This section describes the construction of unbonded concrete overlays over both CRCP and CPCD. The Portland cement concrete (PCC) overlay pavement system, consisting of a concrete layer over existing PCC pavement with an interlayer between them to break the bond, is called an unbonded concrete overlay (UBCO).

UBCOs have been used successfully in many parts of the country. UBCOs are typically thicker and will generally cost more than a BCO. While the existing PCC pavement should be in good condition for a BCO to work, UBCO can be successfully used where the existing pavement is in poor condition. This is a significant advantage of UBCO over BCO or other rehabilitation methods. Also, very little preparation work is needed, except for repairing shattered slabs in the existing CPCD or punchouts in CRCP.

The performance of an UBCO is highly dependent on the thickness and quality of the interlayer. It’s been shown that bituminous mixtures provide the best materials for interlayer. Part of the reason is that bituminous mixtures have lower modulus, thus reducing curling and warping stresses in the overlaid slab. Also, the bituminous layer provides some protection against distresses in the existing pavement affecting the overlaid concrete. The recommended mixtures for interlayer are Dense-Graded item 340/341 TY-C and TY-D, and Superpave mixtures item 344 SP-C and SP-D.

If the existing concrete pavement is overlaid with an HMA layer already, the existing HMA layer can be utilized as the interlayer. The minimum thickness of structurally sound HMA required for bonding is 3 in. Milling can be used to remove the surface distresses of existing HMA. The amount of HMA removed depends on the types and severity of distresses and the thickness of the HMA. If a stripped or debonded layer of HMA is encountered, it must be completely removed to provide a sound structural layer for bonding. All unsound areas should be removed prior to performing any further operations.