Unbonded Concrete Overlay

The PCC overlay pavement rehabilitation technique consisting of a concrete layer over existing PCC pavement with an interlayer between them to break the bond is called an unbonded concrete overlay. The UBCO has been used successfully in many parts of the country.

While the existing PCC pavement should be in good condition for the BCO to work, an UBCO can successfully be used where the existing pavement is in a poor condition. This is a significant advantage of the UBCO over BCO or other rehabilitation methods. Also, not much preparation work is needed, except for replacing/repairing shattered slabs in CPCD or punchouts in CRCP.


For the BCO, proper bond was the most critical factor affecting its performance. For the UBCO, the slab thickness and quality of the interlayer are key. It has been shown that bituminous mixtures are the best materials for interlayers. Part of the reason is that bituminous mixtures have a significantly 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.

In an UBCO, the existing PCC pavement, whether it’s CPCD or CRCP, takes the role of a stabilized subbase like cement treated base (CTB).


Even though the UBCO is a rehabilitation method, the design and construction of the UBCO is very similar to that of new construction. The UBCO procedures are as follows:

Evaluate whether the project is a good candidate for an UBCO

This process is identical to the BCO procedures. If the analysis shows that a BCO is not a good rehabilitation candidate, an UBCO could be a good alternative.

Since the UBCO will increase the thickness of the total pavement system, projects where vertical clearance at numerous locations is an issue may require an alternate rehabilitation strategy.


Develop adequate slab and interlayer thickness and steel designs

Once the UBCO is determined to be a good candidate, the slab and interlayer thicknesses are determined considering future traffic loading and the conditions of the existing PCC pavement. There are several methods available for this analysis. The most widely used method is the method included in the AASHTO 1993 Design Guide. The thickness designs of the PCC slab and interlayer are similar to the new pavement design. The major difference is the assessment of the support provided by the existing concrete pavement. As with standard pavement slab design, the thickness of the overlaid PCC slab is sensitive to design traffic, but not terribly sensitive to the conditions of the existing pavement.

Repair distresses in the existing pavement

If the UBCO is placed over CPCD, shattered slabs need to be removed and replaced. If the UBCO is placed over CRCP, any punchouts should be repaired with FDR. Even though spalling, whether they are partial depth or full depth, does not require repairs, some agencies choose to repair them before applying the interlayer and UBCO.

Place concrete and provide optimum curing

Once all the distresses requiring repairs are completed, the interlayer is placed. The rest of the construction is very similar to a new PCC pavement.


References

Original article content and pictures contributed by TxDOT.