The AASHTO guide for designing rigid pavements considers several factors: the effective modulus of subgrade reaction, concrete elastic modulus and modulus of rupture, load transfer coefficient, reliability and standard deviation, traffic load application, and serviceability loss. The document then proceeds to describe the process for determining each of these factors to complete the pavement thickness design using design charts.

1.  The AASHTO, 1993 Guide for Design of Pavement
Structures considers the following factors in the
design of rigid pavements:
 Effective modulus of subgrade reaction
 Concrete elastic modulus
 Concrete modulus of rupture
 Load Transfer coefficient
 Reliability and standard deviation
 Traffic load application
 Serviceability loss
Design considerations for the AASHTO Rigid
Pavement Design
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2. Effective modulus of subgrade reaction
 The AASHTO 1993 includes an intricate procedure
for evaluating the level of slab support, expressed as
the modulus of subgrade reaction.
 The table shown in next slides, is illustrating the
process for developing an effective modulus of
subgrade reaction.
 Resilient modulus of roadbed material and seasonal
elastic modulus values for the subbase should be
determined and entered in the table 20.8.
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4.  The composite modulus
of subgrade reaction, K
value, pci, is
determined from figure
and entered in the
table 20.8
 If there is no subbase,
the composite modulus
of subgrade reaction
may be determined
from following equation
K = MR/19.4
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5. If a rigid foundation exists within 10 ft of the surface, its
effect on the k-value can then be determine by the chart and
values are entered in table 20.8.
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6. Assuming the thickness of
slab that will be required,
determine the relative
damage, uf and enter the
value in table 20.8
Determine the average
value of all the determined
uf values
The effective modulus of
subgrade reaction can be
determined from the figure
20.11
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7. The effective
modulus of subgrade
reaction is adjusted
to account for the
potential loss of
support caused by
subbase erosion
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8. Concrete Elastic Modulus Ec
 The elastic modulus of the concrete may be
computed from the following equation
Concrete Modulus of Rupture Sc
 The modulus of rupture (flexure strength) of
portland cement concrete required by the design
procedure is the mean value determined after 28
days using third point
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9. Load – Transfer Coefficient, J
 The load transfer coefficient, J, is used in rigid pavement
design to account for he ability to transfer or distribute load
across discontinuities, such as joints and cracks.
 Table 20.10 shows the AASHTO recommended load transfer
coefficients for various pavements types and design conditions.
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10. Drainage Coefficient, Cd
 The effects of drainage coefficient, Cd which varies from 0.7
for vaer poor drainage to 1.25 for excellent conditions. Table
20.11 gives recommended values of Cd for rigid pavement design.
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11. Reliability and Standard Dviation
 It provides a predetermined level of assurance (R) that
pavement sections will survive the period for which they were
designed.
 It is the function of the overall standard deviation (So), the
chance variation in traffic prediction and the normal variance in
pavement performance.
 In rigid pavement the standard deviation values commonly range
from 0.30 to 0.40.
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12. Traffic Load Application
The cumulative expected 18-kip ESAL (W18) during the design life
in the design lane is then determined.
 
1
1
1
T
r
r
T
n





 


Where, T = total volume of traffic expected during the
design period
T1 = traffic volume during first year
r = growth rate
n = design life
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13. E. Serviceability loss (Δ PSI)
 The serviceability loss is the difference between the
initial serviceability index (Po) and the terminal
serviceability index (Pt)
Δ PSI = Po - Pt
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14. Design Procedure
The pavement thickness design can be carried out using the
design charts shown in two segments.
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