pavement design report stabilised

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REPUBLIC OF ARMENIA

MINISTRY OF TRANSPORT AND COMMUNICATION

PAVEMENT SPECIALIST FOR DETAILED DESIGN OF TRANCHE 1

PROJECT UNDER THE NORTH-SOUTH ROAD CORRIDOR INVESTMENT

PROGRAM

PROJECT NO. 42145

PAVEMENT DESIGN REPORT

Bent Kjeldgaard Larsen 30-10-2013


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Introduction

The term Pavement Design or in English terminology Structural Design are used in the meaning of

determining the required thicknesses of the various layers in the pavement. It does not include the

asphalt mix design or the concrete mix design, although these issues naturally have an indirectinfluence on the thickness design. Other conditions, e.g. frost sensitive materials, are also not covered

by pavement design, but are dealt with in parallel to the pavement design.

The methods that as of today are used to determine the required layer thicknesses can be grouped in the

following categories.

Sound Engineering Judgment

Standard (Catalogue) Pavements

Empirical MethodsAnalytical-Empirical Methods

Theoretical Methods

Sound engineering judgment is naturally always good to have, even if you are using some of the othercategories of pavement design. In reality it is indispensable with the state of technological level

pavement thickness design have reached today.

When a pavement designer has performed his thickness design, he has to analyze the results based onhis experience, to see if the results coming out of his design model are realistic. The reason for this is

that, in even the most sophisticated theoretically based methods existing today, the conditions of which

the models are based on, are very different from conditions actually existing in real pavements.

In many countries (e.g. Germany, France, in Denmark for minor roads) the thickness design of a

pavement is chosen from a catalogue of standard constructions.

Catalogues are naturally developed based on theoretical considerations and practical experience, but nomatter on what backgrounds a catalog have been based on, the use of a catalog will have a strong

conservative effect on the choose of thickness design. It is not possible to introduce new materials and

it is not possible to change specifications for existing materials, as the consequences for such changescannot be predicted. The thickness design will most often be uneconomically as the catalogue has to

cover a wide range of subgrade and traffic conditions, and therefore are design for worst case of

those conditions.

For, to a higher degree to take into consideration differences in the condition of the subgrade and other

conditions of the materials in the pavement, a long range of empirical methods have been developed.By investigating a large number of roads that have failed, and compare this to what parameters there

existed of the subgrade and traffic at the same time, dimensioning diagrams have been developed to

determining the required thicknesses of the pavement layers. The drawbacks with using such methods

is that they can go terrible wrong when used under conditions they have not been developed under(especially different climatic conditions, and if the actual design traffic exceeds the traffic from where

the empirical model was developed under).



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Intermediate between empirical models and analytical-empirical models is the AASHTO designmethod. It is actually an empirical model, but because it has been developed studying a vast number of

test pavement, there were tightly controlled with respect to knowledge of pavement thicknesses, quality

of the different materials in the pavement and exact knowledge of the axle loading conditions and

repetitions of loads, the method require special attention.The basis of the method is results of the AASHO Test Road experiment (Ref. 1), set up in Illinois, USA

in the late fifties. Hundreds of test pavements, asphalt and concrete, were build, and during two years

of service these test pavements were monitored closely under tightly controlled traffic conditions.The cost of setting up such an experiment today would be prohibitive, and it will never be repeated.

The AASHTO method, for flexible pavements and for rigid pavements, are still widely used in USA

today and also in many other countries.

By dimensioning after theoretical methods, you are trying to attack the problems in the same way as is

used in other fields in engineering science. You try to calculate the stresses and strains that are exertedon the pavement layers, and from the knowledge of the strength and deformation properties the varying

pavement materials possess, you calculate the necessary layer thicknesses of the pavement.Such type of pavement design is called Analytical-Empirical design methodology and is known in the

west, e.g. The Shell Method (Ref 2) and many others, and in the Former Soviet Union, the VSN-46design method.

In principle logic and simple, but in the real world it has some complexity.

A couple of examples: The allowable horizontal tensile strain, for a given number of load repetitions,that will cause the asphalt to crack, is one number in the laboratory (where the beam is tested in free

air) and another number in real pavements (where the asphalt is supported by a granular layer). That is

why we call such methods theoretical-empirical models, because to predict when the asphalt will crackin a real pavement, we have to apply shift factors to get the laboratory results to match up with real

asphalt behavior in the field.

Another example: The stress or strain level that under a given number of load repetitions will cause a

subgrade material to develop a permanent deformation of say 2 cm, might say us something about howthe rutting of the pavement will progress, but doesnt even relate to why and how a road pavement gets

uneven. We have to assume that what our theoretical models can predict of permanent deformations is

somehow related to what is observed in the field with regard to development of unevenness.To make the whole thing ten times more complicated than already described, most of the pavement

materials available (asphalt, granular materials, subgrade) have deformation and strength parameters

that change over the season influenced by stress conditions, temperature, precipitation and frost, just tomention a few causes, that a designer do not have any reasonable control over.

If we want go one step further up, to purely theoretical models that actually can calculate and predictwhat happens in a road pavement, such models does not exist for flexible pavements.

What come closest is mathematical modeling of asphalt pavement behavior, where the variation of

pavement layer thicknesses, material properties, climatic changes over the season etc. are taken into

account. Such models require vast amount of input parameters and are for research only.When it comes to concrete pavements, analytical models can to a certain degree of confidence predict

cracking of concrete slabs. There exists however no models that in any convincing way can explain or

predict how and why a concrete pavement gets uneven.



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How does theoretical pavement design models predict deterioration of pavements and how does it

conform to actual pavement deterioration?

Asphalt pavements:

In all analytical-empirical models, even the most advanced models existing today, there are the

following general assumption: All layers do not vary in thickness. All layers are homogeneous,

isotropic and are linear elastic.It is in the model anticipated that when the allowable horizontal strain in the bottom of the asphalt is

exceeded, the asphalt will start to crack and propagate up through the asphalt layer. That happens

sometimes and will result in longitudinal cracking eventually developing into crocodile cracking.But as all layers in the pavement in the models are assumed homogeneous, isotropic and linear elastic,

the first day the cracking would be visible on the surface would be on the last day of the design period

and it would happen everywhere along the length of the road. The road would collapse, just like abridge collapses. This never happens in real life.

Another problem: The majority of the cracking of asphalt you will observe in cold climates is caused

by temperature contraction of the asphalt in the coldest periods and is transverse low-temperaturecracking. A phenomenon that is unrelated to the traffic and structural design, and is not calculated in

any structural design model as it is a mix design problem.

Thirdly: Pavement specialists often drill samples in cracks in asphalt pavements to examine the cause

of the cracking. According to the theory used in their models, cracks should always initiate in the

bottom of the asphalt and propagate upwards, as described above. However, quite often they find thatthe cracking is initiating at the top of the asphalt and then propagates downwards.

The reason for this is again to be found in the limitation in their models. It is assumed, in all models,

that the design tire load is round or elliptical in shape and that the forcing stresses are uniformly

distributed over the load. In reality, the stresses at the rim of the tire are many times larger that directlyunder the load. Finite-element calculations have proved that this easily can cause asphalt to crack from

the top of the asphalt and then propagate downwards.

More: In analytical design methods it is assumed that: All layers do not vary in thickness. All layers are

homogeneous, isotropic and are linear elastic.

If that really was the case, an asphalt pavement would never be uneven. It would slowly develop ruttingin the wheel path due to permanent deformations in the layers, but at the end of the design period, the

rutting would be 20 mm, but the road, in the wheel path, would be perfectly smooth.

Concrete Pavements:

The main reason to why concrete pavements gets uneven by time is significantly different from why

asphalt pavements get uneven and are much more easy to explain, but even more difficult to calculate.

When a wheel load moves over a concrete plate (the departing plate) and leaves the plate to go on the

approaching plate, the load is exerted momentarily at the joint of the approaching plate. An the joint ofthe departing plate, the stress from the wheel load is building up steadily during the wheel loads travel



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over the plate and reach its maximum value just before the wheel load is leaving the joint to enter theapproaching plate.

The cause of unevenness on concrete roads are caused by what is called faulting (the approaching plate

is always lower than the departing joint. This builds up during the design period for 0 mm in thebeginning to 10 mm faulting at each joint at the end. When the faulting is 10 mm, the road is so

uncomfortable to drive on that its design life is considered expired. As layers are not homogeneous,

isotropic and linear elastic, some places the faulting will be 5 mm, other places it will be 15 mm.No convincing model has however so far been presented of how to calculate faulting.

When it comes to the prediction of either interior, edge or corner cracking in concrete pavements, thereexist mathematical model that can predict such occurrence relatively precisely. The analytical

calculation of edge interior, edge, corner and warping stresses were published in the twenties by H. M.

Westergaard (he was a Dane although it is common belief that he was American). And these equationshave been verified with actual concrete behavior for more than 80 years, and have practically not

changed.However, as most modern concrete pavements are connected from plate to plate with dowels, precise

determination of the occurrence of cracking cannot be calculated or predicted as the load transfercoefficient is not known. As it is with flexible pavement, shift factors, or safety factors have to be

introduced to get theoretical considerations to match up with practical experience and to ensure that the

concrete do not crack unexpectedly or premature.

Pavement Design Models used in this study.

As explained in the previous section, Pavement Design models is not nearly as mathematical as

existing in other parts of engineering science, as for example structural design of steel bridges.

Structural Pavement Design is to a high degree based on observations in the field combined with

analytical calculations. As one model in certain cases thus can produce misleading results, severalmodels for both asphalt and concrete design have been used.

Asphalt Pavement structural design:

The following models have been used:

1) The German RSTO design method. This is a Catalogue method.2) The English Road Note 29 method. This is a Catalogue method.

3) The AASHTO method. This is an Empirical method.

4) The VSN-46 method. This is an Analytical-Empirical method.

Concrete Pavement structural design:

The following models have been used:5) The German RSTO design method. This is a Catalogue method.

6) The English Road Note 29 method. This is a Catalogue method.

7) The AASHTO method. This is an Empirical method.8) The Portland Cement Association method. This is an Analytical-Empirical method.



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Equivalent number of standard axles (ESAL)

To be able to carry out pavement design with any of the methods described in the previous section, you

need to know the traffic in term of heavy axles, which the road has to carry in the dimensioning period.For flexible roads the dimensioning period is normally 10 years, while it for rigid pavements normally

is 20 years or longer.

As a part of this assignment is to compare an asphalt pavement solution with a concrete pavement

solution, the dimensioning period has to be the same, and has been chosen to be 20 years.

The basis for determining the design traffic in the dimensioning period has been provided in the Padeco

report.

There you can find the anticipated projection of traffic during the next 20 years, the existing percentage

of the traffic that is heavy trucks and the present percentage of heavy trucks that are overloaded and thepresent maximum overloading of trucks.

This information is far from adequate to be able to calculate and predict the number of standard axles

the road is likely to carry in the design period with any precision, and other sources of information has

therefore been brought in to be able to make a reasonable estimate of the equivalent standard axles theroad has to carry in the dimensioning period.

To calculate the traffic in terms of equivalent standard axle loads it is needed to know the spectra ofexisting axle load distribution. No accurate figures exist from Armenia, but axle load spectras from

other countries can be found in highway design books.

The standard axles can be transformed into equivalent standard axles using the fourth power law.This means e.g. that a 20 tons axle is damaging the road 16 times more than a 10 tons load. (20/10)4 =

16. The fourth power law was found from studying the performance from the AASHO Road Test and

is valid for both flexible and rigid pavements. The fourth power law is also used in the Russian VSN-46 design method.

Recording by Armenian Road department shows overloading on interstate roads of 20 30%. If theoverloading is not controlled the pavement structure will deteriorate rapidly. The Armenian Road

Department proposes that legislation be strengthened and a number of weighing stations to be set up. In

the design study it is anticipated that this will happen and overloading will be brought to the same levelas in Western countries. In this study there is assumed 13 % of overloaded trucks.

The anticipated traffic forecast is given in the Padeco Report Table 3-3. In this forecast a traffic growth

of approximately 5 % is expected.

The axle load spectrum is based on table 15 in appendix 3 in the PCA Manual for axle load category 4.



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Figures are for American conditions where the legal axle load is 8 tons. For use in this study the axleloads have been transformed to 10 tons by multiplying the loads by: (10/8) = 1.25.

A B C D E


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In terms of Equivalent 8 tons Axle Loads this becomes = 4.3 million Standard Axles.

The PCA method requires the average number of AADT of trucks (in terms of 8 tons axles). This

becomes = 4454.

The VSN-46 method requires the AADT of 10 tons axles at the end of the design period. This becomes

= 927.

Determination of pavement design parameters.

The consultants of the Padeco report made some geotechnical observations in investigating the existingcondition of the pavement and road base. A total of 20 samples were taken from the existing asphalt

pavement and road base, including sub-base, in order to investigate the thickness of asphalt concrete,

base course, and sub-base. Four samples were taken from designated locations. The results show thatexisting asphalt concrete thickness varies between 10cm and 18cm, and thickness of the existing base

course and sub base is 1218cm in all. Also, the sub-grade materials at the embankment point areconsidered as unsuitable soil, including boulders. Padeco conclusions: Therefore, most of theexisting

asphalt pavement and road base needs to be replaced with properly engineered materials.

In this study, Dorproject has made a number of detailed geotechnical and laboratory investigations, and

these investigations do not support the Padeco conclusions.

Representative sampling and testing of existing asphalt concrete pavement have been carried out and

confirm the Padeco findings. The asphalt layers vary in thickness from 10 20 cm.Supplementary laboratory investigations have been carried out of the grading curve, bitumen content

and air void content of the asphalt and other measurements.

The investigations show that the grading curve is very varying, sometimes insides the Gost

specifications, sometimes not. They also show that the bitumen content is generally in the upper end ofthe Gost specifications or above. This indicates that the plant(s) that have been producing those mixes

either have not been properly set-up or the have not been a sufficient control of the quality of the

materials going into the mix.Analysis of the air void content of the mixes show that it is very variable, mostly outsides of the Gost

specifications. In some places it is very low, indicating post compaction of the asphalt due to too high

asphalt content. In other places the air voids are very high, indicating bad compaction at the time ofconstruction.

In-situ density test within the layers underlying the existing asphalt and 10cm below the level of theestablished existing sub-grade has been carried out. The results show that the layers beneath the asphalt

are compacted to more than 100 %. It is likely that the layers during construction 30 years ago have

been compacted badly, in too thick layers etc., but the own-weight of the overlying pavement layers

and the effect from 30 years of traffic have consolidated and post-compacted the pavement tomaximum compaction. The longitudinal pavement profile have most likely been satisfactory at the time

of construction, but is today, due to consolidation and post-compaction, highly unsatisfactory. The

elastic modulus of the subgrade material is more than 400 MPa, which is higher than expected for acrushed stone base.



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DCP-tests to a nominal depth of 1.5m or more, at an interval of 200m, have been carried out. About the

lowest CBR found, and that was a shallow layer in a depth of more than one meter, was 15. At the end

of the project road there have been found CBR 5, but that is in a depth of 1.6 meter and of no

importance for the structural integrity of the pavement. Typical values of CBR, from the top of thesubgrade right down to the rock bottom, is 30 to 60, and in many cases up to 90. The existing subgrade,

which actually is rock fill with addition of 25 30 % loam, acts like at least a high quality subbase.

CBR test on subgrade material has also been carried out. The CBR equipment is new for Dorproject

and there have been some difficulties in the beginning of the use of this equipment. The subgrade

consist of up to 50% material of stones up to a size of 200 mm, as observed by Padeco. To makerepresentative CBR test of such a material is therefore difficult as only stones less than 19 mm in size

are used for CBR testing. Another factor that has caused trouble is that the optimum water content for

optimum compaction has been determined using Gost methods, where only stones less than 10 mm isused for the test. The first tests therefore failed as the water content the specimens were prepared to

was obviously too high. This caused the samples to be very soft in the top of the moulds, while it wasfound (when the mould were emptied) that the rest of the sample in the mould was very hard. A new

set of tests was then carried out with lowering the water content. This set of tests showed the subgradeto have a CBR of around 60, which conform to the other findings by DCP testing and measurements of

the elastic modulus.

Testing procedures established in Armenia for determining the bearing capacity of the subgrade has not

been able to carry out as no such equipment exist in Armenia

A visual inspection of the entire project road, in both directions, was carried out by the pavement

specialist 27/5-2010. This was done to investigate if the defects found on the road could be supported

by the field investigations and the laboratory measurements.

There were found no structural defects on the length of the road. There are virtually no crocodilecracking (except at potholes), there are no rutting, neither shallow nor wide that would indicate poor

bearing capacity and poor quality of base, subbase or subgrade layers. The poor compaction there most

likely have been at the time of construction have been consolidated during 30 years of traffic, and havecaused the now poor longitudinal profile, but have not caused rutting in the wheel paths. There are

isolated spots with potholes that are a product of nests of poor quality of mix design and poor

compaction of the asphalt mix. The are found large areas of very open structured surface of the asphalt,that is caused by poor compaction, and there are found areas with bleeding that is cased by fat mixes

with too much bitumen content. The unevenness existing on the road is a product of poor workmanship

during construction and rehabilitation and overlays. The cracking is basically transverse cracking and istherefore not associated with structural cracking. Transverse cracking you do find with stabilized bases,

but no stabilized base are to be found here. Transverse cracking here is a result of too stiff bitumen

used in the mix, that causes low temperature cracking, and this phenomena is not influenced by traffic

at all. According to the VSN-46 structural design method, a bitumen pen 60-90 shall be used forclimatic zone IV, where Armenia is situated in according to this method. As the mean lowest

temperature in Yerevan in January is 9 C, and can fall to 20 C, such a hard bitumen will cause

low-temperature cracking to occur.



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The conclusions of the investigations is that the existing subgrade has a quality of at least a goodquality subbase and that the existing asphalt is of poor quality. The existing subgrade can act as

subbase in the new pavement construction. The existing asphalt is too variable and contain too much

bitumen of a too hard grade to be reused as asphalt. However, blended with 50% fresh crushed stone

base material, it could be reused to produce an excellent base material for the new pavementconstruction.

Pavement Design, Asphalt

Method 1: The German RSTO design method (Ref 3). A semi-empirical approach have been used to

derive a series of standardized constructions. The quality of the subgrade does not enter directly intothe system. Guidance is given about the quality of granular materials and considerable attention is paid

to providing an adequate depth of frost-resistant granular materials.

Entering the Design Traffic in terms of 10 tons standard axles, the necessary asphalt thickness is found

from Table 1. 40 mm of asphalt wearing course, 40 mm of asphalt base course and 100 mm of asphaltbound material. Totally 180 mm of asphalt material. As the existing subbase in the Trance 1 project is

frost-resistant, the necessary thickness of crushed stone base is 150 mm. The same which is given bypractical construction considerations.

Method 2: The English Road Note 29 method (Ref. 4). The thickness design are made from designcharts developed from studying pavement performance of a large number of experimental sections in

the UK and from the AASHO Road Test.

The thickness of subbase is determined from the CBR value of the subgrade. If the CBR value is higher

than 30, as for the Trance 1 project, there is no need for any subbase layer.

The required thickness of base and asphalt layers are determined from the below chart, based on thedesign traffic.

The required thickness of asphalt is found to be 100 mm, and the required thickness of base to be 300mm. However, as the existing subgrade is very strong and the existing layers of subgrade and base has



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a thickness of approximately 150 mm, the required thickness of crushed stone base can in this case bereduced to the minimum practical laying thickness 150 mm.

Method 3: The AASHTO method (Ref 5). The design method is based on analysis of pavement

performance from the AASHO Road Test and the original pavement performance equations have beenslightly modified to include design factors not included in the original equations.

Determination of the design structural number (SN) required for specific conditions include: the designtraffic, the reliability (R ), the overall standard variation (S), the effective elastic modulus of the

subgrade (Es) and the design serviceability loss in terms of Present Serviceability Index (PSI).

The reliability level depend of the functional classification of the road. For Interstate highways a

reliability level of 95% is recommended.

A standard deviation should be selected that is representative for local conditions. Performance

predicted errors developed at the AASHO Road Test was 0.35 for flexible pavements, but did notinclude traffic error. A standard deviation of 0.45 is recommended.

The effective elastic modulus of the subgrade have in this study been found to be 400 MPa. To be

conservative a value of 300 MPa is used in the calculations, as this is the maximum value found in the

nomograph in the design manual.

The higher the value of PSI, the smoother the pavement. For new asphalt constructions at the AASHO

Road Test, a mean value of the initial PSI of 4.2 was found (max. is 5.0). The terminal PSI depends onthe functional classification of the road. For Interstate highways a terminal PSI of 2.0 is normally

accepted. The serviceability loss in terms of PSI is therefore 2.2.

From nomograph or equation shown in figure 3.1 in the design manual the required Structural Number

is found to be 3.3.

A Layer Coefficients for asphalt is normally considered as .42 for each inch (25.4 mm) of asphalt. ALayer Coefficient for crushed stone base is normally considered as .14 for each inch.

Constructing the crushed stone base at the minimum practical layer thickness, 150 mm, gives thefollowing requirement for asphalt layer thickness: (3.3-150/25.4*.14)/.42*25.4 = 150 mm.



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Method 4: The VSN-46 method. The design method was developed in the beginning of the sixties and

was first presented in an international forum at the Third International Conference on the Structural

Design of Asphalt Pavements (Ref. 6). The basis is comprehensive theoretical and experimental

research carried out in the Soviet Union. In principle it works like the Shell Design Method and othermodern western design methods. Four examples of pavement designs in the paper show a good

correlation by the method and results from the AASHO Road Test.

The pavement specialist has programmed his own version of the method, which in contrary to the

original VSN-46 system, allows for introduction of techniques and materials not known or in use in the

early eighties (last version was VSN-46-83).

Changes made by the specialist: To allow for the known heavy overloading and the characteristics of

trucks developed after the eighties, a standard axle load of 11 tons, with a tire pressure of 0.7 MPa hasbeen used. The second layer of asphalt have been changed from a porous type to a dense type, as this is

the most common construction type today. However, as comparison, calculations for a porous asphalthas also been carried out. The existing granular layers are very thick, typically more than 1 meter, in

the calculations a thickness of 300 mm has been used. Conservative elastic modulus values has beenused for the granular layers.

The design procedure and results are presented in the following figures.



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The required thickness of asphalt is 90 mm if of dense type, or 120 mm if lower layer is porous.

As the lower dense asphalt layer in practice has characteristics that is in-between a dense asphalt

wearing coarse and a porous asphalt base course, the required asphalt thickness is 110 mm.

Discussion, asphalt pavement thickness recommended.

For all design methods were have the same required thickness of crushed stone base, equal to 150 mm.

The required thickness of asphalt vary from model to model:

Method RSTO RN 29 AASHTO VSN-46

AC thickness mm 180 100 150 110



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The German method quite clearly demonstrate the conservatism of catalogue methods. It is based onpractices, materials and environmental and economic conditions prevailing in Germany and cannot be

recommended to be used in Armenia.

The climatic conditions in the UK, milder winters, cooler summers, might suggest that itunderestimates the required thickness of asphalt as with respect to Armenian conditions.

The results from the AASHTO method shows what can happen if the actual conditions of the pavementto be designed differ vastly from what the AASHTO model is based on. The climate at the AASHO

Test Road site is not very different from climatic conditions prevailing in Armenia. Cold winters and

hot summers, but the AASHTO model is based on data of CBR values of the subgrade in the range 2-15, while the Project road has a CBR value of the subgrade of approximately 60. As flexible pavement

design is very influenced by subgrade conditions it seem obvious that the model overshoots the

estimation of the required thickness of asphalt. The model is simply not build to handle such extremesituations.

The VSN-46 method are designed for climatic conditions in Armenia, and as it is based on analytical

calculations seems as the most trustworthy of the four models.

Conclusion: The required asphalt thickness for the Trance 1 Project is 110 mm

Pavement Design, Concrete:

Method 5: The German RSTO design method. A semi-empirical approach to concrete pavementdesign as described under Method 1 for asphalt pavement design. Entering the Design Traffic in terms

of 10 tons standard axles, the necessary asphalt thickness is found from Table 2. The required slab

thickness is 260 mm. A minimum practical thickness of 150 mm shall be constructed for crushed stone

base.

Method 6: The English Road Note 29 method. As described under method 2 based on results from a

large number of full-scale tests. Evidence from newer experimental sections in the UK has shown thatthe Road Note recommendations are safe except for very heavy loading exceeding 50 million standard

axles.



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The required thickness of concrete is found to be 200 mm.

The requirement for concrete pavements is that the subbase thickness shall be minimum 130 mm for allsubgrades and traffic levels. A minimum practical thickness of 150 mm shall be constructed forcrushed stone base.

Method 7: The AASHTO method. The design method is based on the same foundation as describedunder method 3.

Determination of the design structural number (SN) required for specific conditions include: the design

traffic, the reliability (R ), the overall standard variation (S), the modulus of subgrade reaction (k), thedesign serviceability loss in terms of Present Serviceability Index (PSI), the flexural strength of the

concrete, the elastic modulus of the concrete, the load transfer coefficient and the drainage coefficient.

The reliability level depend of the functional classification of the road. For Interstate highways a

reliability level of 95% is recommended.

A standard deviation should be selected that is representative for local conditions. Performance

predicted errors developed at the AASHO Road Test was 0.25 for concrete pavements, but did not

include traffic error. A standard deviation of 0.35 is recommended.

The existing subgrade can in the AASHTO classification system be classified as A-1-a corresponding

to coarse-grained gravel soils having a k-value of 140 Mpa/m (350 psi/in). The elastic modulus of the

subbase is conservatively set to 300 MPa, and the elastic modulus of the crushed stone base is set to400 MPa. Using figure 3.3 in the design manual with a crushed stone base thickness of 150 mm, the

combined k-value is found to be more than 1500 psi/in. To be conservative is set to 800 psi/in, which is

the maximum value in the nomograph figure 3.7 in the design manual.

For new concrete constructions at the AASHO Road Test, a mean value of the initial PSI of 4.5 was

found (max. is 5.0). The terminal PSI depends on the functional classification of the road. For Interstate



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highways a terminal PSI of 2.0 is normally accepted. The serviceability loss in terms of PSI is therefore2.5.

The flexural strength of the concrete has been set to 4.0 MPa and the elastic modulus of the concrete

has been set to 35000 MPa.

The recommended Load Transfer Coefficient is found in Table 2.6 in the design manual and is 2.9.

The recommended Drainage Coefficient is found in Table 2.5 in the design manual and is 0.85.

From figure 3.7 the required thickness of concrete is found to be 8.7 inches, corresponding to 220 mm.The crushed stone base thickness is as earlier determined 150 mm.

Method 8: The Portland Cement Association method (Ref. 7). The design method is based onknowledge of pavement theory, performance and research experience from theoretical studies of slab

behavior, models and full-scale tests conducted by PCA and other agencies, experimental pavements

subjected to controlled test traffic, mainly The AASHO Road Test, and performance of normally

constructed pavements subjected to normal mixed traffic.



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As accurate axle load data is not available, a simplified procedure described in the design manual

Chapter 3 is used. Entering the deign traffic, and assuming dowelled joints and concrete shoulders, thenecessary concrete slab thickness is found from Table 14a. Assuming a flexural strength of 4.0 MPa of

the concrete we with interpolation find the required slab thickness to be 220 mm. The thickness of the

crushed stone base is found from practical considerations and is a thickness of 150 mm.

Discussion, concrete pavement thickness recommended.

For all design methods were have the same required thickness of crushed stone base, equal to 150 mm.

The required thickness of concrete vary from model to model:

Method RSTO RN 29 AASHTO PCA

PCC thickness mm 260 200 220 220



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The German method again quite clearly demonstrate the conservatism of catalogue methods. It is basedon practices, materials and environmental and economic conditions prevailing in Germany and cannot

be recommended to be used in Armenia.

The climatic conditions in the UK, milder winters, cooler summers, do not have the same affect on thedesign of concrete pavements as it has on flexible pavements. Also subgrade conditions has not nearly

the same influence for concrete pavements as it have for design of flexible pavements. However, in the

UK the daily temperature difference in a slab is 7-8 C, while it in Yerevan is higher, maybe 15 20C. In higher mountains in Armenia it might be 30 40 C. Daily temperature variations causes

warping stress in concrete pavements. Concrete pavement build in Armenia should therefore have a

slightly thicker concrete layer than suggested by using RN 29.

The results from the AASHTO method gives plausible results, especially taken into consideration that

subgrade conditions only has a minor influence on concrete pavement performance.

The PCA method are based on fundamental theoretical principles and extensive studying of actualconcrete pavement behavior. Although not designed for climatic conditions in Armenia it is based on

analytical calculations and seems as the most trustworthy of the four models.

Conclusion: The required concrete thickness for the Trance 1 Project is 220 mm

References:

1) The AASHO Road Test, Report 2 & 5, Special Reports 61B & 61 E, Highway Research Board,Washington D.C., USA, 1962.

2) The Shell Method, Fourth International Conference on the Structural Design of Asphalt

Pavements, University of Michigan, Ann Arbor, USA, 1977.

3) RSTO 01, Recommendations for standardizing the construction of road pavements (InGerman), FGSV Verlag, Kln, Germany, 2001.

4) The Design and Performance of Road Pavements, Second Edition, Chapter 18 & 19, Croney &

Croney, McGraw-hill Book Company, London, U.K.5) AASHTO Guide for Design of Pavement Structures 1993, AASHTO, Washington D.C., USA,

1993.

6) Design of Flexible Pavements for Major Highways, Krivissky, Third International Conferenceon the Structural Design of Asphalt Pavements, University of Michigan, Ann Arbor, USA,

1972.

7) Thickness Design for Concrete Highway and Street Pavements, Canadian Edition, CanadianPortland Cement Association, Ottawa, Ontario, Canada, 1990

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pavement design report stabilised

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