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CASE P1 - Concrete Overlays on Concrete Pavements, M. Jones
Premature cracking of the overlay.
Corner cracking in aircraft wheel paths.
Collection of Facts:  Concrete overlays on concrete pavements are designed
using empirical procedures established by an American Concrete Institute (ACI)
committee.  Three types of concrete overlays are commonly recognized: fully
bonded, partially bonded and unbonded.  The required thickness of the overlay
and consequently its performance under traffic is strongly dependent on the
degree of bond established with the base pavement.
An unbonded overlay normally requires the use of a bond breaking course
such as asphalt concrete or polyethylene sheeting between the base pavement
and the overlay.  This is intended to prevent the reflection of joints and
cracks from the base slab into the overlay.  The properties of the bond
breaking course and its thickness are not taken into account when calculating
the thickness of the overlay.  The assumptions of the design procedure,
however, require that the bond breaking course be kept very thin in order to
obtain maximum structural advantages from the base slab.
In the design of a concrete overlay for a runway at NAS North Island, it
was necessary to place a thick (greater than 1 foot) bond breaking course
along the center lanes in order to raise grade.  The bond breaking course
selected was cement treated aggregate.  The design agency calculated the
required thickness of the concrete slab to be 6 inches using the unbonded
overlay formula.  The slabs were lightly reinforced and were 12 l/2 ft. by 20
ft. in size.  The design loading was an Air Force C-141 having a gross weight
of 320,000 pounds on a twin tandem landing gear.
Within 6 months of completion of construction, random cracking of the
concrete was reported.  Cracking was more pronounced in the aircraft wheel
paths and was concentrated at the slab corners. A survey of the runway
indicated that the cracking was predominantly in the aircraft traffic lanes.
Slab warping measurements were also made which revealed that the slabs had
taken a "dish effect".  Slab corners were warped up and had lost contact with
the underlying course.
An investigation of the cracking distress yielded the
following conclusions:
Cracking occurred in the 12.5 ft. by 20 ft. slabs (length to width
ratio of 1.6) primarily as a result of warping of the slab during the
curing process.
The formula used to develop the thickness of a rigid unbonded overlay
to an existing pavement may yield a slab thickness which hasn't the
inherent strength or weight to resist significant warping, and that
the warping leaves some exterior portions of the pavement unsupported.

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