UFC 3-260-02
30 June 2001
b. All-Bituminous Overlay. The all-bituminous overlay will be composed of hot-mix bituminous
concrete meeting the requirements of TM 5-822-8/AFM 88-6, Chapter 9. A tack coat is required
between the existing rigid pavement and the overlay. The all-bituminous overlay is the preferred
nonrigid type overlay to lessen the danger of entrapped moisture in the overlay.
c. Flexible Overlay. The flexible overlay will be composed of hot-mix bituminous concrete and
high-quality crushed aggregate base with a CBR of 100, provided positive drainage of the base
course is achieved. The bituminous concrete will meet the requirements of TM 5-822-8/AFM 88-6,
Chapter 9 and the minimum thickness requirements of Chapter 8. If the design thickness of
nonrigid overlay is less than that required by the minimum thickness of bituminous concrete and
base course, the overlay will be designed as an all-bituminous overlay.
d. Thickness Determination. Regardless of the type of nonrigid overlay, the required thickness
to will be determined by
&
(17-4)
where
hd = design thickness of plain concrete pavement using the flexural strength R of the
concrete in the existing rigid pavement and the modulus of soil reaction k of the
existing pavement.
The factor hE represents the thickness of plain concrete pavement equivalent in load-carrying
ability to the thickness of existing rigid pavement. If the existing rigid pavement is plain concrete,
then the equivalent thickness equals the existing thickness; however, if the existing rigid pavement
is reinforced concrete, the equivalent thickness must be determined from Figure 13-1. F is a
factor, determined from Figure 17-2, that projects the cracking expected to occur in the base
pavement during the design life of the overlay. Use of Figure 17-2 requires converting passes to
coverages using values shown in Table 17-1. C is a coefficient based upon the structural condition
of the existing rigid pavement. The minimum thickness of overlay used for strengthening purposes
will be 50 millimeters (2 inches) for Air Force type D traffic areas and all overruns, 75 millimeters
(3 inches) for Army Class I, II, and III pavements, 75 millimeters (3 inches) for Air Force types B
and C traffic areas on light-load pavements, 75 millimeters (3 inches) for Navy and Marine Corps
secondary pavements designed for fighter aircraft, and 100 millimeters (4 inches) for all other
Army, Air Force, Navy and Marine Corps pavements. In certain instances, the nonrigid overlay
design equation will indicate thickness requirements less (sometimes negative values) than the
minimum values. In such cases the minimum thickness requirement will be used. When
strengthening existing rigid pavements that exhibit flexural strength less than 3.5 MPa (500 psi) or
that are constructed on foundations with k values exceeding 54 MN/m3 (200 pci), it may be found
that the flexible pavement design procedure in Chapter 10 or 11 may indicate a lesser required
overlay thickness than the overlay design formula. For these conditions, the overlay thickness will
be determined by both methods, and the lesser thickness used for design. For the flexible
pavement design procedure, the existing rigid pavement will be considered to be either an
equivalent thickness of high-quality crushed aggregate base with a CBR = 100 or an equivalent
thickness of all-bituminous concrete (equivalency factor of 1.15 for base and 2.3 for subbase), and
the total pavement thickness determined based upon the subgrade CBR. Any existing base or
subbase layers will be considered as corresponding layers in the flexible pavement. The thickness
of required overlay will then be the difference between the required flexible pavement thickness
17-12
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