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?

 

[jhnblgr]

Pressure wash and repoint joints, or pour a one sided wall against the existing wall. Could probably take stucco netting and anchor it then stucco over top.

 

[TRAK.Structural]

Pressure wash and repoint joints, or pour a one sided wall against the existing wall. Could probably take stucco netting and anchor it then stucco over top.

I'm hesitant to spec pressure washing to be honest. I'd be worried that a contractor would come in and blast all the mortar out of the joints and completely ruin the thing.

The netting is a good idea, just trying to figure out if there is a product (or type of product) that is more suited to very old stone/rubble walls...

 

[Tomfh]

Do you have a photo

 

[TRAK.Structural]

Do you have a photo

Here is a flavor of what I'm contending with. In areas where the bearing is not continuous there will have to be some infill or partial rebuilding to re-support the framing.

 

[LittleInch]

"Having some work done" is rather vague....

As is "mostly intact" ....

Shotcrete maybe?

But the words "rubble / stone" and "foundation" should not really be used together.

They might be stable now right up until you do something to them or give them any sort of loading they haven't seen for the last century.

Those unsupported posts are there for a reason.

 

[jhnblgr]

I’d form up a wall similar to a footing and brace it and pour it, trying to do anything else would be more of a hassle or mess.

 

[TRAK.Structural]

"Having some work done" is rather vague....

As is "mostly intact" ....

Shotcrete maybe?

But the words "rubble / stone" and "foundation" should not really be used together.

They might be stable now right up until you do something to them or give them any sort of loading they haven't seen for the last century.

Those unsupported posts are there for a reason.

The work is replacing damaged wood floor and wall framing

Mostly intact meaning there are some areas that are in tact, and some that aren't. The areas that aren't will be re-built with modern materials

Rubble stone is still permitted for new construction in the IRC. This being an existing condition, and stable, without changing load path or occupancy use, is no less code compliant than before any repairs so per my interpretation of the IEBC would be that these walls are ok to remain (provided the code official doesn't deem it unsafe).

Shotcrete is a potential option, but may be more costly than just rebuilding.

The random wood posts are there to stiffen the floor framing, as far as I can tell the foundations haven't moved.

 

[LittleInch]

Fair enough. The difficulty is in not accidentally disturbing it during the work. Move one bit and you risk the large parts becoming unstable.

 

[gte447f]

It looks OK to me. I say just leave it as is. If there are areas that might be loose or crumbling, just repair/repoint using traditional masonry methods.

 

[phamENG]

just repair/repoint using traditional masonry methods

I agree with this. Have the contractor take a sample of the mortar and send it off for testing to determine composition. Clean the wall, scrape back any loose mortar (within reason - don't destabilize the wall), and repoint with the matched mortar.

 

[TRAK.Structural]

Fair enough. The difficulty is in not accidentally disturbing it during the work. Move one bit and you risk the large parts becoming unstable.

Yes, breaking it loose during construction is a risk

I agree with this. Have the contractor take a sample of the mortar and send it off for testing to determine composition. Clean the wall, scrape back any loose mortar (within reason - don't destabilize the wall), and repoint with the matched mortar.

Have you had the testing done before? Wondering how costly it is and if there is another alternative that may be sufficient and cost effective.

 

[phamENG]

Not personally - on the dream list to set up a historic materials testing lab since I deal with this stuff all the time and I hate trusting the contractor to do it before they buy a bunch of type S and start packing it in that 250 year old wall.....

For mortar, one source is a mail in lab in PA: https://www.limeworks.us/mortar-analysis/.

 

[TRAK.Structural]

Not personally - on the dream list to set up a historic materials testing lab since I deal with this stuff all the time and I hate trusting the contractor to do it before they buy a bunch of type S and start packing it in that 250 year old wall.....

For mortar, one source is a mail in lab in PA: https://www.limeworks.us/mortar-analysis/.

Thanks for the lab reference.

Since you "deal with this stuff all the time" any safe bets for mortar composition that will be reasonably in line with what is likely there? I know that testing is the only way to know what the existing stuff is, but is there a general type/class of mortar these old walls typically have, as opposed to the modern types S/N/M/etc?

 

[phamENG]

any safe bets for mortar composition that will be reasonably in line with what is likely there?

Nope. These were generally mixed up with sand and a local lime source based on the experience of the mason. Quantities and qualities vary widely by region and craftsman.

 

[TRAK.Structural]

Nope. These were generally mixed up with sand and a local lime source based on the experience of the mason. Quantities and qualities vary widely by region and craftsman.

Figures.

May float the idea of mortar testing, expecting raised eyebrows from the contractor.

 

[XR250]

I agree with this. Have the contractor take a sample of the mortar and send it off for testing to determine composition. Clean the wall, scrape back any loose mortar (within reason - don't destabilize the wall), and repoint with the matched mortar.

I thought this was only an issue with historic brick. Why does this matter for rubble stone?

 

[phamENG]

Why does this matter for rubble stone?

Because you're not replacing all of the mortar, I prefer to keep the material consistent so it can continue to behave as it has for the past 50 to 250 years. Introducing Portland based mortars will change the way moisture moves through the wall. There may be situations where that is desirable, but it requires more in depth study of the situation. My general rule with these is to put it back the way it was unless there's a compelling reason to think the way it was is the reason it failed. In this case, I think it's just a matter of age and degradation, not a "design" deficiency.

 

[RWW0002]

Pretty good description of historic mortar, but geared towards mostly historic brick. I think the mortar discussion is still pretty relevant.

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

any safe bets for mortar composition that will be reasonably in line with what is likely there?

I agree that mortar type and strength varies widely based on time of construction and location. I had a mason on a historic masonry project once try to get a Type K mortar, as a good base "weak" mortar, but ended up starting with a Type N and adding lime and sand to that. For small projects where testing is hard to justify, I have had masons start with Type N, and add lime and sand to a desired consistency, comparing to in-place color, texture, and strength of the existing wall until a mortar mix closely matching the existing is found. It is a bit of trial and error and it is good to have a contractor experienced in historic masonry to lean on. Good luck finding that for a small residential project without blowing the budget...

 

[XR250]

Sounds like the OP is just trying to confine the existing mortar. While i appreciate Pham's expertise in this area, honesty, I would just slather it with any modern mortar that reasonably matches the color and call it a day unless the contractor is on board to do testing and find a qualified mason.