STRUCTURAL DESIGN OF PAVEMENT THICKNESS

Introduction - One of the basic requirements for a pavement is that it should be of sufficient thickness to spread

the surface loading to a pressure intensity that the underlying sub-grade is able to withstand, with the pavement

itself sufficiently strong to deal with the stresses incident on it. Thickness is thus a central factor in the pavement

design process. (In order to reflect European harmonization, the names of the various pavement layers have

been altered within the context of thickness design, as seen in the figure).

Flexible pavements

The pavement should be neither too thick nor too thin. If it is too thick, the cost will become excessive. If it is too

thin, it will fail to protect the underlying unbound layers, causing rutting at formation level. A flexible pavement is

defined as one where the surface course, binder course and base materials are bitumen bound. Permitted

materials include hot rolled asphalt (HRA), high density macadam (HDM), dense bitumen macadam (DBM) and

dense bitumen macadam with 50-penetration bitumen (DBM50).

Flexible composite pavements involve surface course and upper base materials bound with macadam built on a

lower base of cement bound material (CBM). Wearing courses are either 45mm or 50mm of hot rolled asphalt or

50mm of porous asphalt (PA). (If PA is used, it is assumed to contribute only 20mm to the overall thickness of

the pavement for design purposes.) The bitumen within dense bitumen macadam road bases and base courses

must be at least 100 penetration grade, with hot-rolled asphalt containing 50 pen binder.

Road Note 29

Pavement thickness design methods have historically been empirically based, with the performance of

pavements being analyzed and design charts being compiled based on the information obtained from the on-site

observations of researchers. This approach led to the publication of Road Note 29 (Department of the

Environment, 1973). This document was based on scrutiny during the 1960s of the behaviour of sections of

highway pavement in UK trafficked by up to 10 million standard axles. It formed the basis for pavement design

philosophy in the UK from then until the mid 1980s.

Road Note 29 (RN29) takes account of increasing axle loads and vehicle numbers while also differentiating

between the performance characteristics of different road base materials. (The road base is assumed to satisfy

the entire strength requirements for the entire pavement, with the surfacing considered to make no significant

contribution to the strength of the pavement. The primary function of the surface material is to provide surface

texture and regularity.) The RN29 procedure is best explained as a series of design steps.

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Step 1 - Determine the cumulative number of commercial vehicles expected to use the highway from its first day

of use to the end of its design life, taken as 20 years.

Step 2 - Determine the cumulative number of commercial vehicles expected to use the ‘design lane’ over its

design lifetime. (The design lane is the most heavily trafficked lane in any given direction.)

Step 3 - Determine the equivalent number of standard axles incident on the road over its design life, based on

the commercial vehicle usage. Based on a standard axle of 80kN, the required value is obtained from the product

of the cumulative number of commercial vehicles and a term called the damage factor which varies for different

road types. The maximum value of this conversion factor is 1.08, used for motorways and trunk roads designed

to cater for over 1000 commercial vehicles per day in each direction.

For a motorway: Equivalent No. of standard axles = No. of commercial vehicles x 1.08

Step 4 - Determine the sub base thickness. This is dependent on both the CBR of the sub grade and the

cumulative number of standard axles over the design life of the highway. For a cumulative number of standard

axles of 1 million (1 msa), a minimum sub base thickness of 150 mm is required where the CBR is greater than

6%, rising to 440mm where the CBR is 2%. Where the CBR is less than 2%, an additional 150mm of sub base

should be added to that required for a CBR of 2%. The CBR of the sub base should be at least 30%. If the CBR

of the sub grade is in excess of this value, no sub base need be used.

Step 5 - Determine the road base and surfacing thickness. This parameter depends purely on the cumulative

number of standard axles over the pavement’s design life. For cumulative standard axles in excess of 10 million,

the surfacing should be 100 mm thick (60 mm base course plus 40mm wearing course). If dense bitumen

macadam is used, a road base thickness of just under 150 mm is required to cater for 10 msa, giving a total

bound thickness of 250mm.

LR1132

Road Note 29 was the sole officially recognized pavement design methodology throughout the 1970s and early

1980s. While it was considered to be generally effective, it had certain inherent deficiencies:

- It was seen as unresponsive both to improvements in the quality of available raw materials and to

changes in construction processes.

- The RN29 method is valid for designs up to 40 msa. Many highways were, by the mid 1980s, well in

excess of 50 msa, with some approaching 150msa over their 20-year design life.

- The 20-year design life implied that, after this period, a surface rut of 20mm or more, or severe cracking

or crazing had developed. The pavement was then considered to be in a failed state and in need of

major strengthening or partial reconstruction. It has been shown that attempting to strengthen a

pavement damaged to such an extent did not automatically result in satisfactory performance

afterwards.

LR1132 revised RN29 by redefining pavement failure, thereby delivering a thicker but longer lasting highway

likely to be in a less deteriorated state after 20 years.

The design criteria adopted by LR1132 were:

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(1) The sub grade must be able to sustain traffic loading without excessively deforming. This is achieved by

limiting the vertical stress at formation level.

(2) Bituminous or cement bound materials used in the flexible pavement must not be subject to fatigue cracking.

This is achieved by limiting the horizontal tensile stresses at the bottom of the bituminous/cement bound road

base.

(3) The load spreading capability of granular sub bases should be enough to provide an acceptable construction

platform.

(4) When a pavement is composed of a considerable depth of bituminous material, its creep must be restricted in

order to stop the rutting which arises from internal deformation.

Some of the stresses referred to above are illustrated in the figure..

In contrast with RN29, where the failure condition was presented as a 20mm rut with severe cracking / crazing,

LR1132 defined the end of a pavement’s design life as indicated by a 10 mm rut depth or the beginning of

cracking in the wheel paths. These less severe indications were chosen on the basis that they are the pre-

cursors of significant structural deterioration. They mark the latest time when the application of an overlay will

have maximum effect and will be expected to make best use of the original structural quality of the pavement. In

other words, the design life as thus defined is the latest time at which the application of an overlay will deliver

another few years of high quality motoring. This is termed pre-emptive overlaying, a process carried out at the

onset of critical structural conditions within the pavement. If application is postponed to a point later in the

pavement’s life, it may well have deteriorated to a stage where extensive pavement reconstruction will be

required.

Since the LR1132 approach maximizes the use of the existing pavement’s strength, a pavement of more uniform

strength will result. In addition, as deterioration can be predicted without too much difficulty, ultimate

reconstruction can be more easily planned. This definition of design life results in LR1132 designing a pavement

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having an additional period of serviceable life before major reconstruction, a period that would not be available if

Road Note 29 were used.

A design life of 20 years is normally employed.

Given the adoption of the design life concept as detailed within LR1132, the cumulative number of equivalent 80

kN standard axles to be carried during the design life of the highway must now be estimated. Observed or

estimated 24 hour commercial vehicle flows must be converted to annual flows. If there is more than one lane in

each direction, an allowance must be made for the proportion of this traffic travelling in the nearside lane,

assumed to be the lane carrying the majority of commercial vehicles (RN29 makes this same assumption). The

annual traffic is then multiplied by the vehicle damage factor – an estimator of the damage effect of an average

commercial vehicle.

The design procedure can be summarized as follows.

Step 1 - Calculate Tn, the total number of commercial vehicles using the slow lane over the n years design life,

as follows:

where

F0 = initial daily flow (base year), r = commercial vehicle growth rate n = design life

P = proportion of commercial vehicles using the slow (nearside) lane

P = 1 if it is assumed that all vehicles use the nearside lane.

Step 2 - Calculate the damage factor, D.

In order to convert Tn into equivalent standard axles, it must be multiplied by the vehicle damage factor, D,

calculated for the mid year of the design life, Fm.

The damage factor is calculated as follows:

where

Fm = number of commercial vehicles per day in one direction (mid-term year)

t = mid-term year minus 1945.

Step 3 - Calculation of N, the cumulative number of standard axles

N = Tn x D

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Sub grade strength

The CBR test is taken as a direct measure of the strength of the in-situ sub grade material. Despite concerns

regarding the limited accuracy of this test, it is utilized on the basis that it is widely used and accepted by both

theorists and practitioners.

Sub base and capping layer

In terms of the overall structural strength of the pavement, the sub base is an extremely important layer. If the

design life traffic volume is less than 2 msa, the CBR of the sub base must exceed 20%. If it is greater than 2

msa, the minimum CBR of the sub base rises to 30%. Use of a capping layer (with a lower specification than the

sub base material) will allow a thinner layer of the more expensive sub base to be used in the pavement.

Table indicates the thickness requirements for both sub base material alone and combinations of sub base and

capping for different CBR values of the underlying sub grade material.

Road base and surfacing

Figures 8.3 to 8.5 are illustrations of the required thicknesses for bituminous, wet-mix macadam and lean-mix

concrete road bases plus surfacing as detailed in LR1132.

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It should be noted from the design charts in LR1132 that not all highway pavement materials are suitable for

traffic levels over 20 million standard axles. A standard long-life design is given for bituminous pavements with a

lean-mix concrete road base. In the case of pavements with bituminous road bases, designs for more than 80

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msa are not included in the standard charts. At these traffic levels, the end result of deep-seated failure is

generally severe traffic disruption. In order to avoid fatigue failure, a layer of rolled asphalt is placed at the

position in the pavement where

resistance to fatigue is most

essential – at the bottom of the

bound layer. This will help ensure

that failure is likely to occur in the

form of surface deformation rather

than deep-seated fatigue failure.

Consequent damage will be less

extensive, confined to the upper

regions of the highway pavement.

Figure 8.6 shows the general

composition of a bituminous

pavement designed to carry traffic in excess of 80 msa. The total thickness of the bound layer should be

approximately 400mm for cumulative traffic in the range of 100 msa.

HD 26/01

The standard HD 26/01 (DoT, 2001) is based on LR1132 for flexible and flexible composite pavements, but with

modifications and amendments to take account of more recent research, new materials and the observed

functioning of in-service pavements.

The standard design life for all types of pavement is assumed to be 40 years, provided appropriate maintenance

programmes are in place. For roads surfaced with asphalt, it is anticipated that surface treatment would be

required every 10 years. (Note: in HD 26/01 all bituminous materials are covered by the generic term ‘asphalt’.)

A 20-year design life may be seen as appropriate for less heavily trafficked schemes.

LR1132 was based on observations of highways over a 20-year period. Later research (Munn et al., 1997)

indicated that cracking and deformation is more likely to occur in the surfacing than deeper in the structure.

Therefore, it was surmised that a well-constructed flexible pavement would have a very long structural life

provided such signs of distress are treated before they affect the structural integrity of the highway. HD 26/01.

notes that, for long-life flexible pavements designed to carry traffic for at least 40 years, it is not necessary to

increase the thickness of the pavement beyond that required for 80 million standard axles.

For flexible composite pavements, the thickness of the cemented lower base can be reduced as the strength of

the CBM is increased. For fully flexible pavements, two design charts apply, one for recipe mixes, one for design

mixes. Figure 8.7 is an illustration of the design thicknesses for four different types of base (road base) material:

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- Dense bitumen macadam/hot rolled asphalt (DBM/HRA)

- Dense bitumen macadam with 50 penetration grade bitumen (DBM50)

- Heavy duty macadam (HDM)

- High modulus base with 35 penetration grade bitumen (HMB35).

The total thickness of a fully flexible pavement depends on the type of road base used. A DBM/HRA base is the

least stiff and therefore requires the greatest pavement thickness. As the stiffness progressively increases from

DBM50 up to HMB35, a reduced pavement thickness provides an equivalent level of structural strength and

integrity.

Similarly for flexible composite materials, the thickness of the pavement decreases as the strength of the CBM

material used becomes greater. The design thicknesses for the asphalt (bituminous) layers within a flexible

composite pavement are given in Fig. 8.8. The design thickness of the lower base varies from 150mm to 250mm

depending mainly on the type of cement bound macadam used and the incident design traffic. A simplified

diagram of the required thicknesses for the lower base of a flexible composite pavement is given in Fig. 8.8.

Example 8.2

A highway is envisaged to carry a traffic loading of 40msa over its design life. Interpret the necessary asphalt

thickness for a wholly bituminous pavement using the relevant chart from HD 26/01 (DoT, 2001).

Solution - From Fig. 8.7, the following required thicknesses can be deduced:

(1) For DBM/HRA base: 350mm (2) For DBM50 base: 310mm

(3) For HDM base: 290mm (4) For HMB35 base: 280mm.

Note: all thicknesses are rounded up to the nearest 10 mm in accordance with the guidance from HD 26/01.

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Example 8.3

A highway is envisaged to carry a traffic loading of 30msa over its design life. Interpret the necessary surfacing

and lower base thicknesses for a flexible composite pavement using the relevant chart from HD 26/01.

Solution - From Fig. 8.8, the following information can be deduced.

The total asphalt layer required is 190mm. Typically, this would be composed of:

_ 40mm HRA wearing course (surface course), overlaying

_ 50 mm basecourse (binder course), overlaying _ 100mm roadbase (base).

For DBM/HRA base, required thickness is 350mm

The lower base layer required on a granular subbase is:

_ 250mm of CBM3G base, or _ 200mm of CBM3R or CBM4G, or _ 180mm of CBM4R or CBM5G, or

_ 150mm of CBM5R.

Note: CBM 5, 4 and 3 are the highest quality cement bound materials, usually prepared at a central plant from

batched amounts of processed crushed gravel or rock. A very similar result is obtained from LR1132. If one

examines Fig. 8.5 detailing required thicknesses for a concrete roadbase plus asphalt surfacing:

For a cumulative traffic figure of 30 msa:

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_ Asphalt surfacing thickness = 200mm _ Lean-mix concrete roadbase = 250mm.

which is within 10mm of the overall thickness recommended by HD 26/01.

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