Ilene,
I will weigh in on your query. Expect no barbs from me.
Going from the assumption that contractor is doing their best my
experience is that they do not always understand the finer points of
mortars, or the special needs of a project, and that the process of
fitting 'practical' considerations to the optimal benefit of the
structure requires a 2-way educational exchange between the architect
and the contractor. A 'good' contractor if you coach them on the reasons
behind your concerns will also work to help you find ways to best meet
the mutually desired requirements.
As to a Spec Mix mix... used to be you could not get pre-made mortars
mixed for historic work. The knowledge, the choices, and the market for
a specialized mix were not as complicated as they are now. There are now
several different sources of prepackaged mortar mixes and if one does
not suit our fancy we can go shop for another. As to the psi, I leave
that to others to debate. In recently consulting on a structure made
with glacial igneous boulders I was asked if the replacement mortar had
to be speced to be softer to which I replied, "I don't really know why
that should be a concern as you would have a very hard time coming up
with a mortar harder than that chunk of basalt." My recommendation there
will be to match the aggregate with a type N using a white Portland with
a touch of a brown tint. Availability of materials is important to
consider. And often local quarried materials are the match for
aggregates of an historic mortar. If the premix comes with a yellow sand
aggregate and your mortar has a brown sand aggregate that is something
you need to think about. Size and proportioning of the sizes of the
aggregate is as important as color, and not all historic aggregates are
purely sand.
As to matching to the existing mortar that is a decision that has to be
made in an informed manner as to what degree it really matters, or not.
This is a question that usually it is best not to bother the contractor
to make them sort it out as it is a design issue and not one of process
in the work. Keep them in the loop if they show curiosity. In many cases
I assume restraints of budget narrow the amount of 'custom' attention to
any structure, and certainly there are structures that deserve a higher
degree of attention to authenticity than others.
From a contractor's perspective a predesigned and prepackaged mix has
several advantages. A major need in the field is to be able to control
consistency of the mixed mortar in order to obtain a material that meets
to desired predictions as to performance. Field blended mortars allow an
opportunity for wide variations and as to the contractor that is doing
their best it becomes a quality control issue for the management of the
project. In the past my experience has gone so far as to prequalify
mechanics to be solely designated as the mixers of mortar, particularly
in cases where custom tinting of the mortar was required. With a
premixed mortar lesser skilled workers can be trusted to properly mix
mortar, and that in the end will reflect on the cost to do the work, and
again goes back to project budget considerations. There is, as your
contractor indicates, logistic issues and the transport of materials to
the point-of-the-work in and of itself can present problems in respect
of the quality of the materials once they arrive to that point of work
(it never helps once you get the bags of white Portland to the staging
area to find they have already gotten wet and are partially hardened and
the mechanics bust them up with a shovel before mixing) -- though a
premix will not avoid problems they are easier for the contractor to
provide the control of the project to avoid problems. There is also the
issue that with a premixed mortar the overall consistency and quality of
the materials, up to the point that it arrives to the distributor, is in
part a responsibility of the manufacturer. So, unless it is a modestly
small project I am in favor of premixed mortars. In the end the greater
the assurance of a desired quality of material with predictable
performance with a reduction in the reliance on the vagaries of an
assortment of mechanics in the field can be balanced out to an optimal
result in favor of the structure and the project as a whole.
Best,
][<en
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