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Section 10: Thin HMA Overlays

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10.1 Introduction

The service life of an older existing CRCP pavement can often be extended many years by the addition of a thin hot-mix asphalt (HMA) overlay, approximately 2 in. thick. This treatment can be applied to pavements that are beginning to show deterioration, are getting rough, are experiencing an increasing number of punchouts, or have experienced a loss of skid numbers.

Thin HMA overlays can dramatically improve the skid resistance of concrete pavements that are still in very good structural condition. The PFC hot-mix is an excellent choice for improving skid resistance and reducing the potential for hydroplaning.

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10.2 HMA Overlay on CRCP

Punchouts and loss of skid resistance have been treated successfully through the use of thin HMA overlays. There are several theories as to the reason for the great success of these thin overlays on pavements experiencing punchouts. The principal reasons usually include that a new smooth surface reduces dynamic loads from trucks driving over rough pavement, and that the new HMA helps keep water from penetrating to the base. These reasons may be true, but it is essential to keep surface water from infiltrating into a CRCP that has significant punchouts. Little or no credit is given to the added structural capacity from the overlay.

As a CRCP ages, it may start developing punchouts, which require a full-depth repair. The HMA overlay will not treat any area where a punchout has started to form. It is essential to perform a full-depth repair of any likely punchout prior to overlaying. If punchouts have developed, it is likely that surface water (rain) infiltrating to the base has greatly contributed to the formation of the punchouts. For the overlay to be successful, the underlying factors causing the distress need to be treated. In this case, the overlay needs to keep any surface water from getting to the base. This requires either a seal coat or a dense graded hot-mix with low permeability. Asphalt rubber seals have had good success. A higher permeability hot-mix, such as a permeable friction course (PFC), may be used as a surfacing if there is something under it that will prevent water infiltration.

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10.3 HMA Overlay on CPCD

HMA overlays have also been used on structurally deficient CPCD. However, the success of those overlays has been marginal to poor because they are an expensive treatment with a relatively short service life. CPCD that has experienced pervasive transverse contraction joint failures with faulting require thicker HMA. As a rule of thumb, reflective cracking progresses at a rate of 1 in. per year. The loss of load transfer at the transverse joints results in independent movement of the CPCD slabs, which causes a crack to form in the HMA that reflects through to the surface. HMA spalls from either side of the joint. If the joint in the CPCD is open wide enough, two cracks will reflect through, one from each side of the joint. Large pieces of HMA can break off and leave an opening above the joint. Rubblization of the existing concrete pavement combined with a thick HMA overlay has also been a successful rehabilitation technique. However, once the existing PCC pavement has been rubblized, it no longer behaves like a rigid pavement and can no longer be considered a rigid pavement. Rehabilitation designs using this technique should use a flexible pavement approach.

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