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RE: Cracks in Masonry

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The cracking is occuring only at the mortar joints.

There is some movement out of plane, but where that occurs the displacement
is for the majority of the wall height, so there is still bearing on both
sides of the cracks - it is not like just a couple of courses are displaced.
And the displacement is not large - perhaps a quarter of an inch.  This
occurs on the rear wall on one abutment.

The larger cracks are in plane and are located on the side walls.  This
makes sense - as the thermal effects occur, the side walls (parallel to the
tee-beam) are being stressed.  Once the bridge expansion has been addressed
by proper joints, then the sidewalls should no longer experience that
loading.

David Finley
M. David Finley, P.E., P.A.
2086 SW Main Boulevard - Suite 111
Lake City, FL  32025
386-752-6400



-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
Sent: Saturday, January 13, 2007 6:05 AM
To: Seaint(--nospam--at)Seaint.Org
Subject: Re: Cracks in Masonry


David,

I assume based upon your description that the cracks basically occur at
the masonry mortar joints primarily (i.e. based upon your conclusion of
thermal effects...presumably thermal effects of the precast t-beam).  Is
that the case or do you have significant cracking of the CMU units
themselves?  Is the cracking/movement causing CMU blocks to basically move
out-of-plane (i.e. pull out from the wall)?  If the later is true, then I
could see some concerns of continuous bearing of the units at the bed
joints at the faces of the units (might be only down to bearing at the
webs if two successive courses don't line up).

I would say that if the "bearing" of the face bed joints is still
basically good and it is just a matter of some mortart joints opening up,
then using mortar might be OK.  But, I can't get a good feel for how bad
things may or may not be from your description.  I would say that the
biggest issue may not be the repair of the cracks, but rather has the
masonry lost enough structural integrity (i.e. out of plum or blocks no
longer sitting directly on top of each other or "buckling" of the the CMU)
to have general structural problems.  If the wall is stabile and doesn't
have any serious overall structural deficiencies, then mortaring the
cracks might be Ok.

Regards,

Scott
Adrian, MI

On Fri, 12 Jan 2007, M. David Finley, P.E., P.A. wrote:

> Old pedestrian bridge consisting of single web precast t-beam.  60'-0"
span.
> Abutments are rectangular (8'-0" wide to match the t-beam and 6'-0" long).
> The front (bridge side) wall bears on a concrete gravity wall.  The side
> walls bear partly on the gravity wall and partly on a continuous strip
> footing.  The rear wall is on a strip footing.  All of the masonry is
> 8"x8"x16 CMU with no vertical or horizontal reinforcing and no grouted
> cells.
>
> The masonry in the abutments has cracks, including some with in/out of
plane
> displacement.  The cracks appear to be caused by the fact that the
original
> construction apparently ignored thermal effects and "locked" both ends of
> the single web t-beam into the front masonry wall.
>
> I am correcting the expansion issue by providing expansion joints at each
> end of the bridge.  No problem there.  And the new expansion joints will
> significantly reduce the forces being transferred into the masonry.
>
> The Owner (small town, so limited funds) wants to repair the masonry by
> patching the cracks with mortar.  Some of the cracks are 3/4" wide.  Can
> cracks that wide be adequately repaired with mortar?  Or should some type
of
> pressure epoxy grouting be used?
>
> Of course this bridge has historical significance, so the "tear down and
> replace" option is not available.
> David Finley
> M. David Finley, P.E., P.A.
> 2086 SW Main Boulevard - Suite 111
> Lake City, FL  32025
> 386-752-6400
>
>

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