Crumbling Old Concrete Basement Wall

[PlumBob]

Hello, All

I have a wonderful old 1917 arts-n-crafts style home in Montana

I'm beginning to see crumbly spots in the concrete basement walls. Each of these is associated with a place where the soil on the outside was damp/moist in the past. The crumbling areas are all associated w/ a good bit of dampness in the concrete. They range in size from 3 - 12 in. in diameter, to 1/2 to 3 in. deep. Total of probably 5 of them.

I've gotten all of the problems fixed on the outside, so that the soil should be drying out over the next summer or so. But now, I need to start planning to repair these crumbling spots on the inside.

The crumbling spots are all open to the basement air - I've pulled out as much of the moist crumbling concrete as I thought prudent. There won't be anyone living in the house over the winter, so there won't be any other moisture generated in the basement. The furnace is on (house thermostats set at 50.) I hope that the spots will dry significantly over the winter.

Does anyone have any experience repairing these kinds of problems? Any suggestions or comments would be appreciated. Any ideas of the best materials?

Thanks in advance for any advice !

PlumBob

[LazyPup]

PlumBob,

I appologize for the delay in my reply and i did receive your email, however, there are many factors involved so before offering you my opinion I took the time to confer with a friend who is a masonary contractor.

Before we examine your problem we need to consider the basic structure of concrete itself. In its simplest form concrete is a mixture of cement and gravel. If left to its own devices when concrete is poured into a form it will produce a very porous surface finish full of nulls and voids. During the pouring process a huge vibrator is inserted into the form to shake the mix, causing the gravel to settle and causing the liquid portion of the mix (the grout) to settle against the form walls which will produce a nice smooth finish. Basically the same thing takes place when concrete flatwork is finished. They use a device called a "jitterbug" that looks similar to a elderly persons walker with an expanded metal plate on the base to tamp the surface forcing the gravel down and leaving a thin layer of the grout on top that can be finished smooth.

While the external grout layer will form to a nice finish, it is rather brittle by comparison to the actual concrete mix.

The problem you are encountering is most likely the result of one of two problems.

Often when concrete is poured in forms, if the mix is not adequately vibrated there are numerous voids in the surface. When the forms are removed the finishers then trowel in a mortar mix and trowel it smooth to fill the voids. It is quite possible that the loose concrete you are seeing is some of those patches that are coming loose, if so, it has very little effect on the overall integrity of the wall.

It is also possible that the areas that are crumbling are natural voids in the rocks in the concrete which filled with the liquid grout when the concrete was poured and vibrated. Under extreme conditions of temperature and moisture those areas of grout that lack the gravel can flake off leaving a small depression in the wall such as you describe.

If the effected areas are small, 2 to 5 inches in diameter and the depths is only an inch or so, it should present no structural problem to the wall.

To repair those holes I would begin with a masonary brick hammer and chip away as much of the loose material as possible, then wet the concrete with a brush or wisk broom saturated in plain water to remove dust and prewet the concrete.

You can then brush on a thin layer of latex binder (available at a masonary supply yard) and finally use a wooded float trowel to fill the voids with mortar mix and trowel it smooth.

Given that you intend to leave the structure unattended with minimal heat through the winter I think i would wait until late spring to make the repairs.

A bit of concrete Trivia- According the the information i recieved from my friend the mason, concrete reaches 90% hardness in about 24 hours. It continues curing and reaches about 95% maximum hardness in 21 days, which is why they take core samples and wait 21 days to test them. It then takes about 99 years for concrete to reach its maximum hardness, then it stays stable about 25 to 50 years, then it begins to deteriorate at a slow pace.

[PlumBob]

OK, LP, Thx....

I'm OK w/ everything you have said. There are a couple of comments, tho.

It may (have been) be a regional thing, but the concrete in the walls (and in other samples I've seen in the same town) that was poured way back when (mine in 1917) is full of debris - sticks, what we would call 2x4 chuncks, etc. "Full of" may be a bit of an overstatement, but of let's say 5 spots that I'm currently looking at in my walls, there's a 2x4x3 chunck in one, and a stick in another. I guess the point is that I'm sure the walls werent't poured, vibrated, etc. w/ the greatest of care or attention to detail. True, when the house was built it was an "upper end" house, but the workmanship (of the concrete, at least) is absolutely shoddy by today's standards, as you've described.

I wasn't aware of the latex binder material, Thanks. Will do.

The other thing that I see here is your use of the term "Mortar Mix." I understand Mortar Mix from some (very little, admittedly) brickwork I've done. I'll use the standard old sack-krete type mortar mix.

Yes, the whole project will wait until late spring, early summer.

BTW, I think I saw you post that you're a ham. I'm WA5YKT.

73. Thx for the note. Thx to your friend for me, pse.

Sam

[LazyPup]

Back in the days when your walls were poured they hadn't yet invented plywood, therefore the forms they used were often made of wooden planking. Quite often they left a small stub of wood between the inner walls of the forms to hold them out. As the concrete was poured the weight of the concrete would then hold the forms apart. During the pour the wood spacers were supposed to be pulled out as they went. But, as is common in many cases today, some things go undone. You may try drilling into one of those wood blocks and see if it goes through the wall, if so, you may want to drill out as much as possible, then pry out the remainder and fill the hole, otherwise the wood will no doubt rot from ground water on the outside and leave a hole through the wall.

73 & GL DE KC8UXZ

[PlumBob]

And finally,

I received the following unsolicited commercial e-mail from a troll.

Some folks just don't have any class.

"See: http://www.sanitred.com/BasementWaterproofing.htm and the other 14 related links w/more info.
Do not hesitate to ask any questions you may have.
Thank You"

Sent by "diane@sanitred.com"

Diane, FWIW, I've added Sanitred to my spam-blocker.

LP, thx for the conversation.

73. WA5YKT QSY