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Lecture 31 AASHTO Design Method
This method developed out of the AASHTO road test in the 1960s and has been refined several times. AASHTO Design Guide . The equation from the 1986 Guide is
[pic]
where
ZR = Standard normal variant
So = overall standard deviation
W18 = 18 kip ESAL
D= thickness in inches
DPSI = serviceability loss
Pi = initial serviceability
Pt = final serviceability
Ec = elastic modulus of concrete
Sc = modulus of rupture of concrete
J = load transfer coefficient = 3.2 (assumed)
Cd = drainage coefficient
K = modulus of sub grade reaction
Data needed to solve the equation for pavement thickness.
Change in performance, (PSI usually 2 to 2.5
Modulus of sub reaction Modulus of Sub Grade Reaction Test
Requires corrections for
Seasonal effects
Thickness of sub base
Loss of support
Location of bedrock
Traffic – by ESAL
Concrete properties
MOR-varies with concrete mix in the range from 500-800 psi
Modulus of elasticity- typical value 4000000 psi
Drainage coefficient as in the flexible pavement design
Reliability- depends on the functional classification 50%-99.9%
Standard deviation-0.4 to 0.5
Load transfer – RJCP = 3.2, PJCP = 3.8
Loss of sub grade support
CTB = 0 to 1
Granular = 1 to 3
Fine granular = 2 to 3
Solution to equation shown in AASHTO Design Guide for Pavement Structures or Fig 21.13
A Calculator for determining concrete pavement thickness is available at Calculator for Concrete Surfaced Road
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