pavement engineering notes 2012

UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING

Pavement Engineering - Module H23P01 Course Notes

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 blocksSoil: unpredictable; water susceptible; sometimes low strengthGranular Material: more predictable; less water susceptible; strongerHydraulically-Bound Material: bound with cement or something similarAsphalt: stones stuck together with bitumen; good quality material

A Typical Pavement Structure

1

Surface course (or Wearing course) – AsphaltBinder 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



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:

2

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100Sieve size (mm)

Per

cen

tag

e p

assi

ng

Upper Limit

Lower Limit

Sample

Shake



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.

3

Dry Density

Water Content

Heavy Compaction (e.g. on site)

Light Compaction (e.g. in laboratory)

0% air voids (i.e. full saturation)

OptimumMaximum suctionDry

Desired in-service condition after drying

out

Saturated

Maximum Dry Density (heavy compaction)

Maximum Dry Density (light compaction)



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 layersPaver = for high quality base layers (consistent thickness and surface level)Compactors:

Vibratory Pneumatic Static

4

Delivery Truck

Delivery Truck

Dozer Motor grader

Paver Compactor

Compactor



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.

5

Stabiliser Compactor

Mixing unit



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.

6

Water Content

Cement ContentRange 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



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.3. Hot-Mix Asphalt (HMA)

7

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



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.

Pave while the mixture is still hot, e.g. 110- 130

8

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



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.

MATERIALS

9

60

70

80

90

100

110

120

0 20 40 60 80

Depth (mm)

Tem

per

atu

re (

°C)

@ 2 minutes

@ 4 minutes20m layer

40m layer

80m layer



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 gravelsCrushed rock aggregateBitumenPortland cementReinforcing steel

5-1020-25

3200-38004500-5000

23000-27000

0.1-0.20.4-0.5

5-812-15

Mixtures Hot-mix asphaltCold-mix asphaltLean concretePavement quality concrete (PQC)Reinforced concrete

600-800150-200450-500750-10001100-1500

12-163-49-1015-2022-30

25-4015-3015-2535-5040-55

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.

2. Unbound Material

10

Initial State After Strain

BB

AA AA

BBF

N



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 .What properties affect shear strength?

11

Initial State After Strain

1

1

22

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)



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.

StiffnessWith 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.

California Bearing Ratio (CBR)

12

Shear Stress

Shear StrainCycle no: 1 2 3 10 100 1000 10000

Ultimate stress (= shear strength)

Applied StressHysteresis loop

– represents energy loss

Approximate shear modulus

Force FDisplacement d

(50mm/min)

152mm diameter 125mm height

Force (kN)

Displacement1.27m

m2.54m

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

Normal stress

Shear stress

Angle of internal friction

Apparent cohesion c

(due to stone interlock)



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

Very, very, very approximate relationships with stiffness: a) E = 10 × CBRb) E = 17.6 × CBR0.64

Confined CompressionA 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.Dynamic Cone Penetrometer (DCP)

13Depth

Number of blows

Depth

CBR (%)

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

Drop weight

ScaleAnvil

Core hole

Granular

Soil

Penetration rate p

(mm/blow)

Load applied via full-face platen

Springs

Locking nuts fix side plates in place

Side plates free to move

Adjustment control for start conditionsSpringbox PUMA (Precision Unbound

Material Analyzer)

Load applied via full-face platenEight wall

segments

Calibrated steel and rubber band



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

Plate TestsNowadays 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.

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

Material Stiffness Modulus (MPa)Triaxial DPT In the Pavement

14

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 EPoisson’s ratio



(confining stress 20kPa; deviator stress 0-100kPa)

(100kPa contact pressure)

(K-Mould, PUMA and Springbox generally give similar results)

Very soft clay soil 10 5 15Firm clay soil 50 30 80

Sandy soil 75 30 50Gravel capping 125 50 80

Sub-base 250 75 150Granular Base 500 100 250

PermeabilityUsually 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

15

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



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]

StrengthStrength can be measured in several ways. Flexural is most realistic for a pavement. Compressive (cube or cylinder) is the most convenient.

Tensile Strength

16

or

Tensile

Indirect Tensile

Compressive

Flexural



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 StrengthThis 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 StrengthThis 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.

17

Max Bending Moment (M) = P L / 6

½P ½P

½P½P

L/3 L/3L/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 / E3. 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



Conclusion: Compressive strength is not a fundamental measure. It should never be relied upon in pavement design.

Strength Gain with TimeAll HBMs become stronger with time. We usually design on a 28-day strength but often use 3-day or 7-day for quality control.

FatigueAll 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.

DurabilityThe 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.

18

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

ress

ive

Str

eng

th (

MP

a)

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

xura

l S

tres

s:S

tren

gth

Rat

io

9.2MPa Compressive strength

32.9MPa Compressive strength

25.9MPa Compressive strength; 0.5% steel fibres

/sf = 1.064-0.064log(N)



Thermal PropertiesHBMs 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

StiffnessHBMs 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 MPaStrong cement-bound base [10-20MPa] 15000-25000 MPaWeak cement-bound base [5-10MPa] 5000-15000 MPaSlag etc. bound base [5-10MPa] 3000-10000 MPaHydraulically-bound sub-base [2-5MPa] 2000-5000 MPaStabilised soil [< 2MPa] 100-300 MPa

The Stiffness of a Discontinuous LayerA 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.

19

1. Curvature = M/EI = 12M/Eh3

2. Strain at top = (h/4)/d ≈ /3 Stress at top = E/3 ( 0 at mid depth) Moment = (3/256)h2 = h2E/256 Curvature = /L = 256M/(h2EL)

3. Moment = hL/2 = ghL/2 Radius of curvature = (L2/4)/2 = L2/8 Curvature = 8/L2 = 16M/ghL3

Combine:Curvature = M[12/Eh3+256/(h2EL)+16/ghL3]Curvature also = M/EeffI = M[12/Eeffh3]

Eeff = 1/[1/E+64h/3EL+4h2/(3gL3)]

Stress distribution

I = h3/12 per m width

Slip stiffness g = /

d ≈ 0.75h

Uneven stress zone

LL L

Lh

h

M

M

Lh

1.

2.

3.

E



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.

Real bitumen behaviour

20

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 bitumenMetal 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



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.

AgeingBitumen 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 ModificationBitumen 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.Bitumen-Filler Mortar

21

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 100Time (years)

Pen

etra

tio

n (

dm

m)

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



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 AdhesionThis 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 ModulusAsphalt does not have a single modulus value; it is both temperature and loading rate dependent because bitumen is temperature and loading rate dependent.

Predicting Asphalt Stiffness Modulus

22

0

1

2

3

4

5

6

7

8

9

10

0.1 1 10

Strain at Failure (%)

Str

ess

at F

ailu

re (

MP

a)

Pure bitumen+5% filler+15% filler+35% filler+50% filler+65% filler

Cold and/or rapid loading

Warm and/or slow loading



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 = 7270MPaMeasuring Asphalt StiffnessHere are three possible tests:

23

Tension-Compression

F

F

E = 4FL/d2

d L



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 ValuesMaterial 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 7000Dense asphalt base (100 pen binder) 3500 5000Surfacing 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)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

24

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[(23L2/4h2)+1+]/hb

F/2

F/2F/2



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 ResistanceThe 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.Permanent DeformationRutting is always a danger. To avoid it you need:

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

25

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

portio

n o

f In

itia

l Stiffnes

s

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

10-6

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



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 DeformationThe 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!

DurabilityAs 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

Mixture Design

Aggregate Particle Size DistributionThe choice depends on where the material is going in the pavement.

26

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

Axi

al S

trai

n (

%)

Mix A - DryMix A - Moisture conditionedMix B - DryMix B - Moisture conditionedMix C - DryMix C - Moisture conditioned

Wheel tracking

Repeated load axial (RLA)

Vacuum repeated load axial

Water bath

Simple Soaking Procedure



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.

Binder ContentDon’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.

27

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Sieve size (mm)

Per

cen

tag

e p

assi

ng

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

Critical particle size (Hot Rolled Asphalt)



The Marshall Mix Design MethodA 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 MethodThis 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.

28

F

(50mm

/min)

Stability Density

Flow

Binder content Binder content

Binder content

maximum

minimum

optimumoptimumMarshall Test

Marshall Hammer

F

Stability

Flow

60C

10kg

100mm

0.45m

50mm

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)



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 fatigueHigh temperature danger of ruttingDesirable working range of binder viscosity ≈ 5 103 to 107 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.

FillerFiller 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.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,

29



- 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 EmulsionFoamed Bitumen

Bitumen EmulsionBitumen 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).

30

The EmulsifierEmulsifiers 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)

+

_

Bitumen droplet

Charged emulsifier molecules

Water + free emulsifier



Typical proportions: 40-70% bitumen.Big advantage of emulsions: they can last for months (with the occasional stir).

Foamed BitumenBitumen 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 WorksFor 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.

In detail:

31

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)



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.

AdditivesBecause 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.

VolumetricsWe 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).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.

32

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



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 = 6090MPaScold-mix low binder = 5350MPaScold-mix std binder = 1790MPa

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

CuringAn 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);

33

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; standard binder content

Cold mix; open grading; standard binder content

Aggregate FillerBinderAir



- 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 MixSummary of key points:

a) The binder contentbinder content will probably be lowerlower than for a hot mix.b) The void contentvoid content is likely to be higherhigher 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 poorer materialpoorer material!d) Water needs to evaporate before sealing the surface. This means that the

aggregate gradinggrading used should be reasonably openopen.e) It is important to limit early traffickinglimit early trafficking.f) All of these points mean that there is a significant risksignificant risk 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.

34



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.

35

Typical highway traffic – 1 hour:Wt band

0-1T1-2T2-3T3-4T4-5T5-6T6-7T7-8T8-9T

9-10T10-11T11-12T12-13T13-14T14-15T

Average0.5T1.5T2.5T3.5T4.5T5.5T6.5T7.5T8.5T9.5T

10.5T11.5T12.5T13.5T14.5T

Number3875120156432025418926541236517617410

0 or 10

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 2 4 6 8 10 12 14Exponent 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



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 rigid3 axle rigid

3 axle articulated4 axle rigid

4 axle articulated5 axle articulated6 axle articulated

-1.160.391.750.842.021.78

0.401.260.652.801.002.501.69

0.402.301.703.001.702.903.70

0.603.402.504.602.504.405.60

1.162.321.792.852.713.703.94

1.462.391.633.122.263.943.03

0.681.290.682.121.102.651.48

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

Contact PressureThe 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.

36

Example

Fully flexible design (asphalt/

foundation)Composite design –

HBM lower base

Composite design – asphalt upper base, binder course and

surface course



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 foundation2. Granular subbase 100MPa at top of foundation3. Weak HBM subbase 200MPa at top of foundation4. 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 pavementCRCR = continuously reinforced concrete roadbase (i.e. needs an asphalt surface)fr = concrete flexural strengthR = reliability

37



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 PrinciplesThe 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 subgradeRecognising the difficulties involved in soil parameter estimation, most of the current analytical design methods use the so-called the subgrade strain criterion

38



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.

39

30

40

50

60

70

80

90

100

1 100 10000 1000000


Ap

plie

d s

tres

s/fa

ilure

str

ess

(%)

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

exc

lud

ing

th

e fi

rst

cycl

e (%

)

99%

54%

88%

83%

71%

52%73%

ClaySand

90%

90%: ratio of applied stress to failure stress

Subbase

Hot-Mix Asphalt

Subgrade



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 layersIt 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 LaboratoryNf [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:

40

Subbase

Hot-Mix Asphalt

Subgrade

Buses 562-axle trucks 5623-axle trucks 4015 or 6-axle articulated trucks (with semi-trailer) 268Equivalent standard wheel loads/day 263320-year design traffic 2 107

1 10 102 103 104 105 106 107 108

20001000800200100502010

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



Design TemperatureThis 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: 2500MPaAsphalt concrete, 125 pen (= DBM125): 3100MPaAsphalt concrete, 50 pen (= DBM50/HDM50): 5000MPaAsphalt concrete, 15 pen (= DBM15): 7800MPaWeak HBM: 500MPaCrushed rock base: 250-500MPaCrushed rock subbase: 150MPaCapping: 75MPa

Poisson’s ratio - asphalt and unbound material: 0.35HBM 0.2

Issues in Real Design

Thin asphalt layersThe 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 thatthough 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.

41

0

100

200

300

400

500

600

700

800

0 50 100 150 200 250

Asphalt Thickness (mm)

Ten

sile

Str

ain

(m

icro

stra

in)

Computed

For practical design

10000

100000

1000000

0 50 100 150 200Asphalt Thickness (mm)

Lif

e (

nu

mb

er

of

pa

ss

es

)

Modelling propagationFor practical designComputed



Load GroupsYou 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 EffectsWheel 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

42

02468

101214161820

1 1.2 1.4 1.6Ratio: maximun dynamic/mean

Mu

ltip

lier

on

dam

age

Exponent = 4

Exponent = 8

Exponent = 12

Exponent = 4;repeatable load pattern

Maximum dynamicMeanLoad

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

P2P1

v

r

Centre of gravity



Vehicle Speed EffectsVehicle 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 WanderThis 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 AsphaltThe 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.

43

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.



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 StressAll 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.

44

h

L

Temperature << temperature at time of set

Angle of friction

HBM layerCrack



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 LoadingThis 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 CrackingEven 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.

45

Recently paved HBM

Form slots to 2/3 depth

Fill slots with bitumen emulsion

Roll HBM

Subbase

Hot-Mix Asphalt

Subgrade

HBM



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

Westergaard AnalysisSince 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.

46

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 – (Ö2 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’ = Ö(1.6 a2 + h2) – 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



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 AnalysisThe 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 AnalysisA 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).

47



The limit state approach opens the door to solving problems of multiple loads and complex joint geometry that would otherwise be impossible.

Warping StressesThermally-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.

48

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 × (Ö2d/2–0.5×Ö2)/(Ö2d/2) = 4P(1–1/d)

Energy dissipated at cracks = 4Ö2dM × (/(Ö2d/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/h2

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



Joint SpacingThe 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 ReinforcedReinforcement 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 ReinforcedBut 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.

49



SURFACE PROPERTIES

1. Ride Quality

Excellent ride quality is not always needed – but it is on high-speed roads.Typical tolerance limits: 3mm in 3mSo how can this be achieved?

Pavement Quality ConcreteNo 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]

AsphaltTo 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 6mmBinder course 6mmBase 15mmSubbase + 10mm – 30mm

Impact of different types of surface:

Surface Type Vibration generation

Energy absorption

Ride quality ranking

Asphalt concrete medium medium 3Hot rolled asphalt + chippings medium low 4Stone mastic asphalt low high 2Porous asphalt low very high 1Surface dressing high low 5=Concrete (PQC) fairly high very low 5=Block paving very high low 7

2. Material Strength, Durability etc

50



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

Asphalt SurfacesAsphalt 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 mediumHot rolled asphalt (+ chippings) medium low highStone mastic asphalt medium-low high medium-highPorous asphalt low medium-high medium-lowBASE HIGH HIGH MEDIUM

Block PavingAlthough 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

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

51



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.

MacrotextureIf 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.

52

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

Water movement away from contact points

Accelerated Polishing Machine

Rubber-tyred wheel

Polishing Test Specimen

Surface aggregate

Rubber pad

Pendulum Test



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.

5. Noise

53



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.

54

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

dB

(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



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 pavementsa) 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 Pavementsa) 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.

Rutting

55



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 SurveysLaser-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.3. Skid Resistance Surveys

56

0

1

2

3

4

5

6

7

0 1 2 3 4Distance (km)

IRI

(m/k

m)

IRI< 2 m/km excellent2-3 m/km satisfactory3-4 m/km moderately bumpy4-5 m/km bumpy> 5 m/km very bumpy

Suspension movement+ve

–ve Typical Data

Surface course problem Binder course - base problem

Foundation problem



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

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

57

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



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 InformationCores 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 TestsThe 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 TestsThese 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).

5. Ground Penetrating Radar (GPR)

58

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

P

P

Peak shear force

Shear slip at failure

P

Leutner Test

Torque Test



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 BeamThis is the oldest and simplest form of deflection test device and it is successfully used throughout the world.

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

59

x

yDeflection 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

ArmPivot Dial Gauge

6350kg axle load6350kg axle load

Twin tyresTwin tyres



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 DeflectographThe 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.

60Deflection(microns)

Offset (m) 012

DeflectionBowl

FWD – Longitudinal Section

Deflection sensorsFalling WeightRubber buffersLoading Plate

Known layer thicknesses

Layer stiffnesses

Measuredload Calculate

deflections using multi-layer linear

elastic analysis

Adjust layer stiffnesses

Good match to measured deflections?

Finish

no

yes

Load: 10-200kNDuration:

25-50msecsPlaten radius:

150mm

Tow bar

Distance travelled, x

6350kg axle loadTwin tyresWinching

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]



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.

How many layers can be analysed? Two: no problemThree: should be OKFour: be careful; don’t just believe the result

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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 foundationLoad



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 DeflectometersThe 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.

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

62

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



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. 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

63



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 DataSince 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) = average15 percentile (i.e. 85% is better) = for design

Pavement Life PredictionHere 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. 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.

64



For example: predicted fatigue life = 30 106 axle loadspast traffic = 12 106 axle loadstherefore 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 DebondingDebonding 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.

65

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 1Depth of interface/total thickness

Rel

ativ

e st

rain

/ s

tiff

nes

s

Strain at base of asphalt

Apparent stif fness

debonded interface

Zero Bond



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 bleedingBush-hammering = abrade the surface stones – counters polishingJet-blasting = eroding bitumen-filler mortar – counters loss of textureGrooving = cutting slots in the surface – also counters loss of textureSURFACE DRESSING (see next page) – counters everything

66

Cracks

Debonded interfaces

Break out to required depth

Tack Coat

Seal



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);

67

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



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.

68

Is existing pavement visibly

cracked?

There is no need for an interlayer

Typical crack spacing?

Traffic level?

Is foundation water-sensitive?

No

C1: <0.5mYes

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 courseB: 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



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.

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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/InlayGeotextile

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



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 Recyclinga) 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.

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Has theupper subgrade

softened?

Don’t worry about the drainage

Has itaffected 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



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.

71

Recycler Compactor

Milling unit



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 LevelA pavement authority needs to monitor performance regularly so as to plan budget allocation.

Survey Type Information UsefulnessTraffic Count Trends; design traffic HighAxle weight Vehicle damage factors Low-mediumVisual Condition Roughness (approx)

Structural condition (very approx)Skid resistance (possibly)

Very high

Profile RoughnessStructural condition (very approx)

High

Deflectograph Structural condition (approx) MediumSCRIM 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.

Profile surveys, Deflectograph measurements and SCRIM surveys will all give a more accurate evaluation of individual aspects of condition, namely ride quality, structural

72



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-LevelA 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.

Possible tools to use:

73

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



Survey Type Information FrequencyCores; trial pits; laboratory tests

Layer thicknessBound material stiffness modulusFatigue strengthDeformation resistanceFoundation strength

Occasional

Radar (GPR) Layer thicknessHigh moisture locations

Continuous

Detailed Visual Survey

Crack densityRutting locations/severity

Continuous

Falling Weight Deflectometer

Layer stiffness modulusJoint 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.

Project-Level Optimisation

74



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 OptimisationNetwork-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)

75



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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pavement engineering notes 2012

Documents

dynamic cone

nottingham

clear critical

current condition

increases

loading rate

ball softening

critical particle