Section 8: Concrete Overlays
Applying a concrete overlay on a 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 intersections or along open sections of highway. The process of applying a concrete overlay on a 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
- ultrathin whitetopping.
Conventional whitetopping will use a slab at least 4.0 in. thick placed on top of the existing HMA surfaced pavement.
For 4.0 in. to 7.0 in. thick slabs, refer to Chapter 8, Section 8, “Thin Concrete Pavement Overlay (Thin Whitetopping).”
For 8.0 in. and thicker slabs, use the same design procedures as new pavement and refer to Chapter 8, “Rigid Pavement Design.”
Concrete inlays may be placed on thicker HMA pavements that have been partially milled out.
Ultrathin whitetopping will use slabs from 2.0 to 4.0 in. thick that are placed on an HMA surface that has been milled or otherwise prepared to enhance the bond. Concrete overlays thinner than 4.0 in. are not currently 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.
Design of whitetopping slabs that are thinner than what would be derived using conventional rigid pavement design procedures (AASHTO 93, using department input guidelines) must be approved by the Director of CST-M&P. Typically, the thinner the slab, the more critical the uniformity and strength of the underlying structure will be. Thinner slabs may contain fiber reinforcement in place of conventional steel rebar. For jointed PCC overlays that are less than 8.0 in. thick, reduced joint spacing over conventional design is recommended. The ACPA recommends a joint spacing of 12 to 15 times the slab thickness to reduce curling stresses and bending stresses due to loads (adjusted to coincide with lane width/striping). Green cut sawing should be used as soon as possible before the internal stresses of cement hydration begin.