 10.87% 10.79% 10.69% 10.66%22 % Eff. Voids 3.41 4.11 5.00 5.48 *Total Vol. 96.59 95.89 95.00 94.5223 % Voids Fill 76.12 72.42 68.13 66.05-- 70% Void Fill 95.72 95.53 95.29 95.16-- 65% Void Fill 95.00 94.74 94.51 94.3626 Riguez Value 96.56 98.27 100.00 100.87______________________________________
As used in this specification, the expression "#" means the unit of weight "pound", and the expression "CF" or "C.F." means the unit of volume "cubic foot".
The Riguez Value curve of FIGS. 1 and 2 is derived from calculation 26 of points A (96.56%), B (98.27%), and C (100.87%), which represents different ratios between (a) the Riguez Value associated with the respective proportions of coarse and fines, and (b) the Riguez Index (133.74 pounds/cubic foot). The relationships for other proportions are graphically derived by interconnecting points A, B, and C, as depicted in FIGS. 1 and 2. Calculation 25 is the Riguez Value expressed as the S.S.D. weight of fine aggregate per combined volumes of fine aggregate V.sub.FA and effective asphalt cement V.sub.EAC. For example, calculation 25 of point A expresses the Riguez Value as the S.S.D. weight of fine aggregate (59.78 pounds) which occupies the 35.42% (calculation 22) of the unit volume divided by the sum of its own volume V.sub.FA (35.42%) and the volume V.sub.EAC of the effective asphalt cement (10.87%), which equals 129.14 pounds per cubic foot. Thus, the Riguez Value of a mixture of the proportion associated with point A equals the ratio 129.14/133.74, or 96.56% of the original Riguez Index. Similar calculations are illustrated for points B and C.
At the "Balance Point", the Riguez Value equals the Riguez Index, as previously indicated, FIG. 2 shows that the Balance Point is attained at a proportion, or gradation of coarse and fine aggregates, of 51.71% coarse aggregate and 48.29% fine aggregate; which implies that the amount of fines contained in a volume constituted by the fine aggregate and effective asphalt of a Balance Point mixture equals 133.74 pounds per cubic foot. This is the same quantity of fines contained in a unit volume of the Riguez Index specimen. Accordingly, a mixture of such proportion possesses the Balance Point parameters depicted in Table 1, one of which being 5% effective air void Volume V.sub.EAC which I sought to achieve.
CONTROL OF VOIDAGE AT THE MIXING PLANT
The voidage of a blended mixture having a balance point proportion contains the specified amount of air. To maintain continuity, I now explain how the voidage of the mixture can be accurately controlled by varying the quantity of asphalt cement or by varying the gradation over the no. 4 or similar size screen. As used in this specification and claims, "gradation", "gradation index" and "proportion" means the percent of coarse aggregate retained on the no. 4 or similar size sieve and is expressed in relation to the total dry weight of all aggregates in the mixture.
FIG. 2 shows an expanded portion of FIG. 1 near the balance point and sets forth the 65% and 70% voids filled isograms and a series of asphalt cement isograms ranging from 4.4% to 5.6% by weight asphalt cement. Using this chart, percent voids filled or percent effect air voids can be controlled by varying proportion or quantity of asphalt cement to produce a uniform pavement. Conversely, to analyze any mixture of aggregates and asphalt cement, the percent voids filled and the percent effective air voids can be readily ascertained by measuring its proportion over the no. 4 sieve and locating the corresponding point on the chart of FIG. 2.
To illustrate a typical plant control use, assume that it is desired to alter an asphalt mixture of our exemplary aggregates from the balance point to a mixture having 70% voids filled at the specified five percent by weight asphalt cement. The gradation of the mix at the plant would then be increased to 53.4% (the intersection of the 5% asphalt cement isogram and the 70% voids filled isogram). At the gradation of 53.4%, air void volume is readily ascertained as approximately 4.6%. If a 65% void fill value is desired, the gradation would be decreased to 48.8% and the air void volume changes to 5.6%. With the use of the asphalt cement isograms, the air void volume and the percentage of voids filled can be systematically and accurately controlled without the need to perform extended laboratory tests and analytical procedures now required in the paving industry.
To illustrate another use of the chart of FIG. 2, the percentage air voids also can be altered at a fixed gradation by varying the quantity of asphalt cement. For example, suppose that it is desired to maintain a gradation of 50%. It is readily seen that the mixture will contain 5.5% air voids at five percent by weight asphalt cement. By increasing the quantity of asphalt cement to 5.4% by weight, the air voids will decrease to approximately 4.4%. Any quantity of air voids can be obtained merely by interpolation between the asphalt cement isograms depicted in FIG. 2. Thus it now is apparent that a paving contractor can now alter his mix within the predesignated job specifications to achieve uniformity in the pavement. Likewise, the contracting authority can now accurately specify the optimum job specifications merely by examining the fine aggregates to be used in the job and with specific gravity value and absorption value of aggregates. Similarly, the paving engineer can design the mix within given job specification to produce a pavement having predesignated physical parameters with the same knowledge about the aggregate qualities.
FIG. 3 depicts density of the aggregate mixture as a function of gradation. Simply by using density of the compacted mixture at the laydown site as a job control parameter, the voidage parameters of the mixture can readily be ascertained upon correlation with the graph of FIG. 3. Of course, any parameter of the mixture that varies as function of gradation or effective asphalt cement can be selected as a job control parameter. The best job control parameters are those of the Riguez specimen as produced from the fine aggregate samples withdrawn from the mixing plant. The job control engineer may use voidage as determined by conventional testing methods as a job control parameter, but then the advantage of prompt results as provided by the Riguez specimen or other measurements for plant control purposes is sacrificed.
As previously indicated, aggregates which are supplied to the paving plant can simply be controlled over a single screen size, such as the no. 4 screen, for controlling mix proportions the mixture. Instead of using the chart of FIG. 2, other graphical parametric representations and comparisons of the mixture against the Riguez specimen can be used. It would be impractical however to present all such representations for mix design, mix analysis, or job control methods so the illustrations presented herein should be construed as illustrative, rather than as a limitation.
FLEXIBILITY AND STABILITY MIX DESIGN PROCEDURES
The degree of flexibility and stability of a compacted mixture can be designed by determining their respective values as a function of the ratio of crushed fine aggregate to blend sand in the mixture. The optimum proportion for voidage control of the blended mixture of fine aggregate and coarse aggregate can be determined by use of the Riguez Index method described method. The blending ratio of crushed fines to blend sand determines stability and flexibility. Once designing the proper blend of fines, plant control also can be effected by the method set forth above. Accordingly, my uniform pavement also can be produced to be uniform both in flexibility and stability. Furthermore, once the flexibility and stability functions are determined, any mixture of aggregates deviating from the specific ratio of crushed fines to blend sand can be adjusted in accordance with predetermined relationships.
Now, in designing a mixture having the desired flexibility and stability properties, I noted that with the use of the aggregates on hand, stability is increased as the percentage of crushed aggregate is increased and that flexibility (of flow) is increased as the percentage of asphalt cement is increased. These are two very important observations because now, a mix design engineer may proceed directly to the appropriate combination of crushed fines and blend sand to achieve the desired flexibility and stability. The procedure accordingly removes all guess work and trial-and-error which previously exists in mix design practice. Flow and stability are inversely proportional, however, both values may be achieved by increasing the extent of crushing of the crushed fine to produce greater stability for a given flexibility. I suggest crushed limestome to provide increased stability.
I have prepared in FIG. 6 a Riguez Value function for each of the seven different blends of crushed fines and blend sand (river sand). Curves 2 and 3 are examined at a gradation of 57.0% (Point a) and 55.4% (Point b), respectively. Both mixes have 5% asphalt cement, but the mix of Point (a) has 48.10% crushed fines and the mix of Point (b) has 57.45% crushed fines. Actual mixes at the selected gradation points were prepared. The mix of curve 2 had a stability of 3200 and flexibility of 5, while the mix of curve 3 had a stability of 2400 and flexibility of 10. Thus, it is seen that flexibility can be increased by adding more crushed fines in the blend of fine aggregate without changing the quantity of asphalt cement.
In examination of actual mixtures prepared at fine aggregate blends of curves 5 and 6 of FIG. 6, I note that at Point (d) and Point (f), stabilities of the samples were 3116 and 2590, respectively, and flexibilities were 9 and 9.5, respectively. Thus, flexibility can be increased by increasing the amount of asphalt cement. It also is interesting to note that for a higher percentage of crushed aggregate with the quantity of asphalt cement held constant the stability decreased to 2243 and flow increased to 13 (Point c). This observation can be used generally to ascertain the appropriate combination blend sand, crushed fine aggregate and asphalt cement to design a mix having any degree of flexibility, stability, air voidage or other desired property.
Further, I have been able to achieve mixes having a stability exceeding 3000 under the Marshall standards with flexibility values between 8 and 16. Present method only permit the at best may achieve flexibility of 5 to 8 for stabilities over 2000 psi under Marshall standards. Controlled stability and flexibility of my illustrated magnitudes have never been attainable by prior art methods.
FIGS. 4 and 5 show how the Riguez Index and total volume of the mixture varies according to the ratio of blend sand to crushed fines. The specific ratios are indicated in the corresponding examples set forth below. It is seen, from FIG. 5 that the Riguez Index increases as the amount of crushed fines in the total mix increases. Thus a more densse mix with increase structural strength having a greater stability is attained with increased crushed fines.
For seven different combinations of crushed fines and blend sand, I show how the Riguez Index changes in FIG. 5. In FIG. 6, I show the volumetric comparisons as a function of gradation for each of the different combinations. I have produced several samples of mixtures within the constraints indicated on the curves for illustrating the variation of physical properties of the mix and thereby provide the means to guide the design engineer directly to the desire mixture for achieving predesignated physical properties without any guess work or trail-and-error process whatsoever. Several gradation points of some of the curves were selected to have mix parameters within a predetermined job specification (such as percent asphalt cement 5%-5.5% percentage air voids 4%-5%, voids in mineral aggregate 15%-16%) and a mixture of the indicated blends were physically prepared. The physical parameters of some of the mixtures are tabulated below.
__________________________________________________________________________ Curve 1 Curve 2 Curve 3 Curve 4 Curve 5 Curve 6 Curve__________________________________________________________________________ 7Total MixCoarse Rock 60% 59% 45% 37% 40% 41% 23%Crushed Fines 15% 19% 36% 42% 44% 42% 62%Blend Sand 25% 22% 19% 21% 16% 17% 15%% Crushed Fines 39.76 48.10 57.45 58.33 71.16 69.30 75.92Bulk density of Fines 135.58 137.32 141.89 143.13 149.49 148.15 152.37 lbs/cubic foot lbs/cubic foot lbs/cubic foot lbs/cubic foot lbs/cubic foot lbs/cubic lbs/cubic footRiquez Index 129.20 130.87 135.27 135.90 142.97 140.52 144.54Sp. Gravity Fines (SSD) 2.7763 2.7793 2.7989 2.8003 2.8253 2.8242 2.8369Bulk Density Total Mix 154.50 157.70 156.40 155.20 156.90 156.12 156.50% Retained no. 4 screen 58.50 57.00 55.40 49.60 44.50 44.60 37.70% Asphalt Cement 5.0 5.0 5.0 5.5 5.0 5.5 5.5Volume of Rock 47.12% 48.01% 46.23% 40.86% 37.20% 36.96% 31.32%Volume of Fines 36.40% 36.82% 37.68% 42.11 46.78% 46.21% 51.96%V.M.A. 16.48% 15.17% 16.09% 17.03% 16.02% 16.83% 16.72%Effective Air Voids 5.12% 3.54% 4.45% 4.35% 4.41% 4.02% 3.90%Voidless Mix 162.84 163.49 163.68 162.26 164.14 162.66 162.85Riquez Value 101.6 101.0 98.85 98.82 98.90 98.20 98.24__________________________________________________________________________
FRICTION OVERLAYS USING RIGUEZ, STABILITY & FLEXIBILITY DESIGN PROCEDURES
A friction course for covering existing asphalt or portland cement concrete surfaces also can be produced using the teachings of this invention. As previously indicated, prior art friction courses comprises open graded mixtures which subjected the asphalt cement to water stripping and aeration. A friction surface is established by the corners and edges of the aggregated protruding upwardly of the riding surface. A low voidage dense graded friction course is preferred, but production of such mixes having high stability and flexibility have previously been impossible. Aggregate particles of prior art friction surface tend to reorientate or submerge into the underlying pavement, particularly when heated or subjected to impact forces, thereby losing their friction properties immediately after load applications. Friction courses of smaller particles are more susceptable to loss of friction quality because usual level of displacement and reorientation occuring with the permissible wide mix tolerances generally given to paving contractors. Using the above teachings however, a uniform low voidage high strength and highly flexible dense graded friction course can now be produced using aggregates of any size because the stability, flexibility, and air voidage can be accurately designed in the laboratory and controlled at the mixing plant. Friction overlays using top size aggregates as small as one-fourth inch or one-half inch can now be produced. Such pavements have superior life and frictional qualities and can be revitalized decades after initial laying by typical emulsions or reclamite without stripping the asphalt cement from the aggregates.
Any overlay requires the overlay thickness to be at least twice the diameter of the largest particle. Since I now may use smaller particles for my friction overlay, I can produce a thin overlay, such as one-half to one inch thick thus resulting in substantial cost saving in production and consumption of asphalt cement.
To establish mix design for friction overlays, it only is necessary to select the top screen aggregate size which renders the desired friction. Of course, larger crushed particles provide greater frictional qualities than smaller crushed particles. I provide a top size screen for crushed particles of three-fourth inch, one-half inch, three-eights inch, or the No. 4 screen. For highway application, the one-half inch top size screen is recommended. Airport landing and runway pavements require anti-skid frictional qualities depending on the size, speed, and weight of landing aircraft operating from the airfield. For landing speeds exceeding 140 knots for heavy jet aircraft, high impact resistence, stability, and friction is desirable at the point of landing and throughout the entire length of the runway for skin control and breaking. Flexibility exceeding 12 or 14 and stability exceeding 2500 or 3000 under Marshall standards can be designed and produced, as indicated above. Separation of the pavement material must be minimize to prevent flying debris upon takeoff and landing.
In any friction course, the quantity of mineral filler passing the No. 200 screen should be limited to eight percent maximum. Five percent to six percent is ideal. The proportion of blend sand should be selected to give the desired flexibility and stability for the overlay friction surface. Air voids should be minimized to two percent to three percent to minimize aeration and water seepage and to protect the underlay surface, flexibility should be designed between 8 and 16, and stability should be between 200 and 3000 psi under the Marshall standards. Accordingly, the designer of a friction overlay should first identify the top screen size to select level of friction desired, then perform the flexibility and stability design procedure set forth above to select the desired mix proportions (noting that increased crushed fines increases stability and increased asphalt cement increases flexibility) and then perform the volumetric analysis to achieve voidage generally between 2% and 3%.
FIG. 8 shows a conventional asphalt paving mixture plant which includes a number of feeding bins 10, 11, 12, 13 and 14 for storing mineral aggregates of different size. Aggregates within these storage bins are obtained from various stockpiles as previously indicated. The mixing plant shown herein includes five different storage bins which separately store mineral filler, blend sand, crushed fines, intermediate coarse aggregate, and coarse aggregate, respectively. Some mixing plants may comprise only two or three different storage bins in which case the modification of FIG. 3 may be implemented to produce greater blending capacity for the coarse aggregates. The improvement which I disclose in FIG. 2 illustrates another arrangement for measuring different sized aggregates in the storage bins and will be subsequently discussed.
The different sized aggregates in the storage bins 10 through 14 are dumped through either regulated or unregulated gate valves 20, 21, 22, 23, and 24, respectively onto a series of intermediate variable speed feed blets 30, 31, 32, 33, and 34, respectively. The rate of aggregate feed is conventionally controlled by controlling the speed of feed belts 30-34. The different sized aggregates from each of the intermediate feed belts are then supplied to the main feed belt 40 which carries all aggregates to an elevator belt 50 which loads the aggregates into a mixing bin 60. Mixing bin 60 is supported by a frame 61 above a loading station 62 which receives a transport vehicle 63 for transporting the hot mix to the pavement site.
Each of the intermediate feed belts 30 through 34 are equipped with variable speed pulleys that control the rate of aggregate flow onto the main feed belt 40. The surface of the respective feed belts 30 through 34 are sufficiently close to the mouth opening 20 through 24 thereby to allow regulation of aggregate flow, in accordance with the speed of the intermediate feed belts 30 through 34 without spilling aggregates around the area of the feed belts. The spacing between the gates 20 through 24 and the surfaces 30 through 34 of the feed belts may vary in accordance with the size of the aggregates contained in its associated storage bin, usually within a few inches.
The main feed belt 40 generally is a constant speed feed for passing all aggregates through the elevator belt 50 which includes a series of flanges on the surface thereof for carrying aggregates upwardly through the top of the mixing bin 60. The main feed belt 40 also contains a series of support pulleys 44 for supporting the weight of the aggregates on the belt 40, and also includes drive pulleys 45 and 46 for driving the feed belt 40. Similarly, elevator belt 50 is engaged about a respective drive and idler pulley 51 and 52 which both drive and support the elevator belt.
My invention improves this standard hot mix asphalt paving mixing plant by including weight cells 70, or 71, or 72 associated with the respective intermediate feed belts 30, 31 and 32. The weight cells 70 through 72 weigh the quantity of fine mineral aggregates which are supplied to the asphalt mixing bin. Conventional mixing plants contain only one load cell for measuring the total weight of the mix, such as load cell 80, which measures the weight of total aggregates as they are passed along the elevator belt 50. These weight cells measure the pressure caused by the weight of aggregates on the belt as the belt circulates. With the use of my invention, however, I am able to determine the weight of the total mix, as well as the weight of fines directly from readings of the weight cells. I also can determine the weight of the coarse aggregates by subtracting the quantity of fines from the quantity of the total aggregates. Therefore, I am able to more accurately regulate the gradation between the coarse and fine aggregates. When used with teachings of my co-pending application U.S. Ser. No. 23,390, filed Mar. 23, 1979, now U.S. Pat. No. 4,221,603 issued Sept. 9, 1980, I am able to control gradation over the No. 4 screen with the accuracy previously indicated.
Of particular importance, though, is the ability to make adjustments in quantity of different sized fine aggregates in accordance with the different amounts of moisture content. By providing separate weight cells for each different size aggregate bin, I am able to more accurately compensate for quantitative errors induced in the total job mix formula as a result of moisture content. Prior to feeding the fine aggregates into their respective feeding bins, moisture analysis would be performed. During the mixing operation, this analysis can be made in situ at the plant site by standard procedures.
Another embodiment of my invention is depicted in FIG. 9 for a mixing plant of the type illustrated. In this embodiment, it was only necessary to measure the quantity of fines in storage bins 10, 11, and 12. Therefore only one weight cell 75 was used on the transfer belt 40 which measures the force of aggregates only of bins 10, 11 and 12. Weight cell 80 measures all the weight of all aggregates. Thus the weight of coarse can be readily determined for gradation control purposes or for making adjustments for moisture content in the aggregates.
Referring now to FIG. 10, a cutaway section of one feeding bin is shown wherein a metal plate 90 is provided in the storage bin for dividing the bin into two separate compartments 96 and 97. For those mixing plants that contain a minimum number of feeding bins, the metal plate can be provided very economically to meet the demand for proportioning capability. The metal plate 90 extends from the top portion of the mixing bin 91 down to a position above the mouth 92 of the mixing bin. I have used this feature to proportion both intermediate sized aggregate and coarse aggregate contained within a single storage bin with satisfactory results in maintaining the desired proportion of the different sized aggregates. As previously explained, the aggregates contained within the mixing bin 91 are poured through the mouth 92 onto the surface of an intermediate drive belt 93 which is engaged about a drive wheel 94 and an idler wheel 95. A load cell 96, if employed, then measures the weight of the aggregates as they flow along the belt 93. The load cell is slidably coupled with the belt and measures the force of the belt resulting from the weight of the aggregates on the top surface thereof as the belt is driven. The operation of each intermediate belt including a weight cell of FIG. 8 operates in a similar fashion. The separator plate 90 also may be used to separate different sized fine aggregates, such as crushed fine and blend sand. The extent to which the separator plate 90 extends downwardly into the feeding bin 91 will vary according to the size of aggregates being separated and passing through the mouth 92. When feeding finer aggregates to the feed belt 93, a retainer is generally used to confine the mineral aggregates on the surface of the drive belt 93 to prevent spillage. As previously indicated, the distance between the mouth opening 92 and the surface of the drive belt 93 may be adjustable to achieve variable rate of flow of aggregates from the feeding bin 91. Thus, when the speed of drive belt 93 is increased, a greater quantity of aggregates may be taken from the feeding bin 91.
The several makes and models of mixing plants in which my improvement can be employed are Cedar Rapids, Boeing, Steel Strand and the CM1 model. My improvement may be used on a number of other mixing plant equipment.
My invention comprises the provision of separate weight cells on the intermediate feed belts associates with individual storage bins of an asphalt paving mixing plant. Any one of the individual feed belts may or may not include a weight cell. One weight cell may be used in the type of plant illustrated, as shown in FIG. 9. I found it necessary only to separate by weight coarse aggregates and fine aggregates only to make correction for most of the moisture content and most of the degradation of coarse aggregates. I achieved that separation by measuring the combined weight of the fine aggregates and then subtracting that weight of the total aggregates to compute the weight of the coarse aggregates. I provided individual weight cells for each of the fine aggregate feeding bins because of differences in moisture content between these sized aggregates. Finer aggregates have a tendency to hold and retain more moisture than the coarse aggregates, and accordingly, the moisture in the coarse can be estimated reasonably accurately although individual weighing cells can be provided for the respective feed belts of the coarse aggregate feeding bins for improved accuracy.
Likewise, in another aspect of my invention I provided a separator plate for establishing two compartments within a single storage bin to provide increased blending capability of the mixing plant. The storage bin also can be divided into four compartments by providing an arrangement of two intersecting separator plates. The invention designs in providing economical means to convert a mixing plant of limited blending capacity to a mixing plant of a higher blending capacity. I achieved this economical conversion by providing a separator plate, rather than an additional mixing bin.
This disclosure reveals the technical aspects of the Riguez Index method of design and stability and flexibility design. It is both a design method and control method and selects a particular gradation of aggregates or quantity of asphalt cement to render voidage values for producing uniform pavements. Thus substantial cost savings in construction are accorded. Further, the time required to perform the analysis and to ascertain corrective action at the mixing plant are much shorter so that results can be obtained in a few hours, instead of weeks. The principal design and control methods for selecting select proper combinations of aggregates are very easy to execute in the field and in the laboratory. The teachings of this invention will allow paving contractors to design their mixes, evaluate better sources of aggregates, give exact bids, and, more important, to have a very meaningful understanding of the proper design of their product by their own forces and control.
It will now be apparent to those skilled in the art from the above teachings that substantial uniform pavement areas can be produced by mix design, mix analysis, and job control methods other than those specifically indicated in my disclosure. My invention includes the product and the processes disclosed as well as those products and processes that come within the scope of any modifications, adaptations, or extensions of the above teachings.

Number	Name	Date
RE30047	Lees et al.	Jul 1979
2884841	Dickinson	May 1959
3824109	Richards	Jul 1974
3907582	Walter	Sep 1975
3965281	Takase et al.	Jun 1976
4196922	Fabb et al.	Apr 1980
4221603	Trujillo	Sep 1980

	Number	Date	Country
Parent	23390	Mar 1979
Parent	764336	Jan 1977

Adaptive mix proportioning method for use in asphaltic concrete mixing plants

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