Stacked Stone/Rubble Foundation

[TRAK.Structural]

Anyone have any experience with repair mortars for old stacked stone/rubble foundation walls? I have a situation where a contractor is wanting to keep existing foundations for a home that is having some work done on the wood floor framing. The foundations are stable and mostly in tact so I think this is reasonable but I am considering trying to add some sort of parge coating to more or less provide some extra insurance. The existing stone and mortar will still take all the load, but I'm hoping there is something out there that can help to contain/confine these walls from breaking loose, any ideas?

 

[gte447f]

I would literally do nothing unless there are areas that are loose and crumbling or otherwise need repair.

 

[masonrygeek]

Just remember that any time you put a parge coat, shotcrete or coating on the masonry wall, you change how it deals with water and how it can dry out. So there are structural solutions to this problem, but don't forget about absorption, drainage and evaporation. I like fixing the joints (repointing) and just keeping an eye on it.

 

[TRAK.Structural]

Just remember that any time you put a parge coat, shotcrete or coating on the masonry wall, you change how it deals with water and how it can dry out. So there are structural solutions to this problem, but don't forget about absorption, drainage and evaporation. I like fixing the joints (repointing) and just keeping an eye on it.

Good points. Wouldn't repointing have the same effect though, considering that matching the existing mortar is not realistically going to happen?

 

[XR250]

Just remember that any time you put a parge coat, shotcrete or coating on the masonry wall, you change how it deals with water and how it can dry out. So there are structural solutions to this problem, but don't forget about absorption, drainage and evaporation. I like fixing the joints (repointing) and just keeping an eye on it.

Seems like coating it would help keep moisture out of the wall. Is this really going to be an issue on a rubble stone foundation wall?
Maybe more of an issue on a multi-wythe masonry exterior wall?

 

[masonrygeek]

For a foundation wall, the water could be coming from many different directions, especially if there is not foundation waterproofing. If the water is coming from the soil side, then putting on a non-breathable coating then you may end up trapping water in the stone. With most rubble stone walls which are so rough you may have a hard time applying a continuous coating.

For the repointing you could slow down the evaporation and that's why it may be necessary to measure the water vapor transmission of the repointing mortar. Maybe more important for a historical structure, but a little overkill for a residential stone foundation. See NPS Preservation Brief No. 2: https://www.nps.gov/orgs/1739/upload/preservation-brief-02-repointing.pdf (see p. 4).

 

[bIockhead]

I'd be worried about trapping and accelerating degradation if it's mostly held up fine. I would think a decent starting place for an overstrength but breathable mortar would be using an NHL mix like a 3.5 but a Type O would probably be fine.

What kind of climate are you in? Are you able to apply modern drainage to the exterior to keep it from sitting directly against wet soil below grade? Is this wall expecting any kind of significant axial stress (over let's say 0.7 MPa)??

There is probably a lot you could do and be fine. I agree with @masonrygeek and would add this as a resource worth looking at. It's probably far more "in the weeds" than you probably care about, but worth a skim: https://heritagecouncil.vic.gov.au/...e/repointing-mortar-joints-in-older-buildings

 

[milkshakelake]

I haven't had any issues with these types of walls when it comes to remove/replace kind of things. They're very common in 1900's to 1940's buildings in my area. We just repoint with...whatever the contractor throws at it. General repointing is accepted by the building department here. I specify type N mortar, but who knows what the contractor actually uses? Nobody checks. I think if it was an issue, there would be more oversight on the type of mortar by the AHJ, since they seem to be uptight on a lot of things (not in a bad way).

These walls are brittle and sensitive to settlement, and they crack in a major way when that happens, so I avoid putting any new loads on them. It sounds like you're not adding loads so that should be fine. The other issue is loose stones, which is harder to fix in my opinion, though I think it's doable. But if you don't have loose stones, you should be okay.