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PAVenturer 09-16-2011 05:22 PM

More Stone Foundation Repair
Hi all - I am new to this board and am very impressed with the knowledge of folks here. I have read a thread on stone foundation repair which makes sense but I have a new question and didn't want to just continue that thread.

We have a 100 year old home with a fieldstone foundation. We had floods a couple of weeks ago and whatever was used in the past to coat/parge/seal the walls came down in several places. So, I need to get something back up. It seems that above the water line (about 3') the finish is still firmly attached. The finish is seems to be sandy where it has come down so I assuming it is something similar as described in

A person I work with says that what he did was put plywood against the wall and then poured concrete behind the plywood. He let it set and removed the plywood and now has a concrete basement wall. I mentioned that he will have issues if he traps water in the foundation. His response was that concrete is porous. I like the simplicity of his solution. I would like the appearance of his solution. But is it a good solution?

Thank you for any advice!

JoJo-Arch 09-16-2011 07:23 PM

Types of mortar
Mortars in the days before cement production (pre 1880) were lime based as the cohesive element.

Problems that were discovered over hundres of years was that lime mortars were relatively soft, and wet weather would eventual wash out the mortar, or when it dried after rain, it would turn to powder. Look at 100 year old brickwork and you will see recessed mortar between the bricks, if they haven't been repointed.

Then they started using cement based mortars because they were stronger, but more importantly, unaffected by water. The cement mortar must be strong enough not to crumble at the exterior joint and soft enough to allow brick movement without cracking the brick. Too strong a mortar is as bad as not enough.
Bricklayers have mastered the art of laying bricks by a trial and tested method over the past 100 years. If you watch a bricklayer applying mortar to the brick edge, you will notice the mortar sticks to the brick and defies gravity. Also the mortar bed he lays has an exact volume of mortar so that when he tamps a new brick in place, he aligns the top edge with the string line and some mortar oozes out of the joints when he taps the top of the brick with the handle of his trowel (2-3 taps) for final alignment and swoops his trowel upwards, skimming the excess mortar which he flicks down to his mortar board or barrow.

All this happens in less than 3 -4 seconds. The secret why he can do this is that a portion of lime is added to the mix to provide plasticity and higher suction between brick and mortar, so the mortar really sticks, the plasticity allows the mortar to flow better. It also takes a few more seconds to set.

If you lay a bed of plain mortar (no lime) with high cement content on the course below, you have about 6 seconds maximum before the mortar 'sets and becomes unworkable. If you place a new brick after this, it will not bond properly and you cannot get it aligned, no matter if you hit it with a sledgehammer. Bricklayers know exactly how much cement. lime, sand and water to add to achieve a workable consistency, and only mix as much as they can use as it is important the mortar doesn't start to set in the barrow or mixer.

The correct mix for foundation brickwork (up to floor level) is: for strong mortar :

1 portion cement, 1/2 portion lime, 6 portions of clean river (not beach) sand and 1 bucket or less of fresh, clean water

For mortar above floor level, the correct mix should be:

1 portion cement, 1/2 portion lime, 9 portions of clean river (not beach) sand and 1 bucket or less of fresh, clean water.

This mortar combination allows bricks to move and grow without stressing the bricks and cracking them and is self healing, allowing fine cracks to almost disappear with time. Note as bricks grow with time and absorb moisture, clean vertical and horizontal joints are ABSOLUTELY ESSENTIAL, AND DEPENDING ON THE TYPE OF BRICK MUST BE PLACED AT CERTAIN SPACINGS AS RECOMMENDED BY THE MANUFACTURER OR REGULATIONS.

Also the insistence of river sand free of salt is critical. Under no circumstances allow beach sand to be used. Salt efflorescence, where the salt laden moisture is drawn up the brickwork and through the salt impregnated mortar shows up as white crystalline patches when the moisture drys out and leaves the salt behind, builds up tremendous pressure with crystal growth just below the brick surface, which eventuall spalls off.
hence you see old walls with the brick surface eroded and scalloped out.

There are other issues to contend with, and what applies to brickwork applies to stonework, but is less critical. You can souce info on the web.

Cheers! :whistling2:

oh'mike 09-16-2011 08:24 PM

Excellent explanation---

mem 09-17-2011 08:49 AM

Very enlightening JoJo, thanks!

PAVenturer 09-17-2011 09:36 AM

While I find the lesson in brick/stone laying interesting, I am not clear on how that answers my question. Forgive me if I am being ignorant.

I am not building walls. The walls exist but need to be parged or coated in some way. The link to another thread I provided gives a good answer on mixing a mortar to trowel on the walls. I can do this but am interested in a response to my coworkers solution of putting plywood up and pouring concrete behind the plywood to provide a solid and smooth interior finish. If concrete is porous, would this not accomplish the goal of not trapping water in the foundation?

Thank you for your wisdom!

JoJo-Arch 09-17-2011 04:04 PM

Foundation repairs
Hi!, (PaVenturer) while I admit my previous answer wasn't addressing your particular question directly, there seemed to be some misconceptions of what cement and lime do to mortar and their historic role.

That said, I am assuming you want to treat the wall by covering the surface with a layer of concrete. Yes, plywood formwork would help to retain the concrete until it set, note however, you need sufficient gap (at least 4") for the concrete to penetrate to the bottom of the formed trench. I would recommend a light (1/4") reinforcing mesh be pinned to the wall before putting up formwork. Also note, the plywood formwork has to be solidly framed and supported.

The hydrostaic pressure genertated and to hold back the wall at least for the first hour after pouring, is extreme. You don't want the formwork to collapse and you end up with a huge lump of concrete on the floor. The concrete though porous, will retain water to about 3/4 the height of the wall, so once set, water will build up over time if the stone wall is actually part of foundations, and subject to wetting, the water trapped behind the wall must be released You can cast 1" PVC pipes every 18" at the base of the walls, just proud of the plywood formwork to relieve this pressure.

Think of it as a swimming pool in reverse, water pressure pressing against the wall must be relieved, or I can envisage the whole concrete wall getting pushed out, possibly pulling the stone wall with it. I am assuming you're forming a basement and the stone wall is below ground. If it above ground, water problems are almost of no consequence.

In this situation, the concrete will stop water getting in (from rain) but if you want he wall to stay dry, you can ask for waterproof concrete to be supplied. If you are mixing your own concrete, add bondcrete (Basically PVA white glue) to the mix as recommended by the manufacturer.

Another consideration is height of the wall. If the concrete is delivered mixed, you could go to 8'-0" each step. If you are mixing your own, I wouldn't go over 4'-0" and either adding formwork as you go to the next level, or waiting at least a week and use the old form work again. Be sure to first broom clean motor oil on plywood face for easy separation of concrete and plywood. Good luck!:whistling2:

Bud Cline 09-17-2011 04:16 PM


While I find the lesson in brick/stone laying interesting, I am not clear on how that answers my question.
My thoughts exactly. Sure was a long read for such an anti-climatic ending. I think I'll wait for the movie to come out.:)

JoJo-Arch 09-17-2011 05:06 PM

Fondation repairs
To Bud Cline, love your wry sense of humour, it's rare in the building trade.

Bud Cline 09-17-2011 05:11 PM

I've been known to get carried away myself. Some times it is hard to stop once the flatulence gets going.:)

PAVenturer 09-17-2011 07:19 PM

Thank you Jo-Jo. I have some food for thought now. I just hope it doesn't cause the flatulence that seems to result from over thinking.

This is a basement, below ground. There is potentially considerable opportunity for water to get in as is evidenced by two floods in the past 6 years.

My plan is to do this: to the exterior walls and I am still contemplating what to do with the interior. Thanks again and any further thoughts are greatly appreciated!!!

JoJo-Arch 09-18-2011 05:30 AM

Foundation repairs
PAVenturer! Thanks for your kind appreciation. You mention "There is potentially considerable opportunity for water to get in as is evidenced by two floods in the past 6 years."

Now if the water only comes in when you have flash floods, probably nothing will stop it getting in. In fact if you completely water proofed your basement, your house will most likely lift up and float and move downstream. By the way, I've seen this happen to empty swimming pools (concrete type), because the outside water couldn't get into the pool quick enough.

You need to examine where the water is coming from by looking carefully at the lie of the land. Is the house in a hollow, on an old creek or river bed or a filled site once a waterhole?. If it is, it will flood again, so the precautions you would take can only be immediate and temporary not to drown that is, to stay above the water line upstairs and let the basement flood, hoping the house won't float away. the water will stink, but should eventually disappear, leaving a muddy mess. Your local authorities should have drainage plans and accurate locations of river courses and creeks if you are seriously concerned.

If the water trickles in through the stone walls and floor, your choice is to purge the water through the walls and allowed it to flow by gravity into a recessed pit. A suitable electric pump fitted with a float switch will then empty the recess pit when it fills up and stop after it's empty. The flow rate of water into the pit will determine the size of pit and pump. This water seepage usually occurs if the house is cut into the side of a hill, water permeating downhill will float above a hard surface such as shale, slate or even clay, and hit your basement wall on the up hill side. You have little choice but to collect the water or divert it before it hits the house wall, or let it enter and control it via pit and pump.

There is a third option, but expensive. Also it assumes your basement is large. This is for someone wishing to use the basement as livable space (to a point). First you would pour a level slab on waterproof membrane. Before the concrete sets you would scallop out spoon drains around the perimeter leading to the recessed pit. When the concrete hardens sufficiently (10 days-14 days) you would erect formwork and space the new concrete wall using waterproof concrete(on the inside of the stone wall) and leave a gap of at least 6" between the old stone wall and the new concrete wall. Lay an agricultural drain (a pvc pipe with slots) along the cavity base on top of the spoon drains. After the wall hardens, strip formwork both sidesand back fill cavity with 3/4" screenings. The inside skin formwork may be as waterproof as you cam make it. if you can't remove it, you may have to leave it buried in the screenings. You may also fix a waterproof membrane on the inside face of the concrete wall to the "lossy formwork" before fixing the outer skin of formwork.

Your sandwich would consist of: your original stone foundations, 6" of screenings (bluestone chips or scoria) inner skin of formwork, waterproof membrane, 5-6" concrete reinforced wall and the outer skin of formwork which can be removed. This is not really DIY and there are dangers, the formwork has to withstand considerable hydrostatic pressures before the concrete sets. Also you need to have rebar coming up from the slab floor at close intervals (see a structural engineer for design), to bond the wall to the floor slab. Unless you're familiar with concreting it is strongly recommended you have this work carried out by experts.

if you don't think the space gained warrants this expensive option, go with the pacthing/parging idea and if water keeps trickling through, add the recess pit and pump. At least this should be effective most of the time, unless you cop another Irene or Katrina, then only a sound boat will help. I'm sorry if you are in this position, as the only real option you have is to sell up and leave. You may however, have to disclose the flooding aspect to the purchaser.

I apologise for the long splurge, but to explain something properly, I assume there are other people out there who may have a similar or worse problem, and not everyone understands building techniques or lingo. Sorry if I couldn't be of more help. Others may like to share their experiences. The candle has burnt down to the bottom now and I need sleep, Cheers! Joe from OZ :whistling2:

PAVenturer 09-18-2011 07:44 AM

3 Attachment(s)
Thank you JoJo!

The basement is unfinished, 600 sq ft. I do have a sump pump which kicks on a couple of times a year. In this case though, the float got stuck and the pipe separated from the pump resulting in about 3" of water. We have a babbling brook 10' from one wall but the creek also has a retaining wall. The creek has filled it's banks and has become an 8' deep raging river a few times. We rarely, if ever, have had issues as a direct result of this creek though. Our issue comes from water coming down the street from the opposite direction of the creek.

Attachment 38363 [ATTACH]38366[/ATTACH Attachment 38365

The attachments show the source of my biggest problem and the result. The top of our driveway is the low spot on a downhill street. I am going to put a drain in at the top of the driveway that should help considerably in the future. The water comes down the driveway and created a river into the side of the house. The interior shows the wall with a hole in it that wasn't there before last week's flood.

It sounds like the right solution is the trowel and mortar mix.

Thank you for everything. I guess I need to get to work on this now!

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