30 June 2001
two. This equation gives generally conservative results. Unbonded overlays are best suited for
restoring a deteriorated pavement to structural and functional capacity.
(4) Flexible overlay where an asphaltic concrete is placed directly on a rigid base pavement
to restore surface and structural quality. For very thick overlays, a combination of granular base
and asphalt concrete surface can be used provided the granular base is positively drained so that
no water can be trapped in the overlay. When compared to more powerful layered elastic based
overlay analysis, the flexible overlay equation tends to be somewhat unconservative for thin
overlay thicknesses and conservative for relatively thick overlay designs. Because of reflective
cracking problems, flexible overlays are probably best suited as an interim rehabilitation technique
that postpones more comprehensive restoration of a deteriorated pavement.
b. Because of concerns over FOD damage to jet aircraft engines, the empirical rigid and
flexible overlay equations were developed for entirely different failure conditions in the accelerated
traffic field tests upon which they were based. The rigid overlay sections were considered failed
when initial structural cracks appeared, since such cracking was considered the precursor of
spalling and potential FOD problems. Failure for the flexible overlays was taken to be when the
underlying slab was shattered into 35 or more pieces and the subgrade was on the verge of failing.
Because these equations represent two vastly different pavement conditions at the end of the
pavement design life, it is not appropriate to try to make comparative cost comparisons between
flexible overlays and rigid overlays designed using these equations. Also, this extreme terminal
design condition for the flexible overlay equation is empirical and can give anomalous results such
as negative numbers. This simply means the design case is outside the valid conditions, and the
minimum thickness flexible overlay of 100 millimeters (4 inches) should be used.
3. SITE INVESTIGATIONS. Explorations and tests of the existing pavement will be made to
determine the structural condition of the existing pavement prior to overlay, assess the required
physical properties of the existing pavement and foundation materials, and locate and analyze all
existing areas of defective pavement and subgrade that will require special treatment. The
determination of the structural condition and required physical properties of the existing pavement
will depend upon the type of overlay used as described in subsequent paragraphs. An investigation
will be conducted to determine whether there are voids under the existing rigid pavement. This
investigation is especially important if there has been, or is, any evidence of pumping or bleeding of
water at cracks, joints, or edges of the existing rigid pavement. Nondestructive pavement test
equipment has application for this type of investigation. If voids are found under the existing rigid
pavements, fill the voids with grout before the overlay is placed. The results of the investigation,
especially the nondestructive tests, may show rather large variations in the strength of the existing
pavement and may lead to a requirement for more extensive testing to determine the cause of the
variation. It will then be necessary to determine the feasibility and economics of using a variable
thickness overlay, basing the design on the lower-strength pavement section, or removing and
replacing the low-strength pavement areas.
PREPARATION OF EXISTING PAVEMENT.
a. General. The preparation of the existing pavement prior to overlay will vary, depending
upon whether the overlay is rigid or nonrigid.
b. Rigid Overlay. Overlay thickness criteria are presented for three conditions of bond
between the rigid overlay and existing rigid pavement: fully bonded, partially bonded, and
nonbonded. The fully bonded condition is obtained when the concrete is cast directly on concrete