Flexible Pavement Design
Dr. TALEB M. AL-ROUSAN
Pavement Types
1.


Flexible Pavement:
Pavement constructed of bituminous &
granular materials.
A structure that maintains intimate
contact with subgrade and distribute
loads to it, and depends on aggregate
interlock, particle friction, and cohesion
for stability.
Pavement Types Cont.
2. Rigid pavement:
 Pavement constructed of Portland
cement concrete.
 It is assumed to posses considerable
flexural strength that will permit it to act
as a beam and allow it to bridge minor
irregularities in base and subgrade.
Cross Section Components

Cross section consist of (from top):
1.
2.
3.
4.
5.
6.
7.
8.
9.

Seal coat
Surface course
Tack coat
Binder course
Prime coat
Base course
Subbase course
Compacted subgrade
Natural subgrade
The use of various courses is based on either
necessity or economy, and some of the courses may
be omitted.
Typical Cross Section
Seal Coat

Seal coat: Thin asphalt surface treatment used to:
1.
2.
3.
4.

Waterproof or seal the surface.
Rejuvenate or revitalize old bituminous wearing surfaces.
To nonskid slippery surfaces.
Improve night visibility.
Single Surface treatment = single application of
bituminous material that is covered by a light
spreading of fine aggregate or sand (spread
mechanically) then compacted with pneumatic tired
rollers.
Surface Course

Is the top course of asphalt pavement
(Wearing course).
 Constructed of dense graded HMA.
 Must be:
1.
2.
3.

Tuff to resist and withstand wear & abrasive
effects of moving traffic & stable to prevent
permanent deformation.
Provide smooth and skid resistant riding surface.
Water proof to protect the entire pavement from
the weakening effects of water.
If the above requirements can not be met, the
use of seal coat is recommended.
Binder Course

Binder course (known also as Asphalt base
course) is the asphalt layer beneath the
surface course.
 Reasons for use:
1.
2.
HMA is too thick to be compacted in one layer (if
the binder course is more than 3” it is placed in two
layers).
More economical design, since binder course
generally consist of larger aggregates and less
asphalt and doesn’t require high quality.
Tack & Prime Coats

Tack coat: Very light application of asphalt (emulsion) to ensure a
bond between the surface being paved and the overlying course.
Binds asphalt layer to PCC base or to an old asphalt pavement.

Prime coat: Application of low viscosity Cutback asphalt to an
absorbent surface such as untreated granular base on which
asphalt layer will be placed on. It binds the granular base to the
asphalt layer.

Tack coat doesn’t require the penetration of asphalt into the
underlying layer, while prime coats penetrates into the underlying
layer, plugs the voids , and form a watertight surface.

Both are spray application.
Base & Sub Base Courses

Base course: Layer immediately beneath the
surface or binder course.
 Composed of crushed stone, crushed slag, or
other untreated or stabilized materials.
 Has good stability & density
 Distributes & spreads the stresses created by
wheel loads so that the stresses transmitted to
the subgrade will not be great to result in
excessive deformation or displacement of that
foundation.
Subbase Course


Subbase course: Layer beneath the base
course, used mostly for economy purposes
since it can be of lower quality.
Subbases may be used in areas where:
1.
2.
3.
frost action is sever, or
Subgrade soil is extremely weak, or
Where construction working table is needed.
Subgrade
Subgrade can be either in situ soil or a
layer of selected materials.
 The top 6” of subgrade should be
scarified and compacted to the desired
density near the optimum moisture
content.

Full-Depth Asphalt

Are constructed by placing one or more layers
of HMA directly on the subgrade or improved
subgrade.
 Used for heavy traffic.
 When local materials are not available to
minimize the administration and equipment
costs.
 Typical cross section: Asphalt surface, tack
coat, asphalt base, and prepared subgrade.
Full-Depth Asphalt Cross Section
Advantages of Full-Depth Asphalt
1.
2.
3.
4.
5.
6.
Have no permeable granular layers to entrap
water and impair performance.
Reduced construction time.
Construction seasons may be extended.
Provide & retain uniformity in the pavement
structures.
Less affected by moisture or frost.
Little or no reduction in subgrade strength
because moisture do not build up in
subgrade when full-depth asphalt is used.
Elements of Thickness Design
1.
2.
3.
4.
Traffic Loading
Climate or Environment
Material Characteristics
Others: Cost, Construction,
Maintenance, Design period.
Traffic Loading

Pavement must withstand the large umber of
repeated loads of variable magnitudes
Primary loading factors:

1.
2.
3.


Magnitude of axle loads (controlled by legal load limits).
Volume & composition of axle load (Traffic survey, load
meters, & growth rate).
Tire pressure & contact area.
Equivalent Standard Axle Load ESAL (80 kN (18,000
lb or 18 kips) single axle load.
The total no. of ESAL is used as a traffic loading
input in the design of pavement structure.
Climate or Environment

1.
2.
Climate or environment affect the behavior &
performance of materials used in pavements
Temperature: high temp. cause asphalt to
loose stability, low temp. cause asphalt to
become hard & stiff, and frost heave.
Moisture: Frost related damage, volume
changes due to saturation, chemical stability
problems with moisture existence (Stripping).
Material Characteristics

1.
2.
3.



Required materials characteristics:
Asphalt surface: Material should be strong & stable
to resist repeated loading (fatigue).
Granular base & subbase: gradation, stable & strong
to resist shears from repeated loading.
Subgrade: soil classification, strong & stable.
Various standard tests are available for
determination of desired properties.
CBR, Marshal stability, Resilient Modulus, Shear
strength.
Mr (psi) = 1500 CBR or Mr (Mpa) = 10.3 CBR
Asphalt Institute Method

Method is based on two assumed stress –strain
conditions:
1.
Wheel load (W) is transmitted to the pavement
surface through the tire at a uniform vertical
pressure (Po). The stresses are then spread through
the pavement structure to produce a reduced max.
vertical stress (P1) at the subgrade surface.
2. The wheel load (W) causes the pavement structure to
deflect creating both compressive & tensile stresses
in the pavement structure.
Asphalt Institute Method Cont.

1.
2.

This method considers the following strains
as being responsible for the most common
traffic related distresses:
Horizontal tensile strains (Et) on the bottom
of the asphalt layer (causes fatigue
cracking).
Vertical compressive strains (Ec) on the top
of subgrade (causes permanent
deformation).
Et & Ec are used as failure criteria
Asphalt Institute Method Cont.

Asphalt Institute thickness design manual was
prepared using a computer program and
suitable data.
 The manual includes charts for six types of
pavement structures, and three sets of
environmental conditions based on the mean
annual air temp. (45o, 60o, and 75o F).
 Example design chart is shown in the coming
slide.
Materials Evaluation






The design subgrade (Mr) should be based on
expected level of traffic expressed in ESALs.
To ensure more conservative design, lower
value of (Mr) is used for higher volumes of
traffic.
It is recommended that (Mr) is found for (6 to
8) samples of subgrade.
Arrange Mr values in descending order.
Plot as cumulative distribution.
Chose design subgrade (Mr) from the curve
as follows:
Design Subgrade Mr
Mr test
Value
13500
% values
Value >=
>=
1
Subgrade Design Limits
12.5
11900
2
25
11300
3
37.5
10000
4
50
9500
5
62.5
8800
6
75
7800
7
87.5
6200
8
100
Traffic Level
Design Percentile
ESAL
Value
<= 10,000
60
10000 to 1000,000
75
> 1000,000
87.5
Design Mr
120
100
% >=
80
60
40
20
0
0
2000
4000
6000
8000
Mr (psi)
10000
12000
14000
16000
Resilient Modulus (Mr)








AASHTO T292 (Resilient Modulus of Subgrade soils).
(Mr) can be found using repeated loading procedure
test such as (unconfined compression test or triaxial
compression test).
0.1 sec loading and 1 to 3 sec. unloading.
Linear Variable Displacement Transducers (LVDTs)
are used to measure strains.
Elastic modulus based on the recoverable strain under
repeated loading is called the resilient modulus
(Mr) = (Deviator stress/ Recoverable axial strain)
Deviator stress = Axial stress – confining pressure
Recoverable axial strain = Max strain – permanent strain
 See Fig. 16.2
Subgrade Mr Seasonal Variation
Freeze Time
Recovery
Time
Traffic Analysis
Estimate the number of vehicles of
different types (Passenger cars, single
unit trucks, multi unit trucks of various
sizes) expected to use the pavement
over the design period.
Design should make allowance for traffic
growth using historical records or
comparable facilities.
1.
Expected Traffic Volume During
Design Period
T = [( (1 + G)Y -1)/ G ] T1
T: Expected traffic volume during design period.
T1: Traffic volume during first Year.
G: Rate of growth.
Y: Design period (yrs).
[( (1 + G)Y -1)/ G ]: compound rate of growth.
Traffic Analysis Cont.
2. Estimate the (%) of total truck traffic expected
to use the design lane.
Design lane: Lane expected to receive the
severe service.
% of trucks is found by observation or using
some prepared tables. (see Table 16.1).
Axle & Wheel Configurations
Single Axle with
Single Tire
Tandem Axles with Dual
Tires
Single Axle with Dual Tires
Tridem Axles with Dual
Tires
Traffic Analysis Cont.
3. For each weight class, determine the truck factor.
Truck Factor: The no. of ESALs contributed by passage of a vehicle.
TF = [SUM (No. of axles in each wt. class X EALF)] / Total No. of vehicles


Truck factor can be estimated Using Table 16.2
Equivalent Axle Load factor or Load equivalency factor (EALF) presented
in Table 16.3.
 EALF: Defines the damage per pass to a pavement by the axle of
question relative to the damage per pass of a standard axle load (80 kN
or 18-kip)
 EALF depends on type of pavement, thickness or structural capacity, and
failure conditions (based on experience).
 See Figure 16.8.
Truck Factor Example
Traffic Analysis Cont.
4. Multiply Tf by the no. of vehicles in each
group and get the sum for all groups.
ESAL = Sum (TF X No. of vehicles) all
groups.
See Examples 16.1 & 16.2
Example on Computation of ESAL
Computing Design ESAL (Projected)
Traffic Analysis

Goal: To predict the number of repetitions of
each axle load group during design period.
 The initial daily traffic is in two directions over
all traffic lanes.
 Must be multiplied by direction distribution &
Lane distribution to obtain initial traffic on
design lane.
 Traffic to be used in design is the average
traffic during design period (i.e. multiply by
growth factor).
Traffic Analysis
ESAL = (ADT) (T) (Tf) (G) (Y) (L) (D) (365)
ADT: Average daily traffic at the start of the design
period.
(T): % of trucks in the ADT.
(Tf): Truck factor
G: Growth factor
Y: Design period
L: Lane Dist. Factor
D: Directional dist. Factor
Note (G Y) = Combined growth rate… use formula to
calculate.
ESAL Example

A two-lane major rural highways has an AADT of 4000
during the first year of traffic, 25% trucks, 4% annual
growth rate, and 50% on the design lane. Compute the
ESAL for a design period of 20 yrs. Use truck factor of
0.38. Consider lane Dist. = 1.0.
Solution
ESAL = (ADT) (T) (Tf) (G) (Y) (D) (L) 365
(G) (Y) = [(1 + G)y - 1]/ G = [(1 + 0.04)20 - 1]/ 0.04
= 29.78
ESAL = (4000) (0.25) (0.38) (29.78) (0.50) (1.0) 365
= 2, 065, 200
Planned Stage Construction

Involves successive application of HMA layers
according to a predetermined time schedule.
 Beneficial when:


Funds are insufficient for constructing a pavement
with long design life.
Great amount of uncertainty in estimating traffic.
• Concept: Remaining life which implies that the
second stage will be constructed before the
first stage shows serious signs of distress.
Planned Stage Construction Cont.
Pavement is designed for initial traffic &
next stage can be designed using traffic
projections based on traffic in service.
 Stage construction allows weak spots
that develop in the first stage to be
detected and repaired in the second
stage.

Planned Stage Construction Cont.
n1: Actual ESAL for stage 1
N1: Allowable ESAL for initial thickness (h1) selected for stage 1.
Then The damage ratio (Dr) at the end of stage 1 is:
Dr = n1/ N1
Dr < 1.0 ………. When Dr =1.0 pavement fails.



(1-Dr) = Remaining life in the existing pavement at the end of
stage 1.
h1 is obtained based on Dr =1.0.
To keep some life, h1 should be determined based on adjusted
ESAL (N1) > ESAL (n1)
N1= n1/Dr
Planned Stage Construction Cont.
n2: Design ESAL for stage 2.
N2: Allowable or adjusted ESAL to permit selection of (h2) that will
carry traffic n2 and use the remaining life in stage 2.
Then The damage incurred in stage 2 should not exceed the
remaining life.
n2/ N2 = (1-Dr)
N2 = n2/ (1-Dr)


h2 –h1 = Additional thickness required in stage 2.
MS-1 recommended (5 – 10 yrs) stage 1 with 60% Dr.
Planned Stage Construction
Example
Given:
 Full-depth asphalt pavement
 subgrade Mr = 10,000 psi
 Use two stage to construct this pavement
 Stage 1: 5 yrs, ESAL = 150,000 , Dr = 60% at
the end of stage 1.
 Stage 2: 15 yrs, ESAL = 850,000
 Required:


Determine thickness of HMA required for first 5yrs.
Thickness of overlay required to accommodate the
additional traffic expected during the next 15 yrs.
Planned Stage Construction
Example Cont.
Solution:
 n1=150,000 &
Dr = 0.6
Find N1 = n1/Dr = 150,000/ 0.6 = 250,000
From design Chart with N1 & Mr find
h1=7.3 in use 7.5 in.

n2 = 850,000
& 1-Dr = 0.40
Find N2 = n2/ (1-Dr) = 850,000/ 0.4 = 2,100,000
From design chart with N2 & Mr find
h2 =11.0


First stage thickness = h1 = 7.5 in
Overlay thickness = h2 –h1 = 11.0 – 7.5 = 3.5 in.
Planned Stage Construction
Example Cont.
Solution:
 If the design was not divided into 2 stages, the
thickness of the pavement using (Mr = 10,000
psi & ESAL = 1000,000) is :
9.8 in use 10 in.
 The use of stage construction decreased the
thickness in first stage by (10.0 -2.5 = 2.5 in),
but increased the total thickness by
(11.0 - 10.0 = 1.0 in).