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RE: Concrete Slab Cracking

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You have several potential causes of the cracking.
1.  By tying the slab on grade to the tie beams and pile caps you have
restrained the slab from its natural tendency to shrink.  

2.  By tying the slab on grade to the tie beams and pile caps you have
restrained the slab for vertical movement due to expansive soils and
settlement.

Parking large vehicles on the slab on grade (accelerating the gravity
settlement) will do nothing to solve the problems created by the restraints.

The only sure cure for this type of problem is a structural slab and avoid
slabs supported on grade.  

If that is not possible and the expansive nature of the soil allows; the top
3 feet of soil should be removed and a low PI soil should be compacted; the
slab on grade should be reinforced with rebar (rebar will not prevent the
slab from moving, but it will control the cracks).  All restraints should be
eliminated (do not tie the slab on grade to the pile caps or tie beams) and
the slab should be allowed to "float" on the prepared subgrade.

If all the slabs are in place and you are stuck with the existing slab.  The
only solution is to epoxy inject the cracks as they occur.  You might want
to consider a semi rigid epoxy so that you won't form any new cracks as the
slab continues to shrink.  If you do indeed have expansive soils, they will
never be totally stabilized unless you can control the ground water.

Good luck,
Harold Sprague
The Neenan Company


-----Original Message-----
From: Francis.Ang(--nospam--at)toyota.com.ph [mailto:Francis.Ang(--nospam--at)toyota.com.ph]
Sent: Monday, July 05, 1999 6:07 PM
To: seaint(--nospam--at)seaint.org
Subject: Concrete Slab Cracking


Yesterday, I visited the site of a new Toyota Dealership in Manila Bay
(Toyota Manila Bay) which is a reclaimed area in the Philippines. I noticed
that the slabs (which are anchored to the tiebeams and pile caps) are
suffering from cracks. This happened after the painting of the floors have
been finished. I think they used epoxy paint finish for these floors. I
verified with the project manager about the cracks and they said they could
not figure out the source.

According to the Project Manager, they have attained a more than 95%
compaction (Modified Proctor Test). That's one of the specs of the
structural engineer of the project. They are wondering if the cracks are due
to temperature change or due to painting works. I had to comment that if
they were temperature cracking, their widths would just be limited. I saw
cracks ranging from 2-3mm wide.  And also, the floors are covered by the
roof with heat insulation so temperature changes would be a minimal factor. 

The problem is that the project is being rushed for an earlier opening. It's
already delayed for about 2 months. The floors would be loaded by several
equipments like vehicle lifters and auto stuff. Moreso, vehicles like Land
Cruiser and Delivery Trucks would use these floors. It might suffer some
cracks in the future which would become an eyesore. 

I just would like to know some remedies or solution for these problems. Any
suggestion would be highly appreciated. Knowing that the soil is expansive
in nature (the place is a reclaimed area, highly prone to liquefaction in
case of earthquakes), I suggested that we move in some large vehicles in the
area. This would make the soil underneath the slab react and future
crackings can be observed and rectified. 

I need some verification or comments on my suggestion. They would be doing
this in a matter of days. I do think that my suggestion is one that can be
subjected to a lot of comments and rebuttal, but it's the most we can do
since the project is in a rush.

Thanks in advance !

Allan Yango
CE/SE
Toyota Motor Philippines