pre mixed mortar

huangyhg

pre mixed mortar
i have a contractor that is putting up a masonry block building.  he showed up this morning with tubs of mortar already made to lay the block with.  he said this is the method they typically use....and the mortar is enough to last him a couple of days.  i have never seen this before...typically the masson has a mixer and a sand pile on-site to do his mixing.  anyone have thoughts?
is the mortar pre-mixed by the mason, or purchased?  i have never seen this either.  does the specifications call for quality assurance of the mortar..typically it does.  in order to verify quality, a qualified person has to witness the mortar making process once per day.  if they are pre-made tubes, then that can not be verified.  mortar cubes could be made to verify strength.
the contractor makes it himself in his yard.  our specification calls for inspection of all motar mixed on-site.  we are going to have to have a method of verifying the mix....i'm concerned about the time between mixing and useing this material...i know you can retemper the mortar after a couple of hours but this seems different than that
a little bit of history -
there was a generally accepted system using controlled retarders that was used by ready-mix suppliers to supply pre-mixed mortar to projects with a controlled setting/usability time that could be very long (approaching days).
it did produce acceptable performing mortar if the supplier had the necessary controls.
the real reason it is not common now it that, despite the ability to reasonably control the proportions and setting time, there was a common problem where the ready-mix trucks eventually containing a few small stones that rendered the batch useless once the masons complained (efficiency dropped!!!), despite the over-all aggregate specifications being met. it was also batched using wet aggregate weights
subsequently, the advent of factory produced, pre-proportioned mortar in 80# to 3000# bags (spec-mix, etc.) replaced the concept. the pre-packaged mortar is made to much tighter controls using dried aggregates than common or high-tech ready mix, with none of the large aggregate contamination. the pre-proportioned mortar is proportioned using dried aggregates, so guesses/estimates at compensating for moisture are eliminated.
around here, and in much of the country, the real standard of quality mortar is the big bags because of the uniformity and economy.
astm c270, in the appendix, states that the most important factor in the quality of masonry construction is the workability. it has been a long time since i have seen site proportioned mortar used by mason contractors, despite most specifications do not require pre-proportioning. - it is just better, more controlled and more economical in the end. this is especially true in variable climates.
dick