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Re: Non-shrink Grout

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Oshin

Part of my skepticism regarding the need for non-shrink grout is that 27
years ago when I got out of College this was not such a big deal.  Since
then, some manufacturers have emphasized that their grout was non-shrink
and their competitors were not and there was a lot more concern.  This
implies that at that time many buildings were built with grout that was not
non-shrink.  If there was a real problem why are we not seeing problems
with older buildings built with grout that is not non-shrink?

Base plates can be divided up into too categories those that are stronger
than the anchor bolts, in other words the anchor bolt will fail before the
base plate, and those that are not.  In the case of many gravity columns
the anchor bolt size is rather nominal and I would expect the base plates
to be much stronger than the anchor bolts.  If the anchor bolts are
stronger, this may still not be a problem depending on the actual stresses
in the base plate and concrete bearing stresses.

Looking at more reasonable shrinkage values, I estimate that a gap of
0.0018 is more likely.  This assumes a concrete shrinkage rate of 0.0006
in/in and a 3 inch thick grout space.  Assuming an anchor bolt length of 3
inches and assuming a rigid base plate, the stress in the anchor bolt would
be about 17 ksi when the gap closed up.

Given that the shrinkage in the concrete under the base plate will occur
after the grout in placed and realizing that the bond between the concrete
and the anchor bolts may not be that dependable, it seems that the
shrinkage in the concrete may cause a larger gap.  This would be similar to
what happens when a structure is supported on stiff piles and the soil
under the structure settles. 

If we assume that the anchor bolts are rigid and the base plate is loaded
enough to close the gap under the column, the question is what is the
stress in the base plate when the gap closes.  Assuming a 1.5 inch
cantilever rigidly restrained by the column, a 1.5" thick base plate would
have a bending stress of 52 ksi and a 0.75" thick plate would have a
bending stress of 26 ksi.  Even high stress would not cause a problem
assuming normal steel ductility and assuming no crack initiators.  What I
am trying to say is that if we give it half a chance steel yields and
redistributes the loads.

If there is a problem with grout shrinkage, I would expect that the concern
has to do with bearing stresses in the grout.  Here again my feeling is
that if we look at the limit states this may not be a major problem and
that some localy high stress can be redistributed.

I am not suggesting that we should not attempt to minimize the shrinkage in
the grout but rather I am suggesting that a little shrinkage is not
necessarily a problem.  Also when detailing the connections we should also
remember that in spite of our assumptions that the loads are being
redistributed by some of these mechanisms.


Mark Gilligan