challenges in pavements

8/6/2019 Challenges in Pavements

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KEYNOTE ADDRESS

ACHIEVEMENTS AND CHALLENGES IN

ASPHALT PAVEMENT ENGINEERING

Stephen F BrownDepartment of Civil Engineering

University of NottinghamNottingham NG7 2RD

UK

Abstract. This series of international conferences hasfacilitated the development of mechanistic methods forthe design and structural evaluation of asphalt pavementsover the past 35 years. While the record is one of real

achievement, stimulated by initiatives such as the U.S.Strategic Highway Research Program, there are stillproblems to solve and challenges to face including that ofimplementing the results of research. The majorachievements are outlined and the latest developmentsdiscussed. These include more realistic theoreticalmodelling and the availability of higher quality data frommodem testing facilities both in the laboratory and in thefield. The need to incorporate the principles of soilmechanics more effectively in the design and evaluationof pavement foundations is identified. The continuedextensive use of the CBR concept is questioned and theneed for application of more relevant parameters isencouraged. Several other procedures which have long

been used are critically reviewed and attention is drawnto innovative ideas including new concepts for modellingasphaltic materials. The theme of Paving-the-Gapbetween research and practice is considered andexamples given of success. The importance of structuralevaluation is emphasised and the need to develop faster,more convenient data gathering facilities is considered.The concept of Smart Roads incorporating appropriatelow cost instrumentation could help in this context.

Keywords. Design, Evaluation, Materials, Foundations,Theory.

INTRODUCTION

The review of this sequence of internationalconferences conducted by Witczak and Shook (1992) setout a record of real achievement and influence over the30 year period since the first event at the University ofMichigan, Arm Arbor, almost exactly 35 years ago. Thatfirst conference under the leadership of Professor BillHousel was organised by a small but distinguished groupof engineers, many of whose names have become part ofthe folk-lore of asphalt pavement engineering in theUnited States;

Walter J Emmons, whose name lives on in the annualbest-paper award of the Association of AsphaltPaving Technologists

Fred Finn, veteran of the AASHO road test, the datafrom which partly inspired this first conference, andsubstantial contributor to research over many years

Norman McLeod, the distinguished Canadian asphalttechnologist

John Griffith of the Asphalt Institute, which hasstrongly supported these conferences

A C Benkelman, whose famous Beam became thework-horse of pavement evaluation over many years.

Witczak and Shook consulted the pavement

engineering community to assess the impact of theseconferences. The principal findings were:

The written proceedings formed an invaluablereference source.

Improvements to the state-of-the-art on theoreticalmodelling and materials characterisation formed themajor achievement.

Little impact had been made on construction,

A moderate influence had been made on design butgreater implementation required wider verification of

research findings.

At the 1962 conference, intellectual ferment arosethrough the tension between the empirical and theoreticalapproaches to design but a paper of lasting significanceby Gil Dormon of Shell (Dormon, 1962) was presented.This will form the starting point and basis for this lecture.

S. Brown ISAP - 8th International Conference on Asphalt Pavements - Seattle, 1997 1


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In 1967, perhaps recognising the problems ofintellectual ferment, attempts were made to Bridge theGap between theory and practice. By 1972, the ability tocarry out theoretical analysis had been greatly enhancedthrough developments in computer technology but theferment continued. At this conference attention started

to be given to pavement evaluation and overlay design.

In an attempt to demonstrate that mechanistic designmethods were complete and available, several substantialpapers were solicited for the 1977 conference to describesuch systems. Most of them were based on the generalprinciples set out by Dormon in 1962. The potential foraccelerated loading at full-scale was demonstrated at thisconference through the work of the South Africans andprogress on rehabilitation design was apparent.

By 1982, design methods were available in the formof computer programs but the problems of satisfactory rutdepth prediction remained despite having been addressed

at preceding conferences.

Use of the Falling Weight Deflectometer forstructural evaluation of pavements was an importantfeature of the 1987 Conference and the impact ofPersonal Computers for pavement analysis becameapparent.

Conscious of the ongoing criticism that the highquality research described at these internationalconferences was not being communicated to the designand construction community, the organisers of the 1992Conference, under the leadership of the InternationalSociety for Asphalt Pavements, adopted the theme of

Paving-the-Gap between research and practice. Thetitle of the conference was changed and special sessionsarranged to attract contractors and material suppliers.This approach has been continued here in Seattle.

In summing up the 1992 conference, Brown (1992)reported that real progress had been made through theavailability of field and laboratory test equipment andanalytical computer packages which allowed engineers toimplement the results of research in user-friendly ways.Although there were few major break-throughs reportedat this conference, improved communications across theGap had been started and the potential for futureprogress was significant.

Another important review of developments in themechanistic design of asphalt pavements was presentedby Professor Carl Monismith when he delivered the firstTransportation Research Board Distinguished Lecture atthe annual meeting in Washington D.C. in 1992(Monismith, 1992). As a significant contributor to all theAnn Arbor conferences since their inception 30 yearsearlier, Professor Monismith was in an excellent positionto provide a perspective on what had been achieved in

this field over that period. He pointed out that just a 1 %saving in the annual cost of asphalt paving in the U.S.would allow the $50m SHRP asphalt research programmeto be paid for in just four months. He maintained that suchsavings could be made by implementation of the newtechnology which was available from research both under

SHRP and the accumulation of knowledge from earlierinvestigations. Subsequent to the completion of SHRP, ithas become clear that full implementation remainsproblematic but much technology in this field has beenimplemented in many countries and achieved significantimprovements for the user and funder of asphaltpavements.

The main achievement of these conferences has beento focus on research which has contributed generally tothe gradual move from empirical methods for design andmaterial specification to those based on theoreticalconcepts and tests to measure mechanical properties. Asinterest in the maintenance of existing highway

infrastructures has increased, the development oftechniques for structural evaluation, and the design ofrehabilitation, has become increasingly important.

To move from reliance on CBR curves, recipespecifications, Black Magic tests involving needles,rings and balls and the Benkelman beam, to computerbased structural analysis, practical tests to measuremechanical properties of binders and asphalt mixtures andthe use of the laser and the Falling Weight Deflectometerfor pavement assessment represents real achievement.

Development of the South African Heavy VehicleSimulators attracted World-wide interest in full-scale

accelerated testing and provided a basis for verification ofnew designs and the utilisation of new materials. Thedramatic increase in research based on the HVSphilosophy, recently reviewed by Metcalf (1996),particularly in Australia, France and the USA, providesconfidence that the verification of design concepts,identified in 1992 as a major barrier to theimplementation of new technology, will be achieved overthe next few years.

Despite real achievements, some of which have beennoted above, many challenges and opportunities remain.A blend of innovation and steady attention toimplementing what is known while communicating

effectively between research and practice will bring realrewards in future.

This lecture will endeavour to focus on some of thesechallenges, point to some innovations and recognise thebackground achievement on which they are able to build.



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FRAMEWORK FOR THE LECTURE

Fig. 1 is taken from the landmark paper by Dormon(1962) presented to the first international conference. Itrepresents an asphalt pavement as a three layer structureof linear elastic materials subjected to a circular,

uniformly distributed load. The two classical designcriteria, compressive strain at the subgrade surface andtensile strain at the bottom of the asphalt layer, werederived from this paper. The tensile strain criterion wasbased on work done by Professor Peter Pell atNottingham in conjunction with the Shell Laboratories,first reported in 1960 (Saal and Pell, 1960). It paralleledsimilar work by Professor Monismith at the University ofCalifornia, Berkeley, published the following year(Monismith et al, 1961).

Figure 1. Pavement as a 3 layer elasticsystem with design criteria

(after Dormon, 1962)

The remarkable feature of Fig. 1 is that it hasremained the basis for mechanistic design (originallydescribed as rational design) for asphalt pavements eversince. Fig. 2 shows a schematic based on the originalDormon figure which provides the framework for thislecture. The nine topics in this figure, some of which are

intentionally linked, are used to highlight achievementsand challenges in the design and evaluation of asphaltpavements, which, together with the key component partsof these subjects, might be referred to globally asAsphalt Pavement Engineering.

STRUCTURAL DESIGN ANDTHEORETICAL ANALYSIS

Fig. 3 indicates the central part which theoreticalanalysis plays in the mechanistic design process. Theachievements of the past have included incorporation oflinear elastic theory and approximate techniques fordealing with the non-linear behaviour of pavement

foundations within design methods. As computationalpower has developed, more realistic modelling ofpavements has become possible. The finite elementmethod has provided the basis for this, allowing, not onlynon-linear elastic models to be readily incorporated, e.g.the FENLAP program (Brunton and d'Almeida, 1992) butalso visco-elastic and visco-elasto-plastic modelling,particularly for asphalt layers.

Chatti and Yun (1996) have developed the SAPSI-Mprogram which represents the pavement as a series ofhorizontal elements (sub-layers) and a] lows movingwheel loads to be accommodated. Each layer is modelledas a linear visco-elastic material and the effects of

vehicle speed can be computed. Good comparisons withfield data have been reported as noted in Fig. 4.

Figure 2. Topics covered by the lecture



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Figure 3. Simplified flow diagram formechanistic (analytical) Pavement Design

Figure 4. Comparison of calculated andmeasured strains at different speeds.

(after Chatti and Yun,1996)

The VEROAD program developed by Hopman andreported by Nilsson et al (1996) uses 15 horizontal layersover an elastic half-space. Materials are characterised aslinear visco-elastic for the shear response but linearelastic for volume change. Realistic transverse andlongitudinal strain variations are predicted (Fig. 5).

The PACE program described by Rowe et al (1995)uses a conventional axi-symetric finite elementconfiguration but models the materials as elasto-viscoplastic. Visco-elastic and elastic behaviour aretreated as special cases of this generalisation, which isillustrated as a rheological model in Fig. 6. Realisticmaterial response is obtained by overlaying several basicmodels in parallel and ensuring strain compatibility. Inaddition to predicting strain variations resulting frommoving wheel loads, Rowe et al are able to computecontours of dissipated energy, which relates to fatiguecracking failure (Fig. 7) and rut profiles.

Fig 5. Comparison between measured andcalculated strain (after Nilsson et al, 1996)

Figure 6. Rheological Representationof Elasto-visco-plastic Model

(after Rowe et al, 1995)



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Deacon et al (1994) have presented a comprehensiveapproach to accommodate temperature as a primaryparameter in design of pavements and for the testing ofasphalt mixtures. Their concept of TemperatureEquivalency Factors (TEF) is used to convert the numberof standard axles to an equivalent number at a standard

reference temperature. The factors were quantified fornine climatic regions in the USA. Their approach allowsdesign and laboratory materials testing to be carried out atstandard temperatures with extensions to the field madepossible via the TEF's.

This approach differs from that widely used in thepast involving conversion factors for the average annualair temperature to define pavement design temperatureand use of the actual number of standard axles in designcalculations. As an example, for U.K. conditions, Brownet al (1985) devised factors of 1.47 and 1.92 applying topermanent deformation and fatigue design respectively.

The concept of a Critical Temperature forpermanent deformation damage was also defined byDeacon et al (1994) as that temperature within 5C ofwhich the largest amount of permanent deformationdevelops in the field. This is recommended as thetemperature for laboratory testing. Brown and Gibb (1996)had recommended that the test temperature should beselected so that the bitumen stiffness in the test matchesthat in the field. These concepts are compatible but wouldrequire that Deacon et al's Critical Temperature bemodified to allow for different grades of binder.

The work of Deacon et al (1994) was stimulated bythe extensive programme of laboratory fatigue and

permanent deformation testing conducted under SHRPcontract A003A led by Professor Carl Monismith. Theimproved quality of materials data made possible throughmodem instrumentation and computer control ofequipment combined with computer based analytical toolsdemonstrated how effective progress can be made inasphalt pavement engineering. Similar work had beenconducted earlier for U.K. conditions by Brown et al(1984) but the work of Deacon et al demonstrates realprogress at a detailed level. It represents an excellentexample of how pavement analysis and design areintimately linked to asphalt material properties.

Design concepts for thermal cracking are well

established and have been summarised by Monismith(1992). For the U.S., the performance grading of bitumensaccommodates the requirements of low temperatureswhile Superpave levels 2 and 3 mixture design testingprocedures are intended to provide the basis for predictivecomputations. However, recent modelling work as anextension of SHRP has indicated further research isrequired in this field.

The effects of air and water on deterioration ofasphalt mixtures, known as durability, continues topresent a challenge. Although oxidative aging of bindersand water damage (stripping) in mixtures has been wellresearched, validation of laboratory testing procedures inthe context of field performance remains problematic.

This is principally because, by definition, long-term fieldmonitoring associated with laboratory testing is needed.The other effect of water is in the influence it has on themechanical properties of soils and granular materials, asubject dealt with under Pavement Foundations below.

The phenomenon of frost heave has been well studiedbut the problems presented by extended freezing ofsubgrades in northern climates would appear to meritfurther study. In these areas, it is a problem which tends todominate pavement performance, not least because of theweakening effects caused by the Spring thaw.

TRAFFIC LOADING

Prediction of trafficvolume for pavement design hasalways been an inexact science. However, the ability tomodel the effects of different wheel loads on pavementdeterioration and to quantify dynamic effects hasprogressed. The current approach of converting traffic toan equivalent number of standard 80 or 100 kN axle loadsusing a fourth-power law is, at best, approximate and canonly be justified in the context of the vagaries ofestimating numbers of axle loads in various categories.

The interactions between surface profile and vehiclesuspension systems have been satisfactorily modelled. The work of Dr David Cebon and his colleagues at the

University of Cambridge is particularly noteworthy. Fig. 9illustrates the Quarter Car model (Cebon, 1993) used tocompute dynamic loads for single axle truck suspensions.In their whole life pavement performance methodology,Collop and Cebon (1997) use this to compute changes todynamic loads as surface profiles deteriorate.

Figure 9. 'Quarter Car' vehicle model(after Cebon, 1993)



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Today, quite extensive use is made of repeated loadtriaxial tests to determine the resilient characteristics ofsoils and compacted granular materials but few agenciesinclude such tests as a formal requirement. It remainsessentially a research tool despite the availability ofsimplified and relatively low cost equipment which

utilises pneumatic loading systems.

The non-linearity of pavement foundations has beendemonstrated both from insitu measurements of stress andstrain, using field instrumentation, and through back-analysis of surface deflection bowls measured with theFalling Weight Deflectometer.

Fig. 11(a) shows the stress-strain relationship for asilty-clay, based on measurements from insituinstrumentation (Brown and Bush, 1972), while Fig. 1l(b)illustrates the well known stress-dependence of granularmaterial determined from similar data in a crushed rockbase (Brown and Pell, 1967).

(a) Shear stress - strain relationship for silty-clay

(after Brown and Bush, 1972)

(b) Resilient modulus against 1st stress invariant for

crushed rock (after Brown and Pell, 1967)

Figure 11. Non-linearity of soils and granularmaterials from insitu measurements

Recent FWD testing on a site in the U.K. wasconducted as construction proceeded. Table 1 shows theback-analysed values of resilient moduli indicating howthe granular capping and the subgrade mobilised higherstiffnesses when covered by the sub-base and base. Thiswas caused by the consequent changes in stress level.

Table 1

Back analysed effective values of resilient modulusfor road in cutting (after Brown, 1996,a)

These non-linear characteristics have also beenextensively studied using repeated load triaxial facilitiesand various models proposed for use in pavement analysis.Some of these are quite sophisticated. For granularmaterials the use of stress dependent bulk and shearmoduli provides a much sounder basis for analysis thanthe simple K- model in which the resilient modulus isexpressed as a function of the mean normal stress and,usually, a fixed value of Poisson's ratio is adopted,typically 0.3. Fig. 12 demonstrates that both deviatorstress and mean normal stress have an influence onresilient modulus for a typical crushed rock.

Figure 12. Resilient modulus for crusheddolomitic limestone as a function of applied

stresses (after Dawson et al, 1993)



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For fine grained soils, emphasis has been placed onthe relationship between resilient modulus and deviatorstress following the early work done by Seed et al (1962).However, when the effective stress state of the soil istaken into account, sometimes expressed in terms of soilsuction, it is apparent that this parameter too has a major

effect. For saturated silty-clay, Brown et al (1987)suggested the following model based on a series of goodquality laboratory tests:

Gq

C

p '

qr

r 0

r

m

=

(1 )

where Gr = Resilient shear modulus

qr = Repeated deviator stress

po' = Mean normal effective stress

C, m = Constants for the particular soil.

For partially saturated soils with degrees of saturationin excess of 85%, the same equation was valid with p o'

being replaced by the soil suction.

The shortcoming of this model is that unrealisticallyhigh values of resilient modulus are predicted at lowstresses. To avoid this, and to make use of expertisedeveloped in another field, pavement engineers shouldconsider use of the strain dependent stiffness modelsadopted in earthquake engineering and for other smallstrain geotechnical problems. Fig. 13 shows a set ofcurves in which the shear modulus (G) is normalised withrespect to the maximum value (Go), which occurs at very

low strains. The actual relationship between G/Go andcyclic shear strain is shown to depend on the plasticitycharacteristics of the soil (Roblee et al, 1994, Vuceticand Dobry, 1991, Sun et al, 1988). Viggiani and Atkinson(1995) have presented a procedure to estimate Go from

Plasticity Index, effective stress and overconsolidationratio for the soil.

Figure 13. Relationship between normalisedshear modulus and Cyclic Shear Strain

(after Roblee et al, 1994)

The tradition in pavement engineering has been tothinkin terms of stress-dependent rather than strain-dependent resilient moduli. This possibly derives from theidea that subgrades are subjected to a controlled stressenvironment. Some simple linear elastic calculations ontwo layer systems following the principles established by

Monismith and Deacon (1969) in relation to asphaltfatigue indicate that this is not the case.

The simple structures are shown in Fig. 14(a). Athick and thin asphalt layer placed directly on soils ofvarying stiffness was used. The effect of reducing the soilstiffness by 25% in each case led to calculation of theMode Factor, M, where:

M| A | | B |

| A | | B |=

+

in which |A| = % change in shear stress|B| = % change in shear strain

for the 25% change in soil stiffness.

Pure controlled stress conditions are given by M = -1while M = +1 corresponds to pure controlled strain.

Figs. 14 (b) and (c) show the results. For the thinstructures M is close to zero, indicating conditionsmidway between the two extremes. However, for the thickstructure, which is closer to a real situation, M0.5showing controlled strain conditions to be moreappropriate than controlled stress.

For practical use, the following model for resilientmodulus was developed by Dawson and Gomes Correia(1996) based on analysis of laboratory data andrecognising the need for realistic values at low stress (orstrain):

Er = 49,200 + 950 po'- 370 qr - 2,400 wp (3 )

in which Er po' and qr are in kPa and wp is the Plastic

Limit expressed as a percentage. For compacted, partiallysaturated clays, po' could be replaced by the soil suction.

Fig. 15 shows that this model compares reasonably wellwith actual measurements over a practical range ofresilient modulus.

At the 1992 Conference, Brown and Dawson (1992)outlined a two stage approach to pavement design inwhich the first stage involved design of the foundation tocar ry construction traffic. In the second stage, thefoundation was represented as an equivalent elastic half-space characterised by an effective foundation stiffnessmodulus (Ef). This is a parameter which can readily be

determined from some form of dynamic plate loading test



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on site or can be computed using elastic analysis. Designof the asphalt layer can be based on a two layer systemwith the lower layer representing the foundation andcharacterised by Er. The problem in applying these

concepts is to ensure that non-linear effects are taken intoaccount. In practice, this means that Ef will increase

when the asphalt layer is constructed. This is welldemonstrated by the data in Table 2 taken from fieldtesting with an FWD, during construction. It shows how Ef

increases from 30MPa for the exposed subgrade to90MPa for the composite, including 300 mm of generalcapping and 150 mm of crushed rock sub-base.

(a) Simplified structures

(b) Subgrade Mode Factor: Stiff Structure

(c) Subgrade Mode Factor: Soft Structure

Figure 14. Computed Mode Factors forSubgrades in idealised pavements

Figure 15. Computed verses measuredvalues of resilient modulus for clays

(after Dawson and Correia, 1996)

Table 2

Equivalent foundation stiffness values for

road on embankment (after Brown, 1996,a)

The mechanistic design of pavement foundations stillpresents analytical problems, not least because of thehigh stresses relative to failure conditions to which thegranular layer is subjected when loaded by constructiontraffic. The design criterion is clearly rutting and a limit of40 mm has been suggested in work conducted atNottingham (Dawson et al, 1993). Thom et al (1993) haveoutlined a novel design method which includes severalapproximations but attempts to deal both with the ruttingproblem and with the resilience of the foundation. Thislatter is of importance for satisfactory compaction of theasphalt layer as well as for its long-term performance.

The Wedge model shown in Fig. 16 is proposed as anapproximate method for dealing with rutting (Thom et al,1993). In this, the granular layer is characterised by itsangle of shearing resistance ( ') while an allowabledeviator stress on the subgrade is determined from

laboratory testing based on limiting the plastic strainaccumulated in 1000 cycles to 0.6%. An alternativeapproach is to determine the threshold stress for thesoil. This is defined as the maximum repeated deviatorstress which results in negligible accumulation of plasticstrain. Fig. 17 shows how it relates to soil suction for threedifferent clays (Loach, 1987). However, this parameter isnot easy to define for some soils, so a maximumallowable plastic strain of, say, 0.6% is an alternativeapproach.



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Figure 16. Proposed 'wedge model forcalculating rutting in pavement foundations

(after Thom et al, 1993)

Figure 17. Threshold deviator stress as afunction of suction for three clays

(after Loach, 1987)

As more accurate modelling of pavements becomeswidely used with the advent of faster computers and highquality user-friendly programs, the non-linearcharacteristics of soils and granular materials will beroutinely accommodated. However, this depends onreliable models being used and field conditions,particularly the effective stress state, being properlydefined. Further field observations of moisture conditionsbeneath pavements are badly needed to assist with thisprocess.

Pending these developments, it is appropriate toconsider the importance of non-linearity for pavementdesign calculations of the type presently undertaken.Sensitivity analyses reported by Dawson and Plaistow(1996) are helpful in this context. They analysed 14pavement structures with asphalt layers between 50 and250 mm in thickness and granular layers between 300 and700 mm. Non-linear models were used for the granularlayer and the subgrade was modelled with a series ofsublayers for which the stiffness increased with depth. Theasphalt stiffness varied from 2 to 8 GPa. Computationswere performed with the finite element program FENLAP(Brunton and d'Almeida, 1992). The results showed thatthe tensile strain at the bottom of the asphalt layer onlychanged by 2% for a change in subgrade stiffness from 40to 90 MPa at formation level. However, changes to thenon-linear parameters for the granular layer, representinga practical range, caused the tensile strain to change by

70%. Clearly, the resilience of the support directlybeneath the asphalt layer needs to be accuratelyquantified for fatigue cracking design calculations.

Similar computations by Dawson and Plaistow(1996) showed the importance of the resilient

characteristics of the granular layer in the context ofpermanent deformation developing both in this layer andin the subgrade under construction traffic. Some linearelastic calculations by the Author have indicated that thepavement foundation characteristics have much lessinfluence on the potential for permanent deformationdeveloping in the asphalt layer except when that layer isrelatively thin.

From the above discussion it may be concluded thatmore effort should be devoted to correctly modelling andspecifying granular materials for pavement designpurposes while subgrade characteristics are relatively lessimportant.

By contrast, when dealing with pavement evaluation,proper characterisation of the subgrade is vital. This isbecause the magnitude of surface deflection at all radialpositions relative to the load is strongly influenced bysubgrade stiffness. Fig. 18 taken from a contribution to the6th Conference (Brown et al, 1987) shows the largeinfluence which the subgrade has on surface deflection ina typical pavement situation. It is for this reason thatproper non-linear characterisation of the subgrade isneeded for successful back-analysis computations. Suchmodels should reflect the influence of changes in both theeffective stress and the deviator stress with depth and, iffinite elements are used, with radial position too. A back-

analysis program FEAD has been developed to performthese calculations (Brunton and d'Almeida, 1992).

There are many other aspects of pavementfoundations which are of importance. Reference may bemade to Brown (1996,a) for a review of soil mechanicsaspects. The opportunities for correctly modelling partiallysaturated soils (e.g. Wheeler and Karube, 1995) haveimproved as a result of research in recent years and thisopens up opportunities in pavement engineering whichneed to be exploited.

Concepts of drainage and permeability, ofstabilisation and the use of geosynthetics are all of

importance and require further research to fully realisetheir potential. The concepts illustrated by Fig. 19indicate how an improved pavement foundation might beconceived. The ideas involved are:

(a ) An open-graded drainage blanket connected to a sidedrain to remove water ingress from the subgrade orthrough pavement leakage.

(b) A geosynthetic separator to minimise contaminationof the drainage layer.



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(c ) A high quality dense crushed rock sub-basereinforced by a geogrid to mobilise high stiffness andgood rutting resistance.

Figure 18. Typical variation of vertical resilient

strain through pavement structure below centreof single standard wheel load

Figure 19. Cross-section showing idealisedpavement foundation

There are other ways of achieving some of these endsbut the principles of good drainage, trafficability byconstruction traffic and long-term stiffness of support areconsidered desirable principles.

PERMANENT DEFORMATION

Rutting remains a major failure mechanism and recentresearch led by SHRP has concentrated on dealing withthis through improved asphalt mixture technology.Analytical techniques to predict rutting are improving, asnoted in the Section on structural design and theoreticalanalysis which reviews the increasing use of visco-elasticanalysis. The work of Deacon et al (1995) shows howadvanced laboratory testing using the Superpave ShearTester (SST) combined with theoretical analysis canproduce design guidance for rutting in the asphalt layer.They showed that the rut depth (in mm) is approximately2.5 times the maximum shear strain (in per cent)measured in the constant height SST test. While this is

useful, it still requires an expensive test facility but theprinciple could probably be exploited using simplerfacilities of the type discussed in the next Section.

While this concentration on analysis and relatedtesting of the asphalt layer is entirely appropriate forheavy duty pavements, for those with thin asphalt layers,significant contributions to rutting may come from thepavement foundation. This can be minimised by adoptingthe procedures outlined in the preceding Section inrelation to soils and granular materials. For the subgrade,imposed stress levels need to be below the threshold orsimilar value. For the granular layer in a completedstructure, the same principle applies. For such material

the ratio of shear to normal stress should be kept below70% of static failure (Brown, 1996,a). Consequently,specification and measurement of shear strength forgranular materials should be encouraged.

These approaches to the rutting problem indicate thatthe concept of limiting the vertical subgrade strain atformation level through an elastic analysis is outdated.Brown and Brunton (1984) reviewed the use of this semi-empirical criterion. While they improved its applicationto allow for varying rut resistance of different asphaltmixtures, they made it clear that the parameter is only anindicator of the potential for critical rutting to develop asa consequence of permanent deformations developing in

all layers. A common misconception is that the subgradestrain criterion only refers to permanent deformation inthe subgrade. The relationships between allowable strainand numbers of standard wheel loads were developedfrom linear elastic back-analysis of structures with knownperformance in relation to rutting. The parameter is not,therefore, fundamentally based and cannot be expected toprovide reliable design guidance for pavements withcharacteristics that differ significantly from those used inthe back-analysis.



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For heavy duty pavements, the stress levels in thelower layers are unlikely to cause significant rutting and,hence, concentration on limiting permanent strains in theasphalt surfacing is an entirely appropriate approach.Equally, to minimise maintenance operations,preservation of a sound foundation is highly desirable so

that rehabilitation can be limited to a quick resurfacingoperation.

PROPERTIES OF ASPHALT MIXTURES

Thi s is a field in which there have been majoradvances stimulated by SHRP. These are well known andwill not be reviewed in detail here. The ability to testasphalt binders to determine relevant mechanicalproperties and the implementation of specifications basedon such tests represents a major positive achievement.The old empirical Black Magic tests such asPenetration and Softening Point have served a usefulpurpose but they are not relevant to the needs of an

industry increasingly looking at modified binders andspecifying mechanical properties. In this context, Fig. 20shows the kind of interesting data which can be obtainedfrom a Dynamic Shear Rheometer demonstrating the verydifferent characteristics of an EVA modified bitumencompared with the standard product (Airey, 1997).

(a) Unmodified bitumen

(b) Bitumen with 7% EVA

Figure 20. Black diagrams for 701100 Venezuelanbitumen (after Airey 1997)

In the U.S., the Superpave procedures resulting fromSHRP have left a gap between Levels 1 and 2 whichneeds to be filled pending the results of continuing long-term research. The need for a simple test or tests tomeasure key mechanical properties as an adjunct to theLevel 1 volumetric mixture design is essential. Such atest system needs to be economic and practical so that awide range of laboratories on both sides of the industry

can perform the necessary tests. This procedure is beingsuccessfully introduced in Europe and in Australia. Itsadoption in the U.S. could lead to Superpave Level 1.5(Brown, 1996,b) which would greatly enhance presentpractice. General guidance on this matter has beenpublished recently (NAPA, 1997) concentrating on tests

to deal with rutting.

At Nottingham, a suite of tests linked to mixturedesign and accompanied by a set of experimentalprotocols was developed in a joint project involvingacademia, oil companies, contractors and Government(Brown et al, 1996). The results are presently beingadopted by the British Standards Institution and are thesubject of discussion in connection with EuropeanStandards. Trial contracts in the U.K. (Nunn, 1996) haveallowed contractors to use these procedures to developtheir own improved mixtures with enhanced mechanicalproperties relative to current practice. This is givingconfidence to the highway authorities as they seek to

introduce specifications based on measured mechanicalproperties and allows the contractors to use their expertiseto meet these specifications.

The pneumatically operated Nottingham AsphaltTester (NAT) (Fig. 21) is used to measure stiffnessmodulus, resistance to permanent deformation and tofatigue cracking. Procedures derived from SHRP are setout for aging and water damage. The whole packagerepresents an excellent example of Paving the Gapbetween research and practice. Its implementation hasdepended crucially on the low cost and user-friendlycharacteristics of the test apparatus (Cooper and Brown,1989).

For permanent deformation, the original RepeatedLoad Uniaxial Test has been adapted to allow confiningstress to be applied using an internal partial vacuum withthe specimen surrounded by a rubber membrane. Thisarrangement is shown in Fig. 22 and allows moreappropriate testing of mixtures such as Porous Asphalt tobe conducted without much increase in equipment costand little change to the laboratory procedures. Fig. 23shows the huge effect of using a small confining stress(20 kPa) on the development of permanent strain in aPorous Asphalt with unmodified binder. Use of an SBSmodifier produced significantly better performance withunconfined specimens but no enhancement when the

20kPa confining stress was used. These results pose aquestion about the appropriate degree of confinement tobe used when carrying out laboratory tests on pavementelements. Clearly some confinement occurs insitu andthe laboratory test needs to reproduce this realistically forthe results to be valid. Brown and Gibb (1996)demonstrated, for continuously graded mixtures, thatconfined specimens may exaggerate the role of theaggregate structure relative to performance in thepavement.



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(a) The test frame and subsystems

(b) The test facility

Figure 21. The Nottingham Asphalt Tester



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Figure 22. Confined RLA Test Arrangementfor NAT

(a) Unconfined

(b) 20 kPa confining stress

Figure 23. Permanent deformation ofPorous Asphalt at 40C

The future challenge here is to calibrate the resultsfrom this test to field performance, to explore how itcompares with the SST and investigate how data from thetest may be reliably used in predictive computations forrut depth. This is the subject of ongoing research.

The increasing use of wheel tracking apparatus invarious countries represents a pragmatic approach totesting for rutting potential. It also allows, in some cases,an immersed specimen to be tested, thus dealing with theproblem of water damage which is of great concern insome locations. Wheel tracking, however, does not allowany fundamental property of the mixture to be measuredand there are concerns about scale effects since loadedareas are usually small in relation to aggregate size andmuch smaller than the situation on site.

The full-scale accelerated loading work, which iscurrently gathering data for validation of mixture designand predictive methods, will contribute significantly to

knowledge and to the confidence with which laboratorytest methods can be used. Notable amongst these effortsare those at WesTrack, Nevada (Nevada Automotive TestCenter et al, 1996), Richmond, California (Noakes et al,1996) and at the FHWA Turner-Fairbanks Center nearWashington, D.C. (Bonaquist and Mogaver, 1997).

While practical tests are needed to help today'sengineers and to improve quality control and qualityassurance, research to understand fundamentalmechanisms should continue. Fresh ideas using idealisedmodels, both theoretical and physical, and treating theproblem at the micro-level will generate new insights; indue course. The philosophy of the work initiated at

Cambridge under David Cebon and extended toNottingham under Andrew Collop is outlined in Fig. 24 forpermanent deformation. Following detailed study ofbinder properties over a wide range of conditions byCheung (1995) a Deformation Map was produced asshown in Fig. 25 drawing on concepts used in polymerengineering and elsewhere. This Map identifies zones inwhich the bitumen will exhibit different characteristicsand obey different models. Recognising that the aggregatein a mixture is almost rigid, it follows that the binder filmmust be subjected to a wide range of strain conditions.

Fig. 24 shows how understanding of real asphaltmixtures can be approached in stages from binder alone

to simple idealised mixtures with large and with smallparticles using two and three-dimensional conditions.

Proposals for similar work have been made byProfessor Monismith and his colleagues at Berkeley.Typical mixtures can be modelled to understand theirmicromechanical behaviour using finite elementsimulations based on use of image processing methods.Development of a macro constitutive law can befacilitated by studies on idealised mixtures.



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Figure 24. Sequential approach to fundamentalunderstanding of permanent deformation in

asphalt mixtures

Figure 25. A Stress/ temperature deformationmechanism map for bitumen (after Chan, 1995)

(Tg= Glass transition temperature)

One of the significant opportunities presented by theSHRP asphalt programme was to take a fresh look atdissipated energy concepts in fatigue. This idea was firstreported in 1972 but improved test facilities allowed moreaccurate data to be collected in the 1990's. ProfessorMonismith and his team (Tayebali et al, 1995) concludedthat dissipated energy was an equally valid criterion for

predicting fatigue life to crack initiation compared withtensile strain. They also pointed out that fatigueperformance of asphalt mixtures must be considered inthe context of the pavement because of the interactionsbetween layer thickness and stiffness and the influence ofthe supporting layers. When using tensile strain as the

criterion, elastic analysis of the pavement is adequate butdissipated energy calculations require visco-elastictheory.

Rowe et al (1995) have shown that such calculationscan be performed and that, for some pavements,maximum dissipated energy occurs at the surface ratherthan at the bottom of the asphalt layer. This is illustratedin Fig. 7 (Rowe and Brown, 1997).

For heavy duty pavements, field observations agreewith this concept (Nunn, 1997) since cracking is usuallyseen to initiate at the surface. However, there may beother reasons for this, since the asphalt surface is subject

to more aging and can contain various crack initiatorswhich take the form of roller cracks and surface textureprovision.

The need to properly model this form of fatiguecracking is a future challenge. Matsuno and Nishizawa(1992) reported progress on this subject at the lastConference. They argued that aging prevents healing ofsurface cracks and demonstrated that the phenomenondepended on high temperature conditions and that it wasinhibited by shadows cast from over-bridges.

In view of the continuing difficulties of predictingfatigue cracking, partly caused by the differences in

circumstance between the laboratory and the field, theconcept of insitu stiffness deterioration deserves moreprominence and this is discussed in the next Section.

There have been encouraging recent developments inasphalt technology for surfacing, notably in Europe. Thelead came from France, where the partnershiparrangements between the highway authorities and thecontractors has been effective in stimulating innovation.This has been backed by a framework of comprehensivetest methods which have been in use for many years(Bonnot, 1986).

The thin and ultrathin wearing courses pioneered

by French contractors have been based on therequirement for a non-structural surface which providesgood skid resistance, low noise and which is waterproof,durable and rut resistant. A related development involvingsome of the same principles is the widespread use ofStone Mastic Asphalt (SMA) since it offers thecharacteristics noted above but in a structural layer.These principles were set out by Molenaar and Liljedahl(1992) at the previous Conference.



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Table 3

Procedures for pavement evaluation andrehabilitation design

Unfortunately, FWD surveys tend only to be conductedwhen pavements are likely to be in need of somerehabilitation. If more data could be accumulated on theeffective stiffness changes taking place from the time the

pavement is first built, through regular FWD testing, thenrelevant models could be developed to assist withcomputations. The increased use of accelerated loadingdevices can assist with this process, as demonstrated byJameson et al (1992) using the Australian AcceleratedLoading Facility (ALF). Fig. 27 taken from their papershows data with considerable variability but a trend fordecreasing stiffness as cracking developed.

(a) Apparatus

(b) Results

Figure 26. Use of Dynamic ConePenetrometer to identify layer thicknesses


Overlay optionsand/or partialreconstruction

Residualfatigue lifeRuttingpotential

Effectivestiffnesses oflayers

Rehabilitation

Forwardanalysis

Back-analysis

Mechanicalproperties ofasphalt layers

NottinghamAsphalt Tester

Recoveredbinderproperties

Dynamic ShearRheometer

Layerthicknesses

Large voids

Ground RadarSurvey

Unbound layerthicknessesFoundationproperties

Dynamic CorePenetrometerdown core holes

Coring Bound layerthicknessSpecimens forlab. testing

Analysis andDesign

Laboratorytesting

Field testing

Deflectionprofile Data forback-analysis

FWD Survey


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Figure 27. Relationship between effectivestiffness and crack development

(after Jameson et al, 1992)

It is, however, important to recognise that cyclicenvironmental effects on pavements are not acceleratedin these trials. Consequently, properly planned

programmes of FWD testing on in-service roads arealso needed.

Collop and Cebon (1996) report a theoretical approachto prediction of stiffness deterioration. Fig. 28 shows howthey related it to cumulative fatigue damage for some ofthe Australian ALF trials.

Figure 28. Reduction in effective stiffnessof asphalt layer: theory compared with data

from ALF trials (after Collop andCebon, 1996)

Laboratory testing of asphalt beams by Tam (1987)generated the data shown in Fig. 29 which relates thedecrease in effective stiffness of the beam to the growth

in crack length. His results showed that a 50% reductionin stiffness can be expected for a crack which is 15% ofthe layer thickness. He also demonstrated, usingcontrolled deformation testing, that a residual stiffness ofless than 1 GPa may be expected for a fully crackedsection. This would be consistent with an unboundgranular material of good quality and supports the conceptof downgrading cracked layers of bound material to astiffness of 500 MPa for rehabilitation design when theyare overlayed.

(a) Influence of number of cycles

(b) Influence of crack growth

Figure 29. Relationship between crack growthand reduction in effective stiffness of asphaltbeam in four-point bending (after Tam, 1987)

The major future challenge in this field is to develop amoving deflectometer which does not require lane closurebut which can measure deflection bowls to an accuracycomparable with that of the FWD, i.e. to a few microns.Complete non-destructive evaluation is always going toproduce less information for the engineer to conductforensic studies of an existing pavement than intrusivemethods. However, the pressures to keep traffic movingon very busy major highways are real and a challenge.Night time lane closures are already the norm in manycountries. It should, however, be recognised that even themost cosmetic of rehabilitation treatments will need alane closure. For such treatments to be cost effective,they need to be based on the best design information andthis, presently, requires an earlier lane closure forrealistic site investigation.



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The particular problem of reflection cracking throughoverlays placed over pavements with well defineddiscontinuities, such as concrete slab joints remains asubject of research study and field trials. The RILEMConferences in recent years have focussed on thisproblem and useful theoretical work, notably by Scarpas

et al (1996), has provided an improved basis for design.His work has also allowed reinforcement to beincorporated in a finite element analysis that is helpful fordealing with reinforced asphalt, one of the treatmentsused for reflection cracking.

The concept of precracking cement treated layers isbeing implemented and good examples of this have beenreported by Shahid and Thom (1996) and were used onthe main runways for the new Kuala Lumpur InternationalAirport in Malaysia as part of an innovative pavementdesign. This used a granular sub-base, a cement treatedbase (precracked into 3 m square sections), an opentextured bitumen macadam and a two course asphalt

concrete surfacing incorporating modified binders.

The concept of Smart Pavements is receivingattention in the U.K. Investigations are underway toidentify appropriate low cost instrumentation which can,preferably, be retrofitted into pavements to monitor keyparameters. These include critical stresses and strains,temperature, permanent deformation and moisture. Ifsuccessful, computer based systems would allow remotemonitoring of pavement performance. This wouldcertainly be helpful for research and, with appropriatedevelopment, could be part of routine pavementmanagement.

CONCLUDING DISCUSSION

The principal future challenges which have beenidentified in this lecture are:

1 . Capitalise on the opportunities for theoreticalmodelling made possible by innovative ideas andpowerful computers.

2. Replace old ideas such as the CBR concept and thesubgrade strain criterion for rutting.

3. Improve fundamental understanding of asphaltmixtures including durability.

4. Develop the concept of effective s ti f fnessdeterioration to deal with fatigue cracking inpavement design.

5. Provide theoretical understanding and practicalsolutions to reflection cracking.

6. implement simple test methods to measuremechanical properties of asphalt mixtures, i.e.Superpave Level 1.5.

7. Upgrade the design and construction significance ofgranular layers.

8. Improve non-linear resilient modelling for foundations.

9. Incorporate the principles of soil mechanics moreeffectively into pavement engineering including newthinking on partially saturated soils.

10. Make good use of the data from accelerated testingfacilities and field observations.

11. Develop the Smart Roads concept to measure keyparameters routinely.

12. Recognise the different characteristics of heavy duty

and low volume pavements.

13. Present the results of research in a manner which canbe readily used in practice.

This lecture has endeavoured to present a combinationof achievements and future challenges and opportunitiesin the field of asphalt pavement engineering. Inevitablythere are areas which have not been discussed, notablythe need for highway engineering to become moresustainable through imaginative recycling and the use ofsecondary aggregates. In addition, the need to reduceenergy consumption and address alleged health issues ispointing to the need for improved cold-mix technology.

Progress will depend on continuing research,particularly in pursuit of innovation and by takingadvantage of the I.T. revolution. It will also require thatexisting knowledge is implemented in practice by Paving-the-Gap.

Both these sets of activities present challenges andrequire that able people are involved. The pavementindustry internationally is vast and still relativelybackward technically, so there are plenty of opportunities.The realisation of these will require that both the publicand the private sector recognise the need for research anddevelopment to be properly funded. It also requires that

both sides of the industry display a willingness tointroduce new ideas to practice.

The subject must be properly featured at an advancedlevel in University courses and in continuing professionaldevelopment programmes. This will create an educationalcohort of asphalt pavement engineers available to respondto the challenges of the future, some of which have beenidentified in this lecture.



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ACKNOWLEDGEMENTS

The Author is indebted to the International Society forAsphalt Pavements for selecting him as their firstDistinguished Lecturer. He is equally grateful to theTechnical Advisory Committee of this conference for

their invitation to deliver the Keynote Lecture and theirChairman, Professor Gary Hicks, for commenting on thetext.

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