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Slab on Grade Floor Joints

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I'm looking for some advice or recommendations for joint caulking/treatment
for a slab on grade.

It's a 5 inch unreinforced slab on grade in a tilt up concrete warehouse.
Control joints were sawcut 1 inch deep at roughly 15 foot intervals. The
slab has been in service for about a year but the sawcut joints were never
caulked.

Myself and the geotechnical/materials engineer were called out because of
some complaints by the forklift drivers that the slab was noisy to drive on,
sort of like the "thump-thump" you sometimes get driving along a concrete
freeway. There was also supposed to be some vibration noticeable.

We stood straddling one of the joints and had a forklift drive across it.
Both sides of the joint are at the same level (i.e. flush) when the slab is
unloaded but you could very definitely feel the slab on either side of the
joint move up and down as the forklift tires crossed over it. (Forklift
weighs about 6000 lbs.)

The geotechnical engineer is confident that the sub-base is sound he figures
that the movement is pretty normal for this type of slab. He suggests that
tying the slab together across the joint with dowels is the best way to
eliminate the problem.

I think that would be overly expensive given that there is no real
functional problem at this point. But I would like to caulk the joint to
somewhat protect the edges against spalling.

Can anyone recommend a caulking which will be hard enough to resist hard
rubber forklift wheels but still be flexible enough to absorb a small of
vertical movement?

Thanks,
John MacLean