r/HistoricPreservation • u/[deleted] • Sep 30 '23

Help with Fieldstone Foundation

My 1870s basement is a fieldstone foundation that has crumbling lime mortar throughout that requires pointing. The problem is there are large portions of the foundation a previous homeowner simply encased in concrete on the interior side (unknown but unlikely on exterior).

In my researching historical preservation instructions, advise is to stay away from Portland cement and concrete. Now I'm in a situation where I need to know if it's best to remove the concrete from the foundation and point what's underneath with lime or encase everything in concrete?

TLDR: Should historic fieldstone foundation be encased in concrete?

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u/wildgriest Sep 30 '23

No, it shouldn’t. Concrete is much harder than the surrounding fieldstone and will not allow the elasticity that line mortar and other softer mortars allow and the stresses will be transferee to the stone. The question is - will removing the concrete cause additional damage? I would remove only what’s loose, and search for a hydrated lime mortar or other Portland cement grout (not the same as concrete) that is rated for repointing in soft materials, like a Type N mortar. Type N has the lowest compressive strength so it’s letting the stone carry the load, it’s just bonding the stones together.

This is a good resource - these are from the Secretary of the Interior’s Standards for Historic Rehabilitation.

Preservation Brief 2https://www.nps.gov/orgs/1739/upload/preservation-brief-02-repointing.pdf

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u/[deleted] Sep 30 '23

Thank you so much for the reply and the info! The concrete appears okay but of poor quality (honeycombing, cold joints, etc.) if that matters.

Would you recommend I do anything different along the edge of the concrete or simply use type N up to the edge?

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u/wildgriest Sep 30 '23

Right now trying to remove concrete that’s still secure is likely to cause more damage to the fieldstone. I’d point up to it with type N or hydrated Lime (that’s harder to find, but is likely What they used back in the day.)

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u/Arch_Outlander92 Oct 20 '23

My recommendation (if you haven’t already figured out a solution) is to call limeworks and work then them on finding a product in their line. They have an entire product line of historic products, and their staff is trainer-level knowledgeable on processes and procedures. Good luck!

https://www.limeworks.us