Bonded concrete overlay (BCO) consists of a 2-in. to 8-in. thick concrete layer placed on top of the existing concrete pavement with operations conducted to ensure full bond between new and old concrete layers. A BCO with a minimum thickness of 4 in. is one of the most cost-effective ways of enhancing structural capacity of under-designed pavements by reducing deflections and extending service life. A BCO with a thickness of less than 4 in. is typically used to restore pavement surface characteristics, such as ride and friction.

The department maintains many miles of thin PCC pavement that have exceeded their design traffic projection and are still in reasonably good condition. The use of BCO is based on the fundamental design assumption that the old and new concrete layers behave as a monolithic layer. Providing full bond is of the utmost importance. During construction, specific steps are taken to enhance and ensure the full bond between old and new concrete as discussed in Chapter 10.

Bonded concrete overlays over jointed concrete pavements are difficult to construct because all joints must be matched. CRCP-bonded concrete overlays have been constructed and have performed successfully in several districts but have not been used widely throughout the state. Districts considering a bonded concrete overlay can contact MNT – Pavement Asset Management, Pavement Analysis & Design Branch, for assistance.

Unbonded concrete overlay consists of a concrete layer (5 in. or greater) on top of an existing concrete with a HMA interlayer to separate new overlay and existing concrete. An unbonded overlay is a feasible rehabilitation alternative for PCC pavement for practically all conditions. These types of rehabilitation methods are most cost-effective when the existing pavement is badly deteriorated because a reduced amount of repairs were made to the existing pavement prior to constructing the unbonded concrete overlay.

Unbonded CRCP concrete overlays may be used over CRCP, jointed concrete pavement (CPCD), or jointed reinforced concrete pavement (JRCP). Unbonded CRCP overlay uses the same design procedure as new CRCP pavements. This use of unbonded CRCP overlay can be credited for contributing to the structural capacity of the existing concrete pavement and results in a thinner concrete pavement design than required for CRCP constructed on a new location.