1900s masonry

huangyhg

1900's masonry
i looked at a structure that has a foundation consisting of cmu on a concrete wall.  the floor joist are bearing on the cmu and the whole wall has shifted due to lateral soil expansion.  in places the mortar has completely deteriorated from the joints.  the block needs to be repointed, but i am unsure of the mortar mix.  i heard that mortar made in the early 1900's had a high lime ratio.  is this correct?  second, if the wall is straightend and the block repositioned and re-mortared, what other considerations should be taken regarding the cmu?
you're right to have stopped and given this thought.  early mortars were either lime rich or even earlier were essentially purely slacked lime.  these mortar will have much lower compressive strengths and will also creep with time.
you need to get a historic masonry expert in to match the mortar, or you can test the cmus for strength and replace all the mortar, which requires complete demolition and is typically only done if the structure is of serious heritage value and the blocks are archialogically/architecturally irreplacable.  alternatively you might be able to replace the forward 1/3 mortar on each side of the block, one side from start to completion at a time.
modern mortars may be used to repoint historic, however it is a very specialised task to do so.  it involves, essentially, a higher water contect than you would typically expect, pre-hydrating your mortar mix and adding one or more admixtures to encourage creep, reduce water migration through the mortar bed, increase bond, etc.
you should work with an mason experienced in historic masonry.
a durable, quality job with old masonry requires a great deal of care!
questions you should ask and answer:
1.  are they really cmu?  could they be older blocks (such as cinder, terracotta or baked block)?
2.  how badly has the mortar deteriorated?  generally repointing needs to get to "sound" mortar.  if there isn't any sound mortar the wall may need to be replaced, otherwise a proper repair job could be prohibitively expensive.
3.  how accessible is the job?  the existing mortar needs to be completely removed to permit/allow bond between the existing brick and the new mortar.
hope that helps...  please update and let everyone know how it goes.
good luck!
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
youngstructural,
thanks.  i'll let you know what the owner decides to do...
glad to help.
there are also numerous good, a few great (and tons of useless) papers on the possible consequences of the wrong mortar.  suffice it to say that if you ignore the problem your wall, not the problem, will go away with time.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
a paper i prepared some time back... in continuous edit mode.
dik
dik & youngstructural...thanks for the information.  i'll post later on and let you know what the outcome is.
ccb
dik: an excellent paper.  well written, and very informative.  always looking to learn more about historic masonry.  i have a comment or two and am currently refining them for you, if you are curious.
definately worth a star!
cheers and thanks,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
gentlemen,
first, dik, i liked your paper, very informative.  second, after several sit-downs with the owner and a manson expert, the owner has decided to "go another route" (read: demo and rebuild).  thanks for your input, it is greatly appreciated.
dik, with your permission, i would like to post your outstanding paper on the "contributions by others" page of my website. please let me know if this is ok.
sre... ok...
ccbi... if it's an old building, i hate to see them demolished.  any particular feature that should be retained and any historical group that can protect it?  depending on the jurisdiction, there are several heritage groups that can 'step' in... in lindsay, ontario, i was on the local lacac group as their technical 'dude' and in winnipeg, i'm on the apegm (man eng assn) heritage committee.
dik
ccb1:
don't know where you are, but in line with dik's comment, in the states we have something known as "the national historic register".  
i believe that if the building can be, or has been, listed on this register, the owner can get tax credits for repairs made in line with restoring and/or maintaining the original architecture, to include any required structural repairs.  check it out.  it may save the building and the project.
mike mccann
mmc engineering