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Re: Low mortor prism test results

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Alden,

You need to be careful when dealing with mortar strengths as opposed to
masonry assembly strengths.  The section of the MSJC (a.k.a. ACI 530) that
you cite is for dealing with the strength of the masonry assembly (the CMU
block AND mortar...or the brick AND the mortar).

To deal with the mortar itself, you are "forced" into ASTM C270, which
really does not specify strength requirements (it does is psuedo rather
unverifiable manner).  The "intent" of ASTM C270 is that if either the
"proportion specifications" (section 3.1...basically a predetermined
proportion of mortar materials) or "property specifications" (section
3.2...basically a set of _LABRATORY_ determined properties that must be
met tha does include strength, then it is clearly states that these
required properties are for the lab and when in the field significantly
more water maybe required to make the mortar workable which will mean that
FIELD tested properties would not even come close to these properties,
which is expected and permitted) are met then the mortar will be find.

The ASTM C270 section on property specifications includes a note that
provides you with some clear information on the issue.  It basically
points out that in the field more water will be typically added to the
mortar to make it workable.  Thus, the mortar strength will be lower than
one might expect.  But, this is "cured" by the fact that the masonry will
"suck" in a lot of that water/moisture, effectively lowering W/C ratio of
the in situ mortar.  The end result is that in theory the mortar will
increase in strength and more than achieve the needed strength.  The
intent of the required properties in table 2 (of the property
specification in ASTM C270) is to represent the "end" conditions of the
mortar in the field AFTER the masonry units have absorbed much of the
mortars water.

Now, IF the test results that you cite are from a LABORATORY mixed and
cured specimen that had a flow of 110 +/- 5% (as required in the property
specification section...this flow value results in mortar that is NOT
of sufficient workable consistency for laying masonry units in the field),
then the test result WOULD be low and you would have a concern.  If,
HOWEVER, the test results of from a field mixed (i.e. taken during from
the mortar that is actually being used to lay the masonry units) mortar,
then the local masonry rep is dead on.

HTH,

Scott
Ypsilanti, MI

On Wed, 3 Sep 2003, Alden Manipula wrote:

> I have test results for mortor in an 8" CMU wall as 800 psi for type S
> mortor.
>
> I was told by a local masonry representative that he wasn't concerned
> because the mix they used was pre-approved/pre-packaged as type s mortor and
> that they are allowed to add as much water as needed to make it workable.
>
> This doesn't seem right.  Wouldn't adding water incrase the w/c ratio and
> decrease compressive strength?
>
> Also, ACI 530-02 Section 1.14.7.1 says "Compressive strength of masonry
> shall be considered satisfactory if the compressive strength of each masonry
> wythe and grouted collar joint equals or exceeds the value of f'm"
>
> So shouldn't there be a concern if the mortor test results are so low if the
> specified f'm=1500 psi?  ACI318 is clear on acceptance criteria, but 530 is
> not.
>
> Any advice appreciated.
>
> TIA
>
> Alden
>
>
>
>
>
>
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