Section 10: Thin HMA Overlays
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.
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.
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.