Latex additives in repointing mortar

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Author Message David Axt, AIA, CCS, CSI Senior Member Username: david_axt Post Number: 1204 Registered: 03-2002 Posted on Wednesday, June 08, 2011 - 02:32 pm:    Does anyone have recommendation (for or against) using latex additives in repointing masonry mortar? John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1320 Registered: 03-2002 Posted on Wednesday, June 15, 2011 - 04:16 pm:    Don't do it. Read the Park Service's preservation standards for the full story. I can't imagine why you'd want to if the repointing is done to best practice. Brian E. Trimble, CDT Senior Member Username: brian_e_trimble_cdt Post Number: 45 Registered: 08-2005 Posted on Wednesday, June 15, 2011 - 04:41 pm:    Latex modified mortars may make the mortar too strong for the units. I remember the NYC Dept of Design and Construction requiring this in the past - incorrectly of course. As John said, there just isn't any reason to consider this. Look at ASTM C1713 Standard for Mortar for the Repair of Historic Masonry. This lists the appropriate materials to use. ken hercenberg Senior Member Username: khercenberg Post Number: 84 Registered: 12-2006 Posted on Tuesday, July 05, 2011 - 04:20 pm:    David, I pretty much limit my selections to Jahn Mortars or Edison's Rosendale (http://www.rosendalecement.net/) mortars when color match is critical. The old 'K' grade (high lime content) mortars are usually fine for most repointing work unless this is structural or engineered masonry. Typically, softer is better. As John suggests, check out the Preservation Briefs - http://www.nps.gov/history/hps/tps/briefs/presbhom.htm. Dave Metzger Senior Member Username: davemetzger Post Number: 396 Registered: 07-2001 Posted on Tuesday, July 05, 2011 - 05:11 pm:    Speaking of "K" mortar--do you know where the designations for mortar types M, S, N, O,[and K]) come from? MaSoNwOrK Knowing this makes the types a lot easier to remember Lynn Javoroski CSI CCS LEED® AP SCIP Affiliate Senior Member Username: lynn_javoroski Post Number: 1273 Registered: 07-2002 Posted on Tuesday, July 05, 2011 - 05:22 pm:    Oh darn; here I thought it was MicroSoft Network, OK! Tim Werbstein, AIA, CSI, CCS Senior Member Username: tim_werbstein Post Number: 40 Registered: 09-2006 Posted on Wednesday, July 06, 2011 - 07:36 am:    If the masonry is neither historic nor likely to become "historic," the following of historic mortar requirements is unimportant. A portland cement mortar, including natural portland cement (Rosedale), with latex additive should perform well, so long as the repointing mortar is not harder or less porous than the remaining existing mortar or masonry units. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1331 Registered: 03-2002 Posted on Wednesday, July 13, 2011 - 06:26 pm:    Tim, Using the latex still seems unecessary to me. And wouldn't the latex have the potential to reduce the rate of water vaper transmission, with consequent increase in potential deterioration? Tim Werbstein, AIA, CSI, CCS Senior Member Username: tim_werbstein Post Number: 42 Registered: 09-2006 Posted on Thursday, July 14, 2011 - 07:59 am:    Unfortunately, this repointing post digressed into "historic" mortars, but the original post did not mention "historic" or "preservation." I've always found latex unnecessary in masonry wall construction or repointing. I feel that solar heat and dry conditions can be more reliably handled in other ways. Latex can also affect color and wet-dry appearance--at least until the wall has weathered a bit. Regardless of whether or not latex adversely affects vapor transmission, the essential requirement is that the repointing mortar (with or without latex) is neither harder nor less vapor-permeable than the remaining, existing mortar or masonry units. That would spell trouble. For that same reason, portland-cement mortars are discouraged for old buildings constructed without portland cement. Gerard Sanchis Senior Member Username: gerard_sanchis Post Number: 31 Registered: 10-2009 Posted on Thursday, July 14, 2011 - 01:58 pm:    I understand the color issue, but assuming that portland cement and not lime mortar was used for the original construction, we should remember that when installed, the mortar shrunk due to water evaporation from the sand/cement/lime mix while the brick expands in size when exposed to humidity and the elements – the brick is smallest (size-wise) when it leaves the kiln. That’s why the walls are watertight when properly constructed. In an existing building the brick will not expand anymore. Substituting latex additive for part of the water for repointing would in theory diminish the amount of shrinkage in the mortar and I wouldn’t dismiss its use. I’d suggest that a sample panel on the South or West side exposure be tried – 10-foot square should be enough. It should sit for a season or two if the schedule allows, see how it behaves and then make a decision on the use of additives. Tim Werbstein, AIA, CSI, CCS Senior Member Username: tim_werbstein Post Number: 43 Registered: 09-2006 Posted on Thursday, July 14, 2011 - 04:17 pm:    This would be a good research topic. In my own experience, repointing mortar has negligible shrinkage. Unlike mortar for laying masonry, repointing mortar is pretty dry--mixed with just enough liquid to be like putty, and (ideally) compacted in layers in a well moistened joint. Where would the shrinkage come from? The worst problem would be if the joint had not been sufficiently premoistened and the bricks drew water out of the dry mortar.