UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING Pavement Engineering - Module H23P01 Course Notes 1 PAVEMENT ENGINEERING INTRODUCTION A pavement is a structure designed to allow trafficking, usually of wheeled vehicles. Most pavements are roads, but airfields, industrial hardstandings, cycle tracks etc. are all included. Key points: a) Pavements are high-volume constructions; the materials used must therefore be cheap and environmentally acceptable. b) There is no exact definition of failure; they simply have to remain ‘serviceable’. c) The definition of serviceability will vary from application to application. d) Maintaining serviceability is an important part of pavement engineering. The basic building blocks Soil: unpredictable; water susceptible; sometimes low strength Granular Material: more predictable; less water susceptible; stronger Hydraulically-Bound Material: bound with cement or something similar Asphalt: stones stuck together with bitumen; good quality material A Typical Pavement Structure Surface course (or Wearing course) – Asphalt Binder course (or Basecourse) – Asphalt Base – Asphalt, Hydraulically-bound (e.g. Pavement Quality Concrete), or Granular (often in more than one layer) Sub-base – Hydraulically-bound or Granular Capping (or Lower Sub-base) – Hydraulically- bound or Granular (only used over poor subgrade; often in more than one layer) Subgrade (or Substrate) – Soil
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
1
PAVEMENT ENGINEERING
INTRODUCTION
A pavement is a structure designed to allow trafficking, usually of wheeled vehicles.
Most pavements are roads, but airfields, industrial hardstandings, cycle tracks etc. are all
included.
Key points:
a) Pavements are high-volume constructions; the materials used must therefore be
cheap and environmentally acceptable.
b) There is no exact definition of failure; they simply have to remain ‘serviceable’.
c) The definition of serviceability will vary from application to application.
d) Maintaining serviceability is an important part of pavement engineering.
The basic building blocks
Soil: unpredictable; water susceptible; sometimes low strength
Granular Material: more predictable; less water susceptible; stronger
Hydraulically-Bound Material: bound with cement or something similar
Asphalt: stones stuck together with bitumen; good quality material
A Typical Pavement Structure
Surface course (or Wearing course) – Asphalt
Binder course (or Basecourse) – Asphalt
Base – Asphalt, Hydraulically-bound (e.g. Pavement
Quality Concrete), or Granular (often in more
than one layer)
Sub-base – Hydraulically-bound or Granular
Capping (or Lower Sub-base) – Hydraulically-
bound or Granular (only used over poor
subgrade; often in more than one layer)
Subgrade (or Substrate) – Soil
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
2
CONSTRUCTION
A competent pavement designer must understand the practicalities of material production
and pavement construction if sensible decisions are to be taken.
1. Unbound Material
Natural Soils
Check it to see that it is as strong as it was expected to be (CBR test – see later).
Protect it. It’s easy to turn a basically sound material into a muddy soup! So
leave a thin layer of overlying material until the very last moment.
Granular Materials
Make sure you have material that meets the specification.
a) Particle Size Distribution: achieved by crushing larger rocks and/or by
blending materials from more than one source. Put a sample through a set of
sieves to check it.
e.g. typical sub-
base limits:
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100
Sieve size (mm)
Perc
en
tag
e p
assin
g
Upper Limit
Lower Limit
Sample
Shake
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
3
b) Particle Soundness: Use a test such
as the Los Angeles Abrasion test;
sometimes also tests for frost damage
and chemical weathering (MnSO4
soundness).
c) Particle Shape: This isn’t always
specified. Most common requirement
= % crushed faces (trying to make
sure that rounded gravel isn’t used);
sometimes also by limits on flakiness
(% of particles of a given size able to
pass through a special thin sieve
opening) and elongation (% of
particles with one dimension over 1.8
times the nominal size) of particles.
The shape depends on the equipment used to carry out the crushing:
Cone Jaw
Roll Impact
d) Water Content: All unbound materials are sensitive to water.
Dry Density
Water Content
Heavy
Compaction
(e.g. on site)
Light
Compaction (e.g.
in laboratory)
0% air voids (i.e.
full saturation)
Optimum Maximum suction Dry
Desired in-service
condition after drying
out
Saturated
Maximum Dry Density
(heavy compaction)
Maximum Dry Density
(light compaction)
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
4
Low water content: negative pore pressure (or suction). Makes compaction
difficult; BUT good once in the road.
Higher water content: positive pore pressure. Makes compaction easier;
BUT bad once in the road.
So: compact at Optimum Water Content (OMC); let it dry out to develop
suction.
Transport it to site in a suitable delivery truck
Place it and compact it properly
Dozer + grader = good enough for lower layers
Paver = for high quality base layers (consistent thickness and surface level)
Compactors:
Vibratory Pneumatic Static
Delivery Truck
Delivery Truck
Dozer Motor grader
Paver Compactor
Compactor
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
5
2. Hydraulically-Bound Material (HBM)
HBM means any material that needs water to activate a binder – usually cement.
In-situ Stabilised
This is usually just soil improvement. Converts a soft soil into something you can build
a road on. As well as cement, lime and/or fly ash (also called pulverised fuel ash – PFA)
are used.
Plant-mixed HBM base/sub-base
Get the right aggregate. You need a good durable rock, either river gravel or
quarried and crushed. Particle size distribution = similar to granular.
Water Content:
Problem: you must have the right amount for compaction (OMC – similar to
granular materials) BUT you must also have just the right amount for the
hydraulic reaction to take place.
Too little water not all the binder will be activated reduced strength.
Too much water free water after the reaction has finished air voids left after
evaporation reduced strength again.
[This restricts the practical combinations of particle size distribution and strength]
Batch it and mix it (or mix it
continuously)
This is a twin-shaft batch
mixer.
The alternative is a drum
mixer, allowing HBM to
pass through continuously.
This gives higher
productivity.
Stabiliser Compactor
Mixing unit
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
6
Transport it to site – usually in a normal delivery truck.
Place it and compact it. Basically the same as for granular material, except that
you would usually put it through a paver to get good level and thickness control.
Cure it – usually by spraying a bitumen seal to stop the water evaporating.
Pavement Quality Concrete (PQC)
Get the mix design right. This is a real concrete, which means it will be too wet
for roller compaction (usually); it will need vibrating.
Batch it and mix it.
Transport it to site – usually in a purpose-
built concrete truck.
Pave it. Wet concrete needs to be enclosed
by formwork of one sort or another.
Options: Fixed form; checker board
pattern – slow process.
Fixed form; continuous side
rails – much quicker.
Slip form; with a purpose-
built slip-form paver –
quicker still; most commonly
used nowadays.
Water Content
Cement Content Range required to
achieve design strength
Range for
wet-form
workability
Range required to
allow chemical
reaction to take
place
Range of mixture
design options for wet-
forming
Range for
roller-
compaction
workability
Range of mixture
design options for
roller-compaction
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
7
Get the surface texture right
Form joints (usually)
Cure it, usually by means of a colourless aluminium-based sealant but can also
use wet cloth or regular water spray application.
Cure it – usually by spraying a colourless seal to stop the water evaporating.
Grooving:
Brushed finish:
Exposed
aggregate:
Burlap drag:
A set of steel tines is dragged across the surface of
the fresh concrete immediately after paving. [can
also be achieved by sawing hardened concrete]
The fresh concrete surface is brushed using an
appropriately heavy-duty brush to form a ridged
finish.
A retarder is sprayed onto the finished concrete and
loose mortar is brushed away from around the larger
aggregate pieces about 12 hours afterwards.
A sheet of rough fabric is dragged over the surface
of the wet concrete, leaving a rough finish.
Joint Types Joint Forming
Expansion Joint
Contraction Joint
Warping Joint
Bottom crack inducer
Joint former strip
Saw Cut (at early age)
Dowel bar
(smooth)
Filler Board
Joint Seal
Slip Coating
Expansion Cap
Dowel bar (smooth)
Crack Slip Coating
Joint Seal
Tie bar (ribbed)
Crack
Joint Seal
Sealant
groove cut
once joint
has been
formed
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
8
3. Hot-Mix Asphalt (HMA)
Heat the bitumen to 140 to 180C; keep it in a hot storage tank.
Dry the aggregate thoroughly in a drum dryer.
Sieve the hot dried aggregate into size fractions;
store in hot bins.
Mix bitumen + aggregate (about 30 secs)
asphalt.
Batch Mix Plant:
Drum Mix Plant:
Drum mixers give higher productivity. They rely on accurate proportioning of
moist aggregate since the bitumen is fed directly into the drying chamber, which
takes the form of an inclined rotating drum. The drying chamber doubles as the
mixing chamber and the hot mixture is fed out continuously from the drum to a
hot storage hopper before being dropped into the back of a waiting truck.
Transport to site in a thermally insulated truck.
Can combine in a
drum mix plant
Aggregate
feed
Dryer
Elevator
Hot bins
Mixer
Bitumen
tank
Combined Dryer-
Mixer Drum
Hot storage
hopper
Cold bins
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
9
Pave while the mixture is still hot,
e.g. 110- 130
Compact before it cools down too
much; either pneumatic or
vibratory for main compaction,
dead weight steel drum for the
final finish
How quickly does the mat cool? For example, assuming a 110C paving temperature:
The practicalities of compaction mean that layer thicknesses tend to be between 25mm
and 120mm.
60
70
80
90
100
110
120
0 20 40 60 80
Depth (mm)
Tem
pera
ture
(°C
)
@ 2 minutes
@ 4 minutes
20m
layer
40m
layer
80m
layer
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
10
MATERIALS
1. Sustainability and Cost
Pavements have to be cheap – that is an absolute requirement. However, we also have to
try to limit environmental costs. The key concept is embedded (or embodied) energy,
which is the total energy used to manufacture, transport, process etc. every component of
the pavement.
Approximate costs and embedded energies for pavement component materials:
Material Embedded
Energy
(MJ/Tonne)
Cost
(£/Tonne)
Embedded
Energy
Direct
Ingredients Sands and gravels
Crushed rock aggregate
Bitumen
Portland cement
Reinforcing steel
5-10
20-25
3200-3800
4500-5000
23000-27000
0.1-0.2
0.4-0.5
5-8
12-15
Mixtures Hot-mix asphalt
Cold-mix asphalt
Lean concrete
Pavement quality concrete (PQC)
Reinforced concrete
600-800
150-200
450-500
750-1000
1100-1500
12-16
3-4
9-10
15-20
22-30
25-40
15-30
15-25
35-50
40-55
Transport All materials (per journey km) 12-20
The question is: what is a MJ worth? In terms of fuel cost it is only about £0.01-0.02! It
also represents about 65g of CO2, and this might be valued anywhere from £0.002 to
£0.02. To be environmentally conservative, the embedded energy costs in the table are
based on an equivalence of £0.02 per MJ.
So: although the table may be exaggerating, hot-mix asphalt and concrete carry a
significant embedded environmental cost.
BUT what about the issue of traffic, responsible for about 36% of all energy consumed
in the UK? A Nottingham research project found that energy losses attributable to road
stiffness were around 100MJ/m2 for a heavily trafficked concrete pavement over a 40-
year life. For asphalt this went up to around 250MJ/m2. These translate to about 300 and
1000MJ/Tonne assuming normal pavement thicknesses – which as you can see are quite
significant numbers. And this does not include energy loss due to surface roughness,
which is likely to be a much bigger factor and definitely needs researching!
Conclusion: we really should take the environmental cost of road pavements seriously.
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
11
2. Unbound Material
What’s really going on?
Individual stones have to translate and rotate.
stones slide against one another.
this is resisted by friction (typically around 30-35 for crushed rock, less for gravels);
stone shape is also obviously important.
Shear strength
Can be measured in a shear box
Can also be measured in a
triaxal apparatus
sin = ½(1–2) / ½(1+2)
Don’t confuse stone-stone friction angle with .
Initial State After Strain
BBB
AAA AAA
BBB F
N
Initial State After Strain
1
1
2 2
2 increasing 1
Normal stress
Shear
stress Angle of internal
friction
(typically 55 for
a crushed rock
limiting / ratio
(typically 10 for a
crushed rock)
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
12
What properties affect shear strength?
particle shape; angular is good
stone-stone friction; not much effect
stiffness modulus of the rock; not much effect
particle size; big is good
particle size distribution; broadly graded is good
particle packing; dense is good
water content; high suction is good (see p3) – increases effective stresses.
Stiffness
With an unbound material you can’t really
talk about a Young’s Modulus because the
behaviour is so non-linear and stress-
dependent. On the other hand it is
convenient to pretend that it has a Young’s
Modulus, so instead we call it Resilient
Modulus or Stiffness Modulus and we have
to remember that its value changes
depending on the level of applied stress.
Typical values:
Solid rock is approximately linear elastic with a stiffness modulus of 100 and 200GPa;
unbound materials typically have a modulus in the range 20-250MPa.
What properties affect stiffness?
particle shape; not much effect
stone-stone friction; high friction is good
stiffness modulus of the rock; stiff is good
particle size; big is good
particle size distribution; not much effect
particle packing; not much effect
water content; high suction is good (see p3) – increases effective stresses.
Shear Stress
Shear Strain Cycle no: 1 2 3 10 100 1000 10000
Ultimate stress (= shear strength)
Applied Stress Hysteresis loop
– represents
energy loss
Approximate shear modulus
Normal stress
Shear
stress
Angle of internal
friction
Apparent
cohesion c
(due to stone
interlock)
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
13
California Bearing Ratio (CBR)
This is a convenient general measure of quality, but has no fundamental meaning.
Very, very, very approximate relationships with stiffness: a) E = 10 × CBR
b) E = 17.6 × CBR0.64
Confined Compression
A triaxial test is better (more fundamental meaning) but it is complicated; and the stress
conditions are usually not right for a pavement. Confined compression is an alternative.
These tests are designed to give about the right level of stress in the material and so
hopefully about the right stiffness modulus for pavement design. You can also get a
measure of resistance to deformation accumulation under repeated load.
Force F
Displacement d
(50mm/min)
152mm diameter 125mm
height
Force (kN)
Displacement 1.27m
m
2.54m
m
F2
F1
CBR = max {(F1/13.2) ; (F2/20.0)} 100
where F1 and F2 are in kN
50mm
Steel
mould
Plunger
Soil
under
test
Load applied via
full-face platen
Springs
Locking nuts fix side
plates in place
Side plates
free to move
Adjustment control
for start conditions Springbox
PUMA (Precision Unbound
Material Analyzer)
Load applied via
full-face platen Eight wall
segments
Calibrated steel
and rubber band
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
14
Dynamic Cone Penetrometer (DCP)
The DCP is especially convenient for doing down a core hole during evaluation of a
failing pavement.
Plate Tests
Nowadays the usual way of doing these is by means of a portable dynamic plate test
(DPT). It is a quick, practical method for getting the in-situ stiffness of a pavement
foundation. You have to remember of course that it is affected by any layer within about
1m of the surface.
Static plate tests are also possible. The standard test to evaluate an airfield pavement
subgrade is a static 762mm diameter plate.
Depth
Number of blows
Depth
CBR (%)
Equation used in UK: log10[CBR] = 2.48 – 1.057 log10[p]
Drop
weight
Scale
Anvil
Core hole
Granular
Soil
Penetration
rate p
(mm/blow)
Boussinesq’s equation for deflection under a
rigid circular plate load:
= P (1 – 2) / 2rE
Therefore: E = P (1 – 2) / 2r
Peak load (P)
Peak deflection ()
Time delay due
to ground inertia
Load
Deflection
Time
Drop
weight
Rubber
buffers
Loading
plate
(radius r)
Load cell
and velocity
transducer
(geophone)
Modulus E
Poisson’s ratio
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
15
This table illustrates the fact that different stress conditions give different stiffnesses.
Material Stiffness Modulus (MPa)
Triaxial
(confining stress
20kPa; deviator
stress 0-100kPa)
DPT
(100kPa contact
pressure)
In the Pavement
(K-Mould, PUMA and
Springbox generally
give similar results)
Very soft clay soil 10 5 15
Firm clay soil 50 30 80
Sandy soil 75 30 50
Gravel capping 125 50 80
Sub-base 250 75 150
Granular Base 500 100 250
Permeability
Usually it is assumed that a sub-base or capping layer will be ‘free-draining’ – which is
not entirely true so you might want to measure it.
Material
Description
Typical
permeability
range (m/sec)
Well graded gravels 10-5
to 10-3
Poorly graded gravels 510-5
to 10-3
Silty gravels 10-8
to 10-4
Clayey gravels 10-8
to 10-6
Well graded sands 510-6
to 510-4
Poorly graded sands 510-7
to 510-6
Silty sands 10-9
to 10-6
Clayey sands 10-9
to 10-6
Low plasticity silts 10-9
to 10-7
Low plasticity clays 10-9
to 10-8
High plasticity silts 10-10
to 10-9
High plasticity clays 10-11
to 10-9
Water supply Overflow
Specimen
(cross-sectional area A)
Porous end restraints
Measurement of volume
flow Q Head difference
H
L
Permeability k = QL/AH
Lid
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
16
3. Hydraulically-Bound Material (HBM)
The word ‘Hydraulic’ means that the binder needs water in order to be activated. The
most common binder of this type is Ordinary Portland Cement (OPC), but fly ash (a.k.a.
pulverised fuel ash – PFA), lime or ground granulated blast-furnace slag (GGBS) are
often mixed in or even used without OPC to give a slow-setting (and cheaper) material.
HBMs come in a range of different strengths / qualities:
Stabilised soil – insitu mixing process; roller compacted; results in a partially bound
material. [Compressive strength < 2MPa]
HBM subbase – plant mixed; uses gravel or crushed rock aggregate; still roller
compacted [Compressive strength 2-10MPa]
HBM base – fully bound crushed rock; usually roller compacted; also known as ‘lean
So a 40m length between cracks will break into two 20m lengths, but a 35m length will
remain intact. In fact eventual spacing should be between 19m and 38m.
h
L
Temperature << temperature at time of set
Angle of friction
HBM layer
Crack
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
45
Conclusion: expect transverse cracks, but the spacing will depend critically on day-night
temperature changes during the first few days and weeks of life, and these cannot
reasonably be foreseen.
Solution: crack deliberately (i.e. form joints) at much closer spacing, usually 3m.
This process creates weaknesses. Hopefully cracks from at each weakness but because
they are quite close together, each crack should remain very narrow (a hairline crack).
Design against Traffic Loading
This time we need to calculate the tensile stress at the
bottom of the HBM layer, again using multi-layer linear
elastic analysis.
We then need a relationship between calculated tensile stress
and life. Going back to p18, the key quantity is the ratio of
tensile stress to tensile strength – well actually flexural
strength, i.e. strength from a realistic test arrangement. We
can just apply the fatigue equation suggested on p18, namely:
t / flexural strength = 1.064 – 0.064 log10 (N)
So, if the design is for 50 million standard axles and the calculated tensile stress at the
bottom of the layer is 0.8MPa, then the required long-term flexural strength is 1.4MPa,
which equates to a compressive strength of 10-15MPa. If we want to use a weaker
material we must make the layer thicker, or maybe make the foundation stronger.
Reflective Cracking
Even if there are no traffic-induced cracks in the HBM, we know that there will be
thermally induced transverse cracks (or joints – which amounts to the same thing). These
represent discontinuities in the support given to the asphalt layers, which means we are
very likely to find a crack appearing through the asphalt at those points. This is reflective
cracking.
a) Reflective cracks are a nuisance not a real failure; could just keep re-sealing them. b) Could use Highways Agency rule and always have at least 180mm of asphalt. c) Could check asphalt tensile strain t – but if so then reduce EHBM to 500MPa. This
represents the effect of a discontinuity in support to the asphalt.
Recently
paved HBM
Form slots to
2/3 depth
Fill slots with
bitumen emulsion
Roll
HBM
Subbase
Hot-Mix Asphalt
Subgrade
HBM
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
where: p = load; a = radius; h = slab thickness; = Poisson’s ratio; Ls = ‘radius of relative
stiffness’ = [E h3 / (12 k (1-
2))]
0.25; E = stiffness modulus; k = ‘modulus of subgrade
reaction’; b = ‘radius of equivalent pressure distribution’ = (1.6 a2 + h
2) – 0.675 h, if a >
0.72 h; = a, if a < 0.72 h.
Estimating modulus of subgrade reaction k:
Use multi-layer linear elastic analysis:
predict deflection
k = (P/r2)/
Note: the result will depend on the value of r.
P
E1, 1
E2, 2
E3, 3
r
Corner
Edge
Internal
Joints (with
zero load
transfer)
Plan View
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
47
Once you have calculated a worst-case tensile stress you could then apply the fatigue
equation suggested above for HBM. However, it is usual to be on the safe side and use a
more conservative equation, known as the Packard line.
t / flexural strength = 0.96 – 0.0799 log10 (N)
So, if you calculate a worst case stress of 1.6MPa for example, and you want to use a
concrete with a flexural strength of 4.5MPa then the design traffic is about 37 million
load applications.
Problem: real life isn’t just corners, edges and places far from an edge. Joints usually
transfer load, so they aren’t really edges. But then not all joints are the same – look back
to p7. Expansion joints are not far from being edges; contraction joints should have pretty
good load transfer; warping joints should have excellent load transfer.
Solution: Usually ignore the corner case. Often ignore the edge case. Use the internal
case but apply a factor depending on how good you think the joints are; e.g. × 1.2 for
good joints, × 1.5 for poor joints.
Multi-layer Linear Elastic Analysis
The same multi-layer linear elastic analysis as has been introduced for flexible pavements
can also be used here to calculate tensile stress.
Advantage: load combinations can be included, such as dual or tandem wheel sets.
Disadvantage: all layers have to be infinite in extent; there is no way of analysing an edge
or corner situation.
Limit State Analysis
A problem with both Westergaard and multi-layer linear elastic analysis is that concrete
cannot really crack at a single point. If cracking is to occur, then there must be a
mechanism of cracks. What will actually happen is that the point of theoretical failure
will simply reduce in stiffness locally as the first inter-particle fractures begin to occur.
Conclusions based on an elastic analysis will therefore be conservative. The alternative
is a limit state analysis.
Key equation: WORK done by loads = ENERGY absorbed by foundation
+ ENERGY dissipated at cracks
Need to: a) arrange loads on pavement;
b) propose a failure mechanism;
c) calculate work done + energy to foundation based on an assumed set of
deflections and angles;
d) derive the resisting bending moment (per metre) at crack locations;
e) relate this bending moment to a required slab thickness for a given
concrete strength using the equation M = h2/6 (refer back to p17).
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
48
The limit state approach opens the door to solving problems of multiple loads and
complex joint geometry that would otherwise be impossible.
Warping Stresses
Thermally-induced warping stresses result from a temperature difference between the top
and the bottom of a slab; they should not be ignored.
Approach 1: assume that the safety margin in using the Packard line (previous page) is
enough to cover warping stresses – the usual approach in practice.
Approach 2: use equations that were developed to predict warping stresses directly.
These are the Bradbury equations for maximum warping stress in a concrete slab, one
for internal stress and the other for edge stress, mirroring two of the conditions covered
by the Westergaard equations.
You can then add the maximum warping stress to the stress caused by traffic to give a
real maximum value for design – although you then have to make some difficult
decisions about the number of likely combined stress applications during the life of the
pavement.
Example:
Assumptions: a) load uniformly distributed over area of foundation
enclosed by cracks
b) square wheel pattern with 0.5m offsets everywhere
Work done by loads = 4P × (2d/2–0.5×2)/(2d/2) = 4P(1–1/d)
Energy dissipated at cracks = 42dM × (/(2d/2)) = 8M
Energy absorbed by foundation = 4P × (/3)
Energy balance: 4P (1–1/d) = 8M + 4P/3
M ≈ P/3 for large d
Since M also equals fh2/6, therefore: f = 2P/h
2
d
0.5m
0.5m
Load P
Moment to cause cracking
= M per linear metre
Internal loading; stress at base of slab:
Tensile = ½.E..T (Cx + Cy)/(1 - 2)
Edge loading; stress at base of slab:
Tensile = ½.E..T Cx (or Cy)
where: E = stiffness modulus; = coefficient of thermal
expansion; T = temperature difference top-bottom; Cx,Cy
depend on the ratio of slab dimensions x and y respectively to
radius of relative stiffness Ls – see inset.
x/Ls=0, Cx≈0;
x/Ls=2, Cx≈0.05;
x/Ls=3, Cx≈0.18;
x/Ls=4, Cx≈0.50;
x/Ls=5, Cx≈0.73;
x/Ls=6, Cx≈0.89;
x/Ls≥7, Cx≈1
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
49
Joint Spacing
The main factor here is the amount of thermal expansion and contraction – not warping.
Taking a typical coefficient of thermal expansion of 10-5
per C and a 30C difference
between maximum summer and minimum winter pavement temperatures, then a
nominally 10m long slab of concrete will vary in length by 3mm. So, if joints are placed
in a road at 10m spacing, there will be a gap of 3mm or more in the winter.
This is usually too much. Even with dowel bars (refer back to p 7) there won’t be enough
load transfer across joints in the winter because there won’t be any aggregate interlock,
which means the design case becomes almost an edge condition.
In fact, experience suggests that for most climates joint spacings between 3.5 and 6m
represent a reasonable compromise between the cost and nuisance of joint construction
and maintenance and the need to maintain load transfer efficiency. Less and the cost
becomes too great; more and joint problems become increasingly likely.
6. Reinforced Concrete Pavements
Lightly Reinforced
Reinforcement is often not economically justified. However a light reinforcement mesh is
sometimes included near the top of the slab as a means of controlling shrinkage cracking.
It can also be used to cut back on the number of joints. The argument goes like this:
1) joints are a nuisance so let’s have less of them;
2) greater joint spacing increased warping stresses less load transfer at joints;
3) therefore there is a much greater likelihood of cracking;
4) but if reinforcement is present cracks are ‘controlled’; they will remain narrow
and the slab will not break up.
In this sort of pavement hairline cracking is accepted; but there is enough reinforcement
to hold the slab together, giving plenty of aggregate interlock across each crack and so
plenty of load transfer.
Continuously Reinforced
But why have any joints at all? After all, if we can accept hairline cracking then what
putting so much reinforcement in that it never fractures? This is continuously reinforced
concrete (CRC), and it sits right at the top of the range of concrete pavement options.
The principle is simple. There has to be enough reinforcement to resist the forces
generated when the concrete contracts due to cooling.
Note:
a) the concrete will still crack – but these will be hairline cracks, typically every 1m;
b) reinforcement quantity will typically be 0.6-0.8% of the concrete area;
c) slab thickness is usually less than in the unreinforced case – but not by much;
d) there needs to be an anchorage (into the ground) at each end.
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
50
SURFACE PROPERTIES
1. Ride Quality
Excellent ride quality is not always needed – but it is on high-speed roads.
Typical tolerance limits: 3mm in 3m
So how can this be achieved?
Pavement Quality Concrete
No problem achieving the tolerance in a machine-laid wet-formed concrete pavement.
Problem: concrete is hard and unable to absorb much energy from the tyres.
high tyre vibration
relatively high noise
not so pleasant to drive on
[joints just make things worse; good texture, e.g. longitudinal grooving
or exposed aggregate finish, can help]
Asphalt
To get a really good finish, the surface course must be relatively thin (say ≤ 50mm);
otherwise the paver operator will not be able to control levels well enough. But then the
underlying layer must also be reasonably even too – and this principle applies right the
way through the pavement. The evenness of the surface of each layer can be constructed
slightly better than that of the one below, but only slightly.
UK Highways Agency tolerances (absolute maxima) at each level:
Pavement surface 6mm
Binder course 6mm
Base 15mm
Subbase + 10mm – 30mm
Impact of different types of surface:
Surface Type Vibration
generation
Energy
absorption
Ride quality
ranking
Asphalt concrete medium medium 3
Hot rolled asphalt + chippings medium low 4
Stone mastic asphalt low high 2
Porous asphalt low very high 1
Surface dressing high low 5=
Concrete (PQC) fairly high very low 5=
Block paving very high low 7
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
51
2. Material Strength, Durability etc
A PQC surface is of exactly the same strength as the rest of the concrete slab not an
issue.
Asphalt Surfaces
Asphalt surface course has to have relatively small stone size due to its low layer
thickness and, consequently, a relatively high bitumen content. This leads to a less stiff
material but with high fatigue resistance and good durability.
Asphalt surface structural properties:
Mixture Type Stiffness Deformation
resistance
Fatigue
strength
Asphalt concrete medium high medium
Hot rolled asphalt (+ chippings) medium low high
Stone mastic asphalt medium-low high medium-high
Porous asphalt low medium-high medium-low
BASE HIGH HIGH MEDIUM
Block Paving
Although blocks themselves are high-stiffness, the effective layer stiffness is a function
of rotation and shear at joints, which depends on how well the joints are filled. It is
common practice to assume a stiffness of 500MPa for a combined block-bedding sand
layer.
3. Skid Resistance
Microtexture
This term describes the intrinsic frictional properties of the surface.
In an asphalt: it relates to the aggregate particles at the surface.
In a PQC: it relates to the cementitious mortar.
In block paving: it relates to the surface of the block.
The microtexture represents the ultimate skid resistance potential of a surface, the level
applying in dry conditions and without any intervening dirt, bitumen or ice lens. It is
logical therefore to insist on improved microtexture at sensitive locations such as
approaches to pedestrian crossings and roundabouts, and this approach is adopted by
highway authorities all over the world. Certain aggregate types such as gritstones
therefore take on a premium value because of their excellent microtexture.
The problem is that the frictional properties of a surface change under the action of
traffic. In dry weather they are polished by the relative motion of tyre and surface,
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
52
activated partly by tyre vibration. This reduces the microtexture. It is standard practice
therefore to assess the so-called Polished Stone Value (PSV) of an aggregate by first
subjecting it to an accelerated polishing regime before measuring the frictional
properties using the pendulum test.
Microtexture is also seasonal. Polishing occurs mainly in dry weather; wet weather
restores frictional properties to some extent due to the abrading effect of small particles
of grit which are present in surface water. For this reason, skid resistance should
preferably be assessed in summer or during the dry season.
Macrotexture
If it never rained you would need no
macrotexture (the visible texture due to
the arrangement of stones or the presence
of grooves etc). Neither tyre tread nor
visible surface texture make the smallest
contribution to basic skid resistance; they
are only present to ensure that surface
water has somewhere to go. Direct
contact is needed between tyre and
surface in order for friction to be
activated; if a water film remains in
between, the vehicle will aquaplane as
soon as brakes are applied. An optimised
macrotexture therefore ensures that there
is only a short distance between individual
contact points and regions where water
can be accommodated without danger.
Water movement away from
contact points
Accelerated Polishing Machine
Rubber-tyred
wheel
Polishing Test
Specimen
Surface
aggregate
Rubber
pad
Pendulum Test
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
53
Macrotexture is generally expressed
as a texture depth in millimetres.
The basic measure comes from a
procedure known as the sand patch
test, although there are also laser-
based pieces of equipment on the
market for rapid, sometimes traffic-
speed, measurement.
Macrotexture can also deteriorate under traffic loading. The same wet-season abrasion
that restores microtexture also reduces the height of individual aggregate particles,
eventually reducing the texture depth excessively. For this reason, it is necessary to
specify abrasion resistance, for example the Los Angeles Abrasion value.
4. Spray
Spray from surface water is a safety hazard. If water cannot easily flow across the
surface of a pavement then it will be available to form spray. The issue is not texture
depth but barriers to lateral flow.
Traditional UK Hot Rolled Asphalt (HRA) with rolled-in chippings has a particularly
bad reputation for spray since each individual chipping sits in its own small indentation
(negative texture) into the asphalt surface, allowing a small ‘pond’ of water to remain
around it until it either evaporates or is dispersed in the form of spray.
Most other surfaces consist of protrusions (positive texture) from a more general surface
level and water can flow around these protrusions and make its way sideways. Asphalt
concrete and SMA therefore generate much less spray than HRA. Grooved concrete is
also good. However, porous asphalt is undoubtedly the premier material. Porous asphalt
allows water to drain straight into the pavement itself and then to pass laterally through it,
below the level of the tyre-surface contact. The result: virtually no spray at all. Of course,
the pavement has to be able to cope with the presence of water within the porous asphalt.
Usually the porous asphalt surface course has to overlie a dense, impermeable binder
course; otherwise pavement durability problems are likely.
Sand Patch Test
1. Measure out
exact volume of
sand
2. Pour onto
pavement
surface
3. Spread out level
with tops of
aggregate particles
4. Record
diameter of
sand patch
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
54
5. Noise
This can be an important issue in urban areas. It is a highly complex field and it is not
necessary for the pavement engineer to appreciate the exact acoustic mechanisms
involved.
As expected, noise level generally depends on texture depth, i.e. roughness. However,
the picture is clearly more complicated than this, with a 10dB(A) difference – a factor
of about 3 in actual sound pressure magnitude – between block paving and porous asphalt
for the same texture depth. Of the more common asphalt surfaces, SMA is evidently the
quietest at normal texture depths (around 1mm).
Conceptually:
Noise is caused by vibration, principally of the tyre tread elements.
Surface type affects both the amplitude and frequency of tyre tread vibration. [a
rough surface will induce a high amplitude of tyre vibration and therefore high
noise]
Some of this noise will be absorbed by the surface, and this will depend on the
hardness of the surface material. [concrete has poor ability to absorb any sort of
vibration energy including noise and this means that it is difficult to produce a
low-noise concrete surface]
Porous asphalt has very low stiffness and therefore causes little excitation to the tyre
tread elements; it also has excellent noise absorption properties – an ideal low-noise
material.
88
90
92
94
96
98
100
102
104
106
108
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Texture depth (mm)
No
ise d
B(A
)
Porous asphalt
Stone mastic asphalt
Asphalt concrete
Surface dressing
Slurry seal
Exposed aggregate concrete
Textured concrete
Blocks
Porous
asphalt
Surface
dressing
SMA
Other materials
Blocks
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
55
PAVEMENT EVALUATION
1. Visual Condition Surveys
A visual survey is the most basic and yet often the most useful survey type of all. Put
simply, not all cracks are the same; nor are all ruts or surface defects. An experienced
engineer can deduce a great deal about the internal health or otherwise of a pavement just
by inspecting the surface.
Cracking in asphalt pavements
a) Is there more cracking in the wheel path than elsewhere? Yes: traffic is responsible, whatever that cracking may look like.
No: traffic is irrelevant and the cause is environmental or due to a general material defect.
b) Are there transverse cracks right across the pavement? Yes: either low-temperature cracks or reflective cracking from an HBM or PQC base.
c) Are there more transverse cracks in the wheel paths? Yes: either traffic-induced reflective cracking or defects built in during construction.
d) Is there a single well-developed longitudinal crack in the wheel path? Yes: traffic-induced fatigue of a thick flexible/composite pavement (cracks usually top-down).
e) Is there multiple cracking (crazing) in the wheel-path? Yes: shallow failure; either thin asphalt or the upper layer has become debonded.
f) Is slurry pumping up to the surface through cracks? Yes: water has become trapped, either in the bound materials or else in the foundation.
g) Are there localised wheel path depressions where more than one crack is present? Yes: probably a HBM base; localised damage water ingress, loss of support, settlement.
Cracking in PQC Concrete Pavements
a) Is cracking (of a jointed pavement) largely restricted to transverse cracks? Yes: thermally-induced, assisted by traffic; initial joint spacing was excessive.
b) Are significant longitudinal cracks present in or around the wheel path? Yes: traffic-induced damage; cracks will propagate rapidly along the pavement.
c) Are longitudinal cracks narrow, relatively close-spaced and straight? Yes: lightly reinforced concrete; minor defects at the time of construction.
d) Are there corner cracks at joint intersections? Yes: lack of slab support close to joints; damage is limited and will extend no further.
e) Are there regular transverse cracks at 1-2m spacing but no joints? Yes: continuously reinforced.concrete; should be hairline; if wide then pavement is too weak.
Chipping Loss: loss of adhesive properties in the binder due to bitumen ageing.
Ravelling: widespread chipping loss, leading to the development of pot-holes.
Bleeding: excess bitumen in the pavement shiny surface significant safety hazard.
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
56
Rutting
2. Profile Surveys
International Roughness Index (IRI)
IRI is defined by the amplitude of motion of a vehicle suspension system as it travels
along the road, measured in cumulative metres of suspension system movement per
kilometre of travel (m/km or mm/m). The vehicles that measure IRI are known as bump
integrators.
Laser Profile Surveys
Laser-based systems are now very commonly used, usually with an array of lasers
pointing down at the road surface. Reflective waves are monitored. They can be used for
profile measurement, texture depth and rut depth. These types of survey can be
carried out at normal traffic speed.
0
1
2
3
4
5
6
7
0 1 2 3 4
Distan ce (km)
IRI
(m/k
m)
IRI
< 2 m/km excellent
2-3 m/km satisfactory
3-4 m/km moderately bumpy
4-5 m/km bumpy
> 5 m/km very bumpy
Suspension movement +ve
–ve Typical Data
Surface course problem Binder course - base
problem Foundation problem
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
57
3. Skid Resistance Surveys
Skid resistance is a property that is directly related to the safety of users, applying to both
highways and airfield runways. In the case of airfield runways there are international
standards and it is necessary for airport authorities to check skid resistance regularly,
particularly under adverse weather conditions (rain or snow). Highways are governed by
standards set by individual countries, regions and cities.
4. Cores and Trial Pits
20
Sideways
force
Plan View
The Sideways Force Coefficient Routine Investigation Machine (SCRIM)
Plan Views
Trailer-mounted
alternatives
Separation
force
Drag
force
Wheel
under
braking
Water
tank
Surface
course
Binder/Base
course
HBM base
debonding
Crack; reflected
from HBM base
through asphalt
surfacing
Crack; top-down,
penetrating
through about
50% of the asphalt
Bottom-up
reflective
crack
beginning
to grow
Crack in
HBM base
possibly
reflected
from sub-
base
Serious
crack in
disintegra-
ting HBM
sub-base
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
58
Coring, using cutters of 100mm or 150mm in diameter, is a relatively non-destructive
method of sampling. Trial pits represent an alternative, labour-intensive method of
sampling. They are suitable i) where bound layer thickness is low; ii) where samples of
unbound foundation material are required; or iii) where specific information is needed
which demands a larger area than can be afforded by a core.
Construction Information
Cores and trial pits reveal the following: Materials present;
Layer thicknesses;
Visual quality;
Inter-layer bond.
Samples can also be taken back to the laboratory for testing.
In-situ Tests
The relatively small size of most core holes means that there is a limit to the types of test
that can be carried out. In fact, there is only one in-situ test device that is commonly used
and that is the Dynamic Cone Penetrometer (DCP – see p14). Trial pits also allow the
portable Dynamic Plate Test (DPT – also p14).
Laboratory Tests
These tests include:
compressive strength of HBM (height-diameter ratio of at least 1.0 required);
uniaxial stiffness modulus of HBM;
indirect tensile strength (ITS), either of HBM or asphalt;
indirect tensile stiffness modulus (ITSM) of asphalt;
indirect tensile fatigue test (ITFT) for asphalt;
repeated load axial test (RLAT) for asphalt deformation;
inter-layer bond strength tests.
Also, density and void content
can be obtained on specimens
of any convenient shape.
Asphalt specimens can also be
broken down into their
constituents, by means of a
centrifuge, with solvents used to
extract the bitumen. Aggregate
gradation and binder content
can be checked. Binder quality
can also be measured using a
Dynamic Shear Rheometer
(DSR).
P
P
Peak shear force
Shear slip
at failure
P
Leutner Test
Torque Test
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
59
5. Ground Penetrating Radar (GPR)
Thickness determination is the main reason for doing a radar survey and pretty good
data is usually obtained, accurate to around 1cm. However, it’s all down to image
recognition software, so mistakes are possible. Radar can also give data on moisture
(water molecules become excited at radar frequencies), voids (because of the strength of
a solid-air interface), and steel reinforcement (steel interferes with wave propagation).
6. Deflection Surveys
The Benkelman Beam
This is the oldest and simplest form of deflection test device and it is successfully used
throughout the world.
Time
Transmitter
/receiver Surface reflection
Asphalt/HBM reflection
HBM/Granular reflection
Depth
Asphalt
Hydraulically-bound material
Granular material
Individual Received Signal Longitudinal Signal Profile
Interpreted Thickness Profile
Radar wave pulses:
Frequency 0.2-1.5MHz
x
y Deflection due
to wheel load
Deflection Adjust for
temperature
Approximate
construction
Annual
Traffic
Life in years
[using an empirically-
determined set of
equations]
Distance travelled, x Measurement, y
Benkelman Beam
Arm Pivot Dial Gauge
66335500kkgg aaxxllee llooaadd
TTwwiinn ttyyrreess
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
60
The Benkelman Beam is a simple frame with an arm on a hinge, the rotation of which is
read from a dial gauge. The equipment is placed on the ground immediately behind the
twin rear tyres on one side of a goods vehicle loaded to a standard weight, the arm resting
on the pavement surface between the twin tyres. When the operator is ready, the goods
vehicle is slowly driven forward and a maximum reading is taken as the tyres pass the
end of the arm; when it has driven forward some metres, a minimum reading is also
taken. The difference relates to the deflection caused by the loaded wheel.
The Lacroix Deflectograph
The Deflectograph is an extension of the Benkelman Beam idea, initially developed in
France. However it allows the vehicle to travel continuously along the road. The
reference frame is in front of the measurement axle, and it is repeatedly dragged forward
relative to the body of the vehicle and then released. As soon as the rear tyres of the
vehicle have drawn level with the tip of the measurement arm, the frame is winched
forward toward the front of the vehicle ready for the next reading. The result is that a
measurement is taken every 3-4m and that the vehicle can travel continuously at a speed
of 2-3km/hr. Readings are taken in both wheel paths.
The problem with both the Benkelman Beam and the Deflectograph is that they are
relatively low-resolution measurements and rely on empirical interpretation. They also
do not give good data on PQC pavements. They are fine for estimating structural
condition of asphalt pavements for network-level management but they are not really
reliable enough for project-level design. For this we need something a bit more
sophisticated.
The Falling Weight Deflectometer (FWD)
The FWD gives a very precise value of absolute deflection (accuracies of 2 microns
commonly quoted), and that opens the door to a much more sophisticated method of
interpretation.
Distance
travelled, x
6350kg axle load
Twin tyres Winching
mechanism
Reference
frame
Arm
x
Deflection, y
Deflection due
to wheel load
Adjust to equivalent
Benkelman Beam
deflection
Deflection
Adjust for
temperature
Approximate
construction
Annual
Traffic
Life in years
[using an empirically-
determined set of
equations]
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
61
The machine is usually trailer-mounted. Tests are performed with the equipment
stationary and with the loading plate and deflection sensors lowered onto the surface. The
load pulse is then generated by the action of a falling weight onto a set of rubber buffers.
Key advantages:
the load magnitude can be selected to match a typical wheel load;
the pulse duration is similar to that from a moving vehicle;
the deflections are absolute and highly accurate (using velocity transducers);
measurements are taken not only at the load location (through a hole in the centre
of the loading plate) but also at selected distances from it.
The full set of readings describes a deflection bowl (or basin) which can be back-
analysed (or back-calculated) to deduce the combination of layer stiffnesses present.
Back-analysis is done by computer, with the following assumptions:
all layers are of uniform thickness and of infinite lateral extent;
all materials are linear elastic and homogeneous;
the load consists of uniform stress on a circular area;
dynamic effects due to inertia are negligible.
Deflection
(microns)
Offset (m) 0 1 2
Deflection
Bowl
FWD – Longitudinal Section
Deflection sensors
Falling Weight
Rubber buffers
Loading Plate
Known layer
thicknesses
Layer
stiffnesses
Measured
load Calculate
deflections using
multi-layer linear
elastic analysis
Adjust layer
stiffnesses
Good match
to measured
deflections?
Finish
no
yes
Load: 10-200kN
Duration:
25-50msecs
Platen radius:
150mm
Tow bar
Upper Pavement
- affects curvature of central part
- typical indicator: d1–d2 or d1–d3
Base and Sub-base
- affects slope in next region
- typical indicator: d2–d4 or d3–d5
Subgrade
- affects deflection at distance
- typical indicator: d6 or d7
d1 d2 d3 d4 d5 d6 d7
Asphalt over
good foundation
Concrete over
poor foundation
Load
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
62
How many layers can be analysed? Two: no problem
Three: should be OK
Four: be careful; don’t just believe the result
Another advantage over the Benkelman Beam and Deflectograph is that the FWD is
equally useful on concrete or asphalt. It is particularly well suited to measuring load
transfer efficiency across a joint. The loading plate is positioned one side of the joint and
deflections are measured either side.
Rolling Wheel Deflectometers
The ultimate deflection test device would be one that measured a full deflection bowl like
the FWD, but which travelled at traffic speed along a highway, thus combining both
quality and quantity of information. Such a device is the rolling wheel deflectometer,
and several versions have been developed over the years, achieving their goal to varying
degrees. One is currently being trialled by the Transport Research Laboratory. There is an
inevitable trade-off between measurement accuracy and travel speed. Several companies
and research organisations have used lasers to measure either distance from a datum of
vertical velocity of the surface.
So: not used much yet – but watch this space.
7. Diagnosis
Pavements with an Asphalt Surface
Rutting: Check the visual condition. If ruts are narrow with shoulders, the problem is
near the surface (surface course or binder course probably); the wider the rut, the deeper
the problem. Inspect cores carefully. If an asphalt layer appears rich in binder, especially
if that binder is soft, that is likely to be the cause of the problem. Consider carrying out
repeated load axial tests (RLAT) to check whether materials are deformation susceptible.
Also look at DCP data (if it exists). This should relate to rut resistance of foundation
materials. Check FWD data. A subgrade stiffness of 50MPa or less indicates potentially
deformable material.
Load Transfer Efficiency
- affects ‘step’ across joint
- typical indicator: d2–d3 or d3/d2
(note: 0 or 100% even if perfect, due to
distance between sensors 2 and 3)
Slab Support
- affects angle of loaded slab
- typical indicator: d1–d2 / L12
(where L12 = distance between sensors)
d1 d2 d3
Load Joint
Poor load transfer
Moderate load transfer
Good load transfer
L12
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
63
Transverse Cracks: Look at the detailed crack shapes. If they are straight and regularly
spaced, they are reflective cracks from joints in an underlying concrete pavement. If the
shape is less regular, they may either be reflective cracks over a hydraulically-bound base
or else low temperature cracking of the asphalt. If reflective cracking is suspected, check
the crack distribution. If there is a concentration in the wheel paths then traffic is clearly
playing an important part; if not, then the effect is almost entirely thermally driven. Short
transverse cracks may also have originated as construction defects and they may be
progressing due to binder embrittlement. If FWD data is available, check for loss of
asphalt layer stiffness in the wheel paths. This provides evidence of crack severity.
Longitudinal Cracking in the Wheel Path: The fact that this cracking is in the wheel
path proves that it is the traffic which is doing the damage. If there are several cracks
within the zone of the wheel path then the effect is almost certainly shallow, possibly
associated with debonding between asphalt layers. If there is only a single crack, cores
through cracks are advised. In many cases, crack depth is shallow. Cores will also
indicate where debonding between layers is associated with cracking. If an FWD survey
has been carried out, check the asphalt layer stiffness. If it is low or variable then this
implies significant damage and/or debonding. Compare FWD-derived stiffnesses with
those from laboratory testing of recovered samples. If the two measures agree then the
asphalt layers are likely to be intact and well bonded. Check evidence for binder
hardening, e.g. from unusually high stiffness of laboratory specimens, or poor binder
adhesion, e.g. unusually low stiffness, even of undamaged material. Also evaluate the in-
situ stiffness of any HBM layer from FWD evidence. If it is less than expected, this is
evidence that cracking is present. Using the best available evidence for the stiffness of
each layer, carry out multi-layer linear elastic pavement analysis and compute
pavement life. Compare the theoretical life with past traffic numbers – and with current
general pavement condition.
Ravelling: Ravelling (and associated pot-holes) occurs when adhesion between binder
and aggregate breaks down. Consider checking penetration of recovered surface course
binder; or consider measuring surface course stiffness in the ITSM. These should
identify binder hardening. Also check binder and filler contents since excess filler can
contribute to poor adhesion.
Bleeding: There is too much bitumen present. Inspect the cores. Multiple layers of
surface dressing are one common source of excess binder. Otherwise consider
determining the void content of the surface course. Bleeding should only occur at void
contents of 2% or less. Also check the visual condition for rutting since low void content
also leads to asphalt deformation.
Pavements with a Concrete Surface
Transverse Cracks (jointed PQC): Transverse cracking, either at mid-bay or a metre or
so from joints, is common; it implies that the joint spacing was too large for the
thermally-induced stresses and strains which have occurred. If the joint spacing is greater
than about 20 times the slab thickness, joints cannot be expected to function properly.
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
64
Also find out whether the cracking occurred soon after construction. If so, shrinkage
cracking may be the cause.
Transverse Cracks (CRC): Transverse cracks are expected in CRC. They should form
at a spacing of 1-2m but should remain narrow. If there are more or they are no longer
narrow, then the pavement is not functioning as intended and will continue to deteriorate.
Check concrete strength, slab thickness and foundation stiffness. The combination should
reveal whether the pavement as it was constructed should have lasted longer than it has.
If so then the problem lies elsewhere, e.g. reinforcement defects, overloading.
Longitudinal Cracking: This is a sign of overloading and is a very serious mode of
distress. Check concrete strength, slab thickness and foundation stiffness. If the computed
life is less than the current condition suggests, then this implies that one of the input
parameters was more favourable in the past. Possibly the foundation used to be stiffer and
has deteriorated over the years. Check FWD data for poor slab support. Also check
evidence for subgrade softening, from DCP, unbound material samples and/or a drainage
survey. If the computed life is greater than the current condition suggests, then something
has happened which the computation doesn’t take into account. This could be shrinkage
cracking during initial concrete curing. Also investigate the crack distribution. If it is
localised then this suggests other areas may have much longer life.
Faulting across Joints: This has a serious effect on ride quality. If dowels or tie-bars are
present, there should be no faulting. If faulting is present, this can only mean serious
corrosion of the bars and disintegration of the surrounding concrete. Even without
dowels or tie-bars, faulting implies poor load transfer, possibly due to excessive joint
spacing, poor durability aggregate, or a deformable foundation.
Surface Deterioration: Concrete relies on the presence of a balanced combination of
cement mortar and aggregate throughout. Excess cement mortar at the surface results in
a relatively weak surface layer. Once trafficking has removed this excess mortar, there
will be a decrease in ride quality. Another possibility is scaling, which means the loss of
discrete areas of surface. This is usually caused by frost action.
8. Prognosis
Statistical Treatment of Data
Since condition inevitably varies it is usual to work in terms of statistics, for example:
50 percentile (e.g. of FWD back-calculated stiffness, or of thickness) = average
15 percentile (i.e. 85% is better) = for design
Pavement Life Prediction
Here you usually need an analytical computation, typically taking 15 percentile stiffness
moduli. You must include consideration of the realistic long-term properties of
materials. Sometimes it is possible to consult a design guide, but most are not flexible
enough to cope with a deteriorated pavement.
UNIVERSITY OF NOTTINGHAM DEPARTMENT OF CIVIL ENGINEERING
Pavement Engineering - Module H23P01 Course Notes
65
Result = predicted lives to failure.
The next step is to take account of the fatigue damage that has taken place already.
Miner’s Law: This law states that relative damage is cumulative.
For example: predicted fatigue life = 30 106 axle loads