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I-60.NCSA.68

A Simple Guide

for the

DESIGN OF FLEXIBLE PAVEMENTS

USING CRUSHED STONE

National Crushed Stone Association

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FOREWORD

In 1965, the National Crushed Stone Association issuedits Statement of Policy With Respect to the AASHO RoadTest. Since then, many authorities have agreed with NCSA'sposition and refuse to accept the so-called layer equivalencyconcept which evolved from the findings of the Road Test.One in particular, the world-renowned Francis N. Hveem, inhis 1968 NCSA Convention paper said:

'I have always deplored the use of thisterm in the engineering design of pave-ments......It enables salesmen or promot-ers of some particular type of pavementto pose as engineers. Too much dependsupon the circumstances, the nature ofmaterials, climatic conditions and aboveall, the weight of traffic, to justify anysuch concept as an 'equivalency ratio'fixed for all materials and conditions.'

This guide for the design of flexible pavements has beenreviewed and approved by NCSA's Technical Committee.Questions regarding any of the information it contains shouldbe directed to Frank P. Nichols, Jr., NCSA's EngineeringDirector. We feel that this publication will assist you in these1ction of material types and thicknesses for a variety ofpavement design situations.

W. L. CarterExecutive Director

March 1968

INTRODUCTION

The design of flexible pavements appears more and morecomplex as more and more research is performed. Basically,however, the observation of certain fundamental principles indesign and construction can usually result in successful andserviceable structures at modest cost.

The typical flexible pavement should embrace threemain elements. The first—perhaps the most important thoughoften given the least consideration—is the foundation, eithera firm, unyielding subbase or a stabilized or otherwise im-proved suhgrade. The second is the base. whose function is totransmit and cushion wheel load stresses to an extent that thefoundation and subgrade are not overstressed. The third isthe bituminous mat, a combination of layers of aggregatesand bituminous binders whose function is to resist abrasion,tangential stresses from acceleration and deceleration, and theinfiltration of moisture. The following notes have been pre-pared to provide guidance to engineers and architects chargedwith the selection of materials and methods of constructionfor these elements. Emphasis is laid on the advantages andthe proper use of dense graded crushed stone for the founda-tion and base courses.

THE FOUNDATION, OR IMPROVED SUBGRADE

Construction of a proper foundation for a flexible pave-ment structure should involve considerably more than justsmoothing up the native soil and rolling it, although in a fewisolated areas this may be about all that is necessary. Themere fact that the bare soil is smooth and hard and can sup-port heavy loads when fairly dry is poor insurance againstsubsequent loss of support. If the native soil cannot be madepermanently stable and essentially non-resilient under all an-ticipated conditions of load and weather, it should be strength-ened by one of several proven methods. Attempts to "bridge

over" weak subgrades by use of thin semi-rigid bases or bythickening the bituminous mat have resulted in some costlyfailures.

Blanket of Granular Material

The most popular method of foundation preparation in-volves a granular material blanket of such thickness that, it isbelieved, the native soil is beyond the depth of frost penetra-tion or the influence of applied loads. Research has shown,however, that legal wheel loads are often felt to depths of sev-eral feet, deeper in most cases than the frost line. Soils whichare resilient or lacking in shear strength, even though blanket-ed to a depth great enough to protect them from freezing,may still be stressed enough to cause damaging deflections orpermanent deformations under repeated heavy loadings. Fig-ure 1 shows fatigue cracking caused primarily by resiliency ofthe foundation layer.

Figure 1. Surface distress commonly associated with inadequate support at thefoundation level.

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Available test methods, such as the California BearingRatio test, ASTM Standard Method D 1883,* may give someidea of the shear strength of the soils involved, but standardtests for resiliency are still in the developmental stage. Thepresence of a high percentage of silt sized particles, and moreparticularly the presence of shiny flakes of mica, may be adanger sign foreboding critical resiliency that will lead in timeto serious fatigue cracking of the bituminous surface. Perhapsthe only reliable indication of the supporting power of thenative soil is the performance of highways in adjacent areas.

Local granular materials from roadside pits often lackstability or may be resilient themselves and thus will not per-form the functions of a foundation layer. Some areas, how-ever, are blessed with abundant local materials of good qual-ity which, being neither resilient nor frost-susceptible, haveperformed well on other projects. In such areas this type offoundation construction may be the most logical choice.Specifications for such materials should require a reasonablegradation —cohesionless sands are undesirable—and a degree ofstability equivalent to a CBR value of around 20 or more ina soaked condition. Good drainability is desirable, but tooopen a texture may not be a wise choice unless positive out-lets are provided at frequent intervals for the removal of freewater which might otherwise soften the underlying soil.

Crushed Stone Subbase

More reliable granular subbases have been built of crushedstone on extensive sections of the interstate and primary sys-tems in states such as Virginia and Connecticut. Materials forthis purpose may have a variety of gradations, usually openand free draining, with top sizes preferably in the 2 to 3 inchrange (See Figure 2). Such large particles, keyed togetherand confined by denser layers above, permit the attainmentof extremely high stability. The same precautions notedabove must be followed with respect to provision of positive

* See Appendix B

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outlets for drainage. Too open a gradation may also invite in-filtration of soil fines from beneath.

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ur 2C ar e Open Graded Crusher Run Stone of a TypeSuited for Subbase Construction.

Subgrade or Subbase Stabilization

A third method of foundation improvement is availablethrough chemical stabilization of the native soils, usually withcement or hydrated lime. Cement is more suitable in friablesoils where mixing and compacting may be most easily ac-complished; 6 to 10 percent cement by weight is most gener-ally specified. Lime, which reacts more slowly, is more effec-tive in cohesive soils which are not easily pulverized. Eithermethod, where it can be successfully carried out, is extremelyeffective in killing the plasticity, resiliency, and frost suscep-

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tibility of the native soil and in providing a stable workingplatform on which to construct subsequent layers.

On some projects, preference should be given to chem-ical stabilization in the granular subbase rather than in the na-tive soil. Here a reasonably uniform aggregate material suchas stone screenings or crusher run, which may be too high infines for the more usual free draining subbase, is stabilized bythe addition of 4 or 5 percent cement. Such a mixture usu-ally makes a truly superior foundation course for the layersabove and greatly reduces the load stresses on the soil be-neath. Being relatively impervious, it is not so necessary thatthese stabilized subbases be drained.

It is the nature of any cement stabilized soil or aggregatemixture to develop shrinkage cracks. It is important thereforethat the base material above such foundations be of a typethat will absorb these cracks rather than reflect them throughto the surface.

The choice of method of foundation construction de-pends upon types of soils in the grade, the availability of ma-terials in the area, and the depth of probable frost penetra-tion. Again it is emphasized that only in rare cases may foun-dation construction be ignored.

THE BASE COURSE - WHERE CRUSHED-STONE IS THE IDEAL CHOICE

As already noted, attempts to use rigid or semi-rigidbases for flexible pavements often produce undesirable re-sults. Concrete base slabs must have joints or else they willdevelop cracks; either the joints or the cracks will reflectthrough bituminous surfacings, creating maintenance prob-lems as well as an unsightly condition. (See Figure 3)

Soil cement or cement treated aggregates, although quitesuccessfully used in many areas as subbases, often create the

same sort of problems when used as bases (See Figure 4).Even hot mixed asphaltic bases may tend to develop crackswhich are reflected through successive resurfacings, particu-larly where local bank run aggregates are used in poorly de-signed mixes. (See Figure 5)-.--t--- ---- • -..••.&r_4,

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Figure 3. Cracks Reflected Through4 Inches of Asphaltic Concrete fromJoints )background) and RandomCracks )foreground) in a Portland Ce-ment Concrete Base.

Figure 4. Cracks Reflected Through2'/ Inches of Hot Mixed Sand Asphaltfrom a Soil Cement Base.

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Figure 5. Hot Mixed Sand Asphalt Used as Base and Surface 7-inch total thick-ness) on Lightly Traveled Highway is Badly Cracked. Attempts to Fill Cracks andSeal Surface Unsuccessful.

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The ideal material for use as a base for flexible pavementis quality controlled, dense graded crushed stone. Properlygraded, compacted, and supported by an adequate founda-tion, dense graded stone possesses all the attributes requiredof a base course. Through internal friction between its parti-cles, it resists lateral displacement and further densificationunder traffic. Thus it normally needs no cementitious addi-tives and so should not develop shrinkage or fatigue cracks.

Research at the laboratory of the National CrushedStone Association has determined that the following are themost desirable characteristics of a graded aggregate base:

The grading should be continuous from the maxi-mum size down to and including the dust of frac-ture.

2. The maximum size of aggregate should be the larg-est that can be handled without segregation.

3. The quantity passing No. 200 sieve should be be-tween 5 and 12 percent and not over 10 percentwhere frost penetration may be a problem. Evenlower percentages are preferable.

4. The plasticity index should be kept as low as pos-sible: non-plastic fines are the best.

5. The aggregate should be compacted to the maxi-mum density attainable, preferably not less than100 percent of AASHO T-180(ASTM D 1557).

6. The aggregate should consist of crushed stonethroughout the size range to assure the higheststrength and greatest rigidity.

(See NCSA Engineering Bulletin No. 12-Characteristics ofGraded Base Course Aggregates Determined by Triaxial Tests—available from the Association's Washington office.)

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It is emphasized that the grading should be uniform,from a maximum size as large as can be handled properlydown to and including a closely controlled percentage of non-plastic fines. The actual percentage passing one of the inter-mediate sieve sizes is not as important as is the fact that thispercentage should not vary widely from load to load. Varia-bility, even within permissible ranges, is undesirable for sever-al reasons, but principally on account of the effect producedon the density to which the material can be compacted andon the moisture content needed to obtain this density. Themaximum density in standard tests like AASHO T- 180 variesappreciably as the percentages of coarse and fine aggregatevary. (For a more complete explanation of compaction con-trol of graded aggregates, see Nichols' paper in Highway Re-search Board Bulletin 325.)

Research by NCSA has also shown that an increase ofonly six pounds per cubic foot in the density of a given aggre-gate mixture can increase its shearing resistance in the CBRtest around 2 1/2 times, further emphasizing the importance ofcompaction control on base materials. The importance of ad-equate and uniform compaction cannot be overemphasized.Achievement of this goal is facilitated by good constructiontechniques which minimize segregation during laydown. (SeeFigure 6)

Figure 6. Proper Equipment for Handling and Spreading Aids in Ob-taining Uniform Density and Minimizes Segregation.

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Specifications for the materials and methods of con-struction required for a high class crushed aggregate basecourse should contain paragraphs similar to those suggestedin Appendix A. The materials specifications, while permittinga variety of gradations--and consequently maximum econ-omy of production—emphasize the importance of uniformityof gradation from truckload to truckload. This important at-tribute is seldom present in naturally occurring deposits, andcan be assured only by the careful quality control exercisedby reputable commercial producers. It will be noted thatthese specifications have a great deal to say about how thematerials are to be blended, sampled, and tested at the source,hauled to the job, spread, and compacted.

A further note is appropriate for the designer of pave-ments to be subjected to deep frost penetration. For suchservice, the base material's job mix formula should be estab-lished toward the lower limit of the range with regard to theamount passing the 200 mesh sieve. If this requirement can-not be met by the local producers, it may be found necessaryto incorporate an appropriate stabilizing admixture to renderthe minus 200 material innocuous Emulsified asphalt, port-land cement, and lime-fly ash have all been used with successwhere proper attention has been given to the necessary curingconditions.

As noted earlier, stabilization should not be resorted toas a substitute for compaction or for the purpose of makingthe base strong enough to bridge over weak spots in the foun-dation. The addition of more than two or three percent ce-ment, by weight, may cause shrinkage cracking and make thebase brittle. Admixtures to a stone base material should befor the sole purpose of altering the character of the fines tomaintain the mixture's inherent stability under all weatherconditions.

Finally, the importance of achieving the maximum den-sity practicable for the grading selected for the job is againemphasized. It is not the value of density, per Se, that is im-

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portant; higher density values can often be obtained by in-creasing the percentage of fines, but this may create other un-desired effects. The important point is that the materialplaced in the road be compacted so that little or no addition-al densification can possibly occur under traffic. This may beaccomplished more easily with the addition of a very smallpercentage of calcium chloride, which may provide the addedadvantage of combatting potential frost susceptibility.

THE BITUMINOUS MAT

The choice of the type and thickness of the bituminousmat to be used depends upon a number of factors. Forless important rural highways and other locations where traf-fic volumes are apt to be light, there is often a great advantageto be gained through the use of stage construction, by whicha temporary or semi-permanent wearing surface is applied aspart of initial construction and a final surfacing is programmedfor later application when needed. Under this system, thetemporary surfacing should possess a high degree of flexibilityto withstand the relatively high deflections which may occurearly in the pavement's life. Various types of bituminous sur-face treatment have been used successfully as temporary rid-ing surfaces.

Where heavier traffic is expected and where superior rid-ing quality is deemed essential, a bituminous concrete mat isgenerally called for. Standard state highway specificationsare usually the best guide to suitable mixtures for this pur-pose in a given area. Suitable mixes are also defined underASTM Standard Specification D 1663 for Hot-Mixed, Hot-Laid Asphalt Paving Mixtures. Although other types of ag-gregate have been used successfully in some bituminousmixes, crushed stone meeting the quality requirements ofASTM Standard Specification D 692 truly sets the standardof excellence for this purpose.

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On a base and foundation constructed in accordancewith this guide, a thickness of bituminous concrete of fromfour to six inches is most highly recommended. The lowerportion of this thickness is usually termed the binder, andshould contain stone with a top size of 3/4 inch or somewhatlarger. The upper l'/4 to l'/2 inch wearing surface mix gener-ally does not contain aggregate as large as 3/4 inch, but a topsize smaller than ½ inch is not recommended. Finer mixes areoften considerably less durable and develop fatigue cracks atan early age. In addition, although some fine mixes have highskid resistance at low speeds, heavy downpours often pro-duce water films thick enough to cause vehicles to "hydro-plane," causing complete loss of steering control. Coarsermixes tend to break this water film and minimize the prob-lem.

THICKNESSES OF THE BASE AND BITUMINOUS MAT

If the designer has assured himself that a permanentlyfirm, non-resilient subgrade or subbase will be provided as afoundation, the question of thickness of the crushed stonebase and bituminous mat becomes relatively simple. Recallthough that the key to this relative simplicity is the assump-tion that the foundation can be relied on to provide supportequivalent to a CBR value of 20 at all times and under all con-ditions. The total thickness required above this foundationcan 'be estimated from design curves devised by the Corps ofEngineers and many other agencies. To add a factor of safetywithout adding unduly to the cost, the designs may be se-lected to satisfy a slightly lower CBR, say, 15.

Depending only upon the volume and weight of trafficexpected, the total thickness of base and bituminous mat re-quired over a CBR of 15 might vary from 7 to 15 inches. Asan additional factor of safety to account for ever increasingwheel loads and tire pressures, the table of standard designspresented below includes total thicknesses of from 8½ to 18inches.

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The table of designs just referred to gives thicknesses interms of five categories of traffic. Traffic counts ordinarilyare made in terms of the average daily volume of (a) automo-biles and pickups, (b) light commercial vehicles like packagehaulers, (c) more heavily loaded vehicles like dump trucks,and (d) trailer trucks and large buses. For design purposes,however, it is generally necessary to consider only the heaviervehicles of designations (c) and (d).

A typical design table for use where crushed stone aggre-gates meeting the specifications of Appendix A are availablefor the base course is presented next:

Table 1

Standard Designs for Construction on a Non-ResilientFoundation With a Support Value Equivalent to CBR 20

Thickness of HighType BituminousMat

Traffic Category1

IIIIIIIV

2 1/2Y1.,,

V

6"

Thickness of WellCompacted DenseGraded Stone Base6"8"8210 2 122

Note 1. Definition of Traffic Categories:Number Average Daily Volume of Heavy Trucks

and Buses in One Direction

0-15II

16-50III

51-200IV

201-800V

801 and over

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Note 2. Base courses in designs for categories III, IV, and V,must be constructed in two layers; bottom layermay he stabilized with 4% portland cement byweight; top layer should extend through outsideshoulder for emergency traffic use.

Although these thicknesses may not be entirely appro-priate in all areas, they are believed suitable for the Middle toNorth Atlantic States and similar climatic conditions. Wherefrost penetrations are deeper than in these areas, it is sug-gested that the foundation layers be extended to greaterdepths. Where there is little or no frost penetration, slightmodifications may be made in both base and foundationthicknesses, but neither should be reduced drastically.

No substitution ratios are offered for the use of alter-nate materials of lower or higher quality than those describedabove. The use of lower class materials in layers of greaterthickness is seldom justified since failures are likely to origi-nate within the layers themselves. The use of higher quality,more expensive materials like asphaltic concrete to replacesome of the graded crushed stone base cannot be justifiedeconomically. Some reasons for these statements may befound in the following section.

THE FALLACY OF SELECTING DESIGNS BASED ON-"EQUIVALENT THICKNESSES"

Data from the AASHO Road Test have indicated tosome that certain equivalent thicknesses of different types ofbase material might be universally applicable. The fallacy inthis concept has been clearly exposed in two reports by theEngineering Director of the National Crushed Stone Associa-tion.* These reports recognize that at certain levels in theNichols, F. P., Jr.. "Facts and Fallacies Concerning Crushed Stone Bases," pre-sented at the 50th Annual Convention, National crushed Stone Association,Miami Beach, Florida, January 27. 1967, and"A Practical Approach to Flexible Pavement Design," presented at the SecondInternational Conference on the Structural Design of Asphalt Pavements, Uni-versity of Michigan, Ann Arbor, Michigan, August 7-11, 1967.

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pavement structure certain types of material provide betterperformance than other types. They indicate, however, that,at the base course level in the foregoing table, a properlygraded and compacted crushed stone, which usually needs nocementitious additive, possesses all of the attributes of anideal base material with respect to the ability to resist lateraldisplacement, fatigue cracking, further densification undertraffic, and the tendency to soften after being frozen in a sat-urated condition. While other, more costly materials mayalso possess most of these attributes, they are not significant-ly better at the base level than crushed stone—on some countsthey are actually inferior—so that on the basis of pure eco-nomics they are not as good a buy.

As noted earlier, in areas where frost may be a problemand where the aggregates available cannot be processed tomeet the suggested requirements in the Appendix, with par-ticular regard to the percentage passing the No. 200, it maybe found advisable to add a small percentage of cement or asimilar stabilizing agent to overcome these deficiencies. Ce-mentitious additives should be added in limited quantities,however, and only for this purpose. They should not be add-ed to make the base strong enough to bridge over weak spotsin the foundation—only to maintain the material's inherentstability under all weather conditions.

The pavement designer who has access to good qualitycrushed stone for his base course is in a fortunate position in-deed. If he can also be assured of the required uniformity,which is a function of the quality control practices of reput-able producers and contractors, he can be confident that aquality job will result.

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APPENDIX ASuggested Items for Specifications for Materials and construction

of Graded Crushed Aggregate Bases or SubbasesA. MATERIALS

1. The material should consist essentially of the products of crushingoperations down to the including the dust of fracture.2. Coarse aggregate retained on the No. 4 sieve should preferably consist ofhard, durable particles of stone, capable of withstanding the effects ofhandling, spreading, and compacting without serious degradation productiveof deleterious fines. Where stone is unavailable and economics dictate the useof other materials, at least 75 percent of the particles larger than 3/8 in. shallhave two or more freshly fractured faces."3. Fine aggregate passing the No. 4 sieve shall consist primarily ofmanufactured (crushed) sand and finer mineral matter. The fraction passingthe No. 200 sieve of either the final blend or any added fine mineral mattershall not exceed 80 percent of the fraction passing the No. 50 sieve. Thefraction passing the No. 40 sieve of either the final blend or any added finesshall not have a liquid limit greater than 25 or a plasticity index greater than4.

4. The composite blend shall be free from vegetable matter and lumps or ballsof clay. It shall conform to the overall grading requirements as well as the jobmix tolerances which follow:

Percentages Overall Job Mix TolerancesPassing Sieve SizePercentages PassingBaseSubbase

3m. 100 - 01 1/2 in. 90-100±2 ±53/4 in. 60-97 ±5 ±83/8 its. 40-75 ±8 ±10No. 4 25-60 78 ±8No. 16 15-40 ±5 +8No. 50 8-22 ±5 ±5No. 200 010* ±3* +3

*Samples shall be tested by wet sievingThe job mix formula chosen shall be suitable for the traffic, climatic

conditions and the type of aggregate available, and shall be approved by theengineer prior to the start of work. (A plotted graph of the job mix formula fromthe No. 4 sieve size down should lie essentially parallel to the plot of mid-rangevalues of the overall master range). Normally if these materials are to be used forbase course, a 1/i In. top size should be specified; if for subbase, a larger top sizemay be permi'tted. Variations from the applicable job mix tolerance or anyvariations outside the overall master range shall not be permitted without penalty.B. CONSTRUCTION CONTROL

Description

Graded crushed aggregates meeting the requirements of Section A(Materials) may be specified as the base course under relatively light bituminouswearing surfaces or as a subbase under heavier bituminous mats which includeeither a "black base" or a penetration macadam layer. If used as the base course,the job mix formula will normally prohibit particles which would be retained on al'/s inch sieve.

Construction MethodsI. Foundation

Graded crushed aggregate base or subbase courses shall be constructed onlyon a properly prepared, firm non-resilient foundation which shall have beencompleted at least 500 feet ahead of the aggregate spreading operation. Suhgrade

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drains extending through the shoulders shall have been installed, when directed bythe Engineer, at low points in the road profile, spaced no more than 50 feet apartfor at least 150 feet each side of such points and at such other locations as may bedesignated on the plans.

2. Mixing

All materials shall be accurately proportioned and mixed at the source in apugmill or other device approved by the Engineer. A quantity of water shall beadded sufficient to assure the optimum moisture content in the material whenplaced. Calcium chloride, portland cement, emulsified asphalt, lime and fly ash, orany other additives which may be specified shall be accurately metered into theaggregate material prior to its entry into the mixer.

3. Sampling and Preliminary Approval

The final mixture shall be sampled from the plant output at intervalscorresponding to no more than 3000 tons, or at least daily. Sampling methodsshall ensure the cutting of increments from the lull cross section of the processflow; each sample shall be a composite of three or more increments taken atrandom from the volume of material required to make up a truckload. Testportions shall be obtained by quartering or splitting the sample to appropriatesize.

Preliminary approval of the material as produced shall be based upon thefavorable results of tests performed by the purchaser or his authorizedrepresentative on the plant output samples.

4. handling and Transporting

Mixed materials shall be handled and transported so as to minimizesegregation and loss of moisture or volatile materials.

5. Spreading

Material that has been satisfactorily mixed and approved shall be spreaduniformly over the surface by means of self-propelled mechanical spreaders of anapproved type capable of cutting the loose material to a string-line grade with aminimum of segregation. In no case shall the material be dumped directly on thefoundation or on the preceding course.

6. Compaction

Compaction shall be by means of appropriate rollers or vibrators to anaverage density equal to at least 98 percent of the density obtained on the samematerial by AASHO Standard Method T-180 (ASTM Standard Method D 1557)or, if elected by the contractor, 98 percent of the density obtained on a controlsection constructed over a cement stabilized foundation and compacted with theapproval of the purchaser or his authorized representative.

When completed, the surface shall be broomed so as to produce a distinctlygranular texture free from loose material or excess fines worked up by thecompaction operation.

APPENDIX B

Typical CBR Values for Typical Soil Types

Soils Classification( Typical Range inAASHO Standard Designation M-145) CBR Values

A-l .aorA-1 . b 20+A-2-4 or A-2-5 25+A-2-6 or A-2-7 8 to 25A-3 12+A-4 4to25A. 5 Oto7A-6 Otol5A-7-5 or A-7-6 0 to IS

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For additional information on crushed stone for use in pave-ment construction, write to:

Frank P. Nichols, Jr.Engineering DirectorNational Crushed Stone Association1415 Elliot Place, N. W.Washington, D. C. 20007

Also available from NCSA headquarters is a 16mm soundcolor motion picture entitled "Graded Aggregate Base CourseConstruction," covering the best of recent production andconstruction practices with respect to this type of material.Reservations for the use of this film for showing to interestedgroups may be made by writing to the above address.

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