Section 8: Concrete Overlays
Applying a concrete overlay on an HMA-surfaced
pavement may be a viable rehabilitation strategy under certain circumstances.
Where existing distress in an HMA-surfaced structure is confined
to the HMA itself (mix rutting, shoving, washboarding, cracking),
but otherwise the existing substructure is sound, a concrete overlay
can offer a durable replacement surface. These circumstances may present
themselves at controlled intersections or along open sections of
highway. The process of applying a concrete overlay on an HMA-surfaced
pavement is sometimes referred to as “whitetopping,” a term used
by the American Concrete Pavement Association (ACPA). The ACPA further divides
whitetopping into three sub-categories:
- conventional whitetopping,
- concrete inlay,
- thin whitetopping.
Conventional whitetopping will use a slab with a thickness
designed by the Rigid Pavement Design procedure (Chapter 8) placed
on top of the existing HMA surfaced pavement. Minimum thickness for
this overlay option is 7.0 in.
Concrete inlays may be placed on thicker HMA pavements that
have been partially milled out. Depending on the proposed slab thickness,
requirements might follow either those of conventional whitetopping
or thin whitetopping.
Thin whitetopping will use slabs from 4.0 to 7.0 in. thick
that are placed on an HMA surface that has been milled or otherwise
prepared to enhance the bond. Refer to Chapter 8, Section 8, “Thin Concrete
Pavement Overlay (Thin Whitetopping),” for the thickness design,
slab size selection, and existing HMA support requirement. Concrete
overlays thinner than 4.0 in. are not allowed under department guidelines.
This may restrict the use of concrete overlays at certain curb and
gutter intersections where vertical profile may not allow direct
placement on top of the existing HMA structure, and milling to the
appropriate depth may leave insufficient support.