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Cast-in-place Segmental box bridge

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Do any of the bridge engineers out there have any thoughts or know of a good
resource in any of the state DOTs to offer some insight into the following
problem?  Any suggestions or contact information would be greatly
appreciated.

BACKGROUND:
We are working on a rehab job of a CIP segmental box bridge, constructed
originally in the early 1980s, that is comprised of nearly identical twin
structures approximately 1 1/4 miles in total length (mainline road and
several access ramps).  General layout of the bridge is as follows:  Typical
spans between 180'-265' long utilizing single boxes (9' tall x 17' wide) and
double boxes (9' tall x 35' wide) reinforced with mild steel and
longitudinal PT strands.  Typical boxes have 15" thick webs, 9" thick top
slab (thickening over 4'-0 length to 18" thick at the webs) and 8" thick
bottoms.  Project is located in the Midwest and subject to heavy snow and
deicing salts in winter.

Most of our work is straightforward, consisting of upgrades to barrier
rails, cleaning/repairing drains and replacement of expansion joints.  The
one unique aspect of the job is how to deal with longitudinal hairline
cracks we are observing in the top of the boxes.  These cracks are located
mid-span of the top slabs, extend full depth thru the box's top slab and
topping and occur along the entire length of the job.  So far we've gained
access inside the boxes on one of ramp sections and have observed
efflorescence and minor rust stains along the cracks.  We've observed these
longitudinal cracks in both the dead of winter and the heat of summer and
they always appear to remain hairline in nature.  Subsequent to our
inspections, one of the ramps was rehabbed at which time the entire topping
was removed and replaced.  Since that time the longitudinal cracks in the
box tops migrated thru the new topping slab with no noticeable difference
between the rehabbed section and the original structure and topping.  

One of our initial thoughts on the origin of these longitudinal cracks is
excess deflection of the top slab.  After some additional research, its been
suggested that perhaps a temperature differential due to the sun hitting the
outside of the boxes while the inner half is shaded by the adjacent twin
structure could also be causing these cracks.  To maintain the useful life
of the structure and avoid any further deterioration of the top slab
reinforcing, we are considering injecting the cracks with a flexible
sealant.  However, there are now questions being raised whether any
remediation to the boxes is necessary at all.  The toppings are all in
excellent condition with maybe 0.2 % delamination and the only real
structural problem evident is the hairline cracks.  The rest of the job is
all maintenance work and modifications to bring the bridge up to current
design standards.

At first look, sealing the cracks appears to be the best answer since
cutting off water from migrating into the boxes prevents deterioration of
the reinforcing from occurring.  However, we're talking about a lot of money
to remove the topping, install injection ports along the cracks, inject the
sealant and finally replace the topping (which didn't need replacement to
begin with) not to mention the maintenance of traffic nightmare we would be
creating.  

QUESTION:
What are the ramifications of waiting a few years before doing anything with
the cracks?  Water will continue to migrate thru the crack but the cracks
remain tight after 20+ years and all that has resulted is some efflorescence
and minor rust staining.  Are longitudinal cracks commonly found on
segmental boxes and shouldn't be a real concern?  Do they possibly signify a
larger problem that we've all overlooked?  Most importantly, our DOT client
is interested in finding precedent from similar projects before deciding how
to proceed: are there any?

Thanks for any insight you might have.

Troy Madlem, P.E.


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