Cinder block stem walls questions..

Started by black_edelweiss, October 19, 2018, 05:27:25 PM

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black_edelweiss

I have a few questions about cinder block stem walls. I have heard of people not using mortar to lay the block, but instead dry stacking them, but filling every core with concrete/rebar. What strength does mortar add to a wall? It seems to me mortar is primarily used to make it so you can adjust the leveling/spacing of the blocks as a typical foundation is not going to be perfectly level, and cinder blocks themselves vary slightly in length/height. Without mortar, you would not be able to adjust for this. Is this correct or am i missing something?

Also, I have heard of people not filling their blocks with concrete, but using sand instead. Of course they are using mortar to lay the blocks, but it seems to me like not having your cores filled with concrete would make the wall extremely weak, especially a stem wall thats over 8ft tall. Am i missing something?

When i was building my 9ft tall cinder block stem wall, i accidentally pushed the wall with a mini excavator bucket as i was trying to dig some of the dirt out. The stem wall had mortar joints and all the rebar in it, but no concrete. As the mini excavator bucket pushed the wall, the wall bent, but did not crack or break anywhere, and went back to its previous form as soon as i moved the bucket. I imagine without rebar, the force from the tractor would not have spread evenly through the wall, but would have instead broken the blocks that were directly in contact with the tractor bucket. I can see how adding concrete would make the wall 100x stronger...so why wouldnt people fill their cores with concrete?




NathanS

Your interpretation about laying block without mortar is the same as mine. I think it would be more work.

Definitely a free standing stem wall of 9' with 8" block needs rebar and concrete core fill. There are some scenarios where you don't need to core fill or you don't need rebar. The block on my foundation is 42" tall and is above grade only 8-12". The wall is held in place by stone on one side, and dirt on the other.

If you use wide block - 10" or 12" you can also avoid core filling and rebar depending on how tall it is, and how much unbalanced fill there is. That might seem a little worrysome, but think about all the hundreds of year old masonry walls there are that are nothing but stone and lime. A common way to build a brick house was 2 rows of interlocking brick all the way up to the roof.

There are a fair number really old non-reinforced masonry houses that will be around longer than what we are building today.


MountainDon

Surface bonding cement

Never used it myself but there was at least one person on here several years ago who did that process.  Not sure that it saves work but for those who are afraid if learning to mortar blocks it may be an alternative.....
Just because something has been done and has not failed, doesn't mean it is good design.

black_edelweiss

Quote from: NathanS on October 19, 2018, 06:07:51 PM
Your interpretation about laying block without mortar is the same as mine. I think it would be more work.

Definitely a free standing stem wall of 9' with 8" block needs rebar and concrete core fill. There are some scenarios where you don't need to core fill or you don't need rebar. The block on my foundation is 42" tall and is above grade only 8-12". The wall is held in place by stone on one side, and dirt on the other.

If you use wide block - 10" or 12" you can also avoid core filling and rebar depending on how tall it is, and how much unbalanced fill there is. That might seem a little worrysome, but think about all the hundreds of year old masonry walls there are that are nothing but stone and lime. A common way to build a brick house was 2 rows of interlocking brick all the way up to the roof.

There are a fair number really old non-reinforced masonry houses that will be around longer than what we are building today.

I guess thats true about brick houses, so i imagine cinder blocks stacked with mortared joints would be quite a bit stronger. Im a little concerned that i may not have gotten the concrete fill all the way down to the bottom of my 9ft tall cinder block stem wall. Looking back, i probably should have filled the blocks with concrete half way through, and another fill when the other half of block is done. At least that way the concrete would have a hellavu lot easier time getting to the bottom. Lots of rebar and mortar debris looking down those holes. The only indication of concrete making it all the way down was the lowest cinder blocks were visibly damp from absorbing moisture from the concrete. Other than that there's no way to know for sure without drilling into the blocks. This was my first time laying block, so the mortar joints looked like utter shit and some of the blocks were not perfectly vertical, with some overlapping the lower by a bit, and some bow in the wall that i hit with the tractor. I figure despite all those beginner errors, if the concrete got to the foundation blocks, it will be able to withstand a 7.0 earthquake ( i hope)

JRR

I have tried the stacking method: it is not for me.   Cinder/concrete blocks are not that perfect in shape, not a perfect cube.  The mortar joint allows for corrections of this imperfection ... and also allows for insertion of steel wire reinforcements.  I also like surface bonding cement on top/side of regularly laid mortar-block, for extra strength and seal.