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UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING

Pavement Engineering - Module H23P01 Course Notes

1

PAVEMENT ENGINEERING

INTRODUCTION

A pavement is a structure designed to allow trafficking, usually of wheeled vehicles.

Most pavements are roads, but airfields, industrial hardstandings, cycle tracks etc. are all

included.

Key points:

a) Pavements are high-volume constructions; the materials used must therefore be

cheap and environmentally acceptable.

b) There is no exact definition of failure; they simply have to remain ‘serviceable’.

c) The definition of serviceability will vary from application to application.

d) Maintaining serviceability is an important part of pavement engineering.

The basic building blocks

Soil: unpredictable; water susceptible; sometimes low strength

Granular Material: more predictable; less water susceptible; stronger

Hydraulically-Bound Material: bound with cement or something similar

Asphalt: stones stuck together with bitumen; good quality material

A Typical Pavement Structure

Surface course (or Wearing course) – Asphalt

Binder course (or Basecourse) – Asphalt

Base – Asphalt, Hydraulically-bound (e.g. Pavement

Quality Concrete), or Granular (often in more

than one layer)

Sub-base – Hydraulically-bound or Granular

Capping (or Lower Sub-base) – Hydraulically-

bound or Granular (only used over poor

subgrade; often in more than one layer)

Subgrade (or Substrate) – Soil



2

CONSTRUCTION

A competent pavement designer must understand the practicalities of material production

and pavement construction if sensible decisions are to be taken.

1. Unbound Material

Natural Soils

Check it to see that it is as strong as it was expected to be (CBR test – see later).

Protect it. It’s easy to turn a basically sound material into a muddy soup! So

leave a thin layer of overlying material until the very last moment.

Granular Materials

Make sure you have material that meets the specification.

a) Particle Size Distribution: achieved by crushing larger rocks and/or by

blending materials from more than one source. Put a sample through a set of

sieves to check it.

e.g. typical sub-

base limits:

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Sieve size (mm)

Perc

en

tag

e p

assin

g

Upper Limit

Lower Limit

Sample

Shake



3

b) Particle Soundness: Use a test such

as the Los Angeles Abrasion test;

sometimes also tests for frost damage

and chemical weathering (MnSO4

soundness).

c) Particle Shape: This isn’t always

specified. Most common requirement

= % crushed faces (trying to make

sure that rounded gravel isn’t used);

sometimes also by limits on flakiness

(% of particles of a given size able to

pass through a special thin sieve

opening) and elongation (% of

particles with one dimension over 1.8

times the nominal size) of particles.

The shape depends on the equipment used to carry out the crushing:

Cone Jaw

Roll Impact

d) Water Content: All unbound materials are sensitive to water.

Dry Density

Water Content

Heavy

Compaction

(e.g. on site)

Light

Compaction (e.g.

in laboratory)

0% air voids (i.e.

full saturation)

Optimum Maximum suction Dry

Desired in-service

condition after drying

out

Saturated

Maximum Dry Density

(heavy compaction)

Maximum Dry Density

(light compaction)



4

Low water content: negative pore pressure (or suction). Makes compaction

difficult; BUT good once in the road.

Higher water content: positive pore pressure. Makes compaction easier;

BUT bad once in the road.

So: compact at Optimum Water Content (OMC); let it dry out to develop

suction.

Transport it to site in a suitable delivery truck

Place it and compact it properly

Dozer + grader = good enough for lower layers

Paver = for high quality base layers (consistent thickness and surface level)

Compactors:

Vibratory Pneumatic Static

Delivery Truck

Delivery Truck

Dozer Motor grader

Paver Compactor

Compactor



5

2. Hydraulically-Bound Material (HBM)

HBM means any material that needs water to activate a binder – usually cement.

In-situ Stabilised

This is usually just soil improvement. Converts a soft soil into something you can build

a road on. As well as cement, lime and/or fly ash (also called pulverised fuel ash – PFA)

are used.

Plant-mixed HBM base/sub-base

Get the right aggregate. You need a good durable rock, either river gravel or

quarried and crushed. Particle size distribution = similar to granular.

Water Content:

Problem: you must have the right amount for compaction (OMC – similar to

granular materials) BUT you must also have just the right amount for the

hydraulic reaction to take place.

Too little water not all the binder will be activated reduced strength.

Too much water free water after the reaction has finished air voids left after

evaporation reduced strength again.

[This restricts the practical combinations of particle size distribution and strength]

Batch it and mix it (or mix it

continuously)

This is a twin-shaft batch

mixer.

The alternative is a drum

mixer, allowing HBM to

pass through continuously.

This gives higher

productivity.

Stabiliser Compactor

Mixing unit



6

Transport it to site – usually in a normal delivery truck.

Place it and compact it. Basically the same as for granular material, except that

you would usually put it through a paver to get good level and thickness control.

Cure it – usually by spraying a bitumen seal to stop the water evaporating.

Pavement Quality Concrete (PQC)

Get the mix design right. This is a real concrete, which means it will be too wet

for roller compaction (usually); it will need vibrating.

Batch it and mix it.

Transport it to site – usually in a purpose-

built concrete truck.

Pave it. Wet concrete needs to be enclosed

by formwork of one sort or another.

Options: Fixed form; checker board

pattern – slow process.

Fixed form; continuous side

rails – much quicker.

Slip form; with a purpose-

built slip-form paver –

quicker still; most commonly

used nowadays.

Water Content

Cement Content Range required to

achieve design strength

Range for

wet-form

workability

Range required to

allow chemical

reaction to take

place

Range of mixture

design options for wet-

forming

Range for

roller-

compaction

workability

Range of mixture

design options for

roller-compaction



7

Get the surface texture right

Form joints (usually)

Cure it, usually by means of a colourless aluminium-based sealant but can also

use wet cloth or regular water spray application.

Cure it – usually by spraying a colourless seal to stop the water evaporating.

Grooving:

Brushed finish:

Exposed

aggregate:

Burlap drag:

A set of steel tines is dragged across the surface of

the fresh concrete immediately after paving. [can

also be achieved by sawing hardened concrete]

The fresh concrete surface is brushed using an

appropriately heavy-duty brush to form a ridged

finish.

A retarder is sprayed onto the finished concrete and

loose mortar is brushed away from around the larger

aggregate pieces about 12 hours afterwards.

A sheet of rough fabric is dragged over the surface

of the wet concrete, leaving a rough finish.

Joint Types Joint Forming

Expansion Joint

Contraction Joint

Warping Joint

Bottom crack inducer

Joint former strip

Saw Cut (at early age)

Dowel bar

(smooth)

Filler Board

Joint Seal

Slip Coating

Expansion Cap

Dowel bar (smooth)

Crack Slip Coating

Joint Seal

Tie bar (ribbed)

Crack

Joint Seal

Sealant

groove cut

once joint

has been

formed



8

3. Hot-Mix Asphalt (HMA)

Heat the bitumen to 140 to 180C; keep it in a hot storage tank.

Dry the aggregate thoroughly in a drum dryer.

Sieve the hot dried aggregate into size fractions;

store in hot bins.

Mix bitumen + aggregate (about 30 secs)

asphalt.

Batch Mix Plant:

Drum Mix Plant:

Drum mixers give higher productivity. They rely on accurate proportioning of

moist aggregate since the bitumen is fed directly into the drying chamber, which

takes the form of an inclined rotating drum. The drying chamber doubles as the

mixing chamber and the hot mixture is fed out continuously from the drum to a

hot storage hopper before being dropped into the back of a waiting truck.

Transport to site in a thermally insulated truck.

Can combine in a

drum mix plant

Aggregate

feed

Dryer

Elevator

Hot bins

Mixer

Bitumen

tank

Combined Dryer-

Mixer Drum

Hot storage

hopper

Cold bins



9

Pave while the mixture is still hot,

e.g. 110- 130

Compact before it cools down too

much; either pneumatic or

vibratory for main compaction,

dead weight steel drum for the

final finish

How quickly does the mat cool? For example, assuming a 110C paving temperature:

The practicalities of compaction mean that layer thicknesses tend to be between 25mm

and 120mm.

60

70

80

90

100

110

120

0 20 40 60 80

Depth (mm)

Tem

pera

ture

(°C

)

@ 2 minutes

@ 4 minutes

20m

layer

40m

layer

80m

layer



10

MATERIALS

1. Sustainability and Cost

Pavements have to be cheap – that is an absolute requirement. However, we also have to

try to limit environmental costs. The key concept is embedded (or embodied) energy,

which is the total energy used to manufacture, transport, process etc. every component of

the pavement.

Approximate costs and embedded energies for pavement component materials:

Material Embedded

Energy

(MJ/Tonne)

Cost

(£/Tonne)

Embedded

Energy

Direct

Ingredients Sands and gravels

Crushed rock aggregate

Bitumen

Portland cement

Reinforcing steel

5-10

20-25

3200-3800

4500-5000

23000-27000

0.1-0.2

0.4-0.5

5-8

12-15

Mixtures Hot-mix asphalt

Cold-mix asphalt

Lean concrete

Pavement quality concrete (PQC)

Reinforced concrete

600-800

150-200

450-500

750-1000

1100-1500

12-16

3-4

9-10

15-20

22-30

25-40

15-30

15-25

35-50

40-55

Transport All materials (per journey km) 12-20

The question is: what is a MJ worth? In terms of fuel cost it is only about £0.01-0.02! It

also represents about 65g of CO2, and this might be valued anywhere from £0.002 to

£0.02. To be environmentally conservative, the embedded energy costs in the table are

based on an equivalence of £0.02 per MJ.

So: although the table may be exaggerating, hot-mix asphalt and concrete carry a

significant embedded environmental cost.

BUT what about the issue of traffic, responsible for about 36% of all energy consumed

in the UK? A Nottingham research project found that energy losses attributable to road

stiffness were around 100MJ/m2 for a heavily trafficked concrete pavement over a 40-

year life. For asphalt this went up to around 250MJ/m2. These translate to about 300 and

1000MJ/Tonne assuming normal pavement thicknesses – which as you can see are quite

significant numbers. And this does not include energy loss due to surface roughness,

which is likely to be a much bigger factor and definitely needs researching!

Conclusion: we really should take the environmental cost of road pavements seriously.



11

2. Unbound Material

What’s really going on?

Individual stones have to translate and rotate.

stones slide against one another.

this is resisted by friction (typically around 30-35 for crushed rock, less for gravels);

stone shape is also obviously important.

Shear strength

Can be measured in a shear box

Can also be measured in a

triaxal apparatus

sin = ½(1–2) / ½(1+2)

Don’t confuse stone-stone friction angle with .

Initial State After Strain

BBB

AAA AAA

BBB F

N

Initial State After Strain

1

1

2 2

2 increasing 1

Normal stress

Shear

stress Angle of internal

friction

(typically 55 for

a crushed rock

limiting / ratio

(typically 10 for a

crushed rock)



12

What properties affect shear strength?

particle shape; angular is good

stone-stone friction; not much effect

stiffness modulus of the rock; not much effect

particle size; big is good

particle size distribution; broadly graded is good

particle packing; dense is good

water content; high suction is good (see p3) – increases effective stresses.

Stiffness

With an unbound material you can’t really

talk about a Young’s Modulus because the

behaviour is so non-linear and stress-

dependent. On the other hand it is

convenient to pretend that it has a Young’s

Modulus, so instead we call it Resilient

Modulus or Stiffness Modulus and we have

to remember that its value changes

depending on the level of applied stress.

Typical values:

Solid rock is approximately linear elastic with a stiffness modulus of 100 and 200GPa;

unbound materials typically have a modulus in the range 20-250MPa.

What properties affect stiffness?

particle shape; not much effect

stone-stone friction; high friction is good

stiffness modulus of the rock; stiff is good

particle size; big is good

particle size distribution; not much effect

particle packing; not much effect

water content; high suction is good (see p3) – increases effective stresses.

Shear Stress

Shear Strain Cycle no: 1 2 3 10 100 1000 10000

Ultimate stress (= shear strength)

Applied Stress Hysteresis loop

– represents

energy loss

Approximate shear modulus

Normal stress

Shear

stress

Angle of internal

friction

Apparent

cohesion c

(due to stone

interlock)



13

California Bearing Ratio (CBR)

This is a convenient general measure of quality, but has no fundamental meaning.

Very, very, very approximate relationships with stiffness: a) E = 10 × CBR

b) E = 17.6 × CBR0.64

Confined Compression

A triaxial test is better (more fundamental meaning) but it is complicated; and the stress

conditions are usually not right for a pavement. Confined compression is an alternative.

These tests are designed to give about the right level of stress in the material and so

hopefully about the right stiffness modulus for pavement design. You can also get a

measure of resistance to deformation accumulation under repeated load.

Force F

Displacement d

(50mm/min)

152mm diameter 125mm

height

Force (kN)

Displacement 1.27m

m

2.54m

m

F2

F1

CBR = max {(F1/13.2) ; (F2/20.0)} 100

where F1 and F2 are in kN

50mm

Steel

mould

Plunger

Soil

under

test

Load applied via

full-face platen

Springs

Locking nuts fix side

plates in place

Side plates

free to move

Adjustment control

for start conditions Springbox

PUMA (Precision Unbound

Material Analyzer)

Load applied via

full-face platen Eight wall

segments

Calibrated steel

and rubber band



14

Dynamic Cone Penetrometer (DCP)

The DCP is especially convenient for doing down a core hole during evaluation of a

failing pavement.

Plate Tests

Nowadays the usual way of doing these is by means of a portable dynamic plate test

(DPT). It is a quick, practical method for getting the in-situ stiffness of a pavement

foundation. You have to remember of course that it is affected by any layer within about

1m of the surface.

Static plate tests are also possible. The standard test to evaluate an airfield pavement

subgrade is a static 762mm diameter plate.

Depth

Number of blows

Depth

CBR (%)

Equation used in UK: log10[CBR] = 2.48 – 1.057 log10[p]

Drop

weight

Scale

Anvil

Core hole

Granular

Soil

Penetration

rate p

(mm/blow)

Boussinesq’s equation for deflection under a

rigid circular plate load:

= P (1 – 2) / 2rE

Therefore: E = P (1 – 2) / 2r

Peak load (P)

Peak deflection ()

Time delay due

to ground inertia

Load

Deflection

Time

Drop

weight

Rubber

buffers

Loading

plate

(radius r)

Load cell

and velocity

transducer

(geophone)

Modulus E

Poisson’s ratio



15

This table illustrates the fact that different stress conditions give different stiffnesses.

Material Stiffness Modulus (MPa)

Triaxial

(confining stress

20kPa; deviator

stress 0-100kPa)

DPT

(100kPa contact

pressure)

In the Pavement

(K-Mould, PUMA and

Springbox generally

give similar results)

Very soft clay soil 10 5 15

Firm clay soil 50 30 80

Sandy soil 75 30 50

Gravel capping 125 50 80

Sub-base 250 75 150

Granular Base 500 100 250

Permeability

Usually it is assumed that a sub-base or capping layer will be ‘free-draining’ – which is

not entirely true so you might want to measure it.

Material

Description

Typical

permeability

range (m/sec)

Well graded gravels 10-5

to 10-3

Poorly graded gravels 510-5

to 10-3

Silty gravels 10-8

to 10-4

Clayey gravels 10-8

to 10-6

Well graded sands 510-6

to 510-4

Poorly graded sands 510-7

to 510-6

Silty sands 10-9

to 10-6

Clayey sands 10-9

to 10-6

Low plasticity silts 10-9

to 10-7

Low plasticity clays 10-9

to 10-8

High plasticity silts 10-10

to 10-9

High plasticity clays 10-11

to 10-9

Water supply Overflow

Specimen

(cross-sectional area A)

Porous end restraints

Measurement of volume

flow Q Head difference

H

L

Permeability k = QL/AH

Lid



16

3. Hydraulically-Bound Material (HBM)

The word ‘Hydraulic’ means that the binder needs water in order to be activated. The

most common binder of this type is Ordinary Portland Cement (OPC), but fly ash (a.k.a.

pulverised fuel ash – PFA), lime or ground granulated blast-furnace slag (GGBS) are

often mixed in or even used without OPC to give a slow-setting (and cheaper) material.

HBMs come in a range of different strengths / qualities:

Stabilised soil – insitu mixing process; roller compacted; results in a partially bound

material. [Compressive strength < 2MPa]

HBM subbase – plant mixed; uses gravel or crushed rock aggregate; still roller

compacted [Compressive strength 2-10MPa]

HBM base – fully bound crushed rock; usually roller compacted; also known as ‘lean

concrete’. [Compressive strength 5-20MPa]

Pavement Quality Concrete (PQC) – strong, fully bound concrete; wet-formed; vibratory

compaction. [Compressive strength 30-50MPa]

Strength Strength can be measured in several ways. Flexural is most realistic for a pavement. Compressive

(cube or cylinder) is the most convenient.

or

Tensile

Indirect Tensile

Compressive

Flexural



17

Tensile Strength

In the end HBMs fail in Tension. The problem is that tensile strength is not easy to

measure. You could do Indirect Tension. This is much easier and can be done on a core

of material taken from a road – but the stress conditions in the test are a little complicated

and it is hard to be certain what the result means.

Flexural Strength

This is much closer to what happens in a pavement.

It all adds up to the flexural strength (i.e. the tensile strength deduced from a flexural test)

being 10-15% higher than the real tensile strength. But this doesn’t really matter since the

pavement will behave more or less like a flexural test specimen.

Compressive Strength

This is the least meaningful strength test – but the most common, because it is so

convenient. For purposes of quality control it is therefore ideal.

Adding all this uncertainty together, the compressive-tensile

strength ratio can be anything from around 5 to 15.

Max Bending Moment (M) = P L / 6

½P

½P

½P

½P

L/3 L/3

L/3

h

[beam width = b]

Low friction graphite

applied to platens

Idealised

shape

Real shape

Analysis:

1. Failure occurs when t,failure is reached.

2. If zero friction: t = c = c / E

3. But things aren’t quite linear, so:

t > t / E

4. So, combining: c > t /

5. And if friction ≠ zero: c >> t /

c

t

>

Assumption:

Reality:

M = 2 b(2x/h)x dx [0 to h/2]

= 4(b/h).[⅓x3]

= h2/6

= 6M/h2

The non-linear relationship between

and in tension means that:

M > h2/6

< 6M/h2

Compression

Tension

x



18

Conclusion: Compressive strength is not a fundamental measure. It should never be

relied upon in pavement design.

Strength Gain with Time

All HBMs become

stronger with time. We

usually design on a 28-

day strength but often use

3-day or 7-day for quality

control.

Fatigue

All HBMs tend to follow a

similar fatigue characteristic

when plotted as numbers of

load applications to failure

against the ratio of applied

stress to failure strength.

Durability

The key to durability is a low permeability – which means small non-interconnecting

voids. The principal cause of damage is water; so if water can’t penetrate then damage

won’t occur. The danger comes when voids become filled with water and there is no clear

escape path. So be careful in design to avoid water becoming trapped.

Frost is another potentially serious problem. Water expands when it turns to ice, which

means that if the water is inside a nearly-closed void at the time it freezes then the

expansive pressure is likely to fracture the surrounding material. In PQC this can be seen

as areas of surface flaking, giving a very rough ride quality to vehicles. Measures to

combat frost damage include:

Air entrainment; introduce tiny air bubbles (e.g. 5% by volume) using a

chemical additive in the mix.

Keep the strength high.

Rule of thumb:

1. Limestone aggregate results in low compressive-tensile strength ratio (e.g. 7).

2. Granite aggregate results in moderate compressive-tensile strength ratio (e.g. 10).

3. Gravel aggregate results in high compressive: tensile strength ratio (e.g. 12).

0

10

20

30

40

50

60

1 10 100 1000

Age (days)

Co

mp

ressiv

e S

tren

gth

(M

Pa)

0.5

0.6

0.7

0.8

0.9

1

1 10 100 1000 10000 100000 1000000

Number of Load Applications to Failure

Fle

xu

ral

Str

ess:S

tren

gth

Rati

o

9.2MPa Compressive strength

32.9MPa Compressive strength

25.9MPa Compressive strength; 0.5% steel fibres

/sf = 1.064-0.064log(N)



19

Thermal Properties

HBMs are rigid solids; they are therefore susceptible to thermal expansion and

contraction. This is the reason for the use of joints in concrete pavements. The key

property is the coefficient of thermal expansion ().

Aggregate Coefficient of thermal expansion (per degree C)

River gravel 13 10-6

Igneous rock 10 10-6

Limestone 7 10-6

Stiffness

HBMs are more or less linear elastic. Stiffness is not usually measured in the laboratory

because it is more difficult to do + a bit less important than strength. Here are some

typical values:

Mixture Type [compressive strength] Typical stiffness

Pavement quality concrete [40MPa] 30000-40000 MPa

Strong cement-bound base [10-20MPa] 15000-25000 MPa

Weak cement-bound base [5-10MPa] 5000-15000 MPa

Slag etc. bound base [5-10MPa] 3000-10000 MPa

Hydraulically-bound sub-base [2-5MPa] 2000-5000 MPa

Stabilised soil [< 2MPa] 100-300 MPa

The Stiffness of a Discontinuous Layer

A HBM is often designed to end up in a cracked state; sometimes it is deliberately

cracked when joints are formed. The effective stiffness in-situ will therefore be less than

that of the intact material. A HBM base with an initial stiffness of 10000-20000MPa can

easily end up with an apparent in-situ stiffness of no more than 5000MPa.

1. Curvature = M/EI = 12M/Eh

3

2. Strain at top = (h/4)/d ≈ /3

Stress at top = E/3 ( 0 at mid depth)

Moment = (3/256)h2 = h

2E/256

Curvature = /L = 256M/(h2EL)

3. Moment = hL/2 = ghL/2

Radius of curvature = (L2/4)/2 = L

2/8

Curvature = 8/L2 = 16M/ghL

3

Combine:

Curvature = M[12/Eh3+256/(h

2EL)+16/ghL

3]

Curvature also = M/EeffI = M[12/Eeffh3]

Eeff = 1/[1/E+64h/3EL+4h2/(3gL

3)]

Stress distribution

I = h3/12 per m width

Slip stiffness g = /

d ≈ 0.75h

Uneven stress

zone

L L L

L h

h

M

M

L

h

1.

2.

3.

E



20

4. Asphalt

Bitumen (otherwise known as binder)

Bitumen is a liquid, even in service. This means that we need to worry about its

viscosity, which is a function of temperature. The problem is that viscosity is a little

complicated to measure directly, so the following two substitute tests are commonly used.

Penetration: the penetration of a needle into a container of bitumen.

Ring and ball softening point: the temperature at which a steel ball drops through a

prepared disk of bitumen.

0.01

0.1

1

10

100

1000

10000

100000

1000000

10000000

100000000

0 25 50 75 100 125 150 175

Temperature (Celsius)

Vis

co

sit

y (

Pa

.s)

50 pen

100 pen

200 pen

Compaction

Mixing

15g Allowed to fall

for 5 seconds

p

Bitumen

at 25C

Penetration p measured in

tenths of a millimetre

Penetration (pen)

Penetration Index (PI): (20-PI)/(10-PI) = 50 (log10[100/pen(at temperature T)])/(SP-T)

Heat at 5C

per minute

Disk of bitumen Metal

ring

Softening point is the

temperature at which

the bitumen disks sag

by 25mm (touching

the plates positioned

beneath them)

Water bath (stirred

continuously in

most specifications)

Ring & Ball Softening Point (SP)

Steel

balls



21

Real bitumen behaviour

There are two main components to

bitumen behaviour, elastic and

viscous, with a visco-elastic bit

between them just to make things

awkward.

Bitumen can also fracture. Tensile stress at failure is generally around 2-3MPa.

Ageing

Bitumen is that it changes with time due to oxidation and absorption by aggregate.

Short-term ageing occurs

during mixing, transporting,

placing and compacting while

the bitumen is at high

temperature; pen goes down

by about 25%.

Long-term ageing occurs

gradually such that bitumen

becomes ever harder during its

lifetime.

Binder Modification

Bitumen chemistry is a complicated subject and there are many different products on the

market that are claimed to improve the properties of bitumen. You should be aware of:

Polymers: Styrene-Butadiene-Styrene (SBS), Styrene-Butadiene Rubber (SBR)

and Ethyl Vinyl Acetate (EVA) will all increase the viscosity at high temperatures

but not at low temperatures. They may also increase fatigue life, particularly SBS.

Natural rubbers: This has been driven by the need to recycle vehicle tyres.

Sulphur: enhances workability at high temperature (>115C); becomes solid at

lower temperature.

Manganese: increases the cross-linkage between molecules and thereby increases

viscosity and stiffness. The problem is that the bitumen becomes brittle.

Load

Displacement

Viscous

Viscous

Visco-elastic

Visco-elastic

Elastic

Elastic

Time

Time

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Time (years)

Pen

etr

ati

on

(d

mm

)

Texas (Benson, 1976)Texas (Benson, 1976)Michigan (Corbett and Schweyer, 1981)



22

Bitumen-Filler Mortar

Filler is the term used for silt-size particles (2-75μm) which are added to enhance the

performance of the bitumen. It stiffens and strengthens the bitumen + gives extra

resistance to fatigue cracking.

Typically filler is added at slightly more than the mass of bitumen – which means about a

2:1 bitumen to filler ratio by volume.

Bitumen-Aggregate Adhesion

This is a vital property if an asphalt is going to work properly. A lack of adhesion will

mean that fracture can take place along the interfaces between aggregate particles and the

bitumen-filler mortar – and this will shorten the fatigue life of the asphalt; i.e. it will

crack.

But what causes poor adhesion?

The problem is that all aggregates would really prefer water to bitumen – hence the need

to ensure that particles are absolutely dry before mixing with bitumen. Aggregate

chemistry determines how easily the bond with bitumen is broken down, but it all

happens much more rapidly if there is water about.

What can be done about poor adhesion?

Answer: include an additive, usually hydrated lime.

Asphalt Stiffness Modulus

Asphalt does not have a single modulus value; it is both temperature and loading rate

dependent because bitumen is temperature and loading rate dependent.

0

1

2

3

4

5

6

7

8

9

10

0.1 1 10

Strain at Failure (%)

Str

ess a

t F

ail

ure

(M

Pa)

Pure bitumen

+5% filler

+15% filler

+35% filler

+50% filler

+65% filler

Cold and/or

rapid loading

Warm and/or

slow loading



23

Predicting Asphalt Stiffness Modulus

Step 1 – Binder stiffness: The most widely adopted approach is to use van der Pohl’s

nomograph, a chart which relates binder stiffness (Ebinder) to temperature (T), softening

point (SP), penetration index (PI), and load pulse duration (t). The following formula

matches the nomograph over a restricted range of input parameters.

Ebinder [in MPa] = 1.157 10–7

t-0.368

2.718–PI

(SP – T)5

Step 2 – Mixture Stiffness: The other controlling parameter here is Voids in Mixed

Aggregate (VMA), = the percentage of the mixture which is not aggregate = binder % +

air %. Many different equations have been proposed. Here is one.

Emixture [in MPa] = Ebinder [1 + (257.5 – 2.5VMA)/(n (VMA–3))]n

where: n = 0.83 log10[4 × 104/Ebinder]

VMA = Vbinder + Vair in %;

Ebinder is in MPa

These are easy enough to use, but the problem is that we don’t normally know the

volume of binder, only the mass percentage; so there is more calculating to be done.

Example: predict the stiffness modulus of an asphalt with 5% binder by mass and 7% air

voids at 10C under fast highway traffic if the softening point of the binder is 49C and

the penetration index is -0.5.

a) What loading time? Fast highway traffic: say 100kph; i.e. 27.8ms-1

. Tyre contact

is typically 300mm long, therefore loading time at the surface = 0.3/27.8 =

0.0108s. Rule of thumb: loading time (secs) = 1/speed (kph); i.e. 0.01s.

So: Ebinder = 1.157 10–7

0.01-0.368

2.7180.5

(49 – 10)5 = 93.7MPa

b) What VMA? We need to estimate the density of the rock in the asphalt, generally

in the range 2500-2900kg/m3, say 2700 here. We also need the density of

bitumen, typically 1030kg/m3.

In 1000kg of asphalt: 5% binder = 50kg; binder volume = 50/1030 = 0.0485m3.

95% rock = 950kg; rock volume = 950/2700 = 0.3519m3.

Combine: 0.4004m3.

But 7% air voids total volume of (100/93) × 0.4004 = 0.4305m3.

binder volume percentage of 0.0485/0.4305 × 100 = 11.3%

Therefore VMA = 11.3% + 7% = 18.3%

So: n = 0.83 log10[4 × 104/93.7] = 2.183

Emixture = 93.7 [1 + (257.5 – 2.5 18.3)/(2.183 (18.3–3))]2.183

= 7270MPa



24

Measuring Asphalt Stiffness

Here are three possible tests:

The tension-compression test is not very practical to carry out. The indirect tensile test

is very quick and easy, but has complex stress conditions; nevertheless it is widely used.

The four-point bending test is usually only carried out when testing for fatigue, but it

also gives a good stiffness measurement.

Note: stiffness depends on loading rate. Therefore you usually have to correct the result

before using it in pavement design.

Typical Stiffness Values

Material Stiffness Modulus (MPa) at 20C

In the Laboratory

(e.g. 125 milliseconds

to peak load)

In the Pavement

(e.g. 10 milliseconds

to peak load)

Dense asphalt base (50 pen binder) 5000 7000

Dense asphalt base (100 pen binder) 3500 5000

Surfacing 2000 3000

These values are for new asphalt, immediately after laying. But bitumen ‘ages’, which

means that the stiffness of a mixture will increase throughout its life as the viscosity of

the binder increases. For example, if a new dense asphalt base with 50 pen binder has an

initial stiffness (in the road) of 7000MPa at 20C (by which time the penetration of the

bitumen has already decreased to around 35 purely as a result of mixing and laying), then

this will probably have increased to about 9000MPa after 10 years in a climate such as

the UK, with much more rapid stiffness increase in hotter climates.

Note approximate temperature correction suggested by the Transport Research Laboratory:

log10(ET) = log10(E20C) – 0.0003 × (20-T)2 + 0.022 × (20-T)

Tension-

Compression

F

F

E = 4FL/d2

d L

Indirect Tensile

Diameter = d

F

E = F(+0.27)/b

Thickness = b

Four-point Bending

(Flexure)

L L L

F/2

h

Thickness = b

E = FL[(23L2/4h

2)+1+]/hb

F/2

F/2 F/2



25

Fracture and Fatigue of Asphalt

Low-temperature fracture can occur in continental climates. Asphalt expands and

contracts with temperature changes, with a typical thermal expansion coefficient of

around 1.8 10-5

per degree C. If it is too brittle at the low point of the temperature cycle

then it may simply break.

More usually, an asphalt

cracks due to fatigue under

millions of load applications.

Localised fractures begin to

form at particle-to-particle

contacts and then slowly

grow, eventually joining to

form proper cracks. Growth

rate is primarily controlled by

the magnitude of strain in the

mixture under load. You can

see the effect in a loss of

stiffness even before any

cracks can actually be seen.

Measuring Fatigue Resistance

The following three tests are commonly used.

Usually what you do is to

carry out a series of tests at

different stress/strain levels

and to plot the lives to failure

against strain (under load at

early stages of the test).

Failure is generally taken to be

a 50% loss in stiffness.

The results tend to be only

slightly affected by

temperature.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Proportion of Life to Failure

Pro

po

rtio

n o

f In

itia

l S

tiff

ness

10°C

20°C

30°C

Indirect

tensile 4-point bending Trapezoidal

(or 2-point bending)

10

100

1000

100 1000 10000 100000 1000000

Number of Load Applications to Failure

Init

ial

Str

ain

(x 1

0-6

)

Fatigue characteristicFatigue characteristic: Slope typically about -0.25 in log-log space



26

Permanent Deformation

Rutting is always a danger. To avoid it you need:

Good angular aggregate

A sensible gradation (i.e. particle size distribution of the aggregate)

Good compaction

A hard binder (so rutting only happens in hot weather)

At least 2% air voids, otherwise you begin to lose particle-particle contacts.

Measuring Permanent Deformation

The most usual forms of test are:

Typical RLA data:

(100kPa, 40C)

Note the effect of

moisture conditioning

– i.e. soaking in water

Rule-of-thumb: 1%

strain is a safe limit;

2% spells danger; 3%

means trouble!

Durability

As mentioned already, bitumen ageing means asphalt becomes

stiffer – a good thing – but also more brittle – not so good!

But we also have to worry about water damage, leading to loss of

adhesion between aggregate and bitumen.

So: test one batch – usually indirect tensile strength;

soak a second batch for a while (many different specifications);

test the second batch;

express the result as retained strength, i.e. ratio of soaked to unsoaked (in %);

if > 75% (specifications vary) then OK

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 500 1000 1500 2000 2500 3000 3500 4000

Number of Load Cycles

Axia

l S

train

(%

)

Mix A - Dry

Mix A - Moisture conditionedMix B - Dry

Mix B - Moisture conditionedMix C - Dry

Mix C - Moisture conditioned

Wheel

tracking

Repeated load axial (RLA)

Vacuum repeated load axial

Water

bath

Simple Soaking

Procedure



27

Mixture Design

Aggregate Particle Size Distribution

The choice depends on where the material is going in the pavement.

Base: usually larger sized particles – because layers are thick; also less bitumen.

Surface: usually smaller particles – better ride quality; better durability; thin layers;

high binder content, therefore expensive.

Gradation – usually go for a broadly graded mixture to give optimum aggregate

packing, deformation resistance, stiffness + minimise binder content. For example the US

Superpave specification uses a Fuller curve with n = 0.45 for what is termed asphalt

concrete (sometimes called dense bitumen macadam in the UK).

Fuller curves: % passing size d = (d/D)n where D = maximum particle size

Another option is gap-

grading. This means that

there are large stones and

smaller particles but not

much in between. Hot

rolled asphalt and stone

mastic asphalt are both

gap-graded mixtures.

A very different surface

material is porous

asphalt, with a near

single-sized gradation

designed to allow easy

water drainage.

The concept of a critical particle size can be useful. This is the point where the actual

gradation just touches a Fuller curve. The gradation to the right of the critical size is less

steep than the Fuller curve, which means that coarser particles are always separated by

plenty of smaller-sized particles, right down to the critical size. However, the gradation to

the left of the critical size is steeper than the Fuller curve, which means there are never

enough small-size particles to fill the gaps between larger ones. This means that particles

above the critical size form the aggregate skeleton; particles smaller than the critical size

are really just floating in the binder.

So: Asphalt concrete – similar to the Fuller curve, therefore no clear critical size.

Hot rolled asphalt – very clear critical size at around 1mm; needs good sand-sized aggregate.

Stone mastic asphalt – should be a large critical size if well designed, but with almost

enough particles to fill the gaps; therefore very sensitive to errors in gradation.

Porous asphalt – definitely a large critical size.

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Sieve size (mm)

Perc

en

tag

e p

assin

g

Asphalt ConcreteHot Rolled AsphaltPorous AsphaltStone Mastic AsphaltFuller Curves (n = 0.45)

Critical particle size

(Hot Rolled Asphalt)



28

Binder Content

Don’t forget: binder content as a percentage by mass is quite different from its

percentage by volume – because the specific gravity of bitumen is so much less than that

of rock. The volume percentage (typically 8-12%) determines mixture properties; the

mass percentage (typically 4-6%) is most easily measured and therefore specified.

The Marshall Mix Design Method

A practical technique developed in the 1950s:

Select a gradation.

Make up a series of mixes at different binder contents.

Prepare specimens using a Marshall hammer for compaction.

Measure achieved densities.

Carry out Marshall tests to derive stability and flow values.

Determine optimum binder content based on stability, flow and density.

The Superpave Mix Design Method

This design approach grew out of research in the US in the 1990s and is principally

concerned with optimising mixture volumetrics:

Select a gradation (only broadly graded mixtures covered; filler-binder ratio by

mass between 0.6 and 1.2).

Make up a series of mixes at different binder contents.

Prepare specimens using a gyratory compactor.

Measure achieved densities.

The optimum binder content is the one that gives a void content of 4%.

Prepare further specimens at the optimum binder content.

Check voids at light and heavy compaction.

F

(50mm

/min)

Stability Density

Flow

Binder content Binder content

Binder content

maximum

minimum

optimum optimum Marshall Test

Marshall Hammer

F

Stability

Flow

60C

10kg

100mm

0.45m

50mm



29

The additional checks at the end ensure that compaction doesn’t occur too easily, an

indicator of poor aggregate interlock, and that void content will never fall below 2%,

even under heavy trafficking. Both checks are intended to avoid the danger of rutting.

Binder Grade (i.e. Penetration or similar measure)

This depends largely on climate. The key point is that the binder should be able to

perform satisfactorily over the full range of temperatures experienced in the pavement.

Low temperature danger of fracture and fatigue

High temperature danger of rutting

Desirable working range of binder viscosity ≈ 5 103 to 10

7 Pa.s.

e.g. 50 pen binder gives a working temperature range of around -10 to +45C.

In some climates it is just not possible to find a conventional binder which covers the

expected temperature range satisfactorily. In these cases there are two options:

Accept that damage will occur and plan accordingly.

Pay extra and use a modified binder, extending the working temperature range.

Filler

Filler is an extremely important part of the mixture. It is a very effective binder

additive, multiplying stiffness, fracture and fatigue strength by a factor of up to about 3.

The successful use of a good quality filler will:

enhance mixture stiffness and fatigue strength;

assist chemically in promoting aggregate-bitumen adhesion;

inhibit drainage of hot binder off the aggregate during transportation;

not prevent proper mixing;

not prevent proper bitumen-aggregate contact.

For best results, filler % by mass ≈ bitumen % by mass, maybe a little more.

Void content check on

specimens at ‘design’

number of gyrations (50-

125 depending on traffic)

Gyratory Compactor

Void Content

Binder content

optimum

4%

Check void content after

‘maximum’ level of

compaction (75-205 gyrations

depending on traffic)

>2%

Check void content after

‘initial’ compaction (6-9

gyrations depending on

traffic)

> 11% (unless traffic < 3msa)



30

5. Cold-Mix Asphalt

Conventional ‘Hot-Mix’ materials have excellent mechanical properties BUT:

- aggregate has to be heated and dried thoroughly,

- this means that certain potential aggregates are excluded,

- the material has to be placed and compacted before it cools.

It would therefore be extremely useful to find an alternative which could be mixed with

cold, wet aggregate. There might be significant environmental benefits as well as a

reduced energy demand.

Options: Bitumen Emulsion

Foamed Bitumen

Bitumen Emulsion

Bitumen emulsion is a ‘suspension’ of bitumen droplets in water, created as follows:

Break the bitumen into very small droplets, typically 1-20 microns in size. This requires a

Colloid Mill (which unfortunately is expensive and uses up significant energy!)

The emulsion works because the polymer parts of the emulsifier molecules attach

themselves to the bitumen droplets. This leaves each droplet surrounded with charge and

means that droplets repel each other. These forces are enough to prevent droplets

coalescing (combining) since bitumen and water have very similar specific gravities (1.00

and 1.03 respectively).

The Emulsifier

Emulsifiers are hydrocarbon chains

with positively or negatively

charged ions at the end of the

chain.

Cationic: positively charged

Anionic: negatively charged

Water + emulsifier

Hot bitumen

(100-140C)

1000-6000rpm

Emulsion (<90C)

+

_



31

Typical proportions: 40-70% bitumen.

Big advantage of emulsions: they can last for months (with the occasional stir).

Foamed Bitumen

Bitumen foaming is an alternative technique to produce a binder which is workable at

normal ambient temperatures. This is done as follows:

Advantage of foamed bitumen over emulsion: it doesn’t need so much water.

Disadvantage: you have to use it within about a minute! (i.e. straight into the mixer)

How Cold Mix Works

For both emulsion and foamed bitumen to achieve reasonable mixing in of the binder the

aggregate must be wet. The water content required is typically 2-3%.

Problem: the binder (bitumen droplets in emulsion or flakes of bitumen foam) heads

straight for the water, which is mostly found amongst the fine aggregate particles.

Result: coarse particles often don’t get coated properly with bitumen, leaving a partially

bound material.

Bitumen droplet

Charged emulsifier molecules

Water + free emulsifier

Hot Bitumen

(140-200C)

Water (2-6% of bitumen)

Air

Foam

Time

Expansion Ratio

Half lives (typically 15-30 seconds)

Maximum expansion ratio (typically 10-20)



32

In detail:

Look in even more detail:

Compaction squeezes

particles together, forcing

the water away from

contact areas and creating

bitumen bonds.

The final trapped water content is usually 0.5-1.0% (by mass). The rest of the water can,

in theory, evaporate; but this is highly weather dependent. Cold-mix therefore needs good

weather.

Additives

Because the UK is not always warm and dry, it is common to add a small percentage (1-

2%) of cement, lime or fly ash to the mixture in order to take up some of the water,

helping the bitumen to attach itself to the aggregate.

Volumetrics

We need good compaction,

and this depends on the

‘fluid’ content (‘fluid’

means water or emulsion or

– more debatably – foam

residue). Optimum

compaction is achieved at

optimum fluid content. This

is typically about 6% (by

mass).

1. Particles

coated with a

film of water before mixing

3. Most of the

water

evaporates; some is trapped

Dry Density

Fluid content

Air voids

Optimum Fluid content

10% 5%

0%

2. Bitumen

droplets or flakes of

foam coalesce onto the aggregate



33

So, say 2% water content is present in the aggregate already. This means that we can add

4% extra fluids. In an emulsion, this can be up to 70% bitumen, which means that the

upper limit on bitumen content is just under 2.8% (compared to a typical 4-5% for a hot-

mix asphalt). If you want more bitumen then you just have to accept a lower density.

Foamed bitumen needs a slightly higher initial aggregate water content to get good

mixing. The result is that you end up with more or less the same limit as for emulsion.

Illustration of volumetrics:

We can use the same equations for predicting the stiffness modulus of a cold-mix as for

hot-mix. A trapped water pocket has just the same effect as an air void.

Equation: Emixture [in MPa] = Ebinder [1 + (257.5 – 2.5VMA)/(n (VMA–3))]n

where: n = 0.83 log10[4 × 104/Ebinder]

VMA is in %; Ebinder is in MPa

Say, for example, Sbinder is 25MPa for hot-mix at 20C but reduces to an average of

20MPa for cold-mix because of non-coated areas and the effect of the trapped water.

Therefore n = 2.66 for hot-mix and 2.74 for cold-mix.

Taking the volumetric examples above: SDBM = 6090MPa

Scold-mix low binder = 5350MPa

Scold-mix std binder = 1790MPa

Fatigue and deformation resistance are both also likely to be poorer than for hot mix.

By Volume: 78% 7% 10% 5%

By Volume: 78% 4% 6% 7% 2%

By Volume: 73% 6% 9% 10% 2%

By Mass: 87.5% 8% 4.5% 0%

By Mass: 90% 4.5% 2.5% 3% 0%

By Mass: 84.5% 7% 4% 4.5% 0%

By Volume: 69% 6% 9% 10% 6%

By Mass: 84.5% 7% 4% 4.5% 0%

Hot mix; dense grading;

standard binder content

Cold mix; dense grading;

low binder content

Cold mix; dense grading;


Cold mix; open grading;


Aggregate Filler Binder Air



34

Curing

An important difference between cold-mix and hot-mix is the time taken to gain strength.

Hot-mix: - fairly high strength as soon as it has cooled down (e.g. 2 hours);

- quite rapid stiffening for a few weeks as traffic slightly reorientates

particles for maximum effectiveness;

- slow stiffening thereafter due to binder ageing.

Cold-mix: - initially little more than a granular material;

- binder effect clear within 24 hours;

- continuing slow strength gain over a

period of up to 6 months;

- simulate in lab with 5 days at 40C

Unfortunately, it is not really practical to

keep traffic off the road for 6 months

while the material stiffens up! Therefore

there is a danger of early life damage

taking place – but this is very hard to

predict. So long as the trafic is light, no

permanent damage will occur. The

question is: just how much is too much?

Practical Use of Cold Mix

Summary of key points:

a) The bbiinnddeerr ccoonntteenntt will probably be lloowweerr than for a hot mix.

b) The vvooiidd ccoonntteenntt is likely to be hhiigghheerr than an equivalent hot-mix.

c) This means that cold-mix will usually be less stiff, less resistant to deformation

and have lower fatigue life. It is a ppoooorreerr mmaatteerriiaall!

d) Water needs to evaporate before sealing the surface. This means that the

aggregate ggrraaddiinngg used should be reasonably ooppeenn.

e) It is important to lliimmiitt eeaarrllyy ttrraaffffiicckkiinngg.

f) All of these points mean that there is a ssiiggnniiffiiccaanntt rriisskk associated with cold-mix.

So why would anyone use cold-mix?

a) The range of possible aggregates is extended. For example, construction and

demolition waste, incinerator ash, crushed concrete and recycled asphalt

planings (RAP) can all be used.

b) Cold-mix technology is ideal for in-situ recycling.

c) Cold-mix can have a storage life of several months.

d) Lower energy usage gives environmental benefits.

For these reasons, cold-mix is a popular choice for minor roads.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 50 100 150 200 250 300 350 400

Sti

ffn

es

s a

t 2

0 d

eg

ree

s (

GP

a)

Days since Construction

Basalt FoamMaster

Blast furnace slag Asphalt planings

RAP Limestone

Average



35

PAVEMENT DESIGN

1. Traffic

Load Magnitude

a) asphalt fatigue cracking depends approximately on the 4th

power of tensile strain

– see graph on p25;

b) concrete fatigue cracking depends approximately on the 10th

to 12th

power of

tensile stress – see graph on p18;

c) subgrade rutting depends approximately on the 4th

to 7th

power of subgrade

compressive stress (or strain?).

So it’s all a bit complicated. The American Association of State Highway Officials

(AASHO) undertook a series of pavement trials during the late 1950s using controlled

trafficking with known loads. Conclusion: use a 4th

power law.

No. of equivalent design axles (Neq) = [axle load (P) / design axle load (Pdes)]4

i.e. if P is twice the design axle load Pdes, it will do 16 times the damage of a design axle.

How much difference does the choice of power law really make?

So, for highways we convert traffic to an equivalent number of 8T (80kN) axles (called

standard axles) which means 40kN wheel loads. For airfields we have to choose a

design aircraft and convert to numbers of equivalent design aircraft; similarly for port

pavements etc.

But, for highways at least, we usually don’t know the detailed numbers in each weight

band, so it is common to use a wear factor (or damage factor) for each vehicle type.

Typical highway traffic – 1 hour:

Wt band 0-1T 1-2T 2-3T 3-4T 4-5T 5-6T 6-7T 7-8T 8-9T 9-10T 10-11T 11-12T 12-13T 13-14T 14-15T

Average 0.5T 1.5T 2.5T 3.5T 4.5T 5.5T 6.5T 7.5T 8.5T 9.5T 10.5T 11.5T 12.5T 13.5T 14.5T

Number 3875 1201 564 320 254 189 265 412 365 176 17 4 1 0

0 or 1

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 2 4 6 8 10 12 14

Exponent n

Eq

uiv

ale

nt

no

of

8T

ax

les

Without 14-15T axle

With 14-15T axle

Convert to equivalent 8T axles:

Neq = N (Wav/8)n



36

Vehicle Type Wear Factors (= conversion factor to standard axles)

Hakim

(1998b)

Frith et

al

(1997)

UK Highways

Agency (HD24)

Collop (1999)

Flexible Rigid

Maint-

enance

New

road

Rutting Fatigue

2 axle rigid

3 axle rigid

3 axle articulated

4 axle rigid

4 axle articulated

5 axle articulated

6 axle articulated

-

1.16

0.39

1.75

0.84

2.02

1.78

0.40

1.26

0.65

2.80

1.00

2.50

1.69

0.40

2.30

1.70

3.00

1.70

2.90

3.70

0.60

3.40

2.50

4.60

2.50

4.40

5.60

1.16

2.32

1.79

2.85

2.71

3.70

3.94

1.46

2.39

1.63

3.12

2.26

3.94

3.03

0.68

1.29

0.68

2.12

1.10

2.65

1.48

Note: these keep changing as vehicle and tyre design changes.

Contact Pressure

The contact zone between a pneumatic rubber tyre and the road surface is obviously

complicated. But should we worry? By the time we go down a few centimetres the

pressure will have evened out so the exact details certainly won’t affect the lower layers –

probably not the base either. Result: don’t worry about it except for high pressure, usually

aircraft loads, when we need to choose a suitably strong surface course.

Typical values: 250kPa for a car, 700kPa for a large truck, 1000-1500kPa for commercial

aircraft and up to 3000kPa for some military planes.

2. Standard Pavement Designs

As an example, here is the main chart from UK Highways Agency standard HD26.

Example

Fully flexible design (asphalt/

foundation) Composite design –

HBM lower base

Composite design –

asphalt upper base,

binder course and

surface course



37

Note: Traffic: (msa = millions of standard axles)

HBM (hydraulically bound material) category:

A. 9-12MPa (gravel)

B. 9-12MPa (crushed rock); 12-16MPa (gravel)

C. 12-16MPa (crushed rock); 16-20MPa (gravel)

D. 16-20MPa (crushed rock)

Foundation class:

1. Capping only 50MPa at top of foundation

2. Granular subbase 100MPa at top of foundation

3. Weak HBM subbase 200MPa at top of foundation

4. Strong HBM subbase 400MPa at top of foundation

Asphalt materials (= base layer):

DBM125 = Asphalt concrete with 125pen binder

HRA50 = Hot rolled asphalt with 50pen binder

DBM50/HDM50 = Asphalt concrete with 50pen binder

EME2 = Asphalt concrete with 15pen binder

Here is the equivalent chart for concrete (rigid) pavements

Note: Traffic: (msa = millions of standard axles)

CRCP = continuously reinforced concrete pavement

CRCR = continuously reinforced concrete roadbase (i.e. needs an asphalt surface)

fr = concrete flexural strength

R = reliability



38

The AASHTO (1993) method is also still widely used

Key Equation: Structural Number (SN) = a1h1 + a2h2 + a3h3

where: a1, a2, a3 = layer coefficients

[Asphalt a ≈ 0.238 × modulus(GPa)0.55

]

[Granular a ≈ modulus(MPa)0.85

/ 470]

h1, h2, h3 = layer thicknesses (inches)

Structural Number (SN) is a measure of the strength of the pavement structure, which is

related to pavement life (in msa) through a complicated equation.

Advantage: pretty simple conceptually

Nowadays the AASHTO Mechanistic Empirical Pavement Design Method (MEPDM)

is also available. This takes several minutes to run on a modern PC but accounts for

detailed traffic distribution + changes in temperature day and night throughout the year.

The problem is that all these methods + many others across the world are basically ‘black

boxes’. You input some parameters and you get a design out – and you just have to trust

that the method applies to your particular case. You calculate nothing!

3. Analytical Pavement Design – Flexible Pavements

Design Principles

The pavement has to fulfil the following roles:

a) Protect the subgrade: Natural ground will not be usually be strong enough to

bear traffic load directly; it would deform and rut.

b) Guard against deformation in the pavement layers: All pavement materials

must themselves be stable enough not to deform too much.

c) Guard against break-up of the pavement layers: The strength of the pavement

layers must be sufficient to prevent excessive cracking from developing.

d) Protect from environmental attack: The materials used must not lose their

properties (too much) under environmental attack.

e) Provide a suitable surface: The design has to be suitable to provide an

appropriate pavement surface.

f) Ensure ‘maintainability’: The design must ensure that it is possible to carry out

necessary maintenance.

Analytical design usually only looks at (a) and (c). The other points are covered by

sensible material specifications and sensible combinations of layers.

Protect the subgrade

Recognising the difficulties involved in soil parameter estimation, most of the current

analytical design methods use the so-called the subgrade strain criterion



39

This is a massive simplification – and relies on a massive assumption, namely:

THE STRENGTH OF A SOIL IS DIRECTLY RELATED TO ITS STIFFNESS

This is not really true!!

Nevertheless, over a limited range of soils, for example UK heavy clays, it may be close

enough to being true to be usable. The argument goes like this:

life (to a limiting rut depth) = fn [stress ÷ strength]

if: strength = fn [stiffness modulus] ….

…. then: life = fn [stress/stiffness modulus] = fn [elastic strain].

The great advantage of this assumption is that it is only

necessary to calculate the elastic strain value in the

subgrade under load, i.e. the vertical elastic strain at the

top of the subgrade under a design wheel load. This can be

done using multi-layer linear elastic analysis – programs

like BISAR, ELSYM, JULEA, CIRCLY etc.

All we need now is a relationship between life and strain.

Here are three:

Nf [in millions] = 3.09 1010

z–3.95

[z in microstrain] – UK Transport Research Laboratory

Nf [in millions] = 8.511 10-3

z–7.14

[z in millistrain] – NAASRA (Australia)

Nf [in millions] = 7.6 108 z

–3.7 [in microstrain] – British Airports Authority

Conclusion: think before you believe any of them! They have all been developed based

on experience. For example, sandy soils will tend to have different relationship between

strength and stiffness modulus from that of clay soils, which means they can carry many

times the number of load applications for a given level of elastic strain.

This is about as good as

it gets unless you really

know something about

the soil in your

subgrade. However if

you know enough to

estimate the shear stress

at failure at the top of

your subgrade you might

be able to be a little

cleverer in your design,

based on data like that

shown here.

30

40

50

60

70

80

90

100

1 100 10000 1000000


Ap

plied

str

ess/f

ailu

re s

tress (

%)

1% strain

2% strain

1% strain

2% strain

possible design line

0

1

2

3

4

5

6

7

8

1 10 100 1000 10000


Sh

ear

Str

ain

exclu

din

g t

he f

irst

cycle

(%

)

99%

54%

88%

83%

71%

52%73%

Clay

Sand

90%

90%: ratio of applied stress to

failure stress

Subbase

Hot-Mix Asphalt

Subgrade



40

This data is from laboratory tests carried out in the triaxial equipment on a soft clay and

an angular sand. Note that when the data is in the form of a stress ratio, the behaviour of

the two soils is fairly similar, allowing a possible design line to be proposed.

Guard against break-up of the pavement layers

It is a fact that a relationship is generally found between

tensile strain in asphalt under load and the number of load

applications until failure occurs; it is therefore quite logical

that the maximum tensile strain in the asphalt layers of a

pavement under a wheel load should be related to cracking.

Cracking can occur:

A – at the bottom of the asphalt immediately under the load;

B – near the surface just outside the loaded area;

C – at the surface in the tyre tread contact zone.

A is generally assumed to be dominant. Even if it isn’t always true it makes life simpler!

It can be calculated relatively easily using multi-layer linear elastic analysis.

We now need a relationship between calculated tensile strain and life. Here are two:

Nf [in millions] = 4.17 10-10

(1/t)4.16

[t in microstrain] – UK Transport Research Laboratory

Nf [in millions] = 0.00432k1'C (1/t)3.9492

(1/E)1.281

[t in microstrain] – AASHTO, US

where: k1' = fn(h); C = fn(mixture volumetrics); E = asphalt stiffness

modulus; h = asphalt thickness.

Putting it all together:

Subbase

Hot-Mix Asphalt

Subgrade

Buses 56

2-axle trucks 562

3-axle trucks 401

5 or 6-axle articulated trucks (with semi-trailer) 268

Equivalent standard wheel loads/day 2633

20-year design traffic 2 107

1 10 102 103 104 105 106 107 108

2000

1000

800

200

100

50

20

10

strain

N

E1, 1

E2, 2

E3, 3

h1

h2

t

h1

h2

t = 150strain

t = 100strain

t = 70strain

t = 50strain

Carry out traffic assessment

Select a fatigue characteristic

Multi-layer linear elastic analysis



41

Design Temperature

This is all OK, but we will get a different answer depending on the temperature of the

pavement since asphalt stiffness modulus depends on temperature. Its stiffness changes

by a factor of about three for every 10C temperature change, and stiffness is a key input

to any calculation of tensile strain.

The problem is far too complex to analyse fully.

So, select a design temperature and calibrate your prediction based on experience. In

the UK, the temperature selected is 20C. The equations for Nf on the previous two pages

assume an asphalt temperature of 20C.

Typical Stiffness Moduli and Poisson’s Ratios

Modulus - Asphalt surface course: 2500MPa

Asphalt concrete, 125 pen (= DBM125): 3100MPa

Asphalt concrete, 50 pen (= DBM50/HDM50): 5000MPa

Asphalt concrete, 15 pen (= DBM15): 7800MPa

Weak HBM: 500MPa

Crushed rock base: 250-500MPa

Crushed rock subbase: 150MPa

Capping: 75MPa

Poisson’s ratio - asphalt and unbound material: 0.35

HBM 0.2

Issues in Real Design

Thin asphalt layers

The problem here is that

deformation (and therefore

curvature) of pavements with

thin asphalt layers is dominated

by the stiffness of underlying

support, which means that it

only increases slightly as

asphalt thickness reduces; and

strain, which is proportional to

curvature but inversely

proportional to thickness,

actually reduces at low

thickness. The trouble is that

though calculated strain may reduce as asphalt gets very thin, experience is that pavement

life does not start increasing!

Suggestion: just extrapolate the design curve derived at greater thicknesses.

0

100

200

300

400

500

600

700

800

0 50 100 150 200 250

Asphalt Thickness (mm)

Ten

sil

e S

train

(m

icro

str

ain

)

Computed

For practical design

10000

100000

1000000

0 50 100 150 200

Asphalt Thickness (mm)

Lif

e (

nu

mb

er

of

pa

ss

es

)

M odelling propagation

For practical design

Computed



42

Load Groups

You don’t often get a truly isolated wheel, in

which case you might have to take account of

effects from neighbouring wheels.

In general: don’t worry about it for asphalt

strain; take it into account for subgrade strain.

Dynamic Effects

Wheel load fluctuates as the vehicle body oscillates vertically. Some suspension systems

are more effective than others at avoiding high dynamic load, but you can’t avoid it

entirely. Usually we ignore it in design, assuming it is included in the calibration. But

let’s take a look at what might really be happening.

Divide into time steps

Work out relative damage in each

time step [ = (W/Wmean)n ]

Average damage over whole cycle

The first three curves make the assumption that the distribution of dynamic load is

random. However, there is plenty of evidence to show that this is often not the case and

that similar vehicles, having similar suspension system characteristics, will tend to apply

peak loads in roughly the same locations. The fourth curve makes the assumption that the

loading pattern is perfectly repeated, in which case the computed damage would occur at

regular intervals along the pavement – and you can sometimes see this in reality.

Cornering

0

2

4

6

8

10

12

14

16

18

20

1 1.2 1.4 1.6

Ratio: maximun dynamic/mean

Mu

ltip

lier

on

dam

ag

eExponent = 4

Exponent = 8

Exponent = 12

Exponent = 4;

repeatable load pattern

Maximum dynamic Mean Load

Sideways force due to cornering = Mv2/r

Vertical force due to gravity = Mg [g = 9.81m/s2]

Balance vertically: P1 + P2 = Mg

Moments about inner wheel path: P1d = (Mv2/r)h + Mg(d/2)

Combine: P1 = M (g/2 + v2h/rd)

P2 = M (g/2 – v2h/rd)

Example: v = 25m/s (approx 60mph); r = 500m; d = 2m; h = 2m;

M = 8T

P1 = 49.24kN; P2 = 29.24kN

M

d

h

P2 P1

v

r

Centre of

gravity



43

Vehicle Speed Effects

Vehicle speed is important for three reasons:

dynamic loads will be higher at high speeds;

asphalt stiffness varies with loading rate;

soils and granular materials at high levels of saturation may suffer from positive

pore pressures (and therefore low strength and stiffness) at high loading rates.

But we still usually ignore it!

Lateral Wander

This refers to the fact that not every wheel follows the same path. On highways, the

distribution across a wheel path commonly has a standard deviation of around 150mm;

on airport runways this is likely to be a metre or more. We could therefore reduce the

design traffic slightly to account for this. On roads this is rarely done; on airfields it is.

Designing with Cold-Mix Asphalt

The trouble with cold mix is that it is neither one thing nor the other. It is partially bound.

Choices:

a) treat it as a hot-mix

b) treat it as a very superior granular material

What stiffness to use? 500MPa if you are really quite unsure;

750MPa for most cases;

1000MPa for well controlled construction in UK;

1500MPa under ideal conditions.

Hot-mix binder course + surface course 3500MPa

Cold-mix base 2500MPa

Calculate asphalt tensile strain at base of cold-

mix layer; use a standard fatigue characteristic.

Assumption: the material achieves full curing without damage [optimistic assumption]

Hot-mix binder course + surface course 3500MPa

Cold-mix base 1000MPa

Calculate asphalt tensile strain at base of binder

course; use a standard fatigue characteristic.

Assumption: the material deteriorates to the

equivalent of an excellent granular material.



44

4. Analytical Pavement Design – Composite Pavements

The first issue here is: what is a composite pavement?

Answer: one with a relatively strong HBM layer in it. A weak HBM is really just like a

good granular material because it will end up broken into small pieces

So, reviewing the design principles listed on p38, the key differences are:

a) Subgrade protection becomes of secondary importance since the subgrade can

never rut until the HBM has broken.

b) We need to make sure the stress in the HBM isn’t enough to break it, or at least

not enough to break it too much.

Thermal Stress

All HBMs expand and contract with temperature changes. They are solids and so this

imposes stresses, mainly in the longitudinal direction. In many climates there is likely to

be at least a 20C difference in HBM temperature between setting and the coldest time of

year, leading to a typical strain of 2 10-4

, and this is around the failure strain for most

HBMs. The HBM will therefore crack at intervals transversely across the pavement. In

fact it will usually crack during the first few night of its life when the strength is still low.

What will the crack spacing be?

Shear stress = gh tan

Force across centre of slab = L/2 = ghL/2 tan (per metre width of slab)

If slab is intact: Tensile stress at slab centre ≈ ghL/2 tan h = gL/2 tan

If this is more than the tensile strength of the HBM, it will break in two

Example: say tensile strength = 0.2MPa when the first relatively cold night occurs,

density = 2300kg/m3 and friction angle is 35.

Stress = strength when: 0.2 106 = 2300 × 9.81 × L/2 × tan(35)

i.e. L = 38m

So a 40m length between cracks will break into two 20m lengths, but a 35m length will

remain intact. In fact eventual spacing should be between 19m and 38m.

h

L

Temperature << temperature at time of set

Angle of friction

HBM layer

Crack



45

Conclusion: expect transverse cracks, but the spacing will depend critically on day-night

temperature changes during the first few days and weeks of life, and these cannot

reasonably be foreseen.

Solution: crack deliberately (i.e. form joints) at much closer spacing, usually 3m.

This process creates weaknesses. Hopefully cracks from at each weakness but because

they are quite close together, each crack should remain very narrow (a hairline crack).

Design against Traffic Loading

This time we need to calculate the tensile stress at the

bottom of the HBM layer, again using multi-layer linear

elastic analysis.

We then need a relationship between calculated tensile stress

and life. Going back to p18, the key quantity is the ratio of

tensile stress to tensile strength – well actually flexural

strength, i.e. strength from a realistic test arrangement. We

can just apply the fatigue equation suggested on p18, namely:

t / flexural strength = 1.064 – 0.064 log10 (N)

So, if the design is for 50 million standard axles and the calculated tensile stress at the

bottom of the layer is 0.8MPa, then the required long-term flexural strength is 1.4MPa,

which equates to a compressive strength of 10-15MPa. If we want to use a weaker

material we must make the layer thicker, or maybe make the foundation stronger.

Reflective Cracking

Even if there are no traffic-induced cracks in the HBM, we know that there will be

thermally induced transverse cracks (or joints – which amounts to the same thing). These

represent discontinuities in the support given to the asphalt layers, which means we are

very likely to find a crack appearing through the asphalt at those points. This is reflective

cracking.

a) Reflective cracks are a nuisance not a real failure; could just keep re-sealing them. b) Could use Highways Agency rule and always have at least 180mm of asphalt. c) Could check asphalt tensile strain t – but if so then reduce EHBM to 500MPa. This

represents the effect of a discontinuity in support to the asphalt.

Recently

paved HBM

Form slots to

2/3 depth

Fill slots with

bitumen emulsion

Roll

HBM

Subbase

Hot-Mix Asphalt

Subgrade

HBM



46

5. Analytical Pavement Design – Rigid Pavements (jointed unreinforced)

Westergaard Analysis

Since the 1920s, the equations developed by H.M.

Westergaard have represented the most widely used

approach to analysis of concrete pavements under load.

The original equations were derived assuming a

concrete pavement to act as a slab in pure bending, and

several subsequent modifications have been made over

the years to increase the accuracy with which a real

pavement can be modelled. The equations are in terms

of the maximum tensile stress in the concrete due to

slab bending and they are for three load locations: i)

internal (i.e. distant from a joint); ii) edge; and iii)

corner. The load is assumed to consist of a uniformly

stressed circular area. Here are commonly applied

versions of the equations.

Westergaard equations for stress in a concrete slab:

Internal loading; stress at base of slab:

Tensile = [0.275 p / h2].[1 + ].[4 log10(Ls/b) + log10(12 (1-

2)) – 0.436]

Edge loading; stress at base of slab:

Tensile = [0.529 p / h2].[1 + 0.54].[4 log10(Ls/b) + 0.359]

Corner loading; stress at top of slab:

Tensile = [3 p / h2].[1 – (2 a / Ls)

1.2]

where: p = load; a = radius; h = slab thickness; = Poisson’s ratio; Ls = ‘radius of relative

stiffness’ = [E h3 / (12 k (1-

2))]

0.25; E = stiffness modulus; k = ‘modulus of subgrade

reaction’; b = ‘radius of equivalent pressure distribution’ = (1.6 a2 + h

2) – 0.675 h, if a >

0.72 h; = a, if a < 0.72 h.

Estimating modulus of subgrade reaction k:

Use multi-layer linear elastic analysis:

predict deflection

k = (P/r2)/

Note: the result will depend on the value of r.

P

E1, 1

E2, 2

E3, 3

r

Corner

Edge

Internal

Joints (with

zero load

transfer)

Plan View



47

Once you have calculated a worst-case tensile stress you could then apply the fatigue

equation suggested above for HBM. However, it is usual to be on the safe side and use a

more conservative equation, known as the Packard line.

t / flexural strength = 0.96 – 0.0799 log10 (N)

So, if you calculate a worst case stress of 1.6MPa for example, and you want to use a

concrete with a flexural strength of 4.5MPa then the design traffic is about 37 million

load applications.

Problem: real life isn’t just corners, edges and places far from an edge. Joints usually

transfer load, so they aren’t really edges. But then not all joints are the same – look back

to p7. Expansion joints are not far from being edges; contraction joints should have pretty

good load transfer; warping joints should have excellent load transfer.

Solution: Usually ignore the corner case. Often ignore the edge case. Use the internal

case but apply a factor depending on how good you think the joints are; e.g. × 1.2 for

good joints, × 1.5 for poor joints.

Multi-layer Linear Elastic Analysis

The same multi-layer linear elastic analysis as has been introduced for flexible pavements

can also be used here to calculate tensile stress.

Advantage: load combinations can be included, such as dual or tandem wheel sets.

Disadvantage: all layers have to be infinite in extent; there is no way of analysing an edge

or corner situation.

Limit State Analysis

A problem with both Westergaard and multi-layer linear elastic analysis is that concrete

cannot really crack at a single point. If cracking is to occur, then there must be a

mechanism of cracks. What will actually happen is that the point of theoretical failure

will simply reduce in stiffness locally as the first inter-particle fractures begin to occur.

Conclusions based on an elastic analysis will therefore be conservative. The alternative

is a limit state analysis.

Key equation: WORK done by loads = ENERGY absorbed by foundation

+ ENERGY dissipated at cracks

Need to: a) arrange loads on pavement;

b) propose a failure mechanism;

c) calculate work done + energy to foundation based on an assumed set of

deflections and angles;

d) derive the resisting bending moment (per metre) at crack locations;

e) relate this bending moment to a required slab thickness for a given

concrete strength using the equation M = h2/6 (refer back to p17).



48

The limit state approach opens the door to solving problems of multiple loads and

complex joint geometry that would otherwise be impossible.

Warping Stresses

Thermally-induced warping stresses result from a temperature difference between the top

and the bottom of a slab; they should not be ignored.

Approach 1: assume that the safety margin in using the Packard line (previous page) is

enough to cover warping stresses – the usual approach in practice.

Approach 2: use equations that were developed to predict warping stresses directly.

These are the Bradbury equations for maximum warping stress in a concrete slab, one

for internal stress and the other for edge stress, mirroring two of the conditions covered

by the Westergaard equations.

You can then add the maximum warping stress to the stress caused by traffic to give a

real maximum value for design – although you then have to make some difficult

decisions about the number of likely combined stress applications during the life of the

pavement.

Example:

Assumptions: a) load uniformly distributed over area of foundation

enclosed by cracks

b) square wheel pattern with 0.5m offsets everywhere

Work done by loads = 4P × (2d/2–0.5×2)/(2d/2) = 4P(1–1/d)

Energy dissipated at cracks = 42dM × (/(2d/2)) = 8M

Energy absorbed by foundation = 4P × (/3)

Energy balance: 4P (1–1/d) = 8M + 4P/3

M ≈ P/3 for large d

Since M also equals fh2/6, therefore: f = 2P/h

2

d

0.5m

0.5m

Load P

Moment to cause cracking

= M per linear metre

Internal loading; stress at base of slab:

Tensile = ½.E..T (Cx + Cy)/(1 - 2)

Edge loading; stress at base of slab:

Tensile = ½.E..T Cx (or Cy)

where: E = stiffness modulus; = coefficient of thermal

expansion; T = temperature difference top-bottom; Cx,Cy

depend on the ratio of slab dimensions x and y respectively to

radius of relative stiffness Ls – see inset.

x/Ls=0, Cx≈0;

x/Ls=2, Cx≈0.05;

x/Ls=3, Cx≈0.18;

x/Ls=4, Cx≈0.50;

x/Ls=5, Cx≈0.73;

x/Ls=6, Cx≈0.89;

x/Ls≥7, Cx≈1



49

Joint Spacing

The main factor here is the amount of thermal expansion and contraction – not warping.

Taking a typical coefficient of thermal expansion of 10-5

per C and a 30C difference

between maximum summer and minimum winter pavement temperatures, then a

nominally 10m long slab of concrete will vary in length by 3mm. So, if joints are placed

in a road at 10m spacing, there will be a gap of 3mm or more in the winter.

This is usually too much. Even with dowel bars (refer back to p 7) there won’t be enough

load transfer across joints in the winter because there won’t be any aggregate interlock,

which means the design case becomes almost an edge condition.

In fact, experience suggests that for most climates joint spacings between 3.5 and 6m

represent a reasonable compromise between the cost and nuisance of joint construction

and maintenance and the need to maintain load transfer efficiency. Less and the cost

becomes too great; more and joint problems become increasingly likely.

6. Reinforced Concrete Pavements

Lightly Reinforced

Reinforcement is often not economically justified. However a light reinforcement mesh is

sometimes included near the top of the slab as a means of controlling shrinkage cracking.

It can also be used to cut back on the number of joints. The argument goes like this:

1) joints are a nuisance so let’s have less of them;

2) greater joint spacing increased warping stresses less load transfer at joints;

3) therefore there is a much greater likelihood of cracking;

4) but if reinforcement is present cracks are ‘controlled’; they will remain narrow

and the slab will not break up.

In this sort of pavement hairline cracking is accepted; but there is enough reinforcement

to hold the slab together, giving plenty of aggregate interlock across each crack and so

plenty of load transfer.

Continuously Reinforced

But why have any joints at all? After all, if we can accept hairline cracking then what

putting so much reinforcement in that it never fractures? This is continuously reinforced

concrete (CRC), and it sits right at the top of the range of concrete pavement options.

The principle is simple. There has to be enough reinforcement to resist the forces

generated when the concrete contracts due to cooling.

Note:

a) the concrete will still crack – but these will be hairline cracks, typically every 1m;

b) reinforcement quantity will typically be 0.6-0.8% of the concrete area;

c) slab thickness is usually less than in the unreinforced case – but not by much;

d) there needs to be an anchorage (into the ground) at each end.



50

SURFACE PROPERTIES

1. Ride Quality

Excellent ride quality is not always needed – but it is on high-speed roads.

Typical tolerance limits: 3mm in 3m

So how can this be achieved?

Pavement Quality Concrete

No problem achieving the tolerance in a machine-laid wet-formed concrete pavement.

Problem: concrete is hard and unable to absorb much energy from the tyres.

high tyre vibration

relatively high noise

not so pleasant to drive on

[joints just make things worse; good texture, e.g. longitudinal grooving

or exposed aggregate finish, can help]

Asphalt

To get a really good finish, the surface course must be relatively thin (say ≤ 50mm);

otherwise the paver operator will not be able to control levels well enough. But then the

underlying layer must also be reasonably even too – and this principle applies right the

way through the pavement. The evenness of the surface of each layer can be constructed

slightly better than that of the one below, but only slightly.

UK Highways Agency tolerances (absolute maxima) at each level:

Pavement surface 6mm

Binder course 6mm

Base 15mm

Subbase + 10mm – 30mm

Impact of different types of surface:

Surface Type Vibration

generation

Energy

absorption

Ride quality

ranking

Asphalt concrete medium medium 3

Hot rolled asphalt + chippings medium low 4

Stone mastic asphalt low high 2

Porous asphalt low very high 1

Surface dressing high low 5=

Concrete (PQC) fairly high very low 5=

Block paving very high low 7



51

2. Material Strength, Durability etc

A PQC surface is of exactly the same strength as the rest of the concrete slab not an

issue.

Asphalt Surfaces

Asphalt surface course has to have relatively small stone size due to its low layer

thickness and, consequently, a relatively high bitumen content. This leads to a less stiff

material but with high fatigue resistance and good durability.

Asphalt surface structural properties:

Mixture Type Stiffness Deformation

resistance

Fatigue

strength

Asphalt concrete medium high medium

Hot rolled asphalt (+ chippings) medium low high

Stone mastic asphalt medium-low high medium-high

Porous asphalt low medium-high medium-low

BASE HIGH HIGH MEDIUM

Block Paving

Although blocks themselves are high-stiffness, the effective layer stiffness is a function

of rotation and shear at joints, which depends on how well the joints are filled. It is

common practice to assume a stiffness of 500MPa for a combined block-bedding sand

layer.

3. Skid Resistance

Microtexture

This term describes the intrinsic frictional properties of the surface.

In an asphalt: it relates to the aggregate particles at the surface.

In a PQC: it relates to the cementitious mortar.

In block paving: it relates to the surface of the block.

The microtexture represents the ultimate skid resistance potential of a surface, the level

applying in dry conditions and without any intervening dirt, bitumen or ice lens. It is

logical therefore to insist on improved microtexture at sensitive locations such as

approaches to pedestrian crossings and roundabouts, and this approach is adopted by

highway authorities all over the world. Certain aggregate types such as gritstones

therefore take on a premium value because of their excellent microtexture.

The problem is that the frictional properties of a surface change under the action of

traffic. In dry weather they are polished by the relative motion of tyre and surface,



52

activated partly by tyre vibration. This reduces the microtexture. It is standard practice

therefore to assess the so-called Polished Stone Value (PSV) of an aggregate by first

subjecting it to an accelerated polishing regime before measuring the frictional

properties using the pendulum test.

Microtexture is also seasonal. Polishing occurs mainly in dry weather; wet weather

restores frictional properties to some extent due to the abrading effect of small particles

of grit which are present in surface water. For this reason, skid resistance should

preferably be assessed in summer or during the dry season.

Macrotexture

If it never rained you would need no

macrotexture (the visible texture due to

the arrangement of stones or the presence

of grooves etc). Neither tyre tread nor

visible surface texture make the smallest

contribution to basic skid resistance; they

are only present to ensure that surface

water has somewhere to go. Direct

contact is needed between tyre and

surface in order for friction to be

activated; if a water film remains in

between, the vehicle will aquaplane as

soon as brakes are applied. An optimised

macrotexture therefore ensures that there

is only a short distance between individual

contact points and regions where water

can be accommodated without danger.

Water movement away from

contact points

Accelerated Polishing Machine

Rubber-tyred

wheel

Polishing Test

Specimen

Surface

aggregate

Rubber

pad

Pendulum Test



53

Macrotexture is generally expressed

as a texture depth in millimetres.

The basic measure comes from a

procedure known as the sand patch

test, although there are also laser-

based pieces of equipment on the

market for rapid, sometimes traffic-

speed, measurement.

Macrotexture can also deteriorate under traffic loading. The same wet-season abrasion

that restores microtexture also reduces the height of individual aggregate particles,

eventually reducing the texture depth excessively. For this reason, it is necessary to

specify abrasion resistance, for example the Los Angeles Abrasion value.

4. Spray

Spray from surface water is a safety hazard. If water cannot easily flow across the

surface of a pavement then it will be available to form spray. The issue is not texture

depth but barriers to lateral flow.

Traditional UK Hot Rolled Asphalt (HRA) with rolled-in chippings has a particularly

bad reputation for spray since each individual chipping sits in its own small indentation

(negative texture) into the asphalt surface, allowing a small ‘pond’ of water to remain

around it until it either evaporates or is dispersed in the form of spray.

Most other surfaces consist of protrusions (positive texture) from a more general surface

level and water can flow around these protrusions and make its way sideways. Asphalt

concrete and SMA therefore generate much less spray than HRA. Grooved concrete is

also good. However, porous asphalt is undoubtedly the premier material. Porous asphalt

allows water to drain straight into the pavement itself and then to pass laterally through it,

below the level of the tyre-surface contact. The result: virtually no spray at all. Of course,

the pavement has to be able to cope with the presence of water within the porous asphalt.

Usually the porous asphalt surface course has to overlie a dense, impermeable binder

course; otherwise pavement durability problems are likely.

Sand Patch Test

1. Measure out

exact volume of

sand

2. Pour onto

pavement

surface

3. Spread out level

with tops of

aggregate particles

4. Record

diameter of

sand patch



54

5. Noise

This can be an important issue in urban areas. It is a highly complex field and it is not

necessary for the pavement engineer to appreciate the exact acoustic mechanisms

involved.

As expected, noise level generally depends on texture depth, i.e. roughness. However,

the picture is clearly more complicated than this, with a 10dB(A) difference – a factor

of about 3 in actual sound pressure magnitude – between block paving and porous asphalt

for the same texture depth. Of the more common asphalt surfaces, SMA is evidently the

quietest at normal texture depths (around 1mm).

Conceptually:

Noise is caused by vibration, principally of the tyre tread elements.

Surface type affects both the amplitude and frequency of tyre tread vibration. [a

rough surface will induce a high amplitude of tyre vibration and therefore high

noise]

Some of this noise will be absorbed by the surface, and this will depend on the

hardness of the surface material. [concrete has poor ability to absorb any sort of

vibration energy including noise and this means that it is difficult to produce a

low-noise concrete surface]

Porous asphalt has very low stiffness and therefore causes little excitation to the tyre

tread elements; it also has excellent noise absorption properties – an ideal low-noise

material.

88

90

92

94

96

98

100

102

104

106

108

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Texture depth (mm)

No

ise d

B(A

)

Porous asphalt

Stone mastic asphalt

Asphalt concrete

Surface dressing

Slurry seal

Exposed aggregate concrete

Textured concrete

Blocks

Porous

asphalt

Surface

dressing

SMA

Other materials

Blocks



55

PAVEMENT EVALUATION

1. Visual Condition Surveys

A visual survey is the most basic and yet often the most useful survey type of all. Put

simply, not all cracks are the same; nor are all ruts or surface defects. An experienced

engineer can deduce a great deal about the internal health or otherwise of a pavement just

by inspecting the surface.

Cracking in asphalt pavements

a) Is there more cracking in the wheel path than elsewhere? Yes: traffic is responsible, whatever that cracking may look like.

No: traffic is irrelevant and the cause is environmental or due to a general material defect.

b) Are there transverse cracks right across the pavement? Yes: either low-temperature cracks or reflective cracking from an HBM or PQC base.

c) Are there more transverse cracks in the wheel paths? Yes: either traffic-induced reflective cracking or defects built in during construction.

d) Is there a single well-developed longitudinal crack in the wheel path? Yes: traffic-induced fatigue of a thick flexible/composite pavement (cracks usually top-down).

e) Is there multiple cracking (crazing) in the wheel-path? Yes: shallow failure; either thin asphalt or the upper layer has become debonded.

f) Is slurry pumping up to the surface through cracks? Yes: water has become trapped, either in the bound materials or else in the foundation.

g) Are there localised wheel path depressions where more than one crack is present? Yes: probably a HBM base; localised damage water ingress, loss of support, settlement.

Cracking in PQC Concrete Pavements

a) Is cracking (of a jointed pavement) largely restricted to transverse cracks? Yes: thermally-induced, assisted by traffic; initial joint spacing was excessive.

b) Are significant longitudinal cracks present in or around the wheel path? Yes: traffic-induced damage; cracks will propagate rapidly along the pavement.

c) Are longitudinal cracks narrow, relatively close-spaced and straight? Yes: lightly reinforced concrete; minor defects at the time of construction.

d) Are there corner cracks at joint intersections? Yes: lack of slab support close to joints; damage is limited and will extend no further.

e) Are there regular transverse cracks at 1-2m spacing but no joints? Yes: continuously reinforced.concrete; should be hairline; if wide then pavement is too weak.

Chipping Loss: loss of adhesive properties in the binder due to bitumen ageing.

Ravelling: widespread chipping loss, leading to the development of pot-holes.

Bleeding: excess bitumen in the pavement shiny surface significant safety hazard.



56

Rutting

2. Profile Surveys

International Roughness Index (IRI)

IRI is defined by the amplitude of motion of a vehicle suspension system as it travels

along the road, measured in cumulative metres of suspension system movement per

kilometre of travel (m/km or mm/m). The vehicles that measure IRI are known as bump

integrators.

Laser Profile Surveys

Laser-based systems are now very commonly used, usually with an array of lasers

pointing down at the road surface. Reflective waves are monitored. They can be used for

profile measurement, texture depth and rut depth. These types of survey can be

carried out at normal traffic speed.

0

1

2

3

4

5

6

7

0 1 2 3 4

Distan ce (km)

IRI

(m/k

m)

IRI

< 2 m/km excellent

2-3 m/km satisfactory

3-4 m/km moderately bumpy

4-5 m/km bumpy

> 5 m/km very bumpy

Suspension movement +ve

–ve Typical Data

Surface course problem Binder course - base

problem Foundation problem



57

3. Skid Resistance Surveys

Skid resistance is a property that is directly related to the safety of users, applying to both

highways and airfield runways. In the case of airfield runways there are international

standards and it is necessary for airport authorities to check skid resistance regularly,

particularly under adverse weather conditions (rain or snow). Highways are governed by

standards set by individual countries, regions and cities.

4. Cores and Trial Pits

20

Sideways

force

Plan View

The Sideways Force Coefficient Routine Investigation Machine (SCRIM)

Plan Views

Trailer-mounted

alternatives

Separation

force

Drag

force

Wheel

under

braking

Water

tank

Surface

course

Binder/Base

course

HBM base

debonding

Crack; reflected

from HBM base

through asphalt

surfacing

Crack; top-down,

penetrating

through about

50% of the asphalt

Bottom-up

reflective

crack

beginning

to grow

Crack in

HBM base

possibly

reflected

from sub-

base

Serious

crack in

disintegra-

ting HBM

sub-base



58

Coring, using cutters of 100mm or 150mm in diameter, is a relatively non-destructive

method of sampling. Trial pits represent an alternative, labour-intensive method of

sampling. They are suitable i) where bound layer thickness is low; ii) where samples of

unbound foundation material are required; or iii) where specific information is needed

which demands a larger area than can be afforded by a core.

Construction Information

Cores and trial pits reveal the following: Materials present;

Layer thicknesses;

Visual quality;

Inter-layer bond.

Samples can also be taken back to the laboratory for testing.

In-situ Tests

The relatively small size of most core holes means that there is a limit to the types of test

that can be carried out. In fact, there is only one in-situ test device that is commonly used

and that is the Dynamic Cone Penetrometer (DCP – see p14). Trial pits also allow the

portable Dynamic Plate Test (DPT – also p14).

Laboratory Tests

These tests include:

compressive strength of HBM (height-diameter ratio of at least 1.0 required);

uniaxial stiffness modulus of HBM;

indirect tensile strength (ITS), either of HBM or asphalt;

indirect tensile stiffness modulus (ITSM) of asphalt;

indirect tensile fatigue test (ITFT) for asphalt;

repeated load axial test (RLAT) for asphalt deformation;

inter-layer bond strength tests.

Also, density and void content

can be obtained on specimens

of any convenient shape.

Asphalt specimens can also be

broken down into their

constituents, by means of a

centrifuge, with solvents used to

extract the bitumen. Aggregate

gradation and binder content

can be checked. Binder quality

can also be measured using a

Dynamic Shear Rheometer

(DSR).

P

P

Peak shear force

Shear slip

at failure

P

Leutner Test

Torque Test



59

5. Ground Penetrating Radar (GPR)

Thickness determination is the main reason for doing a radar survey and pretty good

data is usually obtained, accurate to around 1cm. However, it’s all down to image

recognition software, so mistakes are possible. Radar can also give data on moisture

(water molecules become excited at radar frequencies), voids (because of the strength of

a solid-air interface), and steel reinforcement (steel interferes with wave propagation).

6. Deflection Surveys

The Benkelman Beam

This is the oldest and simplest form of deflection test device and it is successfully used

throughout the world.

Time

Transmitter

/receiver Surface reflection

Asphalt/HBM reflection

HBM/Granular reflection

Depth

Asphalt

Hydraulically-bound material

Granular material

Individual Received Signal Longitudinal Signal Profile

Interpreted Thickness Profile

Radar wave pulses:

Frequency 0.2-1.5MHz

x

y Deflection due

to wheel load

Deflection Adjust for

temperature

Approximate

construction

Annual

Traffic

Life in years

[using an empirically-

determined set of

equations]

Distance travelled, x Measurement, y

Benkelman Beam

Arm Pivot Dial Gauge

66335500kkgg aaxxllee llooaadd

TTwwiinn ttyyrreess



60

The Benkelman Beam is a simple frame with an arm on a hinge, the rotation of which is

read from a dial gauge. The equipment is placed on the ground immediately behind the

twin rear tyres on one side of a goods vehicle loaded to a standard weight, the arm resting

on the pavement surface between the twin tyres. When the operator is ready, the goods

vehicle is slowly driven forward and a maximum reading is taken as the tyres pass the

end of the arm; when it has driven forward some metres, a minimum reading is also

taken. The difference relates to the deflection caused by the loaded wheel.

The Lacroix Deflectograph

The Deflectograph is an extension of the Benkelman Beam idea, initially developed in

France. However it allows the vehicle to travel continuously along the road. The

reference frame is in front of the measurement axle, and it is repeatedly dragged forward

relative to the body of the vehicle and then released. As soon as the rear tyres of the

vehicle have drawn level with the tip of the measurement arm, the frame is winched

forward toward the front of the vehicle ready for the next reading. The result is that a

measurement is taken every 3-4m and that the vehicle can travel continuously at a speed

of 2-3km/hr. Readings are taken in both wheel paths.

The problem with both the Benkelman Beam and the Deflectograph is that they are

relatively low-resolution measurements and rely on empirical interpretation. They also

do not give good data on PQC pavements. They are fine for estimating structural

condition of asphalt pavements for network-level management but they are not really

reliable enough for project-level design. For this we need something a bit more

sophisticated.

The Falling Weight Deflectometer (FWD)

The FWD gives a very precise value of absolute deflection (accuracies of 2 microns

commonly quoted), and that opens the door to a much more sophisticated method of

interpretation.

Distance

travelled, x

6350kg axle load

Twin tyres Winching

mechanism

Reference

frame

Arm

x

Deflection, y

Deflection due

to wheel load

Adjust to equivalent

Benkelman Beam

deflection

Deflection

Adjust for

temperature

Approximate

construction

Annual

Traffic

Life in years

[using an empirically-

determined set of

equations]



61

The machine is usually trailer-mounted. Tests are performed with the equipment

stationary and with the loading plate and deflection sensors lowered onto the surface. The

load pulse is then generated by the action of a falling weight onto a set of rubber buffers.

Key advantages:

the load magnitude can be selected to match a typical wheel load;

the pulse duration is similar to that from a moving vehicle;

the deflections are absolute and highly accurate (using velocity transducers);

measurements are taken not only at the load location (through a hole in the centre

of the loading plate) but also at selected distances from it.

The full set of readings describes a deflection bowl (or basin) which can be back-

analysed (or back-calculated) to deduce the combination of layer stiffnesses present.

Back-analysis is done by computer, with the following assumptions:

all layers are of uniform thickness and of infinite lateral extent;

all materials are linear elastic and homogeneous;

the load consists of uniform stress on a circular area;

dynamic effects due to inertia are negligible.

Deflection

(microns)

Offset (m) 0 1 2

Deflection

Bowl

FWD – Longitudinal Section

Deflection sensors

Falling Weight

Rubber buffers

Loading Plate

Known layer

thicknesses

Layer

stiffnesses

Measured

load Calculate

deflections using

multi-layer linear

elastic analysis

Adjust layer

stiffnesses

Good match

to measured

deflections?

Finish

no

yes

Load: 10-200kN

Duration:

25-50msecs

Platen radius:

150mm

Tow bar

Upper Pavement

- affects curvature of central part

- typical indicator: d1–d2 or d1–d3

Base and Sub-base

- affects slope in next region

- typical indicator: d2–d4 or d3–d5

Subgrade

- affects deflection at distance

- typical indicator: d6 or d7

d1 d2 d3 d4 d5 d6 d7

Asphalt over

good foundation

Concrete over

poor foundation

Load



62

How many layers can be analysed? Two: no problem

Three: should be OK

Four: be careful; don’t just believe the result

Another advantage over the Benkelman Beam and Deflectograph is that the FWD is

equally useful on concrete or asphalt. It is particularly well suited to measuring load

transfer efficiency across a joint. The loading plate is positioned one side of the joint and

deflections are measured either side.

Rolling Wheel Deflectometers

The ultimate deflection test device would be one that measured a full deflection bowl like

the FWD, but which travelled at traffic speed along a highway, thus combining both

quality and quantity of information. Such a device is the rolling wheel deflectometer,

and several versions have been developed over the years, achieving their goal to varying

degrees. One is currently being trialled by the Transport Research Laboratory. There is an

inevitable trade-off between measurement accuracy and travel speed. Several companies

and research organisations have used lasers to measure either distance from a datum of

vertical velocity of the surface.

So: not used much yet – but watch this space.

7. Diagnosis

Pavements with an Asphalt Surface

Rutting: Check the visual condition. If ruts are narrow with shoulders, the problem is

near the surface (surface course or binder course probably); the wider the rut, the deeper

the problem. Inspect cores carefully. If an asphalt layer appears rich in binder, especially

if that binder is soft, that is likely to be the cause of the problem. Consider carrying out

repeated load axial tests (RLAT) to check whether materials are deformation susceptible.

Also look at DCP data (if it exists). This should relate to rut resistance of foundation

materials. Check FWD data. A subgrade stiffness of 50MPa or less indicates potentially

deformable material.

Load Transfer Efficiency

- affects ‘step’ across joint

- typical indicator: d2–d3 or d3/d2

(note: 0 or 100% even if perfect, due to

distance between sensors 2 and 3)

Slab Support

- affects angle of loaded slab

- typical indicator: d1–d2 / L12

(where L12 = distance between sensors)

d1 d2 d3

Load Joint

Poor load transfer

Moderate load transfer

Good load transfer

L12



63

Transverse Cracks: Look at the detailed crack shapes. If they are straight and regularly

spaced, they are reflective cracks from joints in an underlying concrete pavement. If the

shape is less regular, they may either be reflective cracks over a hydraulically-bound base

or else low temperature cracking of the asphalt. If reflective cracking is suspected, check

the crack distribution. If there is a concentration in the wheel paths then traffic is clearly

playing an important part; if not, then the effect is almost entirely thermally driven. Short

transverse cracks may also have originated as construction defects and they may be

progressing due to binder embrittlement. If FWD data is available, check for loss of

asphalt layer stiffness in the wheel paths. This provides evidence of crack severity.

Longitudinal Cracking in the Wheel Path: The fact that this cracking is in the wheel

path proves that it is the traffic which is doing the damage. If there are several cracks

within the zone of the wheel path then the effect is almost certainly shallow, possibly

associated with debonding between asphalt layers. If there is only a single crack, cores

through cracks are advised. In many cases, crack depth is shallow. Cores will also

indicate where debonding between layers is associated with cracking. If an FWD survey

has been carried out, check the asphalt layer stiffness. If it is low or variable then this

implies significant damage and/or debonding. Compare FWD-derived stiffnesses with

those from laboratory testing of recovered samples. If the two measures agree then the

asphalt layers are likely to be intact and well bonded. Check evidence for binder

hardening, e.g. from unusually high stiffness of laboratory specimens, or poor binder

adhesion, e.g. unusually low stiffness, even of undamaged material. Also evaluate the in-

situ stiffness of any HBM layer from FWD evidence. If it is less than expected, this is

evidence that cracking is present. Using the best available evidence for the stiffness of

each layer, carry out multi-layer linear elastic pavement analysis and compute

pavement life. Compare the theoretical life with past traffic numbers – and with current

general pavement condition.

Ravelling: Ravelling (and associated pot-holes) occurs when adhesion between binder

and aggregate breaks down. Consider checking penetration of recovered surface course

binder; or consider measuring surface course stiffness in the ITSM. These should

identify binder hardening. Also check binder and filler contents since excess filler can

contribute to poor adhesion.

Bleeding: There is too much bitumen present. Inspect the cores. Multiple layers of

surface dressing are one common source of excess binder. Otherwise consider

determining the void content of the surface course. Bleeding should only occur at void

contents of 2% or less. Also check the visual condition for rutting since low void content

also leads to asphalt deformation.

Pavements with a Concrete Surface

Transverse Cracks (jointed PQC): Transverse cracking, either at mid-bay or a metre or

so from joints, is common; it implies that the joint spacing was too large for the

thermally-induced stresses and strains which have occurred. If the joint spacing is greater

than about 20 times the slab thickness, joints cannot be expected to function properly.



64

Also find out whether the cracking occurred soon after construction. If so, shrinkage

cracking may be the cause.

Transverse Cracks (CRC): Transverse cracks are expected in CRC. They should form

at a spacing of 1-2m but should remain narrow. If there are more or they are no longer

narrow, then the pavement is not functioning as intended and will continue to deteriorate.

Check concrete strength, slab thickness and foundation stiffness. The combination should

reveal whether the pavement as it was constructed should have lasted longer than it has.

If so then the problem lies elsewhere, e.g. reinforcement defects, overloading.

Longitudinal Cracking: This is a sign of overloading and is a very serious mode of

distress. Check concrete strength, slab thickness and foundation stiffness. If the computed

life is less than the current condition suggests, then this implies that one of the input

parameters was more favourable in the past. Possibly the foundation used to be stiffer and

has deteriorated over the years. Check FWD data for poor slab support. Also check

evidence for subgrade softening, from DCP, unbound material samples and/or a drainage

survey. If the computed life is greater than the current condition suggests, then something

has happened which the computation doesn’t take into account. This could be shrinkage

cracking during initial concrete curing. Also investigate the crack distribution. If it is

localised then this suggests other areas may have much longer life.

Faulting across Joints: This has a serious effect on ride quality. If dowels or tie-bars are

present, there should be no faulting. If faulting is present, this can only mean serious

corrosion of the bars and disintegration of the surrounding concrete. Even without

dowels or tie-bars, faulting implies poor load transfer, possibly due to excessive joint

spacing, poor durability aggregate, or a deformable foundation.

Surface Deterioration: Concrete relies on the presence of a balanced combination of

cement mortar and aggregate throughout. Excess cement mortar at the surface results in

a relatively weak surface layer. Once trafficking has removed this excess mortar, there

will be a decrease in ride quality. Another possibility is scaling, which means the loss of

discrete areas of surface. This is usually caused by frost action.

8. Prognosis

Statistical Treatment of Data

Since condition inevitably varies it is usual to work in terms of statistics, for example:

50 percentile (e.g. of FWD back-calculated stiffness, or of thickness) = average

15 percentile (i.e. 85% is better) = for design

Pavement Life Prediction

Here you usually need an analytical computation, typically taking 15 percentile stiffness

moduli. You must include consideration of the realistic long-term properties of

materials. Sometimes it is possible to consult a design guide, but most are not flexible

enough to cope with a deteriorated pavement.



65

Result = predicted lives to failure.

The next step is to take account of the fatigue damage that has taken place already.

Miner’s Law: This law states that relative damage is cumulative.

For example: predicted fatigue life = 30 106 axle loads

past traffic = 12 106 axle loads

therefore relative damage = 40% (18 106 axle loads remaining)

The concept of relative damage becomes important for strengthening designs since,

whatever the predicted life of the strengthened pavement, up to 40% of it may have to be

discounted straight away due to past damage.

For example: required future life = 30 106 axle loads

design traffic = 50 106 axle loads

(since 40% has to be discounted)

Rutting is a bit different. You have to look at what has happened in the past and use that

to calibrate your future prediction. For example, if you predict rutting due to excess

subgrade strain, but there is no sign of rutting in the past – then ignore your prediction!

Calculations should never be believed without question, especially when so many

assumptions are being made. This is especially true of concrete pavements, where

predictions can change dramatically with a small change in one of the input parameters.

The Effect of Debonding

Debonding between bound pavement layers significantly decreases the overall apparent

stiffness of the bound layers.

In the figure the debonded interface is assumed to transfer no shear stress at all, which is

an extreme case. However, it is common to find that the apparent stiffness of the bound

pavement layers is no more than half that expected when debonding is present. Note the

tensile strain at the bottom of the asphalt. It can be up to 60% higher than it would have

been in a non-debonded pavement.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1

Depth of interface/total thickness

Rela

tive s

train

/ s

tiff

ness

Strain at base of asphalt

Apparent stif fness

debonded interface

Zero Bond



66

MAINTENANCE & REHABILITATION

1. Localised Cracking

First option = just seal the cracks to stop water getting in.

Second option = patch repairs

Problem: it is impossible to get the patch

to be as good as the original pavement. It

is difficult to compact, especially in the

corners and it never bonds perfectly to

surrounding materials.

2. Surface Deterioration

Surface problems are:

Ravelling = pieces of stone becoming detached from the surface;

Bleeding = excess bitumen coming to the surface (of an asphalt surface course);

Polishing = stones losing their friction properties due to tyre action;

Loss of texture = surface high-points wearing away or being pushed down into an

asphalt.

Possible solutions:

Gritting = spreading fine stone (e.g. 3mm) over the surface – counters bleeding

Bush-hammering = abrade the surface stones – counters polishing

Jet-blasting = eroding bitumen-filler mortar – counters loss of texture

Grooving = cutting slots in the surface – also counters loss of texture

SURFACE DRESSING (see next page) – counters everything

Cracks

Debonded

interfaces

Break out to required depth

Tack Coat

Seal



67

Surface Dressing (also known as Chip Seal)

This is really a brand new thin surface layer (sometimes put straight onto a new road).

a) spray a layer of bitumen (usually in the form of emulsion) over the surface;

b) spread a single layer of aggregate particles, usually 10-15mm in size;

c) watch the traffic compacting it.

A ‘racked-in’ surface dressing includes a second application, of much smaller aggregate

size, designed to fill the gaps between larger particles.

Design:

Problems:

Can’t be done in cold, wet weather;

The surface isn’t particularly nice to drive on.

3. Reflective Cracking

This is the phenomenon of cracks appearing in a surface course directly over cracks in the

layer underneath. It happens for 2 reasons:

a) Thermal expansion/contraction opening and closing of joints/cracks;

b) Traffic loads causing high stress/strain over a joint/crack.

What can you do about it?

At least 180mm of asphalt overlay (UK Highways Agency);

A geogrid or strong geotextile, providing actual enhanced strength (A in next figure);

A standard geotextile, acting as a separation layer (B in figure);

A high-durability asphalt layer (C in figure);

An open-graded asphalt layer (D in figure);

Single surface dressing

Racked-in surface dressing

Surface

hardness

Skid

resistance

Surface

dressing type

Aggregate

selection and

spread rate

Season/

weather

Durability

requirements

Local

economics

Binder

selection and

spread rate



68

A granular interlayer (E in figure);

Crack and seat an underlying concrete layer (F in figure).

A B C D E F

4. More serious cracking or rutting

We are now thinking about: a) overlays;

b) inlays;

c) deeper reconstruction;

d) some combination.

Question: how

deep is the

problem?

Rutting might be

entirely due to

asphalt

deformation –

but then it

depends which

layer is

responsible.

Is existing

pavement visibly

cracked?

There is no

need for an

interlayer

Typical crack

spacing?

Traffic level?

Is foundation

water-sensitive?

No

C1: <0.5m Yes

High-durability

asphalt

Open-graded

asphalt

Granular

interlayer

Geogrid

Geotextile

C2: 0.5-2.0m

C3: >2.0m

T1: <5 106

T2: any

F1: no

F2: yes

C1-2, T2, F1

C1, T1, F2

C3, T2, F2

C2, T2, F1

C2-3, T2, F1

Rutting in original

pavement

A: mainly in surface course

B: mainly in binder course

0

10

20

0 10 20 Years

Rut depth (mm) Rehabilitation

40mm inlay; A

40mm overlay; B

40mm inlay; B

40mm overlay; A

Rutting in rehabilitated

pavement



69

Cracking also may not penetrate too far from the surface. In thick pavements it is

common to find cracks to 50-100mm depth only.

just replace the surface course? Or maybe the binder course as well?

Question: how do we deal with broken materials?

Generally, for any material that is going to be left in the

pavement we have to:

assign a long-term stiffness – for analytical

design;

estimate an equivalent thickness of new intact

material – if using design manuals.

We can also use geogrid reinforcement or geotextiles,

especially if the problem is likely to involve reflective

cracking.

If a PQC pavement is in reasonable condition

we can just bond an extra thickness onto the

surface – used in USA particularly.

Occasionally, usually on airfield pavements, it

makes sense to put an entire new PQC slab on

top of an existing cracked layer.

Question: what if the problem is in the subgrade?

Well, it may be OK to use an overlay to increase the thickness of the pavement. That will

reduce the stress on the subgrade.

However it may also be possible to improve its strength by adding/repairing drainage.

Overlay/Inlay

Intact asphalt

Failed asphalt

Damaged HBM

Sub-base

Subgrade

Treat as intact;

predict cracking of

combined layer

Downrate stiffness,

e.g. to 500MPa

Take realistic long-

term in-situ stiffness

Take realistic

stiffness

Take realistic

stiffness

Failed asphalt

Overlay/Inlay Geotextile

Failed asphalt

Overlay/Inlay

Damaged HBM

Predict cracking of

geogrid-reinforced

layer

Overlay/Inlay

Cracked Concrete

Predict cracking of

geogrid-reinforced

layer

Interlayer

HBM Sub-base

Capping

Subgrade

HBM Sub-base

Capping

Subgrade

Intact PQC

Thin-bonded overlay

Failed PQC

PQC overslab



70

5. Recycling

Most pavement materials can be recycled off site. For example, concrete can be crushed

to generate new aggregate. Recycled asphalt planings (RAP) can be added to new asphalt

– depending on quality and consistency of the source.

Problem: you can’t heat the RAP directly because it would give off toxic fumes.

Solution: superheat conventional aggregate; mix with RAP; heat is transferred.

Consequence: there is a limit to the proportion of RAP that can be used – 15-30% say.

Hot In-situ Recycling

Limitations:

Be careful with the heating – toxic fumes [flame, infra-red, superheated gases]

Impractical to heat more than say 50mm, usually less

Needs a reasonably consistent pavement with shallow damage only

Cold In-situ Recycling

a) Break everything up to a depth of up to 350mm;

b) Mix in a binder of some sort [emulsion, foamed bitumen, cement & water];

c) Compact;

d) Preferably leave exposed to the air for a few days;

e) Apply a new surface course.

Has the

upper subgrade

softened?

Don’t worry

about the

drainage

Has it

affected pavement

peformance?

Is the drainage

defective?

Is improvement

technically feasible?

Think about

drainage

improvement

No

No

No

No

Yes

Yes

Yes

Yes

Preheater Units Remixer Compactor

Mixing drum

Hopper for fresh

asphalt

Milling

unit

Screed



71

Limitations:

The ingredients will be relatively uncontrolled, including the water content.

Mixing will be imperfect, meaning there may be less binder at depth.

This means that properties will be poor relative to a plant-mixed material.

The surface will not be particularly smooth.

Consequences:

You generally need a plant-mixed, paver-laid surface course.

If emulsion or foamed bitumen are used, the resulting material will be like a high-

quality granular material; long-term stiffness may be around 1000MPa – but there

is a large uncertainty margin on this.

If cement and water are used then the material will be like a fairly weak (and

highly variable) HBM.

Recycler Compactor

Milling unit



72

PAVEMENT MANAGEMENT

Pavement management refers to the decision-making process as to what maintenance to

do and when – which depends on the data available from condition monitoring. So the

first question is: what type of monitoring should be carried out and how frequently?

1. Managing Pavement Monitoring

Network Level

A pavement authority needs to monitor performance regularly so as to plan budget

allocation.

Survey Type Information Usefulness

Traffic Count Trends; design traffic High

Axle weight Vehicle damage factors Low-medium

Visual Condition Roughness (approx)

Structural condition (very approx)

Skid resistance (possibly)

Very high

Profile Roughness

Structural condition (very approx) High

Deflectograph Structural condition (approx) Medium

SCRIM Skid resistance Medium

Traffic counts can be automatic, using piezo-electric strips buried in the road surface, or

manual. The advantage of automatic counts is that they can continue day and night for

long periods with little expenditure. The advantage of manual counts is that traffic can be

classified accurately into different vehicle types (cars, buses, light goods, heavy goods –

different axle configurations). Both are extremely useful in working out priorities.

Axle weight surveys can be carried out either by stopping and weighing wagons on a

fixed weighbridge or by installing a weigh-in-motion (WIM) device into the pavement.

Such surveys would normally only be carried out at state or country level, in order to

monitor trends in goods traffic development through the years, enabling wear factors to

be updated as necessary.

Visual survey data is essential. The information which results relates to ride quality,

structural condition and also safety-related features such as skid resistance. It can be

processed to give single numbers (e.g. Pavement Condition Index – PCI) that can be used

(approximately) to assign likely remaining life and future maintenance costs. Visual

survey data also allows minor maintenance to be programmed.



73

Profile surveys, Deflectograph measurements and SCRIM surveys will all give a more

accurate evaluation of individual aspects of condition, namely ride quality, structural

condition and skid resistance. However, the high productivity achievable from a profile

survey, together with the fact that structural condition tends to correlate very roughly

with profile, makes it the favourite for network-level monitoring. On major highways

they would be carried out reasonably frequently (e.g. annually or biannually).

Project-Level

A pavement management system (PMS) will include ‘triggers’ to indicate when

detailed evaluation at project level is needed, based on individual indicators (visual,

profile etc.) or a combination of them all. But when is the best time to do it?

It is sensible to carry out a detailed (project-level) evaluation well before the optimum

intervention level is reached – according to network-level survey data. For example, if

optimum intervention level is assumed to occur when the network-level condition

indicator falls to 60% of its original value, it would be worth programming a detailed

evaluation when the survey data indicates 70-75% because the real level may already be

at 60%!

Principle:

Decide on one or more network-level condition indicators.

Estimate what condition (according to the condition indicators) represents the

optimum time for major maintenance.

Set triggers for project-level surveys at condition indicator values significantly

higher than those for optimum major maintenance.

Aims of project-level suvey:

to establish the deterioration mechanism(s);

to provide parameters for analysis and rehabilitation design.

Condition

Condition for cost-effective

major maintenance

Condition according to coarse

network-level surveys

Range of possible

actual condition

Time range for optimised

major maintenance

Time

Initial condition

Projected performance

following major

maintenance



74

Possible tools to use:

Survey Type Information Frequency

Cores; trial pits;

laboratory tests

Layer thickness

Bound material stiffness modulus

Fatigue strength

Deformation resistance

Foundation strength

Occasional

Radar (GPR) Layer thickness

High moisture locations

Continuous

Detailed Visual

Survey

Crack density

Rutting locations/severity

Continuous

Falling Weight

Deflectometer

Layer stiffness modulus

Joint condition

@10-100m

Drainage survey Drainage efficiency Targeted

Basically, you have to decide what you need to know and choose accordingly. Project-

level surveys need ‘designing’ rather than following a set procedure. Cores or trial pits

are almost essential for every investigation, together with a detailed visual inspection of

the pavement. Other surveys depend on situation. If analytical design procedures are to

be used then the FWD (or rolling wheel deflectometer) is a key tool, supported by

appropriate laboratory tests and by use of the DCP in core holes or trial pits. Radar is

less essential and its use should depend on the likely variability in construction. Drainage

surveys are important where water-related problems are suspected.

2. Managing Maintenance

Practical Constraints

Multi-Lane Highways: Clearly one cannot raise (or lower) the surface level of one lane

without doing the same to all other lanes on a carriageway. This means that a simple

overlay solution may not really be as cost-effective as it looks.

Highway Structures: Clearance to overbridges places an upper limit on raising levels.

The carrying capacity of underbridges also places a limit; even a thin overlay adds a

significant extra load.

Kerbs and Barriers: Both kerbs and barriers can be raised – but at a cost. In both cases

there is usually a degree of flexibility, allowing small increases in pavement level, and it

is therefore important to know just how much flexibility is available in a particular case.

Airport Runways: It is generally only the central 20m width which is damaged by

aircraft and it is often permissible to ‘ramp down’ from a relatively thick overlay over the

central part of the runway to little or zero thickness at the edge.



75

Project-Level Optimisation

This is not always easy in practice. The problem is that budget constraints may mean that

the theoretical minimum ‘whole-life cost’ solution may not be affordable in the short

term! There therefore has to be some interaction between project-level design and overall

network-level management. In practice, the usual simplification is for the authority to tell

the engineer what design lives to use (e.g. 20-year; 40-year).

Usually therefore the designer will be asked to come up with the best solution to

strengthen the road to take another x years of traffic. In this case, it is simply a matter of

costing up all the practical alternatives, taking into account any add-on costs due to

practical constraints. Indirect costs (delays to road users, safety-related issues) may also

be considered.

One key question is: what should x be?

This is not an easy question to answer. There has been a trend to increase design life over

the years so that 40 years is now common, and the key parameter is the so-called

discount rate. The idea is that a certain sum of money today is assumed to increase in

purchasing power with time due to continuing economic growth (!!). The corollary is that

future costs can therefore be ‘discounted’ at a certain annual rate. A debate rages as to the

correctness of this assumption – and discount rates range from zero to about 10%. One

further issue is that no-one really knows how long the need for any particular pavement

will last. So, just accounting for this uncertainty, some sort of discount is probably

justified.

Network-Level Optimisation

Network-level pavement management forces non-pavement costs to be taken seriously,

including costs which are not strictly financial:

User costs (speed restrictions, vehicle wear and tear, fuel consumption, delays);

Accident costs;

Environmental costs (air pollution, energy usage).

The most common method of dealing with user costs is to link them to International

Roughness Index (IRI). The following are examples of equations used for this.

Vehicle operating cost (VOC) = VOClow IRI (1+0.06[IRI–3]) [IRI > 3]

Time cost (TC) = TClow IRI (1+0.03IRI)

There is nothing fundamental about these two equations. The economic equation is so

different in different countries that calibration is always required.

Accident costs can be linked to skid resistance coefficient () and to pavement

condition, but only very approximately. Example equations are:

Skidding accident cost Traffic flow 10(1-1.8)

Accident cost due to pavement Traffic flow (rut depth – 10mm)



76

A good management system will include predictive equations for future deterioration –

based on experience, possibly also on the history of each length of road.

It is then possible to run ‘what if?’ scenarios. For example, what if a particular length of

road was strengthened? Or just surface-dressed? Then the up-front cost of the treatment

can be balanced against the reduction in ongoing and future cost.

Does the perfect Pavement Management System exist? NO; it’s just too complex and

there are too many different factors to take account of. In the end, network-level

management is often simply a matter of avoiding foolish mistakes!

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