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RE: tuckpointing

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Bruce,

 

The purpose of tuckpointing is to restore the masonry to its condition when it was built by removing deteriorated mortar and replacing it with new mortar packed into head and bed joints.  Tuckpointing can’t make it better than new.  Tensile strength should be ignored; pointing won’t improve or worsen it – it is zero.  Compressive strength is the important property of old masonry [it is good at carrying gravity loads], and tuck pointing may be counted on to restore it.  Shear strength was generally ample if the building was not built in earthquake country, and pointing will restore it.  Unreinforced masonry structures that are hundreds – even thousands – of years old are still standing and in use; to continue, they require only regular periodic observation, maintenance and repair [mainly repointing of deteriorated mortar joints, maybe 2 or 3 times per century].

 

To someone unfamiliar with old URM buildings, and used to modern masonry, almost any old mortar will seem to be in very poor condition, soft enough to be eroded from the joints with a finger – but it is probably O.K..  Except for damage caused by periodic wetting and drying, mortar is nearly inert and undamaged by the passage of time.  To determine the quality of mortar to be matched by the tuckpointing mortar, match what you find in the joints in a protected location [under a roof or eaves, or near a reentrant corner.  Expect to find that It is unlike modern mortar; it is usually much softer than the masonry units.  Resist the urge to replace old mortar with a harder material.  With passage of time, after pointing with a modern mortar, changes of temperature and atmospheric humidity will slowly lead to damage to the old bricks by the hard modern mortar.

 

A good guide for tuck pointing is Preservation Briefs 2, Repointing Mortar Joints in Historic Masonry Buildings from http://www.cr.nps.gov/hps/tps/briefs/brief02.htm

 

Nels Roselund, SE

South San Gabriel, CA

njineer(--nospam--at)att.net


From: Bruce Holcomb [mailto:bholcomb(--nospam--at)brpae.com]
Sent: Wednesday, July 05, 2006 1:30 PM
To: seaint(--nospam--at)seaint.org
Subject: tuckpointing

 

How does tuckpointing affect mortar tensile, compressive and shear strengths?  This is for 3-wythe brick walls in two separate buildings… one is about 80 years old, the other is 120 years old.  Any thoughts on mortar tensile, compressive and shear strength of these old buildings?

 

 

Bruce D. Holcomb, PE, SE

Butler, Rosenbury & Partners

www.brpae.com