04_aashto1993_flexible

AASHTO 1993
Flexible Pavement Design Equation

Date goes here


Outline
1.
2.
3.
4.

AASHO Road Test
Present Serviceability Index (PSI)
Equation and terms
Example


AASHO Road Test

AASHO Road Test (1)
1958 - 1961

Picture from: Highway Research Board Special Report 61A-G


AASHO Road Test

AASHO Road Test (2)
• Construction:
August 1956 - September 1958

• Test Traffic:
October 1958 - November 1960
• Special Studies: Spring and early summer 1961


AASHO Road Test

Test Loops (1)

Picture from: Highway Research Board Special Report 61A-G


AASHO Road Test

Test Loops (2)

Picture from: Highway Research Board Special Report 61A-G


AASHO Road Test

Environment
•
•
•
•

Mean Temperature (July)
Mean Temperature (January)
Annual Average Rainfall

Average Frost Depth
(for fine-grained soil)

76°F
27°F
34 inches
28 inches


AASHO Road Test

Flexible Materials
• HMA

– Dense-graded
– 85-100 pen asphalt

• Base Course

– Crushed limestone
– 10% passing No. 200
– Average CBR = 107.7

• Subbase Course

– Sand/gravel mixture
– 6.5% passing No. 200
– CBR = 28 – 51

• Subgrade

–
–
–
–

A-6 soil (silt/clay)
82% passing No. 200
Average CBR = 2.9
Optimum wc = 13%


AASHO Road Test

Flexible Sections
• HMA

– 1 to 6 inches thick

• Base Course

– 0 to 9 inches thick

• Subbase Course

– 0 to 16 inches thick

• Thickest section
–
–
–

–
–

6 inches HMA
9 inches base
16 inches subbase
Used for heavy loads
2.6 to 3.6 PSI at test
end

• Thinnest section

– 1 inch HMA
– Used for light loads
– 8 to 25 ESALs to failure


AASHO Road Test

Flexible Performance
• Majority failed
• Even thickest sections sustained
appreciable damage
• Most failed during spring thaw

– Frost action was a major contributor
– Thicker base & subbase helped to mitigate
frost action

AASHO Road Test

Rigid Materials
• Cement

– Type I
– 564 lb/yd3

• Portland Cement Concrete
–
–
–
–
–

Maximum w/c = 0.47
14-day compressive strength = 3500 psi
14-day flexural strength = 550 psi (1/3 point)
Slump = 1.5 to 2.5 inches
Maximum aggregate size = 1.5 and 2.5 inches

• Subbase and subgrade were the same as
flexible sections


AASHO Road Test

Rigid Sections
• Slabs

– 2.5 to 12.5 inches
thick

• Subbase Course

– 0 to 9 inches thick

• Dowel Bars

– All had dowel bars
– Sizes varied

• Thickest section
–
–
–
–

12.5 inch slab
9 inches subbase
Used for heavy loads
4.2 to 4.5 PSI at test
end

• Thinnest section

– 2.5 inch slab
– Used for light loads
– 4.2 to 4.4 PSI at end

AASHO Road Test

Rigid Performance
• Majority did not fail
• Most sections PSI at the test end was
around 3.8 to 4.4


AASHO Road Test

Trucks

Picture from: Highway Research Board
Special Report 61A-G


Picture from: Highway Research Board Special Report 61A-G

AASHO Road Test

Subgrade Support Variation


Test Tracks Today

NCAT Test Track


Present Serviceability Rating (PSR)

Definition:

"The judgment of an observer as to the
current ability of a pavement to serve the
traffic it is meant to serve"


AASHO Road Test

Present Serviceability Rating (PSR)

Picture from: Highway Research Board Special Report 61A-G


AASHO Road Test

Present Serviceability Rating (PSR)

Picture from: Highway Research Board Special Report 61A-G


Present Serviceability Index (PSI)
• Calculated value to match PSR





PSI  5.411.80log 1  SV  0.9 C  P
SV = mean of the slope variance in the two wheelpaths

(measured with the CHLOE profilometer or BPR Roughometer)
C, P = measures of cracking and patching in the pavement surface
C = total linear feet of Class 3 and Class 4 cracks per 1000 ft2 of pavement area.
A Class 3 crack is defined as opened or spalled (at the surface) to a width of
0.25 in. or more over a distance equal to at least one-half the crack length.
A Class 4 is defined as any crack which has been sealed.
P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.


Basic Equations

Serviceability (PSI)

Basic Idea
p0

p0 - pt

pt
Time


Basic Equations

Basic Relationship
W
po  p   p0  pt 









•  and  depend on pavement
structure (thickness and stiffness) and
loading
•  determines the shape of the graph
•  is the number of loads at which p =
1.5


Basic Equations

Basic Equation
 PSI 
log

 4.2  1.5   2.32 logM   8.07
log W18  Z R  S 0  9.36 log( SN  1)  0.20 
R
1094
0.40 
SN  15.19

• Choose these values

– Reliability (ZR and S0)
– p0, pt ΔPSI

• Measure MR


Explanation of Terms
 PSI 
log

4.2  1.5 

log W18  Z R  S 0  9.36 log(SN  1)  0.20 
 2.32 logM R   8.07
1094
0.40 
SN  15.19

W18
Base 10 logarithm of the predicted number of ESALs over the
lifetime of the pavement. The logarithm is taken based on the
original empirical equation form from the AASHO Road Test.


Explanation of Terms
 PSI 
log

4.2  1.5 

log W18  Z R  S 0  9.36 log(SN  1)  0.20 
 2.32 logM R   8.07

1094
0.40 
SN  15.19

SN
Structural number. An abstract number expressing the structural
strength of a pavement required for given combinations of soil
support (MR), total traffic (ESALs) and allowable change in
serviceability over the pavement life (ΔPSI).