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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