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Section 5: Unbonded Concrete Overlay

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 unbonded concrete overlay (UBCO).

UBCO has been used successfully in many parts of the country. While the existing PCC pavement should be in good condition for bonded concrete overlay (BCO) to work, UBCO can be successfully used where the existing pavement is in a poor condition. This is a significant advantage of UBCO over BCO or other rehabilitation methods. Also, not much preparation work is needed, except for shattered slabs in concrete pavement contraction design (CPCD) or punchouts in continuously reinforced concrete pavement (CRCP).

For BCO, proper bond was the most critical factor determining its performance. For UBCO, it’s 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 for distresses in the existing pavement from affecting the overlaid concrete.

In TxDOT, the layer right underneath the new CRCP slabs is always a bituminous layer, whether the subbase is cement treated or asphalt stabilized. The 1-in. bituminous layer over cement stabilized subbase is called “bond breaker.” Its primary contribution to the performance of CRCP is to reduce warping and curling stresses, thus prolonging the life of CRCP. In UBCO, the existing PCC pavement, whether it is CPCD or CRCP, takes the role of a stabilized subbase like cement treated base (CTB). In this sense, from a structural standpoint, UBCO is similar to TxDOT’s new CRCP system with CTB and bond breaker.

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UBCO Procedures

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

  1. Evaluate whether the project is a good candidate for UBCO.
  2. Develop adequate slab and interlayer thickness and steel designs.
  3. Repair distresses in the existing pavement.
  4. Place concrete and provide optimum curing.

1. Evaluate whether the project is a good candidate for UBCO.

This process is identical for the BCO procedures. If the analysis shows that BCO is not a good rehabilitation candidate, UBCO could be a good alternative. TxDOT is currently sponsoring a research study on BCO and guidelines will be developed to help TxDOT engineers determine the optimum PCC overlay options. The guidelines will provide information on whether the project is a good candidate for UBCO as well.

Since UBCO will increase the thickness of the total pavement system, if vertical clearance at numerous locations is an issue, UBCO might not be a good candidate.

2. Develop adequate slab and interlayer thickness and steel designs.

Once 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 PCC slab and interlayer are similar to the new pavement design. The major difference is the assessment of the support provided by existing concrete pavement. Therefore, the thickness of the overlaid PCC slab is sensitive to design traffic, but not as sensitive to the conditions of the existing pavement.

Steel design for UBCO should be in accordance with TxDOT design standards CRCP (1)-03.

3. Repair distresses in the existing pavement.

If the UBCO is placed over CPCD, shatter slabs must be removed and replaced. If it is placed over CRCP, any punchouts should be repaired with full depth repair (FDR). Even though spallings, whether they are partial depth or full depth, do not require repairs, some districts repair them before UBCO.

4. 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 the new PCC pavement which should be constructed according to Item 360 and TxDOT design standards.


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