Hi Ilene, I wouldn't mind weighing in. First off I'm assuming we're talking
about mortars postdating around 1930. We rarely find the 1:1:6 blends in
mortars earlier than that. If your lab is finding them regularly for
earlier mortars, make sure that they are not simply performing a chemical
analysis and calculating the numbers assuming a portland-lime (I see that
very often in my document reviews).
With respect to alkalinity, I'm going to move your decimal places over
assuming you want less than 0.6% equivalent alkalis (i.e.; passing the
optional ASTM C150 requirements). This requirement should not limit you to
white cement as many gray portlands will pass muster due to the need to
minimize alkali-aggregate reactions in concrete. Nevertheless, there's
nothing necessarily wrong with white cement. With respect to in-kind
replacement, they are less common than gray cements historically.
I'm somewhat surprised that SpecMix is being cagey about their formulation.
They often formulate on custom order by proportion. So I am assuming then
that this is a stock blend for them. In this case, my biggest concern would
be sand matching. My experience with SpecMix is that their stock sand tends
to be a fine light colored quartz sand that matches only a small percentage
of historical mortars. Furthermore, the fact that they're trying to pass on
performance suggests to me that they're failing ASTM C144 on the sand due to
fineness. ASTM C144 allows you to fail on the sand gradation if you can
demonstrate you pass the property (or strength) specification in ASTM C270
(have I redlined on the goobledeegook meter yet?). A vast proportion of
available masonry sands for contemporary construction fail on gradation.
As far as their proprietary blends, I doubt they are doing anything fishy.
I have always found them to be on the up and up. I would expect them to
label their bags as masonry cement if they were dumping in crushed
limestone. Every time we've analyzed their portland lime mixes we've found
only portland, dolomitic lime, pigment, and sand.
However, proportions are so much more important than strength. If SpecMix
is blending by property then they are responding to the misguided needs of
the contemporary construction market. Erring on the safe side to satisfy
poorly written job specs means using cements of greater fineness to increase
early strength and going light on lime.
You have every right to demand that SpecMix provide you with batch tickets
as well as mill certificates for their cement, lime, and sand (ASTM C150,
C207, and C144 testing) if you're purchasing their product. I would insist
on those submittals. The batch tickets will be given in weights. If you
get them, I would be happy to convert them to volume proportions for you.
Anyway, that's my 37 cents
John
-----Original Message-----
From: plz practice conservation of histo presto eye blinks
[mailto:[log in to unmask]] On Behalf Of Ilene R. Tyler
Sent: Saturday, January 12, 2008 9:25 AM
To: [log in to unmask]
Subject: [BP] Spec Mix
Anybody want to weigh in on this?
Typically, we specify repointing mortar to match original mortar, based on
laboratory testing for materials and proportions, expecting to see the
ubiquitous Type N, 1:1:6, mix. Further, we specify white Portland and .06
alkalinity. This allows us to reject gray Portland with all its impurities,
and locally St. Mary's cement which has .1 alkalinity. Sometimes we just
specify the Type N mix without testing, based on our experience for age of
building and conditions.
So, when is it possible to allow a pre-mixed and bagged combination of these
essential ingredients? Spec Mix tries to fill that need, but I'm skeptical
of any proprietary additions, which they deny they do, but are intentionally
vague about the proportions. They say their mix is based on performance and
not proportions, so their Type N mix is just white cement and hydrated lime
and clean sand, but they exceed 750 psi in strength. Although typically
their mix reaches 1,250 psi, as long as we test our masonry to be stronger
than the mortar, where am I afoul of restoration practice.
Case in point: our current contractor, highly regarded and very
experienced, is working high on a platform and doesn't want the hassle of
storing and site mixing his mortar ingredients. Spec Mix is his answer.
I stand exposed and vulnerable, but willing to receive all your barbs and
advice, and will do what I need to do, but perhaps with greater wisdom.
Thank you for being gentle and patient, as I strive to better understand
this question of practicality in the field. I'd just like to close the gap
between what architects specify, and what contractors really do, assuming
we're both doing our best for the building.
Ilene R. Tyler, FAIA
QUINN EVANS | ARCHITECTS
219 1/2 N. Main Street
Ann Arbor, MI 48104
ph: 734 663-5888
fx: 734 663-5044
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