AD-763 912 STABILIZATION FOR PAVEMENTS J. L . R ice …

AD-763 912

STABILIZATION FOR PAVEMENTS

J. L . R ice

Army Construction Engineering Research Laboratory Champaign, Illinois

May 1973

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TECHNICAL REPORT SI 1

STABILIZATION EOR PAVEMENTS

by

/. L Ricv

May 1973

yTTD C ^

D Department of the Army

CONSTRUCTION ENGINEERING RESEARCH LABORATORY P.O. Box 4005

Champaign, Illinois 6IH20

Approved for public release: distribution unlimited.

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ABSTRACT

Rigid and flexible pavement model tests were conducted to evaluate methods for assessing the structural benefits imparted to a pavement structure by stabilized elements. Current Corps of Engineers rigid pavement design and evaluation riKthods ate based on stress in the concrete pavement as calculated by the Westergaard algorithm. This method appears applicable for pavements containing lime and bituminous stabilized layers only. Cement stabilized layers should be evaluated by an elastic layered algoiithm. The California Bearing Ratio method of design and evaluation of flexible pavement structure appeared to yield satisfactory results for flexible pavements containing stabilized elements.

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FOREWORD

This investigation was sponsored by the U.S. Army Corps of Engineers, Office of the Chief of Engineers (ÜCE), Washington, D.C. as part of Project "Investigation for Development of Engineering Criteria," under Subproject, "Soils Stabilization and Stability"

The investigation reported herein was conducted by the U.S. Army Construction Engineering Research Laboratory (CERL), Champaign, Illinois and the Ohio River Division Laboratories (ORDL), Cincinnati, Ohio during the period FY 62 -PY 71.

CERL personnel actively engaged in the investigation were Messrs. J.L. Rice and J.J. Healy. Other personnel involved were: Messrs. P.M. Mellinger (ORDL Director, retired) and J.J Scanlon (ORDL) This report was prepared by Mr. J L. Rice.

Col. R. W. Reisacher is Director of CERL and Dr. L. R. Shaffer is Deputy Director.

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CONTENTS

ABSTRACT FOREWORD

... .V

1 INTRODUCTION 1 Hypothesis to be Evaluated

Problem

Background

Scope and Objective

Previous Attempts to Solve

Theoretical Considerations

Prognosis

2 APPROACH 3

Pavement Performance

Types of Specimens and Tests

Values of Variables and Control Constants

Measurement Methods

Method of Analysis

3 TEST PROCEDURES 4 Specimens Prepared

Instrumentation Set-Up

Construction of the Test Models

Conduct of the Tests

4 TEST RESULTS 6

5 ANALYSIS AND DISCUSSION 10

Analysis

Discussion

6 CONCLUSIONS, RECOMMENDATIONS AND CLOSURE 17 Conclusions

Recommendations

Closure

REFERENCES 1 ^^^J: -— FIGURES JTS^^^ 19

pUtribution DISTRIBUTION DD FORM 1473

FIGURES

Figure Page

1 Completed Subgiade to Placement (it Polyethylene Sheeting

2 Coating of Crack Alarm Grid Wires with Epoxy Resin

Prior to Conr .ie Placement 3 Concrete Placement Approximately One Hall Complete

4 Vibrating Pan Type Compactor used to Compact Crushed

Stone Base Course Material for Flexible Pavement Tests 5 Compaction of the Lime Soil Mixture with the Pneumatic Tamper

6 Overall View of a Completed Flexible Pavement Prior to Test Traffic

7 Linear Variable Differential Transformer Mounting Techniques

8 Typical Layout of Crack Alarm Wire Grid 9 Typical Oscillograph Trace from Traffic Testing of

Model Pavement 12 10 Typical Deflection-Coverage Plot, West Slab, Nonstabilized I I Typical Deflection-Coverage Plot, Hast Slab, Nonstabilized 12 Typical Strain-Coverage Plot, West Slab, Nonstabilized

13 Typical Strain-Coverage Plot, Fast Slab, Nonstabilized 14 Typical Deflection-Coverage Plot, West Slab, Stabilized 15 Typical Deflection-Coverage Plot, Last Slab, Stabilized 16 Typical Strain-Coverage Plot, West Slab, Stabilized 17 Typical Strain-Coverage Plot, East Slab, Stabilized 18 Design Factor versus Coverages foi Initial Slab Failure

19 Typical Summary of Load Transfer Tests Acioss lomt 20 Design Curves for Concrete Airfield Pavements, Single Wheel,

100 Sq. In. Contact Area 21 Nondimensional Plot of Single Wheel Design Curve and

Model Test Data Using Westergaard Analysis 22 Close Up View of a Failed Area on Model Test 6

23 Close Up View of a Failed Area on a Prestressed Pavement

24 Summary of Rigid Pavement Tests 23 Consolidated CBR Curves for 5000 Coverages 26 Percent Design Thickness Versus Coverages- Flexible

Pavement Failure

27 Summary of Flexible Pavement Tests

21 21

24 24 25

25

26 26 27

27

30 30

31 31

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Construction Engineering Research Laboratory P.O. Box 4005 Champaign,'Illinois 61820

2» BEPORT SEtUUlTY CLASSIFfCATION unclassiTied

2b. GROUP

I IM I'OII f TITLt

STABILIZATION FOR PAVEMENTS

4. DESCRIPT IVE NOTES llxfic of rrpoil and mr/ujiic daletl

Technical Report 5 AurHOR(S) (Ftrtl name, middle mtlKil. laut namri

J. L. Rice

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12. SPONSORING MILITARY ACTIVITY

Department of the Army

1 J. ABSTRACT

Rigid and flexible pavement model for assessing the structural benefits stabilized elements. Current Corps of evaluation methods are based on stress by the Westergaard algorithm. This me containing lime and bituminous stabil! layers should be evaluated by an elast Bearing Ratio method of design and eva appeared to yield satisfactory results stabi1ized elements.

tests were conducted to evaluate method Imparted to a pavement structure by Engineers rigid pavement design and in the concrete pavement as calculated

thod appears applicable for pavements zed layers only. Cement stabilized ic layered algorithm. The California luation of flexible pavement structure for flexible pavements containing

14 KEY WORDS

pavement model tests elastic layered algorithm flexible pavement structur

DD FORM 1473 REPLACES DD FORM 1473, 1 JAN 64, WHICH IS 1 NOV 65 OBSOLETE FOR ARMY USE.

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r STABILIZATION FOR PAVEMENTS

1 INTRODUCTION

Hypothesis to Be Evaluated. The perlornunce uf pavements containing stabilized elements has been observed to be superior to the performance of similar pavements without stabilized elements.1 ■2•, It would appiMi ilut this improved performance could be oxploiio' i. design of new pavements and in the «.•valuation of existing pavements. These improvements can resul' in large economic benefits during the life of the pavement.

Problem. The use of various stabilizers to improve soil workability and/or to enhance soil strength has been widely recognized as beneficial to pavement construction and performance. Data and analysis are lacking to determine the actual structural benefits imparted to a pavement structure by the incorporation of a stabilized element in the pavement structure. A need exists to quantify the structural benefts associated with stabilized elements and to provide a rationale which engineers can use to exploit the structural benefits derived from stabilization. Present design and evaluation procedures are limited as to the inclusion of stabilized elements in the subgrade. In the case of rigid pavement, all materials beneath the pavement slab are represented by a "k" value obtained from a plate bearing test run on the material directly below the pavement slab. For flexible pavements, the California Bearing Ratio (CBR) is used to describe the strength of pavement elements. Some stabilized layers possess sufficient rigidity and strength to exhibit some flexural action and, because the CBR test is basically a penetration test, its use in assessing the strength of a flexural type element is questionable.

Background. Various forms of stabilization have been used for some 20 to 25 years to improve soil workability and/or provide a working platform for

construction operations. At times, contractors have elected to construct a stabilized layer in order to maintain construction operations during periods of inclement weather. Stabilized layers are also constructed to provide support for construction machinery during favorable weather since construction operations lesult in a rather high volume of traltic during paving operations.

Generally, the performance of pavements with a stabilized element is superior to the performance of identical pavements without stabilized elements. This has bee'i observed in surveys of actual in-service pavements.4 The designer is eager to exploit the improved performance associated with stabilized layers: cost reductions may be realized if a stabilized layer could be used to replace either a higher cost subbase, a select base, or to reduce the quantity of paving materials. Very little quantitative information h-s been available to provide the pavement designer with a rationale to assess the structural benefits associated with stabilization.

Scope and Objective. An extensive testing program has been initiated to determine the structural benefits of stabilized layers in a pavement structure and allow the designer to exploit IhoM benefits.

Previous Attempts to Solve. Many tests have been conducted on pavements containing stabilized elements. These tests were mainly static type tests in which pavement response was measured by strain and/or detlection transducers. The results of these tests were expressed in terms of equivalency between thickness of stabilization and thickness of the base course. Base course materials are known to improve the supporting capability of subgrades; data are readily available which indicate that an increase in subgrade modulus can be expected when various thicknesses of base course materials are placed over subgrades of various strengths.5 By measuring th* response of

1 E.L. Kawala, Cement Treated Suhhases for Concrete Pave- ments. Technical Bulletin 235 (American Road builders Association, 1958).

1 Ahlherj! and Baienbrrg, Pozzolanic Pavement!, bnnEineenng txperimenl Station Bulletin 473 (University of Illinois, Feb- ruary I "öS).

'M.R I !i-mpson, "Lime-Treated Soils tor Pavement Con- struUH-n." ASCE Proceedings, Highway Division Journal HW-2 (November 1968)

*E.L. Kawala, "("emenl treated Subbase Practice in U.S. and Canada," ASCE Proceedings. Highway Division Journal «If^ (October 1966).

'Soil Testing Services, Im . Thickness Design Procedure for Airfields Containing Stabilized Pavement Components, prepared for the federal Aviation Agency, Contract No. ARDS- 468. AD 607331.

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pavements which contain stabilized elements, a back- calculaiion can be performed to determine an equivalent subgrade support Knowing; the subgrade support of the indigenous materials then allows a calculation of the increase in subgrade support and a corresponding estimate of the equivalency to thickness of base course Investigators have reported from 11 to 1 2 equivalency between stabilized layers and base course layers.6

The equivalency varies due to differing stabilizing materials, the amount of stabilizei and strength of the indigenous material.

Theoretical Considerations. Several analytical solutions are available to calculate deflections. ttiMM and strains in pavement structures. These solutions gener ally differ in the model assumed for the subgrade. The Westergaard solution assumes the subgrade behaves ,is | Wmkler foundation while the other solutions assume H elastic subgrade representation.

Existing Corps of Hngineers criteria for rigid pavements are based on the Westergaard solution with the Winkler foundation. This method also considers onl\ | single plate of constant rigidity resting on a foundation composed of independent linear springs. No provision is made for intermediate layers, and these must be considered either in the spring constant of the foundation or in the rigidity of the overlying plate. The Wester- gaard solution is for a static load. Since pavements are subjected to a range of loadings, from static to dynamic, empirical adjustments have been made to establish allowable flexural stresses in rigid pavements to account for the effects of realistic loadings and environmentally imposed loadings.

Existing Corps of Engineers criteria for flexible pavements are based on the correlation between CBK and pavement performance. This correlation is largely empirical and new materials are difficult to incorporate in the present method without exhaustive performance tests. Different paveiiient layers can be analyzed by the CBR nuthod by performing essentially a new flexible pavement design for each layer encountered. The method provides a total pavement thickness require- ment but does not include a provision lor pavement layer quality.

Some analytical techniques are available to analyze multilayered pavement structures. These analyses can

'/.«at/ tad '"i HIXIJ AirjiflJ AwMMM Otnsirui lea ON Slo hilized loundalions. Unpublished Memo: ndum Report (Corps of hngineers, Ohio River Division Laboratories)

be applied to both rigid and flexible pavement struc- tuies. They require the assumption that all layers behave clastically and the loading is static The solutions generally require the use of a digital computer as the equations are loo unwieldly for hand calculation except at a very few selected points in the structiiie. The layeied structure analyses require, as input, a value of elastic modulus and Poisson's ratio for each layer. Elastic moduli and Poisson's effects for naturally occurring and stabilized soils are extremely difficult to .iscertam due to both nonhneanty and variation with time. The primary advantage m layered structure analysis is tha' the state of stress for the complete stniLture can Iv produced, interface conditions as well as intermediate points can be investigated.

Two techniques are also available to determine the ultimate collapse load of rigid pavements.7-" These techniques are c-scntialK elastoplastic analyses. The lailure is assumed to be progressive and is initiated by the lormation of cracks on the bottom slab surface I i form plastic hinge;., i'urther application of load causes a crack to form in the top slab surface at the point of maximum negative bending which occurs at some distance liom the load. Ihe top surface fibers are in tension rather than compression, and the progression of the negative crack is fiom the top surlace down- ward. In both of these methods, the pavement is represented as a single plate resting on a dense liquid foundation. No capability of specifying more than one layer of paving material is pmvided. The loading is considered to be static. Normally pavements crack on the bottom slab surlace, and upon luither application ol load the cracks migrate trom the bottom to the top surface. This migration of cracking has been noted in both test track and operational pavements. The observed manner of crack propagation tends to cast doubt on the applicability of the ultimate collapse load analysis tvhich requires the crack to propagate from the surface to the bottom of pavements subjected to rolling loads.

Prognosis. The probability of successfully quantifying the performance gams associated with a stabilized layer is good if performance data are provided for both

1 Anders Losberg. Slrxuturalty Rctnjtircrd Comrflc Pave- mails. K'hulmcrs I niveisiry ot lethnoloity. Golehorn. I%0)

"J.J. Myerhoff, "Load Carrying Capacity lor Concrete Pave- ments.' ASCt: PrtxrvJin^. Vol ««, No SM-3 Fart I Uun« 1%2).

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pavements with and without stabili/ecl elements. A direct comparison should provide a basis lor deter- mining the pcrtormance benefit and the opportunity to assess the in-use empiricism for pavement design and evaluation.

2 APPROACH

Pavement Performance. Pavement pcrtormance is the prime attribute to be measured in this study. Unloitu- nately, perfoimance can be a rather nelniluus md elusive characteristic. The current Coips of I HgmeeiN pavement design and evaluation methods incorpontC pavement performance by establishing a sliuciural failure cnteiia arid then by deteiimmng the mai'.mtude and number of repetitions ot load required to piodiKc the various degrees of failure as defined in the criteria Failure of any pavement is a progu'ssive pheiionicno'! as opposed to a catastrophic event such as miglil occur when a structural member .n a building tails V ery few catastrophic pavement failures have been rtpoitcd. And, since failures of this typ' are extremely rare, they are not included in the normiil failure ciit.-na. Tluee conditions of failure have ben established tor rigid pavements as follows:

Initial Failure CunJilion. The initial failure condition is defined as that point in he lite of the pavement when 50 percent of the slab:, in the traffic area are divided into two to three pieces by cracks that extend entirely through the slab and which were the result of traffic loading n.ther than Ji mkage, or othei nontrat- fic-induced stresses. This umdition was defined to mark the beginning of struuiral failme and to signal that the pavement should be closely watched as the progression of deterioration will be lather rapid from this point.

Shattereil Slab Condiiion. The shattered slab failure condition is defined as that point in the life of a pavement when the slabs in th«* traffic area are divided into six pieces b/ cracks extending entirely through the thickness ot tue slab and were the lesult of traffic loading. This condition w;is defined to mark the point at which an overlay or some major strengthening program should be ui dertaken if the pavement is expected to continue to provide service.

Complete I'atlurt' Condition. The complete failure condition is defined as the point at which the

pavement is no longer serviceable and is said to occur when the -dabs m the traffic aica are broken into appioxnii;iii'l> }5 pieces (each piece being about IS 20 Square feet in area) This condition was defined to mark the end ■>! a pavement's useful life. Operations fiom a p.ivemeiil m this condition are no longer Icasible due to safety and comtoil considerations.

flexible I'avement lailuie is co isidered to occur at that f'vel ui tultic wlikh pradVM either of two failure conditkutt when wrftCI upheaval ol the pMNMHl ad|a(erit to the lialtic lane uMchiv one iruh HI ihnrr ui when iitrface clacking piogusses to (he

IHIIIII that rtio povttntni la no lonfti wittrprod

Pfewmtnf iviluiin.iiKi' is imbvd b', ili inn mliulf ol fi.id ami lunnbi'i nl u'|K'<ilioiis n-.|iiiird lo piddme .•in of the londitioiis oi railurc («crlhed above. Kepetitioi.s ol load .lo axpreaeed bi term ufoot r^n

A imnrip la laid Ui b w oi i vtni v^iM • Bk pom' m Ihc Utfik Met i i. betll WlHortid I .ixmnini b> ,i siiiiili- |i.i.. nl |IM luad win. I Ihc OOVMip COOOept I desuilnng liallu volumi' is used bMMM ",

the i.indoin neiurc ot ilrcrafi trafTk rnfTii i nonnel l> dislriliiii.il across a firnti width ol p.ivcmeiit and Ilio

oowmgi method ol deicribhn IMIIK provMei i common base loi iepresenlin>.' liallu intensity.

Types of Specimens and TeMs »icmse ol the lngli cost ISS.M i.iicd wilii OOOStructitlg and testing lull scale pavcinenls and because ol IIK- wcalher dependency for this typo ot testing, small scale model testing w;'1

selected as the most economical method of test. A small scale test apparatus was available from a previous study ot prestiessed pavements.' The appaiatus consisted ot a sod bin S Icet wide, l(> liU long and 4 feet deep which rested on a concietc floor. A loading device was designed to operate in the longitudinal direction on railroad rails located on top ot the soil bin walls. The loading device was powered by a 3-horsepower electric motor. The loading box was moved laterally actoss the pavements by a /4-iiorsepov cr electric motoi. The load was applied to the pavement by a single wheel. For rigid pavement tests, a cushion tire was used lor trafficking. A pneumatic tire was used for flexible pavement testing. The cushion tire is capable of loadings up to .1000 pounds and the pneumatic tire

''Small Stalf MIHJCI SliiJin o] Prcslrcsud RitiJ Have menls Pan I, Development of the Model and Remits oj hxploralof\ TlmU, li-. I.ni.il Report NU. 4 I3/AD2KISHII lOIno Rivi-t division 1 jtmralnru-s. Mjy 1962).

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is cupahlo of loadings up to ISOO pounds. The cushion

lire was not suitable lor use on flexible pavements due

to the high contact pressures developed. The flexible

pavement surface was not stable enough to withstand

the range of contact pressures encountereu f()0 300

psi).

lest pavements were cons'.ructed so as to piovide

a direct comparison between pavement performance

for a pavement with and without a stabilized element.

One halt of the pavement contained a stabiii/ed layer

and the second half did not. The pavements were

trafficked in such manner that direct performance

comparisons were possible by trafficking both the

slahih/ed and unstabili/ed pavements in each pass.

Values of Variables and Control Constants. A laige

number ol vaiiables were considered to affect the

lesults of these tests. To provide a cohesive te-.t series

and to facilitate analysis of results, several of these

variables were held constant in so far as possible:

concrete llexural Mtrength. crushed stone ijualily and

gradation, compactive effoil applied and moisture

content of the crushed stone, subgrade moisture

content, subgrade strength for clay and granular

materials, and loading rig speed. The test variables

included stabilized layer thickness, total pavement

structure thickness, and in some cases percentage of

stabilizer material used.

Measurement Methods. The test pavements were traf-

ficked to failure and the behavior observed. The traffic

passes were counted by an electrical counter. This

method was used for the rigid and flexible pavements.

Rigid pavements were instrumented with electron-

ic transducers to measure deflection of both the elastic

and residual type. Electrical strain gages were also used

to determine strains in the concrete slabs. Strain gages

were mounted on the top slab surface of all slabs and

on the bottom slab surface of two test slabs, 11 and

12. A technique was devised duing the program to

determine bottom surface cracking in rigid pavements.

The principle behind the technique was the interrup-

tion of electrical current by cracks forming in the

concrete slab. Complementary strain data were also

collected from mechanical extensometers during static

loading tests to ultimate collapse. Subgrade strength

was r.-easured in terms of the modulus of subgrade

reaction, k, for the rigid pavements.

Flexible pavements posed a difficult problem for

instrumentation. The flexible pavement materials do

not offer the chance to obtain a rigid mounting for the

linear variable differential transformer (LVÜT) trans-

ducei. Proper operation of the LVDT transducer

requires fnm anchorage in the horizontal plane which

was not available in the llexiblc pavement structures.

Without firm anchorage, some component of horizon-

tal movement may be rellecled in the LVÜI output In

one instance, a transducer was installed in the flexible

pavoment suuctuie In all cases residual deflection was

obtained using conventional surveying ledliliquei.

Mechanical extensometers weie ilso used to measure

deflections during ultimate collapse load tests. Sub-

grade, stabilized layer and base course strength were

measured by the CM tert method.

Method of Analysis, lest results were analyzed by

current Corps of l.ngineeis pavement design and

evaluation methods. This provided an indication of the

applicability of current methods to pavements contain-

ing stabilized elements. Another analysis technique

being used is the layeied elastic. Unfortunately, be

cause of the cost of each individual test item, very little

replicate data are available for a statistical analysis.

Non-dimensional expiessions for data will, howevei,

assist in developing a base for interpolation and

extrapolation of data. A brief presentation of dii len-

sional analysis and similitude as applied to this problem

will assist in tie application of these tests to actual

pavement structures.

3 TEST PROCEDURES

Sprcimens Prepared. A total of 34 pavement tests were

conducted. Of this group, 20 were rigid pavements and

14 were flexible pavements. The rigid pavement test

program included both clay and granular subgrades.

Tests conducted on the clay subgrade were: 7 lime

stabilization tests, Q cement stabilization tests, 2

bituminous stabilization tests. Two tests were con-

ducted on the granular subgrade using cement stabilize

tion. The flexible pavement testing program was

conducted with a clay subgrade and granular subgrade.

Tests conducted with the clay subgrade were: <> lime

stabilization tests, 4 cement stabilization tests, 2

bituminous stabilization tests. Two tests were con-

ducted over the granular subgrade using cement stabili-

zation.

The construction of a typical test rigid pavement is

shown b> the sequence of photographs in Figures I. 2

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und 3. The construction of I typical tlexible test

pavement is shown in Figures 4, 5 and 6.

Construction contiol specimens of concrete and

stabilized soil were prepared as the test pavements wee

constructed to provide preliminary intormation to be

used in establishing trafficking loads. A minimum of 9

concrete control beams and 12 unconfincd compres-

sion test specimens ol slabilimi soil were prepared lor

each concrete test pavement. Twelve unconfmed com-

pression test specimens of slabih/ed soil were prepared

Ibi each tlexible test pavement.

Instrumentation Set Up. The inslruinentation tH up

used for the rigid pavement test series is described for

each transducer used.

Dijh-crion Transiimcrs II 11)1 (itgts), Two differ-

ent mounting schemes were used loi these transducers.

Initially the transformer was housed in a brass fixture

which was cast in the concrete slab and acted as an

integral part of the slab. A ferrous core was mounted

on top of a brass reference rod winch extended

through the subgrade and rested on the concete tloor

henealh the soil bin enc/osure. The MCOnd mounting

scheme positioned ttie translormor on top of the brass

reference rod and the ferrous core was rigidly housed

in the concrete slab. A diawing of the two different

mounting methods is shown on Figure 7. The trans-

former signal was carried fioni the transducei by

shielded cable 'o the west edge ni the testing device.

The shielded cable was then threaded through electrical

conduit to the instrumentalion room which housed the

oscillograph and signal conditioning equipment. For

rigid pavements subjected to interior loading one

LVDT gage was installed in the center of the traffic-

area on the stabilized and unst'.'iilized poitions of the

pavement respectively. Jointed edge pavements were

instnimentcd with one LVDT gage mi eitlier side of the

joint on both the stabilizea and unstabilized portion-

for a total of fo.ir gages. Free edge h-aded pavements

contained a total of four gages, one in the center of the

traffic area and OM a nominal 1M inches from the Irec

edge, on boih the stabilized and unstabilized portions.

A maximum of four LVDT gages could be recorded on

the oscillograDh simultaneously.

Strain liages All strain gages were ol the resistive

wire type construction. The active length of the strain

gages was 1 inch. The maximum aggregate size used in

the portland cement concrete slabs was M inch. The

I-inch gage length was chosen to provide a minimum

2:1 ratio of gage length to aggregate siz^. Strain gages

were attached to the lop slab surface as close as

possible, about l/K inch, to the traffic area. No strain

gdj'es WCK' used during the interior loading studies due

to lack of proper lecordmg ec|uipmenl. The jointed

edge pavements were irislrumented with a total of four

strain ga^es, one on either side of the joint and on both

the stabilized ami iinsiahih/ed portions. The free edge

pavements generally were instrumeiUed with six strain

gages. QM gage was located just outside the tialfic area

toward the slab interior, one gage was located just

outside the traffic area toward the tree edge il space

permitted and a strain gage was attached lo the vertical

free edge as near as possible to the extreme fibeis cm

the tensile side of the natural axis. This scheme was

followed loi both the stabilized and unstabilized

portions of the pavemenl. Strain gage signals were

carried In shielded cables and through electrical

conduit to the instrumeii'ation room. (ontinuous

recording was obtained with (he oscillograph and when

more strain gages were used than could be recorded

on the oscillograph, a precision switch was available

to select gages at will.

Criuk Mann S\\lcm. The crack alarm system was

perlecied about half way through the test program.

The function of this device was to detect cracks in

the bottom surface on the concrele test slabs. The

device was intended to provide data on the length of

lime re(|iiired for a crack m the bottom of the slab to

migrate from bottom to top. Cracks in the top slab

surface were detected visually. The alarm system

consisted of a grid of Hal copper wires cast into the

bottom slab surface. The Hat wires were produced

in-house by cold rolling 20-gage (32 mils) enameled

cop|>ei wires to a thickness of 3 to 4 mils. The cold

lolling produced a brittle wire with practically IM

plastic range. Commercially available Hat copper wire

was tested in the laboratory and exhibited nearly KOO

perccnt elorigalion at rupture. The cold rolled malen..1

exhibited about 20 percent elongation at rupture. Both

materials failed at about 4 pounds total tensile load.

Prior to casting the rigid pavements on the prepared

subgrades, a 4 mil polypropylene sheet was placed over

the subgrade to leduce moisture loss. The crack alarm wire grid was formed on top of the polypropylene

sheet.

Two grid layout schemes were tried The first was

a rectargula' grid with the wire.« placed in the

longitudinal and transveise diiections of the slab. The

sejond was again a rectangular grid but with the wires

placed diagonally across the slab at a 45° angle. The

I grid WJS spucod at one toot Intervall us -.liown in Figure

8. The diagonal grid was considered superior because

transverse and longitudinal cracks could he detected

earlier and with greater certainty than with !l.c

longitudinal and transverse grid. The wire grid was

coated with cpoxy resin just prior to concrete place-

ment. The use ol'epoxy resin was considered necessary

to achieve good bond with tiie concrete slabs.

Construction of -the Test Models. Each lest was

designed to encompass the desired parameters and n M

lo exceed the capacity of the test facility. Stabil ed

layer thicknesses and surface pavement thick' esses

were selected to represent realistic ratios of surface/sta-

bih/ed layer typical of field placement The strength of

the overall pavement was estimated and planned not to

exceed the MOO-lb capacity of the facility for rigid

Pcivements and the IXOO-ib capacity lor llexible pave-

m.-nts.

The stabilized layer placement technii|ues varied

somewhat depending on the material. Prioi to testing

with lime stabilized layers, laboratory tests were

performed to determine the optimum lime content for

the clay material. These tests indicated that 6 percent

lime by dry weight was the optimum lime contem for

this material. All lime stabilized layers were prepared

using 3 percent lime for fixation and 3 peiconl lime for

strength gain. Lime stabilized layers were prepared over

a 3-day period. Lime in sufficient quantity to achieve

fixation was thoroughly mixed with the clay and the

material was then covered and allowed to cure for two

days. On the third day lime was again added in

sufficient quantity for strength gain and was immedi-

ately compacted in place with pneumatic tampers.

Cement stabilized layers were prepared by mixing in a

conventional oncrete mixer and placing on the pre-

pared subgrade m wooden forms. Consolidation ol the

cement stabilized layer was accompli'bed with surface

vibration. Bituminous stabilized lay.rs were mixed in a

conventional drum type concrete mixer with the

aggregate and bituminous material both heated to

about 250nF. After mixing, the material was placed

'inmediately and consolidated with a pan type vibrator.

The pavement surfacing material for the rigid

pavements was a convenlional type of concrete with

small aggregate and high early strength Type III

cement. Tie placement of the material was accom-

plished using surface trowel finish. Moist curing was

used for the first 7 days and the slabs were aif cured

until traffic tested. In the case of jointed edge model

pavements, the joint was always a longitudinal joint

with dowels used for the load transfer mechanism.

Doweled |oinl design loi the model pavement was

based on shearing lorces acting along the joint. The

same cross-sectional aiea ot steel dowels per unit length

of slab was provided m the model pavement as would

be required in a piototypc slab. Dowel diameter was

Controlled by the maximum size of coarse aggiegate

used. Sufficient space was provided between the top

and bottom '.lab smface for the dowel to accommodale

the maximum pailicle si/e of coarse aggregate as well

as to allow additional space equal lo one half that same

panicle size loi cement paste. These dimensions

discouiagcd segregation of the concrete in the vicmily

ol the dowels. The llexible pavements were con-

structed of high quality crushed limestone and placed

at I moisture content of about 6 to 7 percent. The

material was compacted with a pan type vibrator. The

pavement stiucture was allowed to air dry for at least

three days before traffic testing was initiated.

Conduct of the Tests. Traffic was initiated with a single

wheel load, which was determined by using the

stiength of UM construction control specimens and

estimating the load failure relationships at several

thousand passes. An attempt was made each time a

load was determined for trafficking to select a load

which would tail both the stabilized and nonstabilized

portions of the pavement within a reasonable lime

period. Often it became necessary to increase the load

during the test when, after several thousand passes, it

became obvious that failure could not be achieved

within a reasonable length of time. All mstiumentation

was monitored during the trafficking portion by

oscillograph recording.

4 TEST RESULTS

A nairative description of the perlormance of each

of the 34 tests will not be presented; instead, sum-

maries of all test results appear in Tables 1 and 2.

Table I contains the summary information for all

rigid pavement tests. Column captions are briefly

described to define and clarify the terminology.

list Sumhcr and Section. Tests are sequentially

numbered for refeiencing convenience; the section

refers to the stabilized or nonstabilized half of the test

pavement.

*2

Table I Summary of Model Tests Results for Rigid Pavements

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Suhgradc Soil. Two difterent subgrade materials were studied, a low strength clay and a medium strength granular material.

LoaJinn Condition. Three different classes of loading were applied to the rigid pavement test sections: interior loading, where loads were applied at a considerable distance from an edge; jointed edge, where loads were applied in the vicinity of a doweled longitudinal construction joint; and free edge, where loads were applied to within approximately one in^ i of a free longitudinal edge.

Typv of Stabilization. Unit, cement, and bituminous stabilizers were used to produce the stabilized element.

Viickntis (pavement and hasi). The thickness of the rigid pavement slab and the stabilized clement is given in inches.

Subgradc Modulus (suh^radc and haut. The modulus of the subgradc material, and the modulus measured at the top of the stabilized element are given where applicable. These values were determined from conventional plate bearing tests conducted with the 30-inch diameter plate and are in pci. The use of a 3()-inch diameter bearing plate in u 4X-inch deep soil bin was investigated by Ahlberg and Barenberg.10

Ahibctg and Barenberg concluded that the pressure bulb as defined by Terz.aghi does not touch the bottom of a 48-inch deep soil bin and the boundary effects are therefore negligible.

hlcxural Strcnyth. After traffic testing the pavements, beams were sawed from the unfailed portions of the slabs and failed in flexure; the llexural strengths are obtained in psi.

Bottom Slab Cracking. The load and number of coverages necessary to cause an indication of slab cracking with the crack alarm device are shown. The load is the number of pounds on a single wheel, and coverages are dimensionless. The number of coverages is actually an equivalent number of coverages. It the pavement had been subjected to some loading less than the load shown, the prior loadings are accounted for in the number of equivalent coverages by using Miner's hypothesis, winch assumes damage is cumulative and linear.

' Alherj! .md iMMbwi, tin t<>t I Pavrnunl Test Track A To'il!<>>■ t'valuannx MfeAwg AnVHMHfl U'mversily of III ttnoii Knpuiminj; t vpcnincm Suiion, January 1963).

Initial Iaihirc. Identical to Bottom Slab Cracking, except the crack must be visible on the top slab surface. As betöre, equivalent coverage levels are shown.

Complete Failure. Identical to Bottom Slab Crack- ing, except the failure condition is changed and the degree of distress is much higher.

In addition to the collection ot data presented in Table 1, behavioral data were collected from the instrumentation transducers. Strain and deflection were measured on most of the rigid pavement model tests. These data were collected on a recording oscillograph and manually reduced at selected intervals. A copy of a typical recording trace is shown as Figure 9. The trace labeled "vertical movement of load rig" is misleading, in that the model pavement profile was not as rough as this liace would indicate. The loading frame was conlaiiud lather loosely in the unit which provided longitudinal and transverse movement. The trace shown is mainly indicating the rocking back and forth of the loading liame in the power unit, which was due to the mechanical connections between the two. The other traces show deflection or strain as appropriate, and the peak values were obtained when the load wheel was in close proximity to a transducer. Two nearly equal maximum excursions are indicative of a pass of the load wheel heading north then reversing and heading south. During this time the load was also moving laterally a small amount and the two peak readings are nearly equal. Other lesser excursions were recorded when the load wheel moved laterally, east or west, away from the transducer. The bottom line which shows as a straight narrow line with periodic- wide short pulses is a timing line. The time interval between pulses is I minute. These data were collected, reduced and plotted. A series of typical results are shown as Figures 10 through 17. These results are for maximum values only.

Table 2 contains summary intormation for all the flexible pavement tests; column captions are briefly described

Test Number and Section. The tests were sequentially numbered for referencing, and the sections were identified as either stabilized or nonstabilized.

Subgrade Soil. The majority of the tests were conducted on a low strength clay subgradc; two tests were conducted on a medium stiength granular subgrade.

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I.IKHI Wheel, l-.ar'y tests were attempted with the cushion tire which was used foi rigid pevement tests. Liter the loadiiig rig was modified to accept a pneumatic tire winch was operated between hU and 70 psi inflation pressure.

Type of StuhiUxation. Stabilizer used tu produce tlie slabili/ed elenent under investigation

ThUkness fennhed stone, UabiUxetlelement). Pre- sents, in inches, the thicknesses of crushed stone over the natural subgiade anil over the stabilized element, and also the thicknes ol the stabilized element. It slunild be noted that the total pavement Ihiekness ovei the nibgnde was held constant on both, the stabilized and nonslabi'ized portions of the pavement. The stabilized element thus replaced an identical thickness of high quality crushed limestone.

Suhfrailc ( HR (stahtlizcil. crusheti alonej. Values of measured CBR arc tabulated for the natural subgrade, for the stabilized element over the natural subgrade. and lor the suiface of the cuished stone, over the natural subgrade and over the stabilized element.

Failure floaJ and (overages). These values show the load and coverages necessary to produce failure as defined proviouslv. As with the rigid pavement tests, the coverages have been adjusted using Miner"s hypothesis to account for loadings other than the lailure load.

5 ANALYSIS AND DISCUSSION

Analysis.

Rigid Pavement Tests. The current Corps of l-.ngineers rigid pavement design and evaluation criteria are based on the Westergaard analysis for pavement stresses.1' Through full scale traffic tests of rigid pavements, empirical relationships have been established which allow i static analysis to be used for dynamic, fatigue and eiivirciimentalK imposed loadings. Stress in a rigid pavemont structure is Cflkulated by the Westeigaard stalle loading method and compared to the modulus of rupture of the concrete pavmg material. A design factor or, more coimionly, a safety

'MM V iiM|uid, Slftuet "i Cbucfrt» Hvtmnti ('»» puli'J hy TliV(>r<lii ill Anal wn Highway ReMwdi loif4 Pa- pei (DcLfmbct 1*25)

factor is then established by dividing the modulus of mpture by the calculated tlexurai stress, and pavement life predictions are made based on the magnitude ol the design factor. Figure 1 H, which rellects the total (' ol I test track experience, shows the relationship between design factor and coveiages for the initial failuri- condition, For "k" values in the M)0 to 500 pci range, a thickness reduction, and conversely a re- ductiuii m design factor, is allowed based on obser vations of full scale pavements which indicate that while slab .racking will occur, the rate of deterioration is slow due to superior subgrade support.

The model pavements trafficked under mierioi loading conditions were analyzed as follows.

The maximum flcxural stress under load is calculated by the Westeigaard interior load formula:

.-F 10.275 (110 ^ kl';v

(ab) tu 239(1 M) — ,

[Eqt]

where 0 = tlexurai stress in the extreme slab fibers. psi

P ■ wheel load, lbs h ■ slab thickness, inches 1, ■ modulus of elasticity for slab material,

psi JU ■ Poissons ratio tor the slab material,

no units k ■ modulus of subgrade reaction, pci

a.b = major and minor axes of assumed elliptical contact area, inches

The loads used for the calculation of tlexurai stress were the loads which caused initial failure of the slabs. Table 3 lists the results of these calculations for the pavements without a stabilized element.

Ihese analyses compare the performance of the model test pavements with the performance which would be predicted by the current Corps criteria. Referring to Figure 18, it becomes apparent that the model pavements should have all failed at less than 10 coverages. The nbserved performance indicates the design factors are smaller than expected and the induced stresses are approximately one-half what would be expected. The absence of environmental effects undoubtedly caused the model pavements to perform better than anticipated: however, the superior performance observed was considered much betler than that exacted Krause of the absence of environmental effects.

_»u L

Table 3 TMI Wheel Slab Subgrade . l-'lexural Modulus lKsi-ii

Number 1 •■.I'l Thickness MIKIUIU* Stress of Rupture Factor

1 2974 2.09 80 1050 765 .73 2037 145 8(1 1388 653 A: 1899 141 M 1365 676 so 2223 1.77 n 1068 its .72 1005 1.10 HS 1157 752 hS

1155 1.00 11J 1493 880 59 1275 1 12 103 1 **,H HS7 .63

Rigid pavcinenl model tests were also conducted on free edge and jointed edge pavements. The analysis of these tests was identical to that of the interior loaded pavements except that Westergaard's edge loading equations were used in place of the interior loading equations. For free edge pavements, the analysis was straight forward, but the analysis of the jointed edge pavements required the use o( a load transfer factor. The load transfer factor is an indicator of the amount of load which is transferred from the loaded slab to the unloaded slab by the jointing mechanism. Several load transfer measurements were made on jointed rigid pavements. As would be expected, load transfer tended to decrease with increased traffic and deterioration. A typical result of such load transfer tests is shown on Figure I1*. These tests were conducted by placing th'* load tangent to the jointed edge and measuring strain in the loaded and unloaded slabs. The strain readings were used to determine the percentage of the load carried by the loaded and the unloaded slabs respectively. In the analyses of jointed edge pavements, the joints were assumed to transfer a constant 25 percent for stabilized sections and 20 percent for nonstabili/ed sections These values are considered typical for the entire life of the pavement. Current Corps criteria assumes an average Inad transfer value ot 25 percent for operational pavements. The analysis of free edge and jointed edge pavements using the Westergaard analysis yielded results similar to those found for the interior loading conditions. The cal culaled stresses were about twice the observed values, based on pavement performance.

The Corps of Engineers criteria utilize tensile stress generated in the slab in the flexural respoi.se mode as a basis of performance analysis. Tensile stress, whii h is maximized at the edge load position, is assumed to be the controlling stress in rigid pavement performance.

This tensile stress is compared with the modulus of mpture of the concrete paving material to define the relationship between allowable loads and repetitions of load. The modulus of mpture is determined by the conventional third point loading of a beam. This information provides a basis for establishing a relationship between flexural stress expressed as a ratio of the modulus of mpture and repetitions of load.

Assuming the tensile stress due to flexure is the controlling stress in a rigid pavement, a dimensional analysis based on flexural stress is required to model pavement behavior. Langhaar12 has proposed a dimensional analysis of stresses in an airport pavement. Langhaar assumes that stress in a pavement slab is a function of: applied load, F, slab thickness, h. contact pressure, p, subgrade modulus, k, and the modulus of elasticity of the concrete slab, E. Langhaar proposed an equation of the form:

a=- I .n,, ".» (Eq

in which nx = ph2/F TTJ =E/kh fj =p/E

where

o - tensile stress in the pavement slab (psi) F - applied load (lbs) h = pavement thickness (in) p* contact pressure (p.i) E= modulus of elasticity of pavement slab (psi) k= subgrade modulus(psi/in)

'HI I .int:lu.ir. Dimensional Analviis and Thtorv of Mtxl- • Is (John Wiley & Son*. 1951)

11

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.41 ^^L

■

MM

Thus Equation 2 becomes

1 . Ep'h'

kl"1

The rchilionship hclwecn these iliinensiouless factors can be ileleimmeil lioin Westerpaanl analysis of a plate supporletl on an elashc louiuialion. The Wesleipaard solulmn is expressed in liquation I tiom which LanghiBf devdoped lite relalionihipi between /r,, rr., ami V] as:

[Eq4]

Thus Langhaar demonstrated that "Westergaard's theory can be represented by a single curve with Ep'h*/ kF2 as the abscissa and oh'/F as the ordinate."

A single curve of this type was developed for typical prototype pavements using Corps of Engineers design criteria. Figure 20 shows a rigid pavement design curve for a single wheel gear fighter aircraft having a 100 sq in lire contact area and a wheel load of 25,000 lbs. The thickness requirements shown for B and (' Traffic Areas represent varying repetitions and magni- tudes of load. As suggested by Langhaar, a curve was prepared which would indicate the relationship between Ep2h3/kF2 and oh2/F for a series of prototype pavements. Variations in magnitude and repetitions of load are satisfied automatically as these changes are reflected by modifying the allowable stress as some percentage of the modulus of rupture. This unique solution is for jointed edge pavements. Such pavements will be in the initial failure condition at the end of their design life. A comparison of this predicted performance and the performance of the model pavements was then made by calculating the same dimensionless factors for all jointed edge model pavements. The dimensionless plot of the prototype pavements and the model pavements is shown on Figure 21. The line showing the prototype pavements shows the r >mial range of pavement thicknesses, stresses and ubgmk reaction values which would be encountered «ilh a 25.000 lbs single wheel gear load. Extrapolation

■ )l the prototype pavement line to the vicinity of the model pavement points would indicate that the Wester- gaard analysis would be a rather poor predictor for the model pavements.

An earlier study of prestressed model pavements"

conducted with the same lest apparatus and with approximately the same slab IhicklWHCi and siihgi.ide moduli, correlated well with piololype picstressed puvemenls. A comparison of (he lailmcs experienced with the model pavements In this study and the prestressed model pavements shows .some rather striking similarities. Figure 22 shows a close up view of a failed area on model test pavement number 6. Figure 23 shows a failed area from a prestressed pavement from the earlier test series referenced above. In both of the two photographs the failed areas were filled with crushed concrete. The crushed material was carefully removed prior to photographing the failed areas. The two photographs are quite similar except for the absence of the prestressing tendons in Figure 22. The similarity of failures and the superior pertbrmance of die model pavements led to an examination of possible mechanisms by which the model pavements may be experiencing spurious prestressing forces which are not considered in the Westergaard analysis.

The Westergaard analysis does not account for stiesses generated by the elongation of the neutral axis (membrane action) or by the frictional restraint acting at the slab/subgrade interface. Using some simplifying assumptions, an analysis of the effects of membrane action and frictional restrain was performed to obtain an approximation of the magnitude of the stresses involved. The general equation for ext :me fiber stress was assumed as follows:

1 Kp" ) t M • Kf [EqS]

' * Small Scale Model StuUws of PmtrcsseJ Rigid Pai,nu-nls Pan I

where F, h, p etc. are as before and represent the Westergaard analysis

M = membrane stresses due to elongation of the neutral axis

Rt =stress due losubgradc frictional restraint.

The sign convention adopted for Equation 5 assumes tensnn on the bottom of the slab to be positive.

An approximation of the men.' rane stresses. M, was developed as follows. A first approximation of the deflected shape of a pavement on an elastic foundation was assumed to be a sine curve. The generated length of a sine curve over one half the period would be indicative of the total strain experienced along the neutral axis of the pavement. It can be shown that the

12

^J ^1

increased longih of ihc sine cuive, S, subtracted irom ihc liori/oiiial projection is i-qual lu:

s L4-T- Al |Eq6

wiiere S = the length of the sine curve W = |IK amplitude of the sine curve L = th-j period of the sine curve n= 3.14159, transcendental numbei

The same relationship will hold true lop any portion of a sine curve provided I, is adjusted to reflect the proper interval of consideration. The average strain pen- enced along the neutral axis can then he approKimated by:

AL

L 1 W T [Eq 7|

ot the deflection hasin which is ahout MV tunes (In- radius ol relative stiffness.

Equation Scan thus be rewrilien a^ Mlows

(i 0UO73I 'n'i

k [Eq l()|

Equation 10 can be further simplified by sub- stituting the equation for the radius ot relative stiff- nets into Equation 10. The equation toi the radius of relative stillness it:

.h'

ISO u')* |lqll|

iKing 0.IS tor Poiwon's ratio and siinplitying the Imal form for the approximate membrane stress is as lollows

The average stress. M, can be obtained from:

M W IT'

4LJ [Eq8J

where b = modulus ot elasticitv.

The elastic dellections ot the model pavements agreed reasonably well with Wesiergaard's predictions. Tims, the amplitude term W in Equation X can be replaced with Westergaard's expression tor deflection, Westergaard's equation for the maximum deflection of m edged loaded pavement is:

W II 4 n k/! [Eq9]

wlieie W = maximum edge deflection due to I concentrated load

F = applied load k= subgradc modulus

/ • radius of relative stitfness 0.411 = constant involving unit width,

moment of inertia and Pi isson's ratio of 0.15

(MISh F'ff1

k/Mi1 |Kqi:|

When F, II, / etc have been previously defined.

An approximation ot lite stress due tu subgrade frictional restraint. R(, was also developed. The pres SUK' exerted by the subgrade on the bottom of the slab is. by definition:

oN ■ Wk |tql3l

where «N ■ normal SIICNS exerted by subgrade on the slab

W ■ slab deflection k " subgrade modulus

The available frictional resiiaining force should be equal to ihe normal force acting at the slab/subgrade mterfa e multiplied by the coefficient ot friction. Again using the first approximation of a sine curve for deflection and using an average value ot amplitude tor a sine curve of 0.636 and unit pavement width, the expression for Irictional force becomes;

206J KI,Cf

(Eq 14)

In the final form the applied load was multiplied by 0.75 to account for load transfer ami the I. term of Lquation | was set at six times the radius of relative stiffness The 1. te-m is an expression of the diameter

where Pt r frictional force

C( ■ coefficient of friction

L, I .ire as previously defined.

\

13

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Tabk- 4

Availahlo Prototy pi- Kcmlini; Mcmbram- i'rklional

h ll 1 Stress Stress Stress in in/psi

200

Ihs psi psi psi

M 25.(1(1(1 S3fi 22 55 10.3 too 25.(1(1(1 S2b .25 42 1(1.6 50 25,(1(10 526 .31 13 IIJ 25 25.(1(1(1 53f .37 25

M«.iK-l

1.53 200 2JhS I.H34 8.5 2t)K 1.00 120 1 .KII5 3.030 4< : 520 1.23 114 1,503 l.«7K 12.1 268 1.50 '»6 2,520 2,212 152 226

Assuming this force to act across the depth of the pavement and thereby converting to a Irictional stress requires division by the slab thickness, h. Tests conducted by the Public Roads Administration (pres- ently the Federal Highway Administration) show the coefficient of friction is inversely proportional to the square root of slab thicuiess.1-' An empirical equation was developed from force displacement tests in which the slabs were moved hori/.ontally across the subgrade. The coefficient of friction thus obtained represents a RMRtaMM available at the point of impending motion. The expression developed from these tests is as follows:

Cf =585 [EqlS]

where Cf ■ coefficient of friction

A= slab length in feel h = slab thickness in inches

0.585 = empirical factor and units factor to account for A being in feet and h being in inches

Incorporating Equation IS into Equation 14, describing L in terms of /, and allowing for load transfer across slabs yields the final frittional restraint equation:

Rf- 5436 / A_ |Eql6|

'Smalt Scale Model Studies oj Prestressed Rixid Pavrtients Part I

The complete stale of stress in a pavement under a single wheel load can thus be approximated by combining Equations 4, 12, and 16. It should be noted tliat Equations 12 and 16 will yield average values since the avuage value for the sine curve approximation was used in their development. Equation 16 is alsi> depend- ent upon incipient motion occurring at the slab subgrade interface.

Table 4 compares the stresses in prototype pavements with selected model pavements. The prototype pavements were selected from the design curve for single wheel aircraft, Eigurc 20. The model pavements were four arbitrarily selected jointed edge pavements withoul stabilized elements. Tests 15. 16, 17 and 18 (see Table I) were the tests selected.

The table illustrates the approximate range of stresses encountered with the model pavements as compared with those of prototype pavements. It should be noted that the bending stresses shown are maximum values and the membrane and frictional stresses are average values.

The normal range of prestress used for pavements is on the order of 200 to 400 psi. therefore, the values shown in Table 4 are significant for the model pavements. The material presented above was considered sufficient basis for using an analysis which assumes full friction is developed at each interface lo analyze the model pavements. The method ol ana lyzing prestressed pavements was not considered applicable as there is no way lo predict how much of the available ftictional restraint stress is mobilized as prestress. Back calcuiatioi. indicates between 25 and 50 percent of the available frictional stress was mobih/cd

14

Jb^. - S

in the model pavements. The method ol malyzing

preslressed pavements does not hive provitiotl for .1

layered (oimdation whieli was also considered to be I

serious drawback in analyzing the pavements whkh

contained a staliih/ed element.

All rigid pavement lest pavements were analyzed

using an elastic layered computer code developed by

the chevron Company and modified by the Unhrenity

ot Illinois to allow tree form data input Ihis e

layered program allows tm the IIKIIISIOII of the

concrete floor supporting the entire model, the sub-

grade soil, the stabilized element and the paveinenl

slab. The analysis iissiimes that no slippage occun

between the layers, i.e., the strain in the bottom libers

ol the upper layer and strain in the top fibers of the

layers immediately below are equal, The program was

developed to analyze layered pavement structures

under static loading. The solution also assumes the

layers are all infinite In horizontal extent with no

diicontinuities such as cracks or joints. To analyze

pa ements constructed with a free edge -n 1 jmnted

edg.1, it was necessary to use the results ol the

instrumentation readings, strain data in particular.

Strains weie measuied and compared at the Irec edge

and the slab interior. Rath s of these slram- nulkated

the stiain and presumablv the sircss ;ii 1 lin aiir v\,i>

115 percent ot the mtenor stress Therefore, ItnMM

computed by the layered pavement piogram were

adjusted to reflect the increase in stress due la I free 01

jointed edge.

The results of these analyses arc compared with

the full scale prototype design line. The design l.u UN

as the ordmate of Figure 24 is computed by dividing

the tlexural strength ot the paving niatena1 by the

maximum load induced stress. The layered analysis, as

would be expected, showed an iiu rease m flexutal

stress in the stablhzal material as the modulus of

elasticity of the stabilized material mueased. Poi the

cement stabilized material, the tlexural stress m the

stabilized element was the controlling stress ralhei than

the ponland cement concrete stress This indicates the

pavements containing cement stabilized elementl acl as

overlay pavements with the slab and stabilize I dement

bnth contributing tlexural capacity to the pavement

si met me. Iiu' layeied .nuKses of pavements containing

lime and bituminous stabilized elemer,is rfiow practi-

cally no capacity for tlexural stresses but mereh serve

to increase the capacity of the foundaiicn to ucept

noimal forces. This behavioi is charactenzed by an

increase in subgrade modulus rather iran .111 overlay

s^iem.

In the ploi ol design factor versus coverages, the

portland cement concrete stress levels are plotted in all

casei except lor the cement stabilized materials which

show the stress level for the stabilized element. The

nunibcr of coverages on the abscissa is the number of

covciages tweessar) to produce the initial failure or

first top surface crack. These data are presented on

Figure 24. The data, in general, lend to fall below the

prototype design line, as expected. The design line has

been eitabliihed from full scale tests of prototype

pavements and is actually an upper bound for the

observed tailures. B-ing an lippei bound, rather than an

average 01 central lendancy indication, some degree of

conservatiam is provided automatically. Also, the pro-

totype pavements were full scale lest tracks and were

thus subjected to climatic conditions which are not

present in the model. The lack of environmentally

induced stresses and strains 111 the model would tend to

promote bettei than anticipated performance from the

model as the pavements were constructed and tested

under ideal laboratory coinlitioiis.

Flirihlc Pavvmcnt Tests. No dimensional analyses

were performed for the flexible pavement tests as

nondimensional plots have been prepared fron, protiv

type tests of flexible pavements previously. The models

icslul were consistently within the range of value of

diese nondimensional plots and were considered repre-

sentative of prototype bchavio. not requiring an

adjustment for size cllects

1 he flexible pavement model tests weio analyzed

using the standaid CBR method of analysis which is

applicable to flexible pavements. The CBR method of

design and analysis has evolved over the past 20 or so

yuan and haa been substantiated by the testing of

many prototype test pavements under simulated air-

craft loadings Sufllcicnt data have been collected to

establish a nondimensional plot of the ratios of CBR

over applied lire pressure versus the thickness required

lor 5,(XX) coverages over the square root of the tire

contact area. This plot is inchuled In Figure 25. Using

the plot, it is possible to establish the total thickness of

llexible pavemem tequiied over a given subgrade CBR

necessarv to withstand ■s,(K)() coverages of a given

wheel load (idenlitled by pressure and contact area).

Other traffic volumes can be analyzed by using an

empirically developed relationship which relates traffic

volume 10 the penentage of the 5.(KX) coverage

thickness 1 his relalionstiip is shown as Figure 26. The

relationship between («rcent design thickness and

coverages is shown as a dashed line for 25,(XX)

13

^J _ _ ^ - -*

coverages ur more because empirical valiJalion ol the line beyond 25,000 coverages lias not been ntiUiihed, ami this portion is Ml extrapolation of lower Iratfic volume ilata

Tbe model tcsl piivemenls were analyzed using the following procedure as an example

Hexible Teil Pavemenl h

Sut^rade (BR - s Wheel 1 ,..i(l - 1559 lbs

Virr PmiUK = 70 pM

(onlail Area ■ 1559/70 2: 3 in;

(BR/p ■ 5/70 . 0.0714 Irom Hgim t/^A ■ 1.20 inr s.ooo cmwafM

^ = 4.72 i 5,66 inches tor 5.00(1

coverages

Actual t = 2,25 Percent of 5,0(10

2 25 TJi'4* coverage thickness =

W/r thickness would correspond to 1 2 coverages from Figure 2t>. All test pavements were analyzed in this manner using the suhgrade CBR measurement. The results of this analysis are shown graphically on Figure 27, The pavements withoul ;i stabili/ed clement are shown as ciicles and the pavement with slabih/ed elements are shown as triangles, 1 he test results generally agree with the results predicted by ("BR analysis. The pavements winch tall below and to the right of the design line were trafficked to a more severe state of distress than would be tolerated on an operational pavement. Only those pavements which experienced failure in the suhgrade material and the stabilized layer are plotted. Tlv first three Hexible pavemenl tests failed entirely within the base course due to the high contact pressures under the cushion tire whidl exceeded the stability of the base civirsc.

Visual observations of pavement performance and results of CBR tests indicate some benefit is derived from the use of lime stabilized and bituminous stabilized layers in Hexible pavements o\er an equal thickness of crushed stone base course. Some increase in ihe CBR value will be noted and the change in performance will be commensurate with the increased CM value. Elastic layeied analyses of flexible pave ments containing cement stabilized elements show that Ihe hvation if the stabilized element within the pavement structure determines the extent of tlcxural behavior the element will exhibit, Stabili/ed elements

below a layer ol crushed stone base course tend to exhibit less tlcxural action since the load is already spread to some extenl before ll reachei Ihe stabilised element.

Due to the limited capacity of Ihe loading rig. the stabilized elements tested in the program were rather thin. Thicker sections probably would have exceeded the load capacity ol the device and could not have been failed. Ihe standard field CBR test14 was used to evaluate the slrength of the suhgrade and the stabilized layen. The ratio of loaded area radius to slal .nzed layer thickness was much greater than would be encountered in the prototype. The use of a scaled p ston was considered; however, because of the lack of performance data using a smaller piston the idea was dropped. The effects of evaluating a prototype stabi liz.ed layer, where the thickness of the layer is large relative to the radius of the CBR piston, cannot be evaluated from these tests t'BR tests on cement stabilized materials, in particular, may not yield reasonable results where the stabilized elements are three inches or greater in thickness.

Discussion. The poor correlation between the model and prototype rigid pavements using the Westergaard analyses is primarily due to the suhgrade frictional restraint stresses. Analysis of the model tests using a layered mathematical model which assumes full friction at all interfaces and compaiing with a friclionless analysis for the prototype improves the correlainm considerably. Some disparity between 1'ie model and prototype can be expected due to the lack of environ- menial elleds acting on the model slabs. The analysis was performed using R values for Ihe soil as determined from CBR tests, unconfined compression tests, plate bearing tests and engineering! |iidgemenl. Some corre- lations have been established between L value and the tests above by Thompson. Wide ranges ol F values can he obtained from the various correla- tions available and Ihe selection of the proper t becomes a matter of judgement. The test data points can be shifted significantly by altering the E value ol the soil la ers. The elastic lawied approach seems most maonabk foi pavements containing stabilized layers if realistic vah'es for the modulus of elasticity ol the various lavcrs .an be determined.

Vl/V>/i<a/li;'i i>l ihi RtMills Of Rru-arrh le Ihi Sinnlural IhMgn 11/ OMWMI I'UU runls Pr^ivdims ol ihe American Concrete Institute. Vot S3 (Seplcmber 1939).

16

t ■

6 CONCLUSIONS, RECOMMFNDATIONS AND CLOSURE

Conclusion. Bns'-'d on the icsi results reported herein

the following conclusions arc clrawn regarding the

structural benefits imparted to a pavement structure by

various stabilized elements

Rigid I'dvcim-nl Teil Scries

(1) Applicability of Theory. The Westergaard analysis did not yield rcasonahk results foi the model

pavements tested in this study. The m^oi drawback in

applying the Westergaard analysis to ;i pav<

strudure containing a stabilb^d element lies in the

assumption of a Wmklcr U uuJui on and the ability to

analy/c only a single plate on the foundation, im

of a stübili/ed layer can only be iccomptilhed by

modifying the spring conatanl of the subgrade,

lying the thickness of tbc concrete slab 01 by some

combination ol modifications to the Klbgrade ami slab.

(2) Lime Stabilization. Lime stabilization used in

conjunction with a rigid pavement appears to inc-

the bearing capacity of the suhgrade. An analysis of a

ngid pavement containing a Irne stabilized laser by the

Westergaard method should refect the ettcct ol the

lime stabilized element by ;III itvieasc in the subg

modulus. Lime stabilized layers do noi contribute

appieciably to the llexural capjcih ol UM Stru Im

due to the low modulus ot elasticity relative lo thai "I

the pavcmeni sbib.

(3» Cement Stabilization. Cement stabilized layen

possess sulikient rigidity to contribute to ibc flexural

capacity of the slrucnire. Rigid pavemei;' structures

containing cement staMUzed element should be treated

as overlay pavements Cement modified sml layers

appear to merely improve soil workability bin d.

increase the structiiryl capacit;. oi riin.i pavemei i

structures.

(4) Bituminous Stabilization. The bituminous

stabilized layers tested in tins id offered no

advantage over high ijualiu cnished lure-:

courses.

Ihxihlc I'atiimiii list Series

(I) Applicabdiiv of Theory, l-or the tests con-

ducted in this studv. the CBR an ilvsis yielded reason-

able results. Apparentlv tbe stabilized m itciiah b.ul no

greater capacits tor suppoitinu traffk than s convcn

tional base couise of tbe same CBR

(2) Lime Stabilizati m. The lime stabilized layers

seemed lo otter hltle or no performance benefits over

an equal thickness of hnJi quality crushed limestone.

I he performance of the layer should be reflected by

the ( BR value of the layer.

(3) Cement Stabilization. The ability of the ce-

meni stabilized layer to contribute tlexural capacity to

tl e pavement structure depends on its location in the

structure. Cemenl stabilized layers located at or near

the top surface will behave as slabs, however, these

same layers located beneath i base course cover will be

loo lai removed from the load to exhibit tlexural

action. Cemenl modification id soils appears to merely

improve soil workability but does not contribute to the

■ naciin .1 capacity of the pavement.

(41 Bituminous Stabilization. The bituminous sta-

biiized layers offered no advantage over high quality

J bed limestone base couises.

RecomrnenHations. The following recommendations

are offered relative to the design and evaluation of

pavement structures incorporating stabilized elements

based MI the results of this study.

I 11 The elastic layered method of analysis is

mended lor promlype pavements containing

stabilized elements. Field applications should be based

on a Irr. lion free interface assumption.

(2) I lie use of equivalency factors between base

materiafa am! ceineni stabilized layers is dis-

couragl d 1 he behawkM of the two materials under load

is completel) different and a true equivalency is not

possible.

(1) Should an actual application require the use of

a very thin slab supported on a subgrade. exploitation

oi th< IM restraint stress is not recommended,

linviromneiital facton acting on a slab, such as ingress

i I moisture at the subgrade interface and warping due

to temperature gradients, will probably reduce the

tihin n.il h-.iiaint lo a very low level.

Closure. Tl e f illowmg observations were made during

i 'his studs. Wlnle these comments are not

dnectlv supported by definitive data, they are con-

ta bi I i value and aie offered for consider-

stion.

( I) An exhaustive dimensional analysis and simih-

lud( Stud) should piet-ede any model lest of pavement

Rigid |>avemeiit models in which the slab

thickness is less than about 4 inches will result in a

magnification of subgrade frictmnal restraint stresses.

i

17

K9J

(2) Care should be exercised in locahng a stabilized elcmenl in a pavement structure so as not to impede drainage of the structure. In general, stabilization tends to reduce the permeability of materials. Analytically and under idealized conditions, some layer arrangements will appear superior in regard to structural support but will offer formidable drainage prob- lems

(3) The use of biluminoi's stabilization of mar- ginal materials should be investigated. For example, an

aggregate base course material which deviates from the recommended gradation specifications may be im proved sufficiently by bituminous stabilization to offer satisfactory performance.

(4) If the clastic layered method of analysis is adopted for pavement structures, further research is recommended for '.he determination of elastic constants of paving materials with reasonable ease, ac- curacy and reliability.

REFERENCES

AJilberg and Barcnberg, Puzzolanic Pavemml:.. Hngi- neering Experiment Station Bulletin 47.1, (Uni versity of Illinois, February l%5).

Ahlberg and Barenberg. Tlw U of I Pavement Test Track A Tool for h'valuating Highway Pave- ments (University ot Illinois Fngiiicenng Experi- ment Station, January 1963),

Application of the Results of Research to the Struc- tural Design of Concrete Pavements, Proceeilings of the American Concrete Institute, Vol 35 (Sep- tember 1939),

Kawala, E.L, Cement Treated Suhhases for Concrete Pavements, Technical Bulletin 235, (American Road Builders Association, 1958).

Kawala, E,L,, "Cement Treated Subbase Practice in U,S. and Canada," ASCt: Proceedings, Highway Division Journal HW-2, (October l%M.

Langliaar, H.L, Dimensional Analysis and Theory of Modeh. (John Wiley and Sons, 1951),

Toad Tests M Rigid Airfield Pavements Constructed on Stabilized /OM/U'I/^ins,Unpublished Memoran- dum Report (Corps of Engineers, Ohio River Divi- sion Laboratories),

Losberg, Anders, Structurally Reinforced Concrete Pavements, (Chalmers University of Technology, Göteborg, I960).

Military Standard 62/-zl,Tesi Methods for Pavement Subgrade, Subbase and Base-Course Materials.

Myerhol, J.J., "Load Carrying Capacity of Concrete Pavements," ASCT Proceedings. Vol ««, No, SM-3 Part I, (June 1962),

Small .'icale Model Studies of Prestressed Rigid Pave ments Part I, Development of the Model and Results of Exploratory Test,, T^hnical Report No, 4-13/AD 281580(Ohio River Division Labora- tories, May 1962),

Soil Testing Service, Inc. Tluckness Design Pro.edure for Airfields Containing Stabilized Pavement Com- ponents, p/epared for the Federal Aviation Agen- cy, Contract No. ARDS-468, AD607331.

Thompson, M.R., "Lime-Treated Soils for Pavement Construction," ASCh Proceedings, Highway Divi- sion Journal HW-2. (November 1968).

Westergaard, H,M,, Strews in Concrete Pavements Computed by Tfieoretical Analysis, Highway Re- search Board Paper (December 1925).

4

18

*4M

IIIW *m»

Figure 1. Completed subgradc prior to placement of polyethylene sheeting. (Material in DM

foreground is lime stabilized clay. The two small black tubes in the upper center of the photo are instrumentation housings with strings attached to pull wiring.)

I

■aw"

Figure 2. Coating o( crack alarm grid wires with epoxy resin prior to concrete placement.

X iy

f ^

Figure 3. Concrete placement ■pproxinutel) one hall" complete (All crack alarm grid wiics have been coated with epuxy resin.)

Figure 4.

Reproduced from best available copy, ©

Vibiatinii pan type oompactoi used to oimpact crushed stone hUH CCHtne natc rial lor flexibk (lavement I

l«J

Figure S. Compaction of the lime soil mixture with the pneumatic tamper.

Figure 6. Overall view of a completed flexible pavement prior to test traffic.

^J

LVDT MOUNTING

. • PCC

Core

.^1 Cop bcr.-w Cap Screw

r^

i

PCC

N—Transformer Windings

Brass Reference Rod

y Hole Casing

\J

INITIAL Floating Transformer

Fixed Core

Windings

Brass „, Reference Rod

/- Hole Casing

FINAL Floating Core

Fixed Transformer

Figure 7. Linear variable differential Iransformei mounting techniques.

A TroffiC Arto

r

0-

^LL'-lLJJ.L'J-LllLULJl-Ui-LJlLUJJ \ Crock Alorm Wiring ScoH I I in = 2 ft

Figure 8. Typical layout of crack alarm wire jmd.

22

T

*J

I

*****

I I!

If JW »* I

:-r:;,;- t^:::

/ : : : ES=: f • ' • : "■T"H '"t- —

"H''!";"' Ißi:™-^--"-

—^^r— ;: : :: :: ;:: .

■ . | .

■ • ■ • i-j-i . ., i ,

/ ' : - : :: I ; ':■ ■'■'■':'•.': i

'ir'U U Tf B IM- ,. ii. . . .

:::

1 . , .4.

1^—^-

Figure 9. Typical oscillograph trace from traffic testing of model pavement 12.

23

^« -»—♦••'■■""••

1501 -|» I40S -U ?I80 -J CtMrtfn

1000 ?000 ^ <». 4000 5000

M \~^ ^*,^•^ k^

"^ 04

|

Not« OtfltCtiO« GO«« PlotICO Q

TEiT iL»B IB

Gronglo' Bat«

oa N E □ © N W

Aipftalt Bolt

s r ao suKÖcri

O CMtltctior G09« □ Stroir Gag«

( Do«« I ltd Long Joint

Figure 10. Typicyi dcfkcÜon-COWBnft pr.it, west slab, nonslabili/cd.

Load.ibt L ,505 4- 1905 --4« 2I»0 -| Co<«fO««i

lOOO

iieo

?000 JOOJ «OÜO 5000

< Uo«rtil«< Loo« Joint

O O««l«etion bo«« □ Sliom Go»«

Figure II. Typical detleclion-cuveragc plot, cast slab. nonttabilized.

24

^f J J -tn ■ 1 r

10*4. lit [• IJOJ «L- l»OS

Cov«rOf«t

tooo yxo *ooo »ooo

zito -j

0

k rtf

04 u ><

0« 1

Nor« Siro." Cot* Helft ^13

H

TtST SL*8 18

Gronutor Bot*

On N i

Alphnlt Bot«

I t oo

O O«li«inoii Oo«« □ S'ioin Co««

t Oo*«li«d Lonq Join*

Figure 12. Typical strain covorago plol, west slab, nonstabiiized.

Load,ibi

h 1501 - I»04 4» Ji80 -^

C ov«r09«t

I00O ZOOO 500C teOI 5000

M

x

h -^ i \ i

Nott tfrclD Gag« ^ o'tta CS3

TfST —1

SI *H It 1 ,'On , 0' Bat« 1 «iphoK Boi«

ao N t

H ■

1 i [-10

s«OD ( Do««ll«« Ion«

JO'ltl

O 0»»"«ci on Go«« O Slra n Go««

Figure 13 I\i>ical sltain-coverage plol, east slab, non-

stabili/eii

ill . t 1 - m m. »m' •^'

h IMS 1905 4- ?l«0 -| Co*«rQg«s

e I 1000 ?00n JOOO 4000 500()

^

^ .

\

PI

_ (M

' N

M 1

Not« OtdttliSn C09* PlOllt«

TfST SLAB 18

Grtnulor Bot« Aiphall Bo»

OC3 M i C3Ö N«

s t no

O 0«ll«cli»n Go«« □ Slio.r Cog«

t Do«ll«d Lonl Jotit

Figure 14. Typical iletlection-coverüge plot, west slab. itabilized.

LM4,lk« I 4.— 2i«o —4

Not« D«ll«cl.«« -.o»« P.oil««

TEST SL«t It

Gf«»ul«r »«K

OC3 N f

15Ö »••

Atphf.i 8a»«

11 CD« s«00

t Dow«11«« Lou« Joint

O 0«li«clio» 6*9« Q Strain &«««

Figure 15. Typical deflectum-oveiage plot, eust slab, stabilized.

>

A _1 jBp!«Mw>

IS05-

J Of I

i

■ I o

—i« 1905 4* ^'B<' -|

C0V«r0f«t

1000 ?000 5000 4000 5000

^ - I V

** k-- A

\

N»H Strsm COf« Plolltd E3

OfOnulOr Bot«

oa N C □o * •

TEST SLAB IH

Atpholl Bot«

s t DO s «OEJ

O Dttlaciion Cat* CD Sttam Co««

t DO««H«O Long Joint

Figure 16. Typical strain-coverage plot, west slab, stabilized.

-.oa.c

h

I

i 04

■t

HOI r»05 2IB0

EBHMpt

1000 2000 5000 4000

1 ■ ■

Sl-o.n C«t« »lout«

1 TIST $L«» I«

SranulOf Bot« i *t«holl Bot«

"I 5c MQ smOÖ

- —( 0*Mli«< Lao« Jeml

O O«(l«cl>on Got« C3 SliO'n So««

Figure 17. Typical strain-coverage plot, cast slab, stabilized.

27

,

50 100

c 1.0

I

Coveroges

500 lOOD 5000 IC 50,000

Figure 18. Design factor versus coverages for initial slab failure.

40

30

111

M 20 z <

o

! .0

0 0

VS^ V ^s.

^1 f^^^V!

^

Dablliltd

i #1

L MNMMntfi )

IS 20 COVERAGES 110

Figure 19, Typical summary of load transfer tests across joint.

25

'

^J» ■ - - '

Tricycl« Gear

in a

| »■

1?, B 3 K «I

E

Traffic Ar«ai 8 C

-i?

i?

s I

IO - c

- a 9

b

— 8

— 7

L-e

I I

•

Figure 20. Design curves tor concrete airfield pave-

ments, single wheel. 100 sq. in. contact

area.

« - modfl Ml do' o

1

dtugn cur»f • or unglt wheti lood - .^^*^

^

X

X

X

(

X

X

»0 900 1000 »000 10000 50000 100000

Figure 21. Nondimensional plot of single wheel design

curve and model test data using Wester-

gaard analysis.

_*J

Figure 22. Close-up view of a failed area on model test 6. (Crushed surface concrete was removed from the area prior to taking the photo- graph)

Figure 23. Close-up view of a failed area on a pnstre» sed model pavement. (Crushed Mirla.o om crete was removed exposing ihe preslies- Mng steel tend..ns )

^«J

Reproduced 'rox best available copy

I u O

Ü.

IS

4

s Covtragts

0 50 100 500 1000 5000 0,000 50pOO

*

B J / 1 /

1

■ 1

4 4

^ • c •

^ ^ 0 • 0

0 1 ^^ _ ' " •. r

• ■

0.7

, 1 ö" t 4

0 • *

05

0 4

■ ■ • ,-

0 No'o'n. • Lim« * Cvnwnt 3 C«m«ni Modifi«d — « Bilunvnou«

0 2

0.1 1 1

Figure 24. Summary of rigu! pavement tests.

*

QOI 002 .004 006 M ,0

CBR/p

2 04 .06 .06 1 .1 .4 1 .8 1

'5

- .^ '

F"

^^^ ..

5

I

M

I

15

4

M

5

9.»

^ S ^ I

/ T

1 /

/

/

/ -1

J / /

/

/ CBR-Colitofn.o B«onng Ro'io

p Appliffd Pr«ltür« i A - Tir« ContOCl 4r»o 1 - Po»»m«nt Truclin»«« R«qu««a _ /

u p /

(

1 fo< 9000 Co»«f

1 1 1 1 Ll 1 1

_ LL . J 1 Figure 25. Consolidated CBR ourve tor 5,000 cover-

ages.

L

31

i

I u

I I c u

50 IOC

Coverages

500 1000 5000 10,000 50,000

Figure 26. Percent design thickness verus coverages, llexiblc pavement failure.

Cover oges

500 1000

Figure 27. Summary of flexible pavement tests.

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AD-763 912 STABILIZATION FOR PAVEMENTS J. L . R ice …

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