PART II : PAVEMENT DESIGN METHOD

Part II Pavement Design Guide7Pavement Design Method

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PART II : PAVEMENT DESIGN METHOD

1. INTRODUCTION

The structural design guide developed consists of nomographs from which the thickness ofthe asphaltic layers in relation to the thickness of the base and subbase layer of a flexiblepavement can be determined. The nomographs have been developed in terms of the stiffnessof the subgrade, expressed in CBR, and the total number of Equivalent Standard Axles(ESA).

For practical reasons the thickness of the sub-base/base untreated layer have been fixed to200mm, 300mm and 400mm.

Since air temperature affects the mechanical properties of the bituminous mixtures two setsof nomographs have been prepared. One set at the almost lowest average yearlytemperature to occur along Egnatia road which is 13 oC and one set at the almost highestaverage yearly temperature to occur which is 16 oC. For intermediate temperature valuesthe thickness is determined by interpolation.

Nomographs have also been prepared not only for the standard type of bitumen used whichis the 50/70pen bitumen but also for a harder bitumen 40/50pen. Mixtures prepared with40/50pen bitumen may be considered as heavy duty mixtures. The type of bituminousmixtures considered in the development of the nomographs are dense asphaltic concretemixtures.

For the development of the nomographs, the elastic theory has been employed for multi-layer systems in which the materials are characterized by the elastic modulus (Young’smodulus) and the Poisson’s ratio. The materials are assumed to be homogeneous andisotropic and the layers have horizontally infinite dimensions. The calculations of thestresses and strains at the critical interlayer points and the required thickness of each layerto sustain these strains were carried out with the BISAR-PC1 and SPDM-PC2 programmes.

2. DESIGN CRITERIA AND PRINCIPLES

The design criteria used in order to ensure the satisfactory performance of the pavementthroughout the design period are:

a) The bituminous materials should not crack extensively under the influence of trafficloading. This is controlled by the horizontal tensile stress or strain at the bottom of thelast bituminous layer.

b) The subgrade should not deform excessively under the influence of traffic loading. This iscontrolled by the vertical compressive stress or strain developed at the surface of thesubgrade.

c) The thickness of the sub-base, base and the capping layer (if constructed) should be thickenough to withstand the construction traffic without overloading the subgrade.



The traffic is expressed in terms of standard axle loads acting vertically and/or horizontallyon the surface, which are assumed to be uniformly distributed over two circular areas. Thepavement is regarded as three or more distinct layers according to the type of materials ineach layer.

The BISAR computer program incorporated into the SPDM-PC program, for a givenpavement structure, calculates the maximum compressive strain at the top surface of thesubgrade and the maximum horizontal tensile stains, normally occurred, at the bottom of thelast asphaltic layer. If the calculated compressive strain is excessive, permanent deformationwill occur at the top of the subgrade, and this will cause deformation at the pavementsurface. Similarly, if the calculated horizontal tensile strain is excessive, cracking of theasphaltic layer will occur, which soon will show on the surface of the pavement. Thethickness of the pavement is determined by the lowest cumulative number of standard axlesdetermined by the corresponding fatigue equations.

The BISAR computer program was also used to determine the developed compressivestrain at the surface of the subgrade when the pavement consisted only of sub-base and baselayers. By this, it was possible to determine the thickness of the above layers so as there wasno excessive overloading of the subgrade during construction.

3. CHARACTERISTIC EQUATIONS, MATERIALS AND ASSUMPTIONS USED

3.1 Elastic modulus of the subgrade

The elastic modulus of the subgrade (Esubgrade), being one of the major input data foranalytical calculations, was determined from the California Bearing Ratio (CBR). CBR isnot a direct measure of stiffness modulus but is widely used and considerable experiencewith it has been gained throughout the years. Empirical relationships have been developedbetween the elastic modulus and CBR by various international organizations. The mostcommonly used relationships are those developed by the Asphalt Institute3 and TRL4.Graphical comparison of the equations is given in Appendix B, Figure B-1. The equationdeveloped by TRL gives more conservative results for CBR values >5%.

The equation adopted for the development of this design guide is the one proposed by TRLand is as shown below:

subgrade = 17,6 x CBR0,64 (1)

The above equation is considered to by applicable to materials expected to have CBRbetween 2 and 12%. For materials with CBR 15% and 20% the values of 100 MPa and 120MPa were assumed.

3.2 Elasticity modulus of the sub-base/base layer

The elastic modulus of the unbound layers sub-base and base was determined from theelastic modulus of the subgrade in relation to the thickness of the unbound layer. Thisapproach was proposed and used by the Shell pavement design methodology and itsAddendum 19855. The formula used is:



Esb/b = kxEsubgrade (2)

where k = 0,2x(hsb/b)0,45 (limits of k: 2<k<4) (3)

The above formula is valid for a confidence limit of 50%. For the 85% confidence limitused, the k value is corrected according to the Addendum 1985. However, the maximumelastic modulus of this layer was limited to 150 MPa.

3.3 Subgrade strain criterion

The allowable maximum subgrade compressive (vertical) strain to be developed is directlyrelated to the deformation of the subgrade. Extensive laboratory work by variouslaboratories and organizations determined mathematical equations from which it is possibleto determine the allowable maximum strain of the subgrade in relation to the number ofequivalent axle load repetitions until failure. All the equations have the general format of:

c = axNb (4)

where c = maximum allowable compressive strain to be developed at the surface of the subgrade N = cumulative number of standard axles a and b = regression coefficients

and in most cases failure is considered to be a certain deformation of the subgrade whichwill result in a 20mm, approximately, rut depth at the surface of the pavement. Forcomparison reasons the most widely known equations developed are shown in Appendix B,Figure B-2. As it can be seen the equation developed by AASHTO6 gives the mostoptimistic results, i.e. it allows higher subgrade compressive stains to be developed.

In the design guide developed, the equation adopted to be used was the one proposed byTRL and also used in the current British pavement design methodology7. The reason forchoosing this equation is simply the fact that it gives less optimistic results which may addto the probability of the pavement achieving the required life.

3.4 Stiffness modulus of the bituminous mixtures

The stiffness modulus of the bituminous mixtures is determined automatically by theSPDM-PC program. The determination is based on the work reported by Bonnaure et.al.8.In fact, the stiffness modulus of the bituminous mixtures is determined from the propertiesand relative volumes of the constituent materials. In other words, by the stiffness of thebitumen, the percentage by volume of the bitumen and the percentage by volume of themineral aggregate used. The determination of the stiffness of the bitumen was also carriedautomatically by the program, by using the work reported by Van der Poel9. According tothis, the effect of the air temperature, the properties of the bitumen and the time of loadingare all taken into account for the determination of the stiffness of the bitumen.

It must be noted that, as far as the air temperature is concerned, for the determination of thestiffness modulus of the bituminous mixtures, the program does not use the average yearlytemperature but it follows a certain procedure to determine the weighted mean annual



temperature and from that the effective temperature of the bituminous mixture. Theeffective temperature of the bituminous mixture is also dependent on the thickness of thebituminous layer. The effective temperature of the bituminous mixture is always higher thanthe average annual air temperature. Details about the procedure followed can be found inthe relative bibliography5.

3.5 Fatigue of bituminous mixtures

The fatigue of the bituminous mixtures due to the tensile horizontal strains developedresults in cracking of the bituminous layer. The fatigue behaviour is affected by variousparameters and primarily from the type and composition of the mixture, the characteristicproperties of the bitumen, the environmental temperature and the stress conditions. Undergiven conditions the bituminous layers can take a certain number of traffic loading beforethey crack.

Several nomographs and equations have been developed for the prediction of fatigueperformance of the bituminous mixtures. Each one differs in complexity, i.e. in the numberof parameters taken in order to characterize the mixture and the bitumen, the test conditionsetc., and in the type and number of mixtures covered in the experimental data which wereused to device the method.

In the present study the fatigue equation used was the one based on the work reported byValkering and Stapel10. The equation was developed after many years laboratory workusing various types of bituminous mixtures inclusive asphaltic concrete type. The equationused is as follows:

. = (0.856V .+1,08)(S .)-0,36 x (N .)-0,2 (5)

where = horizontal tensile strain V . = volume of bitumen, % S . = stiffness modulus of the bituminous mixture, MPa N = cumulative number ESA to cause fatigue

3.6 Representative bituminous mixtures

The types of bituminous mixtures taken into consideration for the structural analysis of thepavement were dense asphaltic mixtures, used in all pavement construction in Greece. Themixtures are specified by the Greek specifications PTP -265 and PTP -260. Thecomposition properties of the typical mixtures considered are as shown in Table 1.

Other mixtures were also considered but only for the construction of the anti-skidding layer.These mixtures were: dense Asphaltic Concrete (AC) for anti-skidding layer, PorousAsphalt (PA), Slurry Seal (SS) (micro-surfacing), and Stone Mastic Asphalt (SMA). Thecomposition properties of the typical mixtures used are as shown in Table 2.



Table 1 Composition properties of typical dense asphaltic mixtures

Mix property

Type of mixturePercentageof bitumen

(%)

Percentageof voids

(%)

Volume ofbitumen in the mix

(%)

Volume ofaggregates in the mix

(%)265 Wearing course265 Binder course260 Asphaltic base

5,34,64,0

468

12,510,69,0

83,583,483,0

Table 2 Composition properties of hot mixtures for antiskidding layers

Mix property

Type of mixturePercentage of

bitumen

(%)

Percentage ofvoids

(%)

Volume ofbitumen in

the mix (%)

Volume ofaggregates in

the mix(%)

AC for antiskidding layerPorous asphaltStone mastic asphalt

5,34,57,0

4203,5

12,59,216

83,570,880,5

3.7 Types of bitumen

The types of bitumen used in the analysis were: 40/50pen, 50/70pen and 80/100pen, all ofthem straight run bitumen. The typical characteristic values of their properties are:Penetration value 45, 65 and 90, respectively. Ring and Ball 52 oC, 46,5 oC and 43 oC,respectively.

3.8 Other data and variables used

Confidence level : 85%Mean vehicle speed : 50 km/hAsphaltic mix temperature : Varies (see paragraph 3.4)Poisson ratio : 0,35 for all layersEquivalent standard axleAxle stress : 80 kNNumber of tires : 2 twins on each sideContact radius : 105 mm



4. DESIGN PROCEDURE

The steps in the design procedure developed are illustrated schematically by the flowdiagram shown in Figure 2. The steps are:

1. Determine the input data(a) Traffic value, ESA(b) CBR of the subgrade(c) Mean annual air temperature (MAAT)

2. Decide whether capping layer is required (Use appropriate Table 4)

3. Decide on the sub-base/base layer thickness(Use appropriate Table 4)

4. Determine design thickness of the bituminous layers for the specified conditionsdescribed by the input data

(Use relevant nomographs)5. Choose the appropriate anti-skidding layer

(Refer to para. 11)6. Determine the thickness of each bituminous layer

(Use appropriate tables, Table 4 and 7)7. Decide on stage or one-stage construction (Refer to para. 13)

If yes, prepare stage construction design8. Make an economic analysis of the various solutions

(Refer to para. 14)9. Select the final design

5. TRAFFIC ASSESSMENT

In a pavement design the traffic volume should be expressed in terms of EquivalentStandard Axles (ESA=ITA) and not in number of vehicles. Furthermore, the traffic volumeof ESA should be calculated for the whole design period assumed (cumulative trafficvolume) and it should refer to the design lane, per direction of traffic flow.

For the determination of the cumulative number of ESA ( (ESA) per direction in the designlane the following equation is used:

(ESA) = (ESAdaily) x PTDL x 365 x CF (6)

where ESAdaily= number of daily ESA, per direction, in the year ofopening to traffic (base year)

PTDL = percentage of traffic in the design lane CF = Cumulative factor (= [(1+r)n -1]/r) r = mean annual increase of traffic (i.e. 0,03 for 3% mean annual

increase) n = design life, years



DETERMINATIONOF CBR FOR SUBGRADE

(Paragraph 6)

Choose the appropriateantiskidding layer (Para.11)

DETERMINATIONOF ESA

(Paragraph 5)

DETERMINATIONOF MAAT

(Paragraph 7)

Cappinglayer ?

(Para. 8)

FINALRESULT

Determinationof thickness

(Para. 8)

Determination ofsub-base/base

thickness (Para. 9)

Determination ofbituminous layers

thickness (Para. 10)

Economicanalysis (Para. 14)

Adjust, if necessary, theunderlying

bituminous layers

Stageconstruction(Para. 13)

Determinepavement forStage I & II

YES

NO

YES

Figure 2 Flow diagram of the pavement design guide



a) Determination of ESAdaily

In general, for the determination of the number of equivalent standard axles per day, perdirection (ESAdaily) the designer must determine (or know) the following:

a) the number of commercial vehicles per day, per directionb) the percentage classification of commercial vehicles into five categories: Buses, Semi- tracks, Trucks, Heavy trucks and Other vehicles, in the year of opening the road to traffic, andc) the equivalent wear factor per vehicle category.

The commercial vehicle (gross weight >1,5 ton) is defined as any vehicle apart from apassenger car. The description of the commercial vehicle categories, as mentioned above, is:

Buses : all buses and coaches Semi trucks (or light trucks) : all vehicles with two axles Trucks : all vehicles with three fixed axles or three axles articulated Heavy trucks : all vehicles with four or more axles, fixed or articulated Other vehicles : Any other vehicle which is not included in the above four

categories, except passenger car (such vehicles: agricultural vehicles or construction mobile machines)

For the purpose of designing a pavement in Egnatia road, a table has been prepared whichgives the total daily number (in both directions of flow) of commercial vehicles per link ofroad (project), for the opening year 2000 (assumed), Table C-1, column 11, Appendix C.This table was produced from data resulted from the recent traffic simulation study carriedout by EOAE11. It must be noted that the extracted data, and also all the others which willbe mentioned later on, are for the scenario of median GDP growth (i.e. mean annual GDPgrowth 3%) with generated traffic. This choise was made because this scenario wasconsidered to be the most likely to occur in the next years.

The sensitivity analysis that was carried out concluded that even if one of the other twoscenarios materialize, (i.e. mean annual GDP growth of 2,5% or 3,5%) it will not affect thepavement thickness determined by the median scenario to a great extent. In particular, themaximum decrease of the daily ESA, if the scenario of 2,5% GDP growth materialize, willbe -5,64%. This decrease is expected to result in a maximum decrease of the pavementthickness (asphaltic layers) of 0,9mm.

Respectively, the maximum increase of the daily ESA, if the scenario of 3,5% GDP growthmaterialize, will be +7,97%. In this case, the maximum increase of the pavement thickness(asphaltic layers) is expected to be 1,25mm.

Finally, it must be mentioned that the typical commercial/passenger vehicles ratio adoptedwas that of the year 2025.

Additionally, a table has also been prepared which gives the composition of commercialvehicles, in percentage, per classes determined and for each link of road (project), AppendixC, Table C-2, columns (5) to (9). The percentages per vehicle category shown in Table C-2have been derived by an approximation from the work presented by Nikolaides and



Mintsis12. The designer may use different traffic composition, if he proves (by analyticaltraffic studies) that the assumed traffic composition is not representative of the area.

With the help of the data obtained from Tables C-1 and C-2, Table C-3 has been preparedwhich gives the total daily number of commercial vehicles (column 5), the daily number ofcommercial vehicles per category (columns 6 to 10) and the number of daily ESA (column11) (all results per one direction of traffic).

For the determination of commercial vehicles per direction it was assumed that the traffic isequally split in each direction, i.e. 50% and 50%. The designer may use different percentageof split, if he has more accurate traffic information of the area.

The determination of the number of daily ESA (ESAdaily) per direction, (Table C-3), wascarried out using the following general formula:

ESAdaily = CV1xEWF1 + CV2xEWF2 + CV3xEWF3 + CV4xEWF4 + CV5xEWF5 (7)

where CV1, CV2, CV3, CV4 and CV5 = the number of commercial vehicles per category 1,2,3 and 4, respectively

EWF1, EWF2, EWF3, EWF4, EWF5 = the equivalent wear factor for each vehicle category 1,2,3 and 4, respectively (Table 3)

The equivalent wear factors per vehicle category are given in Table 3. The equivalent wearfactors given in Table 3 are those proposed by the work presented by Nikolaides andMintsis12. In this work, the proposed equivalent wear factors for Greece were derived fromthe similar ones proposed and used in the British pavement design methodology13.

Table 3 Equivalent wear factors

Vehicle category Equiv. Wear Factor (EWF)

1 Buses 1,32 Semi-trucks 0,343 Trucks 1,54 Heavy trucks 3,0255 Other vehicles 0,0266 Passenger cars 0,0

The equivalent wear factor represents the structural damage caused to the pavement by avehicle category in relation to the structural damage caused by the standard axle. In otherwords, the damage caused by one bus is 1,3 times more than the damage caused by onestandard axle. For clarification and comparison purposes, the structural damage caused by apassenger car is negligible and is considered to be equal to zero.



b) Determination of PTDL

In the case that the road has two traffic lanes per direction, in most cases Egnatia road hastwo lanes per direction, it is unrealistic to assume that all traffic will use only the one lane.However, more commercial vehicles will use the slow inner lane than the outer fast lane.The distribution of traffic per lane depends primarily on the volume of traffic and on thenumber of traffic lanes available. Other factors, such as type and classification of road(urban or rural, motorway, principal or secondary road, etc.) may also influence thedistribution of traffic per lane.

Taking into account that Egnatia road is a ‘homogenous’ type of road (rural motorway),hence the factors mentioned in the last sentence do not apply, the percentage of traffic in thedesign lane (PTDL), for the sections having more than two lanes, can be determined fromFigure 3.

50%

55%

60%

65%

70%

75%

80%

85%

90%

95%

100%

100 1000 10000 100000

Commercial vehicles per day in one direction

Figure 3 Determination of the percentage of commercial vehicles in the design lane13

c) Determination of mean annual increase of traffic

Traffic flow from day one of opening the road is going to increase, or decrease, or even stayconstant, year by year until throughout the design life selected. It is important, thoughdifficult, to estimate as accurately as possible, the mean annual percentage increase oftraffic. Any great diversions from the resulted percentage may result in an under designed orover designed pavement.

For the purpose of pavement design for Egnatia road, Table C-4, Appendix C, has beenprepared which gives the mean annual increase of traffic for each link of Egnatia road, Table



C-4 (column 8). The designer, for the time being, may use these annual increases when hedesigns a pavement for up to 20 or 30 years. He may also use these figures for designing apavement for 40 years, in absence of any other very long term forecast traffic data.

Summarizing the procedure for determining the cumulative number of ESA the steps tofollow are as follows:

a) Use equation 6b) For the determination of ESAdaily use values given in Table C-3, column 11 If there are more representative data for commercial vehicles distribution and knowing the total number of commercial vehicles per day, calculate the ESAdaily from equation 7 using equivalent wear factors given in Table 3c) For the determination of percentage of traffic in the design lane, use Figure 3d) For the determination of annual increase of traffic use values given in Table C-4, column

8e) The design life, in years, is determined by the designer

5.1 Examples for calculating the cumulative number of ESA

Example 1Calculate the cumulative number of ESA in the link, say Ladochori-Neochori, for a designperiod of 30 years. No traffic data are available other than the ones given in the Appenticesof the design methodology.

SolutionFor the determination of cumulative number of ESA equation 6 will be used.

From Table C-3, column 11, the daily number of ESA per direction (ESAdaily), at the year ofopening, is 300 ESA.The number of commercial vehicles per day/direction is 392, Table C-3, col. 5.

Hence, from Figure 3, the percentage of traffic in the design lane (PTDL) for 392commercial vehicles per day/direction, is 95,5%.

From Table C-4, the average annual increase of traffic in this link is 4,3%.

Thus, the cumulative factor (CF) for a design period of 30 years is:

CF = [(1+0,043)30 - 1] /0,043 = 59

Therefore, using equation 6, the cumulative number of ESA ( )) during the designperiod of 30 years will be:

(ESA) = 300 x 0,955 x 365 x 59 = 6,17x106 ESA, or 6,2x106 ESA



Example 2Calculate the cumulative number of ESA in the link, say Diavata-Efkarpia, for a designperiod of 20 years, when it is known (assumptions made) that the daily traffic at the time ofopening the road is: 6000 passenger cars and 3600 commercial vehicles. The distribution ofcommercial vehicles into categories is not known. However, it is known that in the eastwarddirection the percentage of traffic flow is 55% of the total traffic flow. The average annualincrease of traffic has been forecast to be 3,2%.

SolutionThe ESAdaily will be determined from equation 7.

Since the distribution of commercial vehicles is not known, data given in the Appendices ofthe design methodology will be used.

From Table C-2, col. 5 to 9, assuming that the distribution of commercial vehicles is valid:from a total 31,2% of traffic flow being commercial vehicles, 2% is buses, 11% is semi-trucks, 12,7% is trucks, 5% is heavy trucks and 0,5% is other vehicles.

The percentage of commercial vehicles of the total traffic according to the data given is3600/9600 = 37,5%. Thus, the previously given distribution of traffic has to be adjusted forthe 37,5%. The commercial vehicle distribution then becomes: 2,4% buses, 13,2% semi-trucks, 15,3% trucks, 6% heavy trucks and 0,6% other vehicles.

Using the above figures and the fact that in the eastward direction (the most heavilytrafficked direction) the percentage of traffic is 55% of the total, i.e. 1980 in eastwarddirection, the number of commercial vehicles for this direction is:

1980 x (2,4/37,5) = 127 buses,1980 x (13,2/37,5) = 697 semi-trucks,

1980 x (15,3/37,5) = 808 trucks,1980 x (6/37,5) = 316 heavy trucks and

1980 x (0,6/37,5) = 32 other vehicles

Thus, from equation 7 and using the above figures and the equivalent wear factors fromTable 3, the ESAdaily is equal to:

ESAdaily = 127x1,3 + 697x0,34 + 808x1,5 + 316x3,025 + 32x0,026 = 2571 ESA

From Figure 3 and when the number of commercial vehicles per direction is 1980, thepercentage of traffic flow in the design lane (PTDL) is: 88%.

The cumulative factor (CF) for 20 years design period and 3% annual increase becomesequal to: [(1+0,03)20 - 1]/0,03 = 26,87

Therefore, the cumulative number of ESA is:

(ESA) = 2571 x 0,88 x 365 x 26,87 = 2,22x107 ESA, or 2,2x107 ESA



6. SUBGRADE ASSESSMENT

The strength of the subgrade is of great importance in a pavement design. Weak subgradewill require greater thickness of overlying layer, in comparison to high strength subgrade, inorder the resulted traffic stresses at the formation level to be reduced to a level sustained bythe subgrade. The strength of the subgrade, in this design methodology, is determined interms of the California Bearing Ratio (CBR).

The CBR should be tested in the laboratory according to the Greek specifications E 105-86(in soaked condition) but in all cases, as a representative CBR value for the design, the CBRvalue corresponding to the 90% of the maximum dry density determined by the modifiedProctor test should be taken. As an exception, and only, in the case of an undisturbednaturally cemented soil material in a cut with low water table (>1m down), the CBRcorresponding to the 95% of the maximum dry density by modified Proctor test may betaken.

However, it must be stated that the 90% compaction used for the determination of thedesign CBR should not be confused with compaction requirements of the subgrade duringconstruction. This should always be aimed to be greater or equal to the value determinedfor field compaction (i.e. 95% or any other value, of the maximum dry density determinedby Proctor test).

The design CBR of the subgrade layer, in all cases (cut or embankments), should bedetermined from the worst material found within 600mm down from the decided formationlevel. It is clearly stated that, the range of CBR values of the subgrade layer (bearingcapacity of the subgrade) given in Table 4.5.1 of OSMEO can only be used as indicationvalues and only in a preliminary design. The designer, for the final detailed design, shouldalways use the CBR laboratory values from representative samples taken of the materialsgoing to be used in the construction. In case of no uniformity of the materials, the valueobtained from the worst material is the one to be used for further calculations. The contentof this paragraph should always be related with capping layer requirements and itsthickness, paragraph 8.

In other words, for a pavement going to be constructed in a cut where the subgrade within600mm depth from the formation level consists of materials type E2 and E1, the designer, ifhe wishes to carry out preliminary design, may use the lowest corresponding value of CBRfor each material, given in Table 4.5.2, i.e. >5% (say 5,1%) and >3% (say 3,1%)respectively, and the design CBR for preliminary design will be 3,1%.

For final detailed design, the designer should always use the laboratory CBR values and thedesign CBR will be the lowest of the two materials tested, i.e. if the CBR of material typeE2 was 7% and the CBR for material E1 was 3,5%, the design CBR value is 3,5%. In thisparticular case, according to the requirements of the methodology, a capping layer is alsoneeded, refer to paragraph 8.

For a pavement going to be constructed on an earth or rock embankment, taking intoaccount that the top layers of the embankment are forming the subgrade layer, all itemsmentioned in the previous paragraphs are also valid.



Exemption to all the above is when the pavement is constructed in a cut of rocky ground.Here the designer may choose subgrade CBR values between 15% or 20%, depending onthe soundness of the rock and the material used in the regulating course. For sound qualityrock and a very good quality regulating course use CBR value equal to 20%. For weatheredrock with a good quality regulating course, use CBR value equal to 15%. However, thesecases may not frequently arise since the final thickness of the pavement most probably willbe dictated from the design of the adjacent embankments.

When a drainage layer is used, the design CBR should always be that of the material foundimmediately below the drainage layer. In other words the effect of using granular material inthe drainage layer is not taken into account. The same applies in case of using a frostprotection layer.

Water table and preservation of constant moisture content of the subgradeThe water table should always be kept at least 50 cm from the formation level. To obtainthat refer to paragraph 3.8.3.2 of OSMEO.

In situ testing during constructionIn situ testing during construction must be carried out in order to check the compliance withthe design during construction. It is not intended for altering the design. The in situ testsare, primarily: a) the compacted field density determination (by sand cone or nuclear densityapparatus) and b) the moisture content determination of the soil to be compacted. Referalso to relevant articles of the T .

It is advised to carry out Proctor test every time there is a noticeable change in material.

Field CBR test may be carried out, but the design CBR should always be the onedetermined in the laboratory as mentioned earlier on.

7. ASSESSMENT OF MEAN ANNUAL AIR TEMPERATURE (MAAT)

Air temperature greatly affects the stiffness modulus of the asphaltic layers. It is thereforenecessary to assess the mean annual air temperature and use the appropriate nomographsfor the determination of the thickness of the asphaltic layers.

The assessment of the mean annual air temperature can be done by examining long termstatistical data collected by the nearest to the project meteorological station.

For the purpose of developing appropriate design charts, the MAAT over 10 years (1981-1991) of four distinct locations/cities have been considered along Egnatia road. Theselocations were:

Ioannina, with MAAT = 14,4 oC Kozani, with MAAT = 13,0 oC Thessaloniki, with MAAT = 15,8 oC Alexandroupolis, with MAAT = 14,9 oC



It was assumed that the above locations cover well the temperature variations that mayoccur along the Egnatia road. As a result two distinct sets of nomographs have beenprepared, one for 13 oC (the lowest) and one for 16 oC (the highest).

The designer, depending on the location of the project, has to assess the representativeMAAT of the area and use one of the two available sets of nomographs. In the case of anintermediate temperature, he has to interpolate the pavement thickness by using a third setof nomographs, showing all design curves together, Appendix A, Figures A-7 to A-9 (onlyfor 50/70pen bitumen).

8. DETERMINATION OF CAPPING LAYER

Capping layer is required when the CBR of the subgrade is less than 5%. The thickness ofthe capping layer is determined by the CBR value of the subgrade. This design methodologydistinguishes two CBR values and hence two required thickness’ of capping layer.Specifically when:

• CBR is less or equal to 2,5%, the thickness of the capping layer should be 600mm• CBR is greater than 2,5% and less or equal to 5%, the thickness of the capping layer

should be 300mm

The above required thicknesses of the capping layer are related to the thickness of the sub-base and base layer. Therefore, refer to Table 4 also.

The design CBR value in the cases that the subgrade CBR is less than 3% will be taken asequal to 3%. In all other cases, i.e. when the subgrade CBR 3% -5%, the design CBR isdetermined by the measured CBR of the subgrade.

In the cases that a capping layer in needed and the natural moisture content of the soil isrelatively high during construction, more specifically when the CBR is <2,5%, ageosynthetic material, placed on the exposed surface of the subgrade, after removing thetop 600mm or 300m, may be useful.

If the soil is cohesive, a lime treatment may also be another alternative for improving thebearing capacity of the subgrade. Subject to soil suitability this solution may prove to bemore economic compared to capping layer with graded material.

For materials used in capping layer refer to Part 3.

9. DETERMINATION OF SUB-BASE AND BASE LAYER

The layers of the sub-base and base are seen as one layer of unbound material. For practicalreasons, the thickness of the unbound layer was chosen to have three distinct values:400mm, 300mm and 200mm. Therefore, the designer has to choose one of the threeproposed thicknesses in order to carry one further the determination of the bituminouslayers.



However, the thickness of the unbound layer is related to the type of the subgrade. Hencethe choice of an appropriate thickness should be made according to Table 4.

Table 4 Required and recommended thickness of sub-base/base layer (unboundlayer)

Subgrade CBR(%)

Capping layerthickness

(mm)

Unbound layerthickness

(mm)Required thickness

≤ 2,52,6 - 5,0

5,1 - 10,0

6003000

400400400

Recommended thickness10,1 - 20,0

> 20,000

300200

The granular material used for the construction of the unbound layer, should comply withthe PTP -155 and Article 67 of TSY.

Allowances for the thickness of 400mm can be made so as, the first layer, overlying thesubgrade, of 200mm may be constructed from natural granular uncrushed or semi-crushedmaterial complying with PTP -150 and Article 66 of TSY, and the second 200mm layer ofmaterial complying with the PTP -155 and Article 67. However, even in this case, whenthe design life in cumulative traffic loading is greater than 1x107 cumulative ESA, or inlocations where sub-zero temperatures are predominant during winter months, (in this caseregardless of traffic loading), it is recommended to use, exclusively, only material complyingwith PTP -155 and Article 67 of TSY.

Cement treated sub-base or/and baseThe sub-base and the base layer could be constructed with cement treated materialaccording to Article 26 and 27 of TSY. In case cement treated materials are used, themethodology recommends that the thickness of the treated sub-base or/and base should bethe same as the thickness of the unbound layers.

As far as the effect of using cement treated materials, instead of unbound materials, to thethickness of the bituminous layers is concerned, the following arguments must be stated.Theoretically, a strength increase of the materials used in the sub-base/base layer will resultin thinner bituminous layers.

However, taking into account the resulted strength of the treated mixtures, as describedabove (compressive strength of 7 MPa and 3 MPa, respectively, for 7-day curing), thestresses generated by construction traffic and temperature in the cemented layer will cause itto crack. The severity and extent of cracking will be influenced by the amount ofconstruction traffic, the environmental temperature during construction and the stiffness ofthe subgrade. Long construction sections with few exit/entry points to the construction site,



carry appreciable amount of construction traffic. Construction during days with relativelyhigh air temperatures and on sites with relatively weak subgrade, will act adversely to thedevelopment of the cracks. The degree of subsequent deterioration of the cement treatedsub-base/base and hence their contribution to the structural performance, under less severestress regime of the completed pavement, is uncertain and extremely difficult to simulateand analyze.

Internationally, there is no common acceptance that cement treated sub-base/base layer ofthis magnitude of strength, can result in a satisfactory structural performance of thepavement, if the thickness of the bituminous layer is reduced. On the contrary, it iscommonly acceptable and usual practice, in the absence of acceptable quality material forsub-base/base layer, cement treatment to be used, in order to utilize the marginal orunacceptable available material. Nevertheless, research is carried out, which, it must bestated, should be confirmed by trial road sections, on whether the use of cement orlime/cement stabilized material, with compressive strengths up to 7MPa, can result in areduction of overlying bituminous layers.

Taking into account the above, and considering that:

a) there is very little structural experience in this country,b) the maintenance works in Egnatia road, being a future major highway, should be as less

frequent as possible,c) there is no shortage of good granular material for sub-base/base construction, in the

vicinity of most sections to be constructedd) higher cost will be resulted from the use of cement treated materials, certainly when no

reduction of the bituminous layers is not anticipated,e) investment will be required by the contractors for new machinery and training of the

people to lay these mixtures,

the methodology recommends that cement treated material, as per Articles 26 and 27, maybe used as a substitute to the sub-base and/or base unbound granular material, without anyreduction to the thickness of the bituminous layers. The possible better performance of thepavement, will be a bonus in terms of extended service life.

10. DETERMINATION OF BITUMINOUS LAYERS THICKNESS

The thickness determination of the bituminous layer can be carried out by nomographsshowing design curves at different CBR values. Figures A-1 to A-15, Appendix A.

Description of the design curvesTwo basic sets of nomographs have been developed: one for mean annual air temperature(MAAT) of 13 oC, Figures A-1 to A-3, and one set for mean annual air temperature(MAAT) of 16 oC, Figure A-4 to A-6. Each set contains nomographs for three differentbase/sub-base thickness of 400mm, 300mm and 200mm.

The determination of the thickness for temperatures other than 13 oC and 16 oC is carriedout by interpolation of the results. To facilitate the interpolation procedure, corresponding



design curves obtained for 13 oC and 16 oC were plotted on three additional nomographs.This nomographs are shown in Figures A-7 to A-9.

All the design curves developed are valid for dense asphaltic concrete mixtures (AC),complying with PTP A265 and PTP A260, with 50/70pen type of bitumen.

In case, type 40/50pen bitumen is decided to be used for the production of all asphalticconcrete mixtures, nomographs shown in Figures A-10 to A-15 should be used. It must bestated that the use of harder bitumen is primarily seen as to reduce the expected permanentdeformation of the asphaltic layers due to the inherent properties of the bitumen. The use of40/50pen bitumen may arise in areas with very heavy traffic loading. Environmentaltemperature may also affect this decision. When high mean annual air temperature isexpected (15 oC & 16 oC) the bituminous layers are expected to exhibit more permanentdeformation compared to environment with MAAT of 13 oC or 14 oC.

In case the designer wishes to design a pavement where the wearing course or the top50mm of the binder course is to be constructed with 40/50pen bitumen (this may arise onlyin the case of using porous friction course), the pavement thickness is determined from thebasic nomographs, i.e. Figures A-1 to A-9.

To facilitate the decision where and when type 40/50pen bitumen may be used, a protocolshown in Table 5 has been prepared and may be used. This protocol has been drawn on thebasis of the predicted permanent deformations estimated, for the thickness determined fromthe design curves developed. The predicted estimated permanent deformation using variousscenarios is shown in Appendix D, Figure D-1 and Figure D-2. The criterion used was thatthe bituminous layers should not exhibit excessively more than, approximately, 25mm ofrutting at the end of the design period.

Table 5 Protocol for using the appropriate nomograph

CumulativetrafficESA

Mean annualair temperature

(MAAT)

Type ofbitumen

Which layers willbe constructed with

a certain type ofbitumen

Whichnomograph will

be used

≤ 1x107 13oC to 16oC 50/70 pen All Fig. A-1 to A-91x107 - 5x107 13oC to 14oC

15oC to 16oC50/70 pen40/50 pen

AllThe top layer only

Fig. A-1 to A-9Fig. A-1 to A-9

> 5x107 13oC to 14oC15oC to 16oC

40/50 pen40/50 pen

Top layer onlyAll

Fig. A-1 to A-9Fig. A-10 to A-15

The designer may also use modified bitumen, instead of conventional bitumen 40/50pen, forthe construction of the top layer. In this case, i.e. use of modified bitumen only at the toplayer (40mm or 50mm), the determination of the thickness is carried out by the samenomographs, as given in Table 5.

Finally, if the designer wishes to use bitumen 80/100pen for the production of the mixturesfor the asphaltic base, only, he has to increase the resulted thickness of the asphaltic base



layer by 7%. The latter is not recommended by the methodology, certainly in situationswere the MAAT is >15 oC or the cumulative traffic loading is >1x107 ESA irrespective ofMAATs.

How to use the nomographsKnowing the cumulative number of ESA and the CBR of the subgrade, the total thicknessof all bituminous layers, i.e. wearing course, binder course (base course) and asphaltic base(road base), can easily be determined, see Figure A-1. The final result should always berounded up to the nearest higher 5mm.

In case design CBR value is between those that appear on the relevant nomograph, roundup the value to the nearest lower integer number, i.e. if CBR is 6,7% round it up to 6,0%.For intermediate CBR values, between those shown in the nomographs use linearinterpolation.

For the determination of the thickness of each individual bituminous layers together with thesuggested type of asphaltic mixture, per case, use Table 6.



Table 6 Thickness determination of individual bituminous layers

Totalthickness of

asphalticlayers

Wearing course Binder course(Base course)

Asphaltic base(Road base)

(mm)Thickness(mm)(1,2)

Type ofmix

Thickness(mm)

Type ofmix(3)

Thickness(mm)

Type ofmix(3)

100

125

150

200

250

300

350

400

450

500

550

40mm

or

less,

depending

on the

type of

mixture

used(1)

Refer

to

Table 7

-

-

50

50

50

100

100

100

100

100

100

-

-

265-C/Bind.(N12,5)

265-C/Bind.(N12,5)

265-C/Bind.(N12,5)

265- /Bind.(N19,0)

265- /Bind.(N19,0)

265- /Bind.(N19,0)

265- /Bind.(N19,0)

265- /Bind.(N19,0)

265- /Bind.(N19,0)

60

85

60

110

160

160

210

260

310

360

410

A260-D(N 19,0)A260-D(N 19,0)

260-D(N 19,0)

260-D(N 19,0)

260D(N 19,-0)

260-C(N 25.0)

260-C(N 25.0)

260-C(N 25.0)

260-C(N 25.0)

260-C(N25.0)

260-C(N25.0)

For interim values of total asphaltic layers thickness, the difference is added to theasphaltic base(1) The thickness of the wearing course with antiskidding properties may be as low as,

approximately, 10mm. If, in all cases, its thickness is less than 30mm it is simply added on top,increasing the total thickness of the pavement by the same amount. The underlying layer in thiscase should be 40mm of dense asphaltic concrete type A265B (N12.5). Otherwise, if thethickness of the antiskidding layer is ≥30mm, add the remaining thickness (≤10mm) to thethickness of the binder course or the asphaltic base. The above are applied in conjunction withthe requirements of Table 7.

(2) The thickness of the ACfc, PA or Ogfc, for a new construction, is recommended to be 40mm inall cases.

(3) Mixtures characterized with the letter N are proposed new mixtures, subject to approval byEOAE, for details, refer to Part 3



11. ANTISKIDDING LAYERS

11.1 Provision of wearing course with antiskidding properties

The surface of the top layer of a flexible pavement should always have good and lastingantiskidding properties. The designer has five alternative bituminous mixtures to choosewhich can provide a good antiskidding surface. These mixtures are called friction coursemixtures or mixtures for antiskidding layers and are as follows:

• Dense asphaltic concrete for friction course (ACfc),• Open graded mixture for friction course (OGfc)• Porous asphalt (PA)• Stone mastic asphalt (SMA)• Slurry seal - microsurfacing (SLms)

Some of the above mixtures have different structural capacity and different air permeabilitythan the dense asphaltic concrete mixture, taken as representative mixture for developingthe design curves. Therefore, when they are chosen (in a new construction), slightmodification on the total thickness of the asphaltic layers and the type of underlying layer isnecessary and it should be made. The required changes in the total thickness of the asphalticlayers and in some cases the type of underlying layer, are summarized in Table 7.

Table 7 Required changes due to different friction courses used

Friction coursemixture

Required change in thickness Required change to the underlyinglayer

ACfc None None

(OGfc)orPA

Increase total thickness ofasphaltic layers by 20mm

In all cases, the first 50mm ofunderlying layer should be of a

dense AC mixture, type A265-B (orN 12,5) for wearing course

(see also note 1 & 3)SMA None None

SLms NoneThe underlying wearing course is

constructed with dense ACmixture, type A265-B (or N12,5)

for wearing course(see also note 2 & 3)

Notes:1. When the total thickness of all asphaltic layers is <150mm, see Table 6, introduce a

binder course of 40mm of dense asphaltic concrete type A265/B for wearing course (N12,5) and make the corresponding adjustment to the thickness of the asphaltic baselayer

2. SLms has a thickness of approximately 10mm which does not contribute at all to the structural capacity of the pavement, it is simply added on top of the wearing course3. The A265-B (or N12,5) mix is produced with crushed limestone aggregates

More details on the friction course mixtures can be found in Part 3.



However, the designer may consider any other mixture, apart from the above mentioned,proven to be suitable for antiskidding layer.

11.2 The choice of friction course

The choice of the type of friction course depends on various parameters which may changefrom project to project. So, it is left to the designer, after considering carefully all of them,to decide which friction course he will propose to be used.

The main parameters he has to consider are:

a) skid resistanceb) cost (material and construction cost but taking also into consideration the contribution or

no contribution to the structural capacity of the pavement, since it is a new construction.This means that the resulted effective cost must be taken into account)

c) market availabilityd) economizing on hard aggregate (in relation to availability of hard aggregates and

preservation of natural resources)e) life expectancyf) noise reductiong) easiness to renew or maintain

To facilitate the designer in choosing the most appropriate antiskidding layer, the followingprocedure is recommended:

Firstly, each mixture is rated in relation to each one of the parameters previously mentionedon a scale of 1 to 5. Number 5 characterizes the mixture with the highest skid resistance, theless costly, the most readily available on the market, the lowest requirement for hardaggregate, the highest life expectancy, highest noise reduction and the easiest to renew andmaintain. The rating of the antiskidding layers considered, according to the team of experts,is as shown in Table 8.

Table 8 Rating of the friction courses

Type of

frictioncourse

Skidresistance

Cost Marketavailabi-

lity

Econo-mising on

hardaggregate

Lifeexpectany

Noise

reduction

Easyto

renew

ACfc

OGfc

PA

SMA

SLms

2

4

5

4

4

5

2

2

3

4

5

3

3

1

5

1

2

2

1

5

4

2

3

5

1

1

5

5

3

2

3

2

2

3

5

Secondly, it is determined which of the above mentioned parameters is most important in aparticular project. The parameters are ranked in a descending order of significance and a



weighing factor, from 7 to 1, is assigned to each one of them. For instance, if the order ofsignificance for a particular project is: skid resistance, cost, economy on hard aggregatenatural resulted, life expectancy, market availability, reduction of noise and easiness torenew, the weighing factors are from 7, the most important parameter, to 1, the leastimportant parameter.

The cumulative product of the rates of each parameter, for each antiskidding layer, and therespective weighing factor is the decision making factor for each antiskidding layer:

DM = ( ) ( )WFI x RFi i∑ (8)

where DM = decision making factor for choice of friction course WFI = weighing factor, from 1 to 7 RF = ranking factor, from Table 8 i = parameter concerned

Comparing the numerical result of DM for each alternative friction course considered, thechoise of the optimum friction course is made in a more objective way. The optimumfriction course for the particular project is the one with the highest value of DM. The aboveare exemplified in the example that follows.

It must be stated that the number of parameters considered could be increased, as well asthe number of alternative antiskidding layers. Furthermore, the suggested rating of theparameters for each antiskidding layer could be slightly modified. Preserving the same basicprinciples, though, the procedure for the choise of the optimum antiskidding layer remainsthe same.

With the above procedure a more objective choice of antiskidding layer for a particularproject is being succeeded. However, for the final choice and decision it is advised toconsider, also, the results of the economic analysis of the pavement (life cycle cost analysis),which is explained in paragraph 14. It is believed that this is the most professional way indecision making.

ExampleAssuming that the friction courses to be considered are as those outlined in paragraph 11.1,choose the best solution (friction course) for the project A to B.

ProcedureAssuming that in a given project in rural area the decisive parameters are ranked in order ofimportance as follows:

Skid resistance = 7 Cost = 6 Life expectancy = 5

Economy on hard aggregates = 4 Market availability = 3 Easy to renew = 2 Noise reduction = 1



Using formula (8) and the ranking factors given in Table 8, the decision making factor(DM) for each friction course is as follows:

for ACfc : DM1 = 7x2 + 6x5 + 5x4 + 4x1 + 3x5 + 2x3 + 1x1 = 90

for OGfc : DM2 = 7x4 + 6x2 + 5x2 + 4x2 + 3x3 + 2x2 + 1x5 = 76

for PA : DM3 = 7x5 + 6x2 + 5x3 + 4x2 + 3x3 + 2x2 + 1x5 = 88

for SMA: DM4 = 7x4 + 6x3 + 5x5 + 4x1 + 3x1 + 2x3 + 1x3 = 87

for SLms : DM5 = 7x4 + 6x4 + 5x1 + 4x5 + 3x5 + 2x5 + 1x2 = 104

From the above results and for this particular project, the slurry seal micro-surfacing is thebest alternative for a wearing course with antiskidding properties. Second best comes theasphaltic concrete (ACfc) with marginal difference to PA and SMA.

12. EXAMPLE FOR PAVEMENT THICKNESS DETERMINATION

Determine the thickness of a pavement when:a) Cumulative number of ESA for 30 years design period with annual increase of 3% was found to be 7x106

b) CBR of the subgrade is 5,3%c) Mean annual air temperature (MAAT) is 16 oC

SolutionFrom Table 4, since CBR is >5%, no capping layer is required and the thickness of thebase/sub-base layer should be 400mm.

Since CBR is 5,3%, use CBR=5% for all other calculations

From Table 5, since cumulative number of ESA=7x106 and MAAT=16 oC, thedetermination of the thickness of all asphaltic layers will be carried out from one of thenomographs shown in Figures A-1 to A-9 and the top layer of the wearing course is advisedto be with 50/70pen bitumen. Elimination of relevant figure: since MAAT=16 oC andbase/sub-base thickness = 400mm, use Figure A-4.

From Figure A-4, for ESA=7x106 and CBR=5%,total thickness of bituminous layers = 283mm, rounded up: 285mm

According to Table 6, for a total thickness of 285mm, the thickness of the individual layersare:

Wearing course = 40mmBinder course = 50mmAsphaltic base = 195mm [=160+(285-250)] (see bottom of Table 6)



Here, the designer should decide which type of antiskidding layer is going to propose. Forthe choise of the appropriate antiskidding layer the designer follows the guidelines ofparagraph 11.

Say that the decision was made to use porous friction course.

Referring to Table 7, the following adjustments are necessary: a) to increase the totalthickness of the asphaltic layers by 20mm and b) to provide a 50mm dense AC layer (typeA265-B (or N 12,5)) for wearing course with crushed limestone aggregates and preferablywith 50/70pen bitumen), underneath the porous friction course.

Due to the above, the final design is as follows:

Porous friction course = 40 mm Wearing course = 50 mm

Binder course = 50 mm Asphaltic base = 165 mm Total of asph. layers = 305 mm

Base/sub-base = 400 mm

Total pavement thickness = 705 mm

Alternatively, Slurry sealing-microsurfacing could be chosen as antiskidding layer. In thiscase, according to Table 7, the underlying layer should be a dense AC mixture type A265-Bfor wearing course, with limestone aggregates and preferably with 50/70pen bitumen.

Hence, the final design is as follows:

Slurry seal-microsurfacing = (~10 mm) Wearing course = 40 mm Binder course = 50 mm Asphaltic layer = 195 mm Total of asph. layers = 285 mm



Finally, if dense AC friction course or SMA was chosen, the final design would be:

ACfc or SMA = 40 mm Binder course = 50 mm Asphaltic base = 195 mm

Total asph. layers = 285 mm





13. PLANNED STAGE CONSTRUCTION

13.1 General

In this stage it is examined whether a planned stage construction is preferable than the onestage construction. Planned stage construction is the construction of the bituminous layersin two, most usual, stages according to design and to a predetermined time schedule.

Planned stage construction may be preferable:

a) when there is not enough capital to construct the full design thickness (say for 20 or 30years), and

b) when there is a difficulty or uncertainty, in estimating the traffic for the design periodconsidered

In addition to the above, some authorities experience better pavement performance whenpavements are overlaid after they had been subjected to traffic, than new pavements ofequal design.

However, it must be emphasized that, in case of planned stage construction, the requiredcapital for further works at a later stage must always be readily available. Meanwhile, it isalso important, to monitor the traffic, between the first and second stage of construction,and to make frequent assessments on the structural condition of the pavement. The lattergives the opportunity to intervene earlier in case the pavement starts to deteriorate earlier.In all cases, the planned second stage construction must always take place at a stage whenthe pavement is still in sound condition.

Summarizing, the decision for a stage construction, is a decision with commitments and theactual determination of the second stage of construction by no means is as simple as itlooks.

The design of planned stage construction should not be confused with the design of majormaintenance or reconstruction work of the existing pavement.

13.2 The design method

The design method outlined is similar to the one recommended by the Asphalt Institute ofUSA. It involves three steps: i) first stage design, ii) preliminary design of second stageoverlay and iii) final design of second stage overlay.

i) First stage constructionThe first stage design is based on the remaining life concept14. In this concept, the first stageis designed for a design period less than that which causes fatigue failure. This design periodis considered as 60% of the design period that represents a one stage design. To use thisconcept, the estimated number of ESA, for the first stage design period, ESA1, must beadjusted to provide for the necessary life remaining at the end of the first stage designtraffic,

i.e. the adjusted ESA, (AdjESA1) =100/60xESA1 = 1,67xESA1.



Having determined the traffic for the first stage construction, design the pavement for thistraffic and construct. Say that the thickness of the pavement for the first stage ofconstruction is h1 (consider only the thickness of the asphaltic layers).

ii) Preliminary design of second stage overlayThe thickness of the overlay is also selected on the basis of the remaining life concept. Theidea is to estimate, at the time the original design is prepared, the thickness of the overlaythat will ensure the final pavement structure will last for the whole design period (first andsecond design periods). This is done by first adjusting the estimated second stage designESA2, so as to use the remaining life provided for in the first stage design. In this procedurethe remaining life is 100-60 = 40%.

Thus, the adjusted preliminary second stage traffic (AdjESA2) =100/40xESA2 = 2,5xESA2

With the adjusted preliminary second stage traffic determine the thickness of the pavement,say h2 (consider only the thickness of the asphaltic layers)

The thickness of the second stage overlay (hov) is determined by:

hov = h2 - h1 (9)

The thickness of the overlay must be constructed as planned and before the pavement showsany serious signs of distress. It may also be used for further economic analysis in order toestimate future expenditures. However, because of the random nature of pavementperformance, the pavement may be in a better or worse condition at the end of the firststage time period. For this reason the final design of second stage is considered.

iii) Final design of second stage overlayThe final design of second stage overlay takes care of the possible diversion in the expected(assumed) condition of the pavement at the end of the first stage design period. For thisreason, one year before the end of the first stage design period conduct a condition surveyof the pavement (use any method adopted by EOAE).

If the pavement is showing little distress with few or no visible cracks or distortion and ingeneral there is some remaining life, program another survey in the next year or any otherappropriate time.

When it appears that the pavement is approaching a level of deterioration, but still inreasonable condition, then, either, apply the overlay as determined in the second preliminarystage of design or design a new overlay using any approved overlay design procedure.

All the above are exemplified in the following example.

ExampleApply planned stage construction in a region with 13 oC MAAT and a subgrade CBR of7,2%. The pavement should last for 20 years and the cumulative ESA during the first stageof construction (5 years) was estimated to be 1,1x106 ESA and 4,7x106 ESA during thefollowing 15 years period.Solution



i) First stage designThe adjusted first stage traffic (AdjESA1) is: 1,1x106x1,67 = 1,84x106 ESA.

From Figure A-1, the thickness of the bituminous layers is: 170 mm

Hence, construct a pavement having 170 mm bituminous layers and 400 mm base/sub-base.

ii) Preliminary design of second stage overlayThe adjusted preliminary second stage traffic (AdjESA2) is: 2,5x4,7x106 = 1,18x107 ESA

From the same Figure A-1, the thickness of the bituminous layers is: 250 mm

Hence, for the preliminary second stage the thickness of the overlay (hov) is:

hov = 250 - 170 = 80 mm

iii) Final design of second stage overlayOn the 4th year, after construction, conduct a pavement condition survey and decidewhether the overlay will be placed on the 5th year. If not, arrange another condition surveyin the next year or any other appropriate time. Keep monitoring the condition of thepavement for any signs of unexpected distress, prior to the 2nd survey. Depending on theway the condition of the pavement is determined (visually or by suitable monitoringequipment, i.e. falling weight deflectometer, Benkelman beam), construct the overlay eitherwith a depth of 80 mm or with a depth determined by overlay analytical design.

14. ECONOMIC ANALYSIS

A highway construction must be considered as capital investment. It is therefore necessaryto conduct an economic comparison between alternative pavement designs. A pavementdesign may be affected by the design life considered, i.e. for how many years the pavementis assigned to serve the public, or by the materials and techniques used to build thepavement. The designer should always examine all the design options in order to come upwith the most economic or appropriate solution. The procedures followed are generalaccounting practice usually applied to capital investments.

There are two basic methods for an analysis of project costs. The first involves all costspertaining to the complete construction (highway or pavement alone) and is normally usedto assess the cost of the construction for short term banking. The second method takes intoaccount not only the construction cost but also the maintenance and/or the reconstruction,the user cost resulting from delays during road works, possible accidents’ cost and anyother cost that may arise throughout the analysis period, in which the pavement is going tofunction. Only the latter method is useful and is needed to evaluate and compare alternativepavement designs to determine the most appropriate design for a specific section of ahighway. This type of economic analysis is known as life cycle cost analysis (LCCA).

For a LCCA the basic factors required to be known are:a) the initial cost of the pavement structure (you may or may not include drainage,

shoulders, safety barriers, lighting etc.)



b) the cost of future maintenance, strengthening overlays, reconstruction and other similaractivities

c) the time, in years, from initial construction until each major activity takes placed) the analysis periode) the interest, inflation or discount rate, andf) the salvage value of the last activity or the salvage value of the pavement

The most commonly used method to carry out a LCCA is the Present Worth method.

14.1 Present worth (value) method

In the present worth analysis all expenditures involved are represented in terms of presentday values.

The basic formula for the analysis by present worth method is:

= + EE KA K KX EE YAi tt

t n

t t t i n, ,[ ]=

=∑ + + −0

where = Total present value = Initial construction cost

EEi,t = Discount coefficient for discount rate i in year t, = 1/(1+i)t

KAi = Reconstruction cost in the year t, expressed in present valuest = Maintenance cost in the year t, expressed in present valuest = User cost in the year t during which maintenance or reconstruction

works are carried out, expressed in present values = Salvage value

n = analysis period, in years

Initial costThe initial cost, in this particular case, may be considered only the cost for constructing thepavement. However the designer may include any other initial cost he may wish.

Discount coefficient - Discount rateThe discount coefficient (also known as present worth factor) is to adjust the present daycost of an activity which will be performed at some future date and is related to the discountrate.

The discount rate reflects the real rate of return on invested money and is interrelated toinflation rate and interest rate. However, the discount rate used in the LCCA of a pavementdoes not express the real rate of return of each alternative antiskidding layer because theobjective in a LCCA is to compare the total discounted cost of an alternative againstanother, during the selected analysis period.



User costThe user cost in most cases is very difficult to be determined or even estimated. Because ofthis difficulty quite often it is not included in a LCCA. However, this is wrong, and everyeffort must be made to estimate this cost and include it in the analysis. One design option,although more expensive than others, in terms of present worth, it may be less expensivewhen the user cost is incorporated. For example, if a road work activity in a busymotorway, requires more time to be completed than another, although it may be lessexpensive as construction cost, it may at the end become more expensive, if the user cost isincorporated.

Analysis periodThe analysis period is the length of time (in years) that is selected for consideration of thelife cycle costs. It is not the design life of the pavement and it should not be confused(although some times may be so). The selection of the design period should be such that: a)it is not biased in favor of any particular design or maintenance strategy and b) it should notextend beyond the period of reliable forecasting. Most agencies frequently opt for 20, 30 or40 years in their life cycle costing analysis. It is customary to designate the final year of newconstruction as ‘year 0’ and the following years of operation as ‘year 1’, ‘year 2’, etc.

Salvage valueSalvage value (or also called residual value) is the remaining value of the pavement at theend of the analysis period. At the end of the analysis period some of the materialsincorporated into the pavement still have some value, the sum of which gives the salvagevalue of the pavement. With conservation of materials and energy in mind, the salvage valueof pavement materials for recycling should be taken into account in a LCCA. Theassessment of the salvage value is not an easy task. However, one way of doing so is toconsider the cost of the materials and deduct the cost of reclaiming those materials (bytaking into account the loss or waste), all expressed in present day values.

In conclusion, LCCA is recommended in every pavement design. The analysis period isrecommended to be either 20, 30 or 40 years and the discount rate to vary between 5% to12%, with the 7% being the most realistic figure in a stable economy.



BIBLIOGRAPHY

1. Shell International Petroleum Co. Ltd. BISAR-PC, Stresses and strains calculationsin pavement models, Release R1.0, 1987

2. Shell International Petroleum Co. Ltd., SPDM-PC, Shell pavement design methodfor use on a personal computer, Release 2.0, 1994

3. Asphalt Institute, Thickness design, Asphalt pavements for highways and streets,Manual Series No.1 (MS-1), Lexington, USA, 1981

4. TRRL LR 1132, The structural design of bituminous roads, Transport ResearchLaboratory, Crownthorn, 1984

5. Shell International Petroleum Company, Shell pavement design manual, London,1978 Addendum 1985

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7. The Department of Transport, Design manual for roads and bridges, Vol.7,Pavement design and Maintenance, Section 2, Part 2: HD 25/94, London, 1994

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10. C.P. Valkering and F.D.R. Stapel, The Shell pavement design method on a personalcomputer, 7th International Conference on Asphalt Pavements, Vol. 1, p. 351,Nottingham, 1992

11. Traffic study of Egnatia Road, 1997

12. Nikolaides A. and Mintsis G., Conversion of traffic volume of all Greek roadnetwork into standard axle loads, 2nd National Conference of Asphaltic Mixtures andPavements, Thessaloniki, 1996

13. The Department of Transport, Design manual for roads and bridges, Vol.7,Pavement design and Maintenance, Section 2, Part 1: HD 24/94, London, 1994

14. Aphalt Institute, Research and development of the Aspahlt Institute’s thicknessdesign manual, RR-82-2, 1982

PART II : PAVEMENT DESIGN METHOD

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