chapter 3 rigid pavement - DOT On-Line Publications - Department ...

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Bondbreaker% fir Portland Cement
Concrete Pavement with.Lean Concrete Bases
Chief, Pavement Division
Regional Federal Highway Administrators
Uashinqton, D-C. '20590
Reclly 10
Attn of HHO-12
During the past 2 years, we have reviewed several projects with Portland Cement
Concrete (PCC) pavements constructed over lean concrete bases, which have
experienced premature cracking. We have suspected that the principal cause of
the d$stress was the partial bonding of the PCC slab to the lean concrete
bases, during the period of joint and crack formation in jointed and continuous
PCC pavements. Generally, this bond was believed to be weak, and would be lost
within 6 to 12 months, because of stresses caused by loading and/or temperature
variations. This weak bond would also be broken during coring or following the
development of pavement distress. However, recently on two projects, cores
were retrieved with the slab bonded to the lean concrete base. These projects
which lend support to our theory are described below:
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1. A Continuously Reinforced &Crete Pavement (CRCP) began experiencing
premature punchouts. The pavanent scctfon consisted of 9 inches of -
CRCP over 6 inches of lean concrete base'on a cement treated subgrade.
During coring operations 6-plus years after construction, approximately
30 percent of the cores indicated the slab was bonded to the base.
Failure of this pavement is believed to have resulted because the
amount and location of steel was designed based on the unbonded
condition. When bonding occurred, the slab was significantly under
reinforced, and the reinforcement was located well above the neutral
axis of the composite section. As a result, the steel was overstressed
causing.excessive.crack widths, steel ruptures, and ultimately
punchouts.
2. An &inch Jointed Plain Concrete Pavement (JPCP) over an &inch lean
concrete base experienced random cracking within 6 months after
construction. Coring revealed that the cracks were forming from the top
of the slab downward, and were not reflective cracks. Also, cores of
numerous sawed joints revealed that cracking had not occurred at the
joints. Project records and discussions with project personnel
indicated that sawing was done in a timely manner. There was no
correlation between cracking, and temperature extremes at the time of
construction. A number of the cores taken during the investigation of
the cracking were retrieved with the slab bonded to the lean concrete
base. We now believe that partial bonding during the joint formation
period rejulted in the saw cuts being an inadequate depth to force
cracking at the joints. The depth of the saw cuts was based on the
thickness of the slab in the unbonded condition.
3.12.01