TM 5-822-13/AFJMAN 32-1018
8-5. Rigid Overlay of Flexible and Composite Base Pavements.
This type of design includes rigid overlay of either flexible or composite base pavements. The design procedure
for these types of overlays are contained in TM 5-822-5/AFM 88-7, Chap. 3.
8-6. Flexible Overlay of Rigid Base Pavements.
The flexible overlay thickness for roads and streets can be determined using equation 8-4.
to = 3.0 (Fhd - CbhE)
(eq 8-4)
where
to = required flexible overlay thickness, inches
F = a factor that projects the cracking expected to occur in the base pavement during the design life
of the overlay
hE, hd = defined in equation 8-1 to 8-3
Cb = condition factor
Condition factors for existing plain concrete pavements are assigned based on the following conditions:
Cb = 1.00 - Pavements are in good condition with some cracking due to load but little or no progressive-
type cracking.
Cb = 0.75 - Pavements exhibit progressive cracking due to load and spalling, raveling, and minor fault-
ing at joints and cracks.
Cb = 0.50 - Pavements exhibit multiple cracking along with raveling, spalling, and faulting at joints
and cracks.
Condition factors for existing reinforced concrete pavement are assigned based on the following conditions:
Cb = 1.00 - Pavements are in good condition but exhibit some closely spaced load-induced transverse
cracking, initial interconnecting longitudinal cracks, and moderate spalling or raveling of joints
and cracks.
Cb = 0.75 - Pavements in trafficked areas exhibit numerous closely spaced load-induced transverse and
longitudinal cracks, rather severe spalling or raveling, or initial evidence of punchout failures.
The estimate of condition factor C for plain concrete pavement may be made from the SCI of the existing rigid
pavement. The SCI is that part of the PCI related to structural distress types or deduct values. The relationship
is shown in TM 5-825-3-1/AFM 88-6, Chap. 3, Section A. However, when determining Cb, the only distresses
considered are those associated with structural loading. These include:
a. Longitudinal, transverse, and diagonal cracks of medium to high severity.
b. Corner breaks of any severity.
c. All large patches of load associated failures.
d. Pumping.
e. Settlement or faulting of any severity.
f. Shattered slabs of any severity.
g. Certain types of joint spalls believed to be load-associated.
If the PCI is calculated using only these structural distresses (SCI), Cb can be obtained from figure 8-2.
(1) The F-factor is a function of the foundation k value and design traffic, an is determined as follows.
(2) The modulus of subgrade reaction k may be estimated using the following relationship between subgrade
modulus and static modulus of soil reaction or from table 4-2:
k = 10x
(eq 8-5)
where
X = log10 E - 1.415
1.284
k = modulus of subgrade reaction, pci
E = subgrade modulus, psi
(2) If a base or subbase is present above the subgrade, an effective k is determined from figure 8-3.
(3) The F-factor is then obtained from figure 8-4.
8-7. Flexible Overlay of Flexible Pavements.
The flexible overlay thickness above a flexible pavement is the difference between the existing pavement thick-
ness and a new pavement thickness determined using the layered elastic procedure and the modulus value of the
existing subgrade. For instance, if the existing flexible pavement is 16 inches (i.e., 4-inch bituminous concrete,
6-inch base, and 6-inch subbase) and the new pavement thickness is determined by the layered elastic method to
be 19 inches, the flexible overlay thickness will be 3 inches.
8-3
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