2. Introduction
The main objective of pavement design and
management is to build sustainable pavement
structure with minimum costs during its whole life.
There are many uncertainties in the process of
pavement design relating many of its variables, such
as future traffic estimation, long time behavior of
materials, future weights and types of traveling
vehicles, availability of funds etc.
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3. Introduction
Therefore, it is important to apply pavement
stage construction technique during the
process of pavement design and management
to minimize the risk associated with these
uncertainties.
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4. Stage construction of
pavement
Planned stage construction is a process of
providing fully adequate pavements with the
most effective use of materials, energy, and
funds as reported by Asphalt Institute (1991).
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5. Stage construction of
pavement
Stage construction is the construction of roads
pavement by applying successive layers of asphalt
concrete according to design and to a predetermined
time schedule.
The design of planned stage construction should not
be confused with the design of major maintenance
or the rehabilitation of existing pavements.
The procedure is based on the assumption that the
second stage will be constructed before the first
stage shows serious signs of distress.
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6. advantages of Stage
construction of pavement
When funds are inadequate to construct the full
design thickness, the pavement may be designed
for construction in two stages.
Accuracy problems in estimating traffic for a
period of 20–25 years make planned stage
construction attractive.
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7. Stage construction of
pavement
Experience indicated that pavements overlaid
after they had been subjected to traffic
performed somewhat better than new
pavements of equal design.
Pavement distresses that develop during the first
stage can be restored.
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8. Stage construction of
pavement
Savings may be realized from reduced final thickness
or from extended life of the original pavement.
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10. Stage construction of
pavement
According to AASHTO (1993), if the analysis period
is 20 years (or more) and the practical maximum
performance period is less than 20 years, there may
be a need to consider stage construction in the
design analysis.
When considering in stage construction design
alternatives, it is important to consider the impact
of compound reliability.
The overall reliability for example of two stage
strategy (each stage is designed for 90% reliability)
would be 0.90 × 0.90 or 81%.
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11. Stage construction of
pavement
Therefore, the following equation should be
considered to check the overall and individual
reliability of stage construction:
R (stage) =(R overall) 1/n (1)
Where n is equal to the number of stages included
in the initial pavement design.
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12. Stage construction of
pavement
In the stage construction, the traffic growth should
be estimated for each stage.
The future number of 18 kips equivalent
standard axle load repetitions is estimated by
multiplying the first year 18 kips equivalent
standard axle load repetitions (ESAL) by the growth
factor (G).
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13. Stage construction of
pavement
Growth factor (G) can be obtained from the following
equation, (AASHTO, 1993):
G= [(1+g) n-1]/g (2)
Where G is the growth factor, g is the traffic growth rate
divided by 100, and n is the number of years.
For example, if the traffic growth rate is 5% (g = 0.05)
and n = 20 years, then the growth factor G equals to
33.066.
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14. Stage construction of
pavement
Table 1 shows the AASHTO traffic growth factors as a solution of
Eq. (2) above (AASHTO, 1993).
Increasing the annual growth rate and/or the analysis period (as
the case in perpetual pavement) may increase the AASHTO
growth factor to up to unreasonable value of hundred times to
be multiplied by first year traffic.
For this reason, the consideration of stage construction becomes
more necessary especially if we recall that perpetual pavements
should serve for long period up to 50 years and growth rate
more than 7%.
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17. Stage construction of
pavement
To clarify more, the following example is explained:
Stage Construction / Example:
Time Constraints
The analysis period selected for this design example is 20years.
The maximum performance period (or service life) selected for
the initial flexible pavement structure in this example is 15 years.
Thus, it will be necessary to consider stage construction (i.e.
planned rehabilitation) alternatives to develop design strategies,
which will last the analysis period.
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18. Stage construction of
pavement
Traffic
Based on average daily traffic and axle weight data from the planning
group, the estimated two-way 18-kip equivalent single axle (ESAL)
applications during the first year of the pavement’s= 2.5 x 106 and the
Projected growth rate is 3 % per year.
The directional distribution factor (DD) is assumed to be 50 % and lane
distribution factor (DL) for the
Facility (assume three lanes in one direction) is 80 %.
During the first year (in the design lane)
= 2.5 x 106 x 0.5 x 0.8 = 1.0 x 106 18-kip ESAL applications.
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19. Stage construction of
pavement
A plot of the cumulative18-kip ESAL traffic over
20- Year analysis period.
The curve and equation for future traffic (w18)
are reflective of the assumed exponential
Growth rate (g) of 3 %.
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21. Stage construction of
pavement
Reliability (R)
Although the facility will be a heavily trafficked state highway, it is in a rural
situation where daily traffic volumes should never exceed half of its capacity.
Thus, a 90-percent overall reliability level was selected for design.
This means that for a two-stage strategy
(initial pavement + one overlay), the design reliability for each stage must be
0.90^(1/2) or 95 %.
Similarly, for a three –stage strategy
(initial pavement + two overlays), the reliability for all three stages must be 0.90
^(1/3) or 96.5 %.
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22. Stage construction of
pavement
Standard Deviation (So)
An approximate standard deviation value of 0.35 will be used for the purpose of this example problem.
Serviceability
The overall design serviceability loss (ΔPSI = po – pt)
for this problem is:
ΔPSI = 2.1.
Resilient Modulus
Application of the effective roadbed soil MR
estimation procedure results in a value of 5700 psi.
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23. Stage construction of
pavement
Pavement Layer Material Characterization
Three types of pavement materials will constitute the individual layers of the structure. The
moduli for each determined using the recommended laboratory test procedures, are as follows:
Asphalt concrete EAC=400000 psi
Granular Base EAC=30000 psi
Granular Subbase EAC=11000 psi
Asphalt concrete a1=0.42 (Figure 2 5, part II)
Granular Base a2=0.14 (Figure 2 6, part II)
Granular Subbase a3=0.08 (Figure 2 7, part II)
(From AASHTO GUIDE)
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30. Stage construction of pavement
Overlay Thickness (Second Stage)
Initial Structure
(first sage)
Overlay
(second stage)
Two Stage strategy
(Initial Structure +
Overlay)
Period
(years)
Performance
Period (15 years)
Performance
Period (5 years)
Design Analysis
Period (20 years)
W18
(ESAL)
18.6 x 106 26.9 x 106
SN , ▲SN SN = 5.6 ▲SN = 0.5 SN = 6.1
Asphalt
thickness
20 cm
(8 inch)
SN1 = 3.36
3 cm
(1.2 inch)
SN1 = 0.5
23 cm
(9.2 inch)
Thickness 66 cm
(26 inch)
69 cm
(27.2 inch
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