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26
24
22
,
20
E
18
}
16
"
14
o
12
-
5
10
"
8
>
6
4
2
o
23
100
150
200
250
300
500
1
k=50pci
13 5 AASHTO
Method 639
P = 9000 pounds
a 5.9 inchcs
M
25
2s
" n n
34 35
%
Deflcchon basin AREA. mches
FIGURE 13 21
Detcrmination o[ dynamick-,alue trom d
0
andAREA 1 in. = 25-4 mm, l \b =
4.45 N. I pci
=
271.3 kN/m-'). (From the
AASHTO
Guidc for Design
of
Pavcn1cnt
Structures. Copvright
1993.
American Associalion of Sta
e
Highway and
Transportation Officials. Washington, DC. U sed by pcrmission.)
a = 5_9 inches
''
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2/4
640 Chapt er
3
Design of Overl
ays
cbart
is
the dynamic k-value, whereas tbe
k va
l
ue
to be used wi tb the
AAS
HTO design
equation and ch
art is
the static k-value.The fo
ll
owing relationship may
be
used for the
conversion:
Dynamic k-value
Static k-value =
2
(13.29)
Example 1312:
Much as in example 9.4,
dete
rmine the dynamic k-value and E by Figures 13.21
and 13.22.
So lution:
In Example 9.4,
d
0
= 0.003 in. = 3 mil (0.076 mm) and A
RE
A = 31.0 in. (787
mm). From Fgure 13.21,
k
=
210 pci (57 MN/m
3
) , which is
the
same as Example 9.4(a). Wi lh
AREA =
31.0 in. (787 mm) and k = 210 pci (57 MN/m
3
, from Figu
re
13.22, EcD
3
=
6
X
10
9
Since D
=
10 in . (25 mm), E
=
6 x 10
9
/10
3
= 6 x 10
6
psi (41 GPa), which checks w lh the
5.7
X
10
6
(39
GP
a) obtainc
< 1
in Example 9.4(a).
Remaining Life after Fatigue Damage
by
Tr
atlic To compute the remaining life, the
actual traffic in 18-kip
ESAL
the pavement has carried to
date
and the total traffic the
pavement could be expected to carry to failure, as indicated respectively by
NP
and
N1 s
in Figure 13.15, must
be
determined
firs
t.
NP
m
ay be
estim
ated
by
the
pavement design
equations or nomographs in Secti
on
11.3
or
12.3. To
be
consistent with the
AAS
H
TO
Road
Test aud the development of these equations, a failure PSI equal
to
1.5 an
da
re
liability of 50 percent are recommended .
Th
e
re
maining life, RL in percent, can thcn be
comput
ed
by
RL = 100 1
-
NP )
N1 5
(13.30)
With RL known, a condition factor,
CF
, can
be
obtained from Figure 13.23, and
the effe.ctive structural capacity can be calculated as
SCett = CF X
SC
0
(13.31)
E
xa
mple
1313:
A 10
-in.
(254-mm) concrete pavement with an initial PSI of
4.
5 has been su
bj
ectcd
to
14.5
million 18-kip (80-kN)
ESA
L before being overlaid. G
iv
en that
k =
72 p
ci
(19.5 MN/m
3
,
E
= 5
X
10
6
psi
(34.5 GPa),
Se
= 650 psi (4.5 MPa), J
=
3.2, and
d = 1.0, det
er
mine the effective structura l capacity of the pavement by the remaining
life approach.
Solution: With NP
=
14
,500 ,000, it
is
now necessary to determine N1 sTue data givcn
in
th
is
example are
the
same as those in Example 12.6, except that, for failure to occur,
APSI =
4.5 - 1.5
=
3.0, instead of 1.7, and R
=
50 , instead of
95
.N1 s can be determincd from
Figure 12.17 by the fo
ll
owing steps:
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13 .5 AASHTO Method 64
0.9
~
:
0.8
ir
e
_g
;o
e
0.7
0.6
~ ~ ~ ~ L . ~ ~ . l . . - ~ ~ ~ ~ ~ ~ - - ~ - - - - 1
100
90
80
70
6
50 40
30
2
10
o
Remaioiog Lif
e, RL
, pcrcent
FIGURE
13.23
Relatiooship betwccn condition factor and remaining
He
. (From theAASHTO Guide
for
Design
of
Pavement Structurcs. Copyright 1993.American Association of Sta e Higbwayand
Transportation Official
s,
Washington. DC Used by permission.)
L
Same as
Examplc
12.6.
2. Starting
at
74 on
the
match line
in
Hgure 12.l7b, a linc
is
drawn tbrough APSI
=
3.0
until it intersecls the vertical ax is, tben turns horizontally until it intersects D = 10 in.
(254 mm) , and fioaUy turns vertically until it intersccts the horizontal axis.
3.
From the re liability scale with R = 50 , a line is drawn parallel and over the overall
standard deviation scale until it intersects the
TL
line.
4 The intersecti
on
of a line connecting
th
e last two points. tbe one in step 2 and the
other in step 3, with thc ESAL sca e gives
N1 s =
28 millions.
A more
accurate
ESAL can
be obtained from
Eq.
12.21,
or
log
Nt 5
=
7.35 log
(10
l) - 0.06 (4.22 - 0.32
X
1.5) log{[(650
X
1.)/(215.63
X
3.2)][(10)
75
-
l .132)/l(10)
75
-
18.42/(5
X
10
6
/72)
25
]}
=
7.654 - 0.06 - 0.096
=
7.498
or Nu =
31,500,000. From Eq. 13.30,
RL
= 100(1 - 14.5/31.5)
=
0.54. From Figure 13.22, CF
=
0.9. From Eq. 13.31, D
err =
0.9
X
10
=
9 in. (229 mm).
1353 Future Structural Capacity Analysis
The major objective of futurc structural capacity analysis is to determine the total
structural capacity of a new pavement required to carry
load repetitions during thc
overlay design period, as shown by
S
in Figure 13.15. In
other
words, this step is sim
ply a new pavement dcsign for either a flexible
or
rigid pavement system bascd on the
existing subgrade
or
foundation condition
s.
Conscquent
1y
, the design procedures for
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642 Chapter 13 Design of Overlays
new pavements, as discussed in Section 11.3 or 12.3,
can
be used. However, sorne design
factors which may
be
slightly different from new construction, are discussed below.
Traffic
Anal
ys is The purpose
of
traffic analysis is to determine the 18-kip (80-kN)
ESALs expected over the design life
of
the overlay
in
the design Jane. The estimated
ESALs must be calculated
by
using the appropriate equivalent factors for flexible
or
rigid pavements. Flexible pavement equivalent factors should be used if ali layers are
flexible with no any workable PCC layer underneath. Ri
gi
d pavement equivalent factors
should be used
if
the ovcrlay is PCC
or
if the existing pavement has a
PCC
layer that is
not
subjected to break and seat
or
rubblized procedurcs befare overlay. This principie is
based on the predominant effect of PCC over AC. f there is a PCC layer in the existing
pavement,
it
s effective structural capacity should be in terms
of
PCC thickness,D so the
future structural capacity should also be in terms
of
D so that the structural deficiency,
D
Deff
can
be
determined. f AC is used for
the
overlay, this deficiency in P
CC
thick
ness
can be
converted to AC thickness by a conversion factor to be discussed later.
An
approximate relationship exists between flexible pavement and rigid pavement
equivaJent factors.A factor of 0.67 can be used to converl rigid pavement ESALs to
fl
e
xi-
ble pavement ESALs. For cxample, 15 million rigid pavement ESALs equal 10 million
ESALs. Similarly, a factor of
1.5
can be used to convert flexible pavement ESALs to rigid
pavement ESALs. Failure to use the correct type
of
ESALs will result
in
significant errors
in the overlay design. Methods for traffic computations are preseoted in Chapter
6.
Subgrade The methods used for new pavements to determine
th
e effective resilient
modulus, MR
or
the effective modulus
of
subgrade reaction, k, can also be used for
overlay design. However, if as-constructed soil data are used, their properties may have
changed since construction, due to changes in moisture content
or
other factors.
f NDT
procedures
are
used,
the
methods for calculating
MR
or
the
k-value vary
with the types of ovcrlay and existing pavement as follows:
l.
f both the overlay and the existing pavement are AC, use
NDT
for flexible pave
ments, as described in Section 13.5.2, to backcalculate
MR
by
Eq.1
3.23.
2. l f the existing pavement is
PCC or
AC/PCC, regardless of whether thc overlay is
AC
or
P
CC
, use
NDT
for rigid pavements, as described in Section 13.5.2,
to
back
ca
lculate dynamic k-value by Figure 13.21.The dynamic k-value should be divided
by 2 to obtain
the
static value for use in thickness design.
3. f
the
overlay is
PCC
and
the
existing pavement is AC or fractured PCC, use
NDT for flexible pavement
s
as described in Section 13.5.2, to backcalculate MR
by
Eq
. 13.23 and by Figure 13.17. 1l1en, by considering P as sa and as
Dsa. the dynamic k-value can be fou
nd
from Figure 12.18.
Reliability
An
overlay may be designed for different levels of reliability, as described in
Section 11.3.1 for new pavement
s.
However, reliability level has a large effect on overlay
thickn
ess.
Varyiog reliability leve to determine
SN
or
D
between 65
to 99
percent may
yield a difference in overlay thickness
of
6 in. (152 mm) or more. Based on field testing, it
appears that a design reliability leve
of
95 percent and an overall standard deviation
of
0.49 for any type ofAC overlay and of 0.39 for any type of
PCC
overlay give overlay thick
nesses consistent with thosc recommended for most projects
by
state highway agencies.
