Section 5: Unbonded Concrete Overlay

5.1 Introduction

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.

5.2 UBCO Procedures

Even though UBCO is a rehabilitation method, the construction methods are very similar to that of new concrete pavement construction. The UBCO construction procedures are as follows:
  1. Repairing distresses in the existing pavement.
  2. Placing the interlayer.
  3. Constructing the UBCO.
1. Repairing distresses in the existing pavement
If the UBCO is placed over CPCD, shatter slabs must be removed and replaced. If the UBCO is placed over CRCP, punchouts should be repaired with full-depth repair (FDR) methods. Spallings, whether they are shallow or deep, do not need to be repaired. Sections 2 and 3 detail the methods that should be used to properly repair these distresses.
2. Placing the interlayer
Place the interlayer in accordance with HMA specifications and the details of construction method in Chapter 6.
3. Constructing the UBCO
Once all the distresses requiring repairs are completed and the interlayer is placed, the UBCO is constructed using the same methods and following Item 360 and the pertinent pavement standards as when constructing a new concrete pavement. Chapter 9 discusses these construction methods.