UFC 3-260-03
15 Apr 01
b. Methods of Evaluation. The methods of evaluation for nonrigid overlay on rigid pavement are
presented below. One method, designated as rigid pavement overlay evaluation, uses evaluation curves
for plain concrete pavements discussed in chapter 6. The other method, designated as flexible pavement
evaluation, uses the flexible pavement evaluation curves presented in chapter 5. Normally, the rigid over-
lay evaluation method yields the higher allowable gross weights at a selected pass level for these types of
pavements and will be used. However, when the flexural strength of the rigid base pavement is less than
2.76 MPa (400 psi) or the k value of the foundation is greater than 54 kPa/mm (200 pci), the flexible
pavement evaluation method will sometimes yield the higher allowable gross weight at a selected pass
level, in which case this method should be used. Therefore, when the test results indicate that the flexural
strength of the rigid base pavement is less than 2.76 MPa (400 psi) or the k value is greater than
54 kPa/mm (200 pci), it will be necessary to evaluate the nonrigid overlay on rigid pavement by both
methods to determine which yields the higher allowable gross weight for a selected pass level.
(1) Rigid Pavement Evaluation Method. The first step in evaluating a nonrigid overlay using the
rigid pavement evaluation method is to determine the equivalent thickness of the combined overlay sec-
tion. The equivalent thickness, hE , is defined as the thickness of a plain concrete pavement having the
same load-carrying capacity as the combined overlay section and can be determined by the following
equation:
1
( 0.33 t % hb)
hE '
(eq 6-6)
F
where
t = thickness of nonrigid overlay pavement, millimeters (inches)
hb = thickness of rigid base pavement, millimeters (inches)
F = a factor which controls the degree of cracking in the rigid base pavement.
(figures 6-61 through 6-80)
(a) The factor F in equation 6-6 is related to the controlled cracking in the rigid base
pavement during the life of the pavement and is therefore dependent on the modulus of subgrade or base-
course reaction k and traffic intensity in terms of passes. If a k value greater than 135 kPa/mm
(500 pci) is established, the F value for a k of 135 kPa/mm (500 pci) should be used in computing the
hE value. For certain values of F , the equation will yield hE greater than the combined thickness of hb + t.
When this occurs, use the value of hb + t for hE.
(b) For an evaluation, the equivalent thickness computed by means of equation 6-6, the
concrete flexural strength, and modulus of subgrade or base-course reaction are used in conjunction with
figures 6-1 through 6-60 to determine the allowable gross weight at selected pass levels or the allowable
number of passes for selected loads. However, the determining of allowable number of passes becomes
an iterative procedure since the F factor depends upon the traffic level.
(c) If a condition factor (C) for base pavement is known, then the thickness of the rigid
base pavement (hb) would be multiplied by the condition factor to determine the equivalent thickness.
Since the base pavement has been overlaid, the condition of the base pavement would not normally be
known.
(2) Flexible Pavement Evaluation Method. The flexible pavement evaluation method considers
the nonrigid overlay on rigid pavement to be a flexible pavement, with the rigid base pavement assumed to
be a high-quality base course with a CBR of 100. The nonrigid overlay on rigid pavement is evaluated as
6-11
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