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Rigid Airfield PavementsDesign and Evaluation Methods

GUIDANCE NOTES2


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This guidance note is intended to give advice on the selection

and use of Rigid Pavement design methods for airfield

pavements for:

Those interested in specifying or selecting a design method

suitable for their circumstances Sections 1 Introduction and 2

Selecting a design method only.

Those interested in using the principal UK design methods

Section 3 Using a design method.

Those interested in the background to the principal UK design

methods Section 4 Derivation of design methods.

Summary

Property Services Agency (PSA) A Guide to Airfield Pavement Design & Evaluation (1989) BAA plc Pavement Design Guide for Heavy Aircraft Loading (1993)

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PSA1, BAA2, FAA3 or LEDFAA4; four major design methods (two British

and the two major American) for airfield pavements, including rigid

pavements which is correct, which represents best practice, whichshould you use?

In practice there are numerous other design methods (important

methods have been published in French, Japanese and Russian), and

no major published method is wrong. Differences between design

methods are caused by a number of factors, including:

the condition of the pavement at the end of the design life,

construction practice and material specification,

analysis methods.

There are also important differences in the scope of design methods, e.g.:

they may cater for the evaluation of the strength of existingpavements and the design of rehabilitation and strengthening

requirements.

they may be restricted in the range of aircraft or subgrade

strengths covered.

Finally a design method is often linked to a particular material andconstruction specification; for instance the PSA design guide was

linked to the PSA specification.

Informed selection of a design method requires some knowledge of

these factors and an understanding of how they may affect the capital

and whole-life costs of the pavement design.

This guidance note provides advice on:

The selection of a design method for Clients and Project

Managers (Section 2).

The use of UK design methods for Designers (Section 3).

Section 4 gives some general background to the derivation ofdesign methods

1. Introduction

Selection of a design method will usually depend on three factors:

Construction practice and material specifications. For instance it

would not usually be practicable to choose an American design

method, linked to American specifications and standards, in the

UK; when British design methods are closely linked to British

construction practice, specifications and standards.

The pavement condition expected at the end of the design life

(known as the failure criterion), which governs likely maintenance

requirements. In areas where low downtime for maintenance is

vital a design method with a combination of a conservative failure

criterion and a high design reliability (the probability of the

pavement life equalling or exceeding the design life) may be

appropriate. However, the pavement thickness and capital cost will

be higher, and in most circumstances a less onerous design

method will be suitable.

Whether an evaluation of an existing pavement and rehabilitation

/ overlay design is required.

The failure criterion is usually defined by the proportion of the

pavement area that has failed structurally at the end of the design life,

e.g. structural cracking of 30% to 50% of bays in the trafficked areas.

The capabilities and limitations of the four major design systems in use

in either the UK or USA is shown in Table 2.1.

2. Selecting a Design Method

1. 2.

Rigid Airfield PavementsDesign & Evaluation Methods

Guidance Notes


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A Guide to Airfield Pavement

Design and Evaluation(PSA,

1989)

The BAA Design Guide for

Heavy Aircraft Pavements

(BAA, 1993)

Airport Pavement Design and

Evaluation(FAA, 1995)

LEDFAA (1995)

(NB Must be used in

conjunction with the FAA

design guide3.

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Guidance Notes

Covers a wide range of

pavement types including

the preferred new rigid

pavement option (Fig 1 (i)) and

alternatives generally used for

evaluation and rehabilitation /

strengthening design (Fig 1 (ii)

and (iii)). Generally based on

common UK construction

practice, but allows alternative

details, such as doweled joints,

reinforced / continuously

reinforced pavements, cement-

bound soils in lieu of drylean

concrete and unbound bases /

capping layers. Linked to the

PSA specification7, now

partially embodied in Defence

Estates Functional Standards9.

Limited to jointed

unreinforced concrete

pavements on a cement-

bound base (Fig 2).

Linked to American

construction practice, with

little scope for variation of, for

instance, joint details. Covers a

wide range of pavement types.

Linked to the FAA

specification8.

Linked to the FAA

specification8.

Developed for UK military

airports. Intended to give

minimum whole-life cost for

pavements where the cost of

disruption due to

maintenance is low.

Developed for BAA airports.

Highly conservative, intended

to give minimal pavement

downtime for structural

maintenance on very heavily

used pavements. Intended to

give minimum whole-life cost

for pavements where the cost

of disruption due to

maintenance is high.

Developed for American civil




disruption due to

maintenance is low.

Developed for American civil




disruption due to

maintenance is low.

Comprehensive ability to

evaluate existing pavements

and design rigid overlays.

Very limited capability for

evaluating the strength of

existing pavements. No

capability for the design of

rigid overlays to existing

pavements.

Comprehensive ability to

evaluate existing pavements

and design rigid overlays.

Very limited capability for

evaluating the strength of

existing pavements. Has a

capability for the design of

concrete overslabs to existing

concrete pavements.

Covers a limited range of

concrete strengths.

May under-design some joints

on very heavily trafficked

pavements.

Has graphs for limited ranges

of aircraft and subgrade

strengths, and considers only

one concrete strength

(concrete strengths may be

accounted for by the use of a

separate chart providing an

approximate modification to

the basic design, or by the

use of the associated design

spreadsheets - see Section 3.2).

May not design joints

correctly if the pavement

construction falls outside

common ranges.

Requires new graphs for new

aircraft. Has not been

extended to cover aircraft

such as the Boeing 777 and

Airbus A340.

FAA only recommend the use of

LEDFAA where the aircraft traffic

mix includes Boeing 777 aircraft.

Based on a computer

programme covering fixed

aircraft.

May not design joints correctly

if the pavement construction

falls outside common ranges.

Design Method Construction Practice Failure Condition

Evaluation & rehabilitation

/strengthening design Limitations

Table 2.1 Comparison of design methods


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With a cement-bound base to:

to provide a uniform and substantially improved support,

particularly at joints,

to preserve aggregate interlock and therefore load transfer at

joints by reducing deflections,

to protect moisture sensitive subgrades and act as a working

platform during construction,

to prevent mud-pumping

to reduce the rate of deterioration if cracking of the PQC slab occurs,

to reduce the required PQC thickness for heavy loadings.

JointedUnreinforcedPavement QualityConcrete (PQC)

Induced transversejoints withoutmechanical loadtransfer devices

Drylean Concrete



Drylean Concrete


Jointed

Unreinforced

Pavement QualityConcrete (PQC)

(for evaluation, rehabilitation and strengthening design)

Concrete

Pavement Quality Concrete

Concrete


Figure 1: Rigid pavement constructions included in A Guide to Airfield Pavement Design and Evaluation.

Concrete


Drylean Concrete

Concrete


Drylean Concrete

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Guidance Notes

i. Preferred NewRigid Pavement

On the subgrade or an unbound sub-base (for evaluation)

ii. TraditionalRigid Pavement

Figure 1.

iii. Multiple Slab

Rigid Pavements


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This section provides advice for designers on using the two UK design

methods, A Guide to Airfield Pavement Design and Evaluation1 and The

BAA Design Guide for Heavy Aircraft Pavements2, lists the known

limitations of the two methods and describes the dangers of mixing

design methods.

3.1. A Guide to Airfield Pavement Design and Evaluation

The standard design methods of A Guide to Airfield Pavement Design

and Evaluationcover:

Rigid pavements comprising jointed, unreinforced, concrete slabs

on a cement-bound or bituminous-bound base course, without

mechanical load transfer devices at joints.

The design methods can also be used for:

Jointed, reinforced, concrete pavements.

Doweled concrete pavements.

Concrete pavements on an unbound base or directly on the

subgrade.

Evaluation of all of these pavement types is described, together with:

Multiple slab pavements (bonded, partially bonded or unbonded

concrete overlays on concrete).

Composite pavements (bituminous surfacing on concrete).

Problems with the guide include:

Concrete strengths.

Aircraft Classification Numbers.

Design Charts for new or unusual main wheel gears.

The design charts cover a limited range of concrete strengths. The low

end of the range is rarely a problem; but the high end can be,

particularly when evaluating older pavements. Limited extrapolation is

possible, but otherwise higher strengths cannot be accommodated.

The ACNs given in Appendix B are out of date and need to be

supplemented by manufacturers data for recent aircraft.

The Design Charts do not cover modern 6 wheel main wheel gears (e.g.

Boeing 777 and Airbus A380), or unusual main wheel gears such as the

Lockheed C5, Antonov 124 and Antonov 225. Designs can be obtained by

using the Dual-Tandem charts, although it is currently uncertain as to

whether the results are conservative or not. (NB if designs are un-

conservative the result is a reduction in the design reliability).

When using A Guide to Airfield Pavement Design and Evaluation it is

very important to understand the relationship between the design

flexural strength of concrete and the specified strength, and the design

guide should be read in conjunction with Pavement quality concrete for

airfields9.

3.2. The BAA Design Guide for Heavy Aircraft Pavements

The principal problem with The BAA Design Guide for Heavy AircraftPavements is the limited range of aircraft, subgrade strengths and

concrete strengths covered by the design charts. In particular the

aircraft mix considered typical in 1990 is no longer adequate.

The design guide is accompanied by a spreadsheet for rigid pavement

design (Fig 3) which allows the concrete strength to be varied and also

allows mixed aircraft use to be easily dealt with. However, the

spreadsheet holds the standard range of aircraft and requires

modification for an alternative aircraft mix.

For aircraft other than those in the standard mix, it is necessary to

calculate the maximum flexural stress in the concrete slab for the

standard pavements shown in (Fig 2), and the range of concrete slab

thicknesses included in the spreadsheet, and then transfer the stresses

to the spreadsheet. The aircraft data, including the Pass-to-Coverage

Ratio must also be entered in the spreadsheet. The stresses should be

calculated using the multi-layer elastic analysis program JULEA, using

the pavement model shown in (Fig 6).

Subgrade strengths other than the standard values, and between the

minimum and maximum standard values can be dealt with by using

interpolation, giving a reasonably accurate result. For values outside

the standard range, or accurate results, the stresses should be

re-calculated as described above.

The design guide includes a chart that gives an approximate

modification to the design thickness for alternative concrete

strengths. For an accurate design the spreadsheet should be used.


300mm Unbound Sub-Base150mm Drylean Concrete300mm Unbound Sub-Base




150mm Drylean Concrete

Figure 2: BAA Standard Constructive Practice.







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Guidance Notes

3. Using a Design Method

Modulus of Subgrade Reaction (k) 20MN/m2/m Modulus of Subgrade Reaction (k) 40, 80, 150 MN/m2/m


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Figure 3: Output example from BAA design spreadsheet

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Guidance Notes

BAA plc Rigid Pavement Design ChartsPQ Concrete Flexural Strength N/mm2Design Factor (DF) = a+b LOG CDesign Factor (DF) = MR/STRESSDesign Life Years

Annual Growth Rate %

File RIGCDF2.xls (Originator: John Barling) v.2 issued 10/10/97.k80 stresses corrected to agree with

B777-2000 0 3700 0 0 0 0 0 0 0 0

B747-940 B747-400 MD11 Wing B767-300 B757-200 B737-400 BAel 146-300 B747-600X A3XX 20W A3XX24WAN DepartAircraft

Gross WT lb 592000100 100 100 100 100 100 100 100 100 100 100

940000 851000 621000

0.48a= 0.39b=

6

030

405125 256000

PQC mm 440 390 330 295Subgrade Ult. Low Low Medium High

150500 98000 1250000 1700000 1254000% Gross WT

1000

0.1

1

10

100

Slab Thickness (mm)

Cumulative Damage Factor

CDF

250 350

x

x 450 500 550 600300 400

Ultra-Low

Low

Medium

Highx

It is possible to use the BAA design method for pavement constructions

other than those shown in (Fig 2), as long as the following parameters

are not altered (see Section 4.1):

Elastic stiffness and Poissons Ratio of concrete.

Interface Friction.

CBR or k to Elastic Stiffness relationships and Poissons Ratio of the

subgrade.


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Guidance Notes

A Guide to Airfield Pavement Design and Evaluation

The BAA Design Guide

Longitudinal joints in pavements for heavy aircraft,

when subjected to regular trafficking normal to

the joint

Pass-to-Coverage Ratio

Flexural strength of concrete

Standard longitudinal joints are butt joints without

mechanical load transfer devices. In the majority of

cases they perform well. However, there is someevidence that regular trafficking normal to the

joints by heavy aircraft such as the Boeing 747, e.g.

on some aprons, can cause premature failure.

The formula given in Section A of Pavement Design

Charts and Computer Programsis incorrect for the

common case of traffic normally distributed about a

centreline. The correct formula is given in Appendix

E of A Guide to Airfield Pavement Design and

Evaluation1.

The relationships between flexural and compressive

strength given in Equations 10 and 11 in Performance

of Base Slabs (Flexible Overlays on RigidPavements)are very optimistic and likely to over-

predict the actual flexural strength of the concrete.

Alternative methods are given in A Guide to Airfield

Pavement Design and Evaluation1or paragraph 684

of Airport Pavement Design and Evaluation3.

Design Method Limitation Comments

Table 3.1 Limitations of UK design methods

3.3. Known limitations

In addition to any described in Sections 3.1 and 3.2 there are some

other limitations with both the principal UK design methods, describedin Table 3.1.

3.4. Mixing design methods

Design methods should not be mixed! A good design method, such as the

four described in this document, is a coherent system with components

that are logically dependant on each other. Used out of context the

results can be misleading or completely wrong. Typical examples are:

The use of the PSA graph for the effective Modulus of Subgrade

Reaction on an unbound sub-base under a rigid pavement in

conjunction with the BAA design guide. In practice one of the

reasons for the production of the BAA design guide was a concern

that the PSA graph overestimates the effective Modulus of

Subgrade Reaction under heavily loaded, thick concrete slabs, so

using the PSA graph removes one of the drivers for higher

reliability in the BAA design guide. The correct method for dealing

with sub-bases using the BAA design guide is to model them by

multi-layer elastic analysis.

Mixing material equivalency factors. If a design method includes

equivalency factors to convert one material to an equivalent

thickness of another material, they should not be altered.

Using entirely different techniques from other design

methodologies, e.g. using the Method of Equivalent Thicknesses to

combine pavement layers.

The use of information from road design methods. There is rarely

any justification to show that design information suitable for

roads are correct for airfield pavements where the magnitude of

the loading may be an order of magnitude higher, and the

frequency of trafficking several orders of magnitude lower.

There are some parts of design methods that are external to major

assumptions used in the derivation of the method, and they can be

exceptions to the rule. An example is the statistical method used to

calculate the Pass-to-Coverage ratio.


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Guidance Notes

This section describes the background to the derivation of rigid

pavement design methods, and particular details of the two UK design

methods, A Guide to Airfield Pavement Design and Evaluation1and The

BAA Design Guide for Heavy Aircraft Pavements2.

4.1 The challenge

A rigid pavement design method has to determine a pavement thickness

that will provide the required pavement performance, i.e. an amount of

cracking equal to the failure criterion after the design movements by the

aircraft mix. Unfortunately, it is not possible to directly predict performance.

Design methods work by relating pavement behaviour (deflections,

stresses and strains), which can be approximately calculated, toperformance by a black box function (Fig 4). The black box has to be

obtained by relating measured performance from full-scale testing, or long-

term loading, to calculated behaviour for the actual pavements (Fig 5).

The result for a rigid pavement is an approximate relationship between

something that can be calculated (concrete stress) and performance in

terms of allowable load repetitions.

4.2 Empirical and analytical design methods

A differentiation is often made between so-called empirical and

semi-empirical design methods, and analytical or mechanistic

design methods.

Empirical design methods combine the behaviour calculations and the

black-box, into a single empirical (i.e. derived from testing) relationship

between the required pavement thickness and the loading and subgrade

strength. The term semi-empirical is used when the original empirical

relationship has been extended to other situations by the use of theory.

Analytical or mechanistic design methods use a theoretical method to

calculate the pavement behaviour, before applying the black box to

obtain performance. All of the design methods descried in Section 2 are

basically analytical, although both A Guide to Airfield Pavement Design1

and Evaluation and Airport Pavement Design and Evaluation3 use

empirical relationships to deal with multiple slab pavements.

4.3. Analysis methods

Two methods of analysing the behaviour of a rigid pavement are used:

Westergaard solutions for stresses at the centre, edge of corner of

a finite plate on a dense liquid (Winkler) subgrade (A Guide to Airfield

Pavement Design and Evaluation1and Airport Pavement Design and

Evaluation3).

Burmeister solutions for stresses in a multi-layered semi-infinite

pavement on an elastic half-space (The BAA Design Guide for Heavy

Aircraft Pavements2 and LEDFAA4), often known as Multi-Layer

Elastic Analysis.

Each method has advantages and disadvantages. Multi-Layer Elastic

Analysis allows better account to be taken of bases and sub-bases, and

may have a more accurate subgrade model, but cannot deal with edge orcorner loading. The adequacy of the slab edges and corners has to be

dealt with by theoretical relationships to the stress at the slab centre,

which may not be accurate outside a relatively limited range.

Westergaard solutions can deal with edge and corner loading, but cannot

take accurate account of the effect of bases and sub-bases.

More accurate analysis methods using 2D plate theory or 3D Finite-

Element Analysis are in use in research and development but have not

yet been incorporated into routine design methods for various reasons,

including the requirement for re-calibration against performance data.

4.4. A Guide to Airfield Pavement Design and Evaluation

Rigid pavement design in A Guide to Airfield Pavement Design and

Evaluation is based on an analysis of stresses in the concrete using

solutions of the Westergaard equations, together with an Allowable Live

Load Stress criterion derived from measured long-term performance of

a set of pavements.

4.5. The BAA Design Guide for Heavy Aircraft Pavements

Rigid pavement design in The BAA Design Guide for Heavy Aircraft

Pavements is based on an analysis of stresses in the concrete using

Multi-Layer Elastic Analysis, together with an Allowable Live Load Stress

criterion derived from US Army Corp of Engineers full-scale testing.

Analysis is based on the pavement model shown in (Fig 6), using theprogram JULEA.

4. Derivation of Design Methods

Drylean ConcreteE = 5171 N/mm2, = 0.2

h = 150 mm

Concrete SlabE = 27586 N/mm2, = 0.15

h = variable

SubgradeE = 4.970 x k0.7741, = 0.4

h =

Rigid Pavement

0% Horizontal Shear Transfer

100% Horizontal Shear Transfer

Figure 6: BAA Pavement Model


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Guidance Notes

Pavement Quality Concrete known thickness

Calculated stress

Test Aircraft

Subgradeknown strength

Coverages

Stress

Pavement Quality Concrete known thickness

Test Aircraft

Calculated BehaviourTest Pavements with

measured performanceRelate calculated behaviourto measured performance

(measured Coverages to Failure)



Test Aircraft(measured Coverages to Failure)



known thickness

Calculated stress


known thickness

Figure 5. Derivation of Failure Criteria


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Guidance Notes

1. PSA. A Guide to Airfield Pavement Design and Evaluation. HMSO.

1989.

2. BAA. Pavement Design Guide for Heavy Aircraft Loadings. BAA.

1993.

3. FAA. Airport Pavement Design and Evaluation. Advisory Circular

150/5370-6DD, Federal Aviation Administration. Washington DC. 7

July 1995.

4. FAA.Airport Pavement Design for the Boeing 777 Airplane.

Advisory Circular 150/5320-16. Federal Aviation Administration.

Washington DC. 22 October 1995.

5. WOODMAN G.R. A commentary on A Guide to Airfield Pavement

Design and Evaluation. Proc. Instn. Civ. Engrs, Transp. 1992. 95.

Aug. 167-172.

6. LANE R. WOODMAN G.R. and BARENBERG E.J. Pavement Design

Considerations for Heavy Aircraft Loading at BAA Airports. Proc.

ASCE Speciality Conf. Airport Pavement Innovations Theory to

Practice. Vicksburg, MS. September 1993

7. PSA. Standard Specification Clauses for Airfield Pavement Works.

PSA Airfields Branch, 1989.

8. FAA. Standards for Specifying Construction of Airports. Advisory

Circular 150/537-10A. Federal Aviation Administration. Washington

DC. 1 September 1991.

9. MINISTRY OF DEFENCE. Pavement Quality Concrete for Airfields.

Specification 33. Ministry of Defence. 1996.

5. References

The Britpave Technical Committee would like to thank Graham

Woodman (WSP Group) for assistance in the preparation of this

Guidance Note, and John Cairns, Richard Moore and Glyn Davies of TPS

Consult, Bob Lane of BAA and John Cook of Defence Estates for their

contribution to its preparation.

Further details on Britpave are available at

www.britpave.org.uk

All advice or information from Britpave is intended for those who will

evaluate the significance and limitations of its contents and take

responsibility for its use and application. No liability (including that

for negligence) for any loss resulting from such advice or information

is accepted.

6. Acknowledgements

1. 2.


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Century House, Telford Avenue, Crowthorne, Berkshire RG45 6YS

Tel. 01344 725731 Fax. 01344 761214

www.britpave.org.uk

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