Section 10: Thin ACP Overlays
The service life of an older existing CRCP pavement can often be extended many years by the addition of a thin Asphaltic Concrete Overlay (ACP), approximately 2 in. thick. This treatment can be applied to pavements that are beginning to show deterioration, getting rough, experiencing an increasing number of punchouts, or a have experienced a loss of skid numbers.
There are two principal reasons for adding a thin ACP overlay to an existing PCC pavement; either to greatly retard structural deterioration and formation of punchouts, or to improve skid resistance. Both types of treatments have been very successful. 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 ACP helps keep water from penetrating to the subbase. All the reasons may be true, but it is essential to keep surface water from infiltrating on a CRCP that has experienced 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 ACP 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 subbase has greatly contributed to the formation of the punchouts. For the overlay to be successful, it needs to treat the underlying factors to the distress. 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.
Thin ACP overlays can dramatically improve the skid resistance of CRCP pavements that are still in very good structural condition, with few or no punchouts. The PFC hot mix is an excellent choice for improving skid resistance and reducing the potential for hydroplaning. If the CRCP is older and has not experienced water related distress, an impermeable layer may not be necessary. Once the overlay has been placed, it is more difficult to install an impermeable pavement layer. A PFC by itself may retard the water from draining from the surface and retain a significant amount of water within the PFC matrix. This could lead to a significant increase in water infiltration. This could lead to a softening of an unstabilized subbase and the beginning of punchouts.
ACP overlays have also been used on older jointed concrete pavements. These overlays are frequently much thicker, often with 6 in. or more of ACP added to structurally deficient JCP that has experienced pervasive transverse contraction joint failures with faulting. The loss of load transfer at the transverse joints results in independent movement of the JCP slabs which causes a crack to form in the ACP that reflects through to the surface. ACP spalls from either side of the joint. If the joint in the JCP is open wide enough, two cracks will reflect through, one from each side of the joint. Large pieces of ACP can break off and leave an opening above the joint. The success of those overlays has been marginal to poor. A “rule of thumb” has been cited as to the success – each inch of ACP adds a year of life. This makes ACP overlays of structurally failing JCP an expensive treatment for a relatively short life. One exception to the performance of thin ACP overlays on JCP occurs when the existing pavement remains in excellent structural condition, particularly the joints, and the only distress is a loss of skid resistance.
Rubblization of the existing concrete pavement combined with a thick ACP 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.