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RE: Low compressive strength - grout

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Peter,

You should note that my previous response was strictly looking at it from
the code provisions that I am aware of and in very general terms.

You pointed out the main concern that would come to my mind...the ability
of the grout to transfer/take the loads by way of bond from the rebar.  It
sounds from your description that the grout even in the actual prism test
(btw, the inspector seems to be a little loose with his terms...the grout
test would be a cube test, NOT a prism test...the only purpose of the
blocks in the case of the cube would be as forms to create the cube...the
blocks [forms] would then be stripped and the cube would be tested in
compression very similar to a concrete cylinder would be tested...the
block are only there as a form and also to potentially help wick away some
of the moisture) was not of the best quality if it crumbled in someone's
hand.  This would obviously be a concern for developing the rebar.  If in
fact the grout is crumbling in someone's hand and the grout must develop
rebar, then I would say that you have a problem to deal with.  So, in
response to your second question...yes, I would likely be worried about
the ability of the grout to really develop the rebar.  One possible way to
try to test the ability of the grout to develop  the rebar would be to try
a pullout test on a rebar that is embedded in the grout.  Obviously, this
could prove difficult to do on any of the test specimens since they are
likely already broken in test or don't have a rebar in them.  If you have
a speciman that has not been tested yet, then you could potentially drill
a hole into the grout and epoxy in a rebar and do a pullout type test.  I
have to admit that I am not aware of any ASTM test procedure for something
like this.

In response to your question about non-grouted vs. grouted prism tests,
while I have not read the prism test procedures in awhile, the compressive
stress reported from the test should be a function of the contact area.
So, in otherwords, yes, you could have a prism test done without the grout
and get similar results as the grouted case.  I would, however, suggest
that the results would be different still.  In the case of the grouted
masonry prism test, if the grout was failing resulting in the block and
mortar really taking all the load, then the resulting f'm from the test
should be lower than the non-grouted prism test because in theory both
prism would fail at about the same load, but the grout prism would have
a largers contact area, which would result in a lower stress.

In your respose to your response to Jake Watson, I would be very careful
about using the empirical sections of the code.  If you want to look at
the wall as unreinforced, then I would strongly encourage you to use the
engineered sections for unreinforced masonry to check the wall NOT the
empirical sections.

HTH,

Scott
Ypsilanti, MI


On Sun, 30 Jun 2002, Peter Griem wrote:

> Scott,
> 
> Thank you for your reply - and of course, Go Blue.  
> 
> In this case, two tests were taken - a grout "prism" (which I would have
> referred to as a grout "cube," but that is the term used by the inspector)
> and a grouted masonry prism.  The grout prism/cube was cast in an apparatus
> that has cmu on the sides of the box to wick away the moisture as would
> occur in a grouted wall.  The grouted masonry prism test result was 2315 psi
> (7 days) vs 1350 spec'fd (28).  The grout prism/cube broke at 650 psi (7
> days)vs 2000 psi (28).  But, the inspector noted that even the grout in the
> masonry prism was very weak and he could crumble it in his hand.
> 
> I assume that the the results for a grouted vs a non-grouted prism are
> reported based on different contact areas?  Or put another way, if the
> masonry prism test had been done on a non-grouted prism would I expect to
> still see a high prism strength because the mortar and the concrete masonry
> unit itself are of adequate strength?  In that case, the grout would just be
> extra and you'd never see a low strength break.
> 
> The development length of rebar in concrete is related to f'c.  If the grout
> strength in the masonry is very low, should I worry about a typical 40 db
> rebar lap splice not being long enough? (This is a non-loadbearing back-up
> wall)
> 
> I, too, have only found references that state 'grout shall be 2000 psi
> minimum -period' - but nothing that references how to investigate low
> strengths or what the implications of low strength grout may be.  It is a
> very interesting point that you make re: unit strength method vs prism test.
> I will look at that closely.
> 
> Thanks again,
> 
> Peter
> 
> 
> 
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Friday, June 28, 2002 9:24 PM
> To: 'SEAINT(--nospam--at)SEAINT.ORG'
> Subject: Re: Low compressive strength - grout
> 
> 
> Peter,
> 
> Keep in mind that grout for filling the cells of masonry will typically be
> of a lower compressive strength than natural instinct would dictate (i.e.
> one's gut would say that the grout compressive strength should basically
> match or excede the overall masonry compressive strength).  This is
> largely due to the fact that when the grout is placed in the masonry, the
> CMU units will actually "wick" a lot of the water content of the grout out
> of the grout and into the units.  Thus, the in place grout will end up
> with a lower water-to-cement ratio (and thus higher strength) than just
> the pure grout cylinders/cubes that were tested to get the supposed
> compressive strength of the grout.
> 
> The end result is that I would think that the compressive strength of the
> gourt, while important, is not nearly as important as the finished
> masonry compressive strength (i.e. prism test).  Thus, if you are getting
> test results that show that the masonry compressive strength (f'm) is
> adequate, then I doubt a low compressive strength for the grout would
> overly concern me.
> 
> Now, having said that, I am going to contradict myself.  ;-)
> 
> It is important to note, however, that there is a provision within the
> MSJC masonry code/spec (actually ACI 530.1/ASCE 6/TMS 602) that states:
> 
> "For grouted masonry, the meets one of the following requirements:
> 
> a) Grout conforms to ASTM C 476.
> b) Grout compressive strength equals f'm but the compressive strength is
> not less than 2000 psi.  Determine the compressive strength of grout in
> accordance with ASTM C 1019."
> 
> This provision would seem to indicate that the grout in your case would
> need to be rejected.  HOWEVER, I will point out that this provision is
> only included/required under the provisions for determining f'm by the
> UNIT METHOD.  If you are determining f'm by using the PRISM METHOD, then
> it would appear that the above provision would not apply.  Thus, it would
> seem the my initial opinion would stand.  I must admit, however, that I am
> not sure how the fact that you appear to be under the UBC might affect
> things, since the UBC does not always match the MSJC (or ACI 318 for that
> matter).
> 
> HTH,
> 
> Scott
> Ypsilanti, MI (non-UBC country)
> 
> 
> On Fri, 28 Jun 2002, Peter Griem wrote:
> 
> > If the compressive strength of grout is low (say 1000 psi vs 2000
> > specified), but the grouted and ungrouted masonry prism strengths are
> > greater than 1.33 times the specified f'm - should there be cause for
> > concern?  If so, why?
> > 
> > Where can I quickly get a copy of UBC Standard 21-17?
> > 
> > Thanks in advance,
> > 
> > Peter Griem
> > The S/L/A/M Collaborative
> > 80 Glastonbury Boulevard
> > Glastonbury, CT 06033
> > 
> > (860) 659-1010 x3389
> > griem(--nospam--at)slamcoll.com
> > 
> > 
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