Block Walls

DaHammer

I have a question regarding concrete block walls and lateral strength. I'm usually not involved until well after the foundation is in place, so I'm not familair with the exact order of things. Anyway, I'm about to build a house that will have a raised slab foundation (also called supported or floating slab). The slab will be raised approximately 2 to 3 feet above the surrounding grades, using concrete block walls that are sitting on footers. My question is concerning compacting the dirt fill that will be in the void, under the slab. How do you compact it without bowing or breaking the block walls? The reason I ask is that I stopped by a jobsite today to talk to a foundation contractor and he was filling and compacting the void on one that will be like mine. While compacting it using a jumping jack they bowed one of the walls on one end of the house and were bracing it back in place using 2 x 4s. At least he said that's what caused it, but I'm not sure he didn't get a little to close to the wall with his backhoe or dozer while placing the fill. Anyway, I noticed that none of the concrete block voids had been filled yet. They had vertical rebar in the wall about every 4 feet, but that wasn't helping since the voids weren't filled. He commented that it would be ok once the slab was poured and tied into it with the rebar, and he is probably right, but I think it would be better to prevent it to start with. So how exactly does everyone compact the fill without damaging the perimeter walls? My thoughts were that the walls should be filled with grout or concrete before putting the fill in place. Or at least the voids containing the vertical rebar. Prehaps even use a bond beam or some of the block remesh hortizontally on 1 or 2 of the courses. But if you fill all the block voids before doing the dirt fill, then the plumber would probably need to come out first and put whatever pipes he needed too in before hand. Thoughts?

Subia29

You may have answered theses questions yourself.Yes have your wall grouted solid with proper rebar placement before compacting fill.Dont these foundations have crawl spaces?

concretemasonry

The job you looked at did not have a floating slab if it is supported by the block. This should really be called a structural slab. I hope it is not just 4" thick with no rebar in it.

Usually with proper fill material placed in layers, it is not necessary to use a "jumping jack" and a plate vibrator is best. If he has clay, dirt or other bad soil you also end up with the wrong compaction and compaction equipment. - At least he is try to compact, but soil will probably still settle from under the slab eventually.

I would question the contractor, method of design and if you are considering living the a house for quite a while. If you are just going to build and sell, you can be like many other contractors.

You should grout the block core with rebar in them before compacting. By pushing the walls out he broke all the mortar joints and will have reduced strength since all you can count on is the narrow grout core inside the block.

Obviously and plumbing should be put in before the grouting and backfill.

The best way is to use a real floating slab (wire wire mesh and not rebar) inside the block foundation walls. Have the block walls 3-4" above the slab level so you have a concrete curb to help in clean-up and to keep the wood well above the grade. This allows you to put in the foundation walls and start framing quickly. The slab can be poured at any convenient time.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

Same Old

What does this mean?

Tscarborough

It means the walls/foundation are not tied to the floor (structurally), hence the slab "floats".

concretemasonry

A real floating slab is supported by the soil and may be surrounded by a foundation wall or it may be a "patio" type slab. It is like an independant extension of your driveway and is supported by the soil (hopefully compacted) and not by the foundation.

A structural slab is rigid and carries the all load, so it needs more strength (thickness, rebar) to support the garage floor loads. You can never count on both the soil and the foundation walls to both support the loads at the same time. Compacted fill is never as good as the native soil. Even 1/16" or less of settlement is enough to throw all the load into the slab and rebar that are supported by the foundation, forcing the slab to span between the walls.

With questionable/poor soils, you may see thicker reinforced concrete slabs with thickened edges that have the ability to float over some of the questionable pockets of bad soil.

You may find some local differences in terminology, but the things still work the same no matter where you are.

Usually, a floating slab with wire mesh is adequate if there is decent natural or compacted soil under it. If you come 15 miles north of you, I can show you my townhouse "floating" garage slab that was supported by the usual uncompacted junk a cheap builder throws in before pouring a townhouse garage slab. In my case, the floating slab should have been reinforced with rebar as a structural slab because the drywall and beer cans compacted over 20 years time. Apparently Mr. H????? did not care about the garage slabs when he built in 1980-85.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

Same Old

I got you now. I was confused because I've never poured anything without rebar. A floating slab to me should still be reinforced with a bar grid, if only to keep it together and prevent lippage.

And, what is this mesh that you keep talking about? Microfibers baby, all the time, no more messing with mesh anymore.

DaHammer

The slab itself will be 5" to 6" thick with a 5/8" rebar 2' x 2' grid placed in it.

According to the geotechnical engineer that did my soil test the fill needs to be a clay-sand dirt and needs to be compacted to 95% Standard Proctor Scale. So I need some way of compacting the fill without compromising the perimeter block walls.

This will be my personal home for many years to come, so I want it done right. So you think grouting just the block void that contain the vertical rebar would prevent damaging the walls when comacting the fill?

Well you can put the drain line that will penetrate the walls in and just cut it off a few inches from the wall. That would give you something to hook the rest of the line onto once the backfill is done. But there is really no way to do all the underslab plumbing prior to doing the backfill. The backfill will be 2' deep on one end and almost 4' on the other end and it would have to be in place before you could run the plumbing in it.

Maybe I haven't explained the situation well enough. The wood wall will be a at least 2' above the grade, as it sits on the slab. That's the whole point of using the block walls to raise it. You get a crawlspace look without a crawlspace.

Anyway, what I'm interested in is how to properly construct a concrete block wall so that it can take the lateral pressure of the compacted backfill. The walls will be 3 courses high on one end and 5 courses high on the other end. The end walls are 32' feet long. The front and back walls are 70' long and the block wall will be stepped from 3 courses to 5 courses high along them.

concretemasonry

Is sounds like things are being taken care of.

The 5-6 slab w/rebar is a structural slab.

you had a geptech specify what kind on compaction is necessary for the fill you are using. At least you do not have to go to 95% of the MODIFIED Proctor. Some areas have soil that can only be compacted with a jumping jack, but that can do a job on ebrything around it.

Grout before backfilling. It should only be necessary to fill the cores with grout in them.

Plumbing is always a problem to anticipate what will be needed. You can put it in inadvance or sleeve it or stub it out. It is always questionable what can happen to plumbing when you start compacting the soil under, around and over it. Obviously, it is best to compact the soil, put in the plumbing and then compact what is above, but that is not always possible. Thank goodness there is some flex in piping since there is little "give" in concrete.

Just build your block walls, reinforce and grout them and brace them so compaction does not put too much unbalance load on the walls. If an engineer did it, he probably over-did it some to cover any errors. Don't be too worried about the height since many basement block walls now are built 9'+(13 - 14 courses) with 7-8 feet or so of soil retained.

Same Old -
Fiber mesh is good for mico-cracking and shrinkage, but wire mesh (6x6 - 10x10) is still the best for both strength and shrinkage cracks even though many concrete contractors are too lazy to do wire mesh the right way. If you don't saw the control joints correctly, you will quickly realize how much good the mesh does.

My last personal driveway was 5000 psi, air entrained with fiber mesh and 6x6 wire mesh. If I had to take out one thing, it would be the fiber mesh. Cemstone has a good policy with their Duracrete for driveways.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

DaHammer

You meant "with rebar in them", correct? What type grout would you use? Does grout work better than ready mix concrete for this?

If I can get by with just grouting the cores that contain rebar, then the plumbing thing is not really an issue anymore. There are only a few places where the plumbing would penetrate the perimeter walls and I'm sure the plumber can work around the grouted cores for that. In fact I could grout all the cores, leaving just the ones that will have plumbing hollow until after the plumbing is in.

And yeah, the jumping jack is problematic. I'm not sure what the best tool for compacting the coil next to the walls is. I've read that the vibrating plate compactors don't do so well on anything except sand. You can't get a Rammax or anything like that up next to the walls either. So it's either a jumping jack or a hand tamp I guess. I suppose I could do it with a hand tamp.

Same Old

I have to respectfully disagree on your assertion that mesh adds any reasonable strength to the slab. As far as I am concerned it is only reliable to help control shrinkage cracking and fibermesh does a better job of that. If I had anything to do different on your slab, I'd use an all granite ag and a rebar grid instead of the mesh. I've seen many, many slabs where the frost sheared the mesh clean at cracks, never saw that with bar. Typically I pour 4000psi, 3/4" granite ag, 6% air, fibers with a 3' grid for 4" driveway slabs on decent soil.

I like Ag Ind for mud. Cemstone is good for color though. Ag Ind is more consistent at residential volumes in my experience. I've had too many hot loads from Cemstone, and sometimes it seemed the mix wasn't anywhere close to the batching on the ticket. Better drivers at Ag too it seems.

concretemasonry

Wire mesh does add to the flexural strength if it is not laying in the sand, although you can never to get an engineer to put it in writing (but he sleeps better). In some more advanced countires, they use wider spaced mesh so you can step in the gaps and make the mesh work the way it should.

Fiber mesh is good for micro cracking, but it can be bad if over-mixed.

Nothing is better for crack control than a properly sawed control joint (1/3 of the depth) that is filled with a good caulk after the crack has had a chance to form.

You and I are spoiled since we are sitting on some of the largest and best natural aggregate sources in the country. Even the worst are still good. Some old pits have a very minor problems with shale, but the new reserves to be open jointly with Ag Ind(Holcim Cement), Cemstone and others should be state of the art. I just have a preference in suppliers because of the research laboratory and the technical support. Hopefully, Ag Ind will catch up on that item.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

Tscarborough

My philosophy is that concrete is going to crack, no matter what you do. My job is to control where it does so, so that I can address carrying the load across the crack, preventing the crack from moving, and protecting against water infiltration through the crack.

Fibermesh helps, but continuous reinforcing is still needed, along with properly sealed control joints.

Dik Redi

vert re-bar grouted is a must bom-beam is a good idea,, and for compacting you should compact gravel/crushed stone every 4" ( use dura-wall on evry course to be safe)

masonking02

the block foundation should be filled solid every 4feet with rebar placed inside the grouted cells .

Same Old

Do you mean a "bond" beam? If not, what is a bom-beam?

My engineer tells me dura-wall actually makes for a weaker mortar joint. He insists that you will get a stronger wall without.

DaHammer

Interesting. I assume dura-wall is the wire mesh made specifically for block mortar joints?

What type of mortar and grout makes for the strongest wall? I noticed the mason that built the wall that bowed used Lonestar type S mortar.

ch0mpie

The only advise I would add is pay close attention to the optimum moisture content of your fill material. The closer you are to the optimum moisture content, the less energy needed for compaction, the less passes required to reach 95% proctor, less dynamic forces acting on your block wall

POOLMANinCT

I trust concremasonry will correct me if I'm wrong. I am under the impression the only effective bond beam steel placement is horizontal & what we call "7 iron" the 7 only refers to the shape of the steel not size...

simply put the "fat ring" on top of a dixies cup is a bond beam, break that "fat lip" and the cup will easily fold.

ray

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......Less with the jaw & More with the paw.....

POOLMANinCT

while we ve have you here, concremasonry. would you elaborate on fibermesh slabs. I have no quantitative data, but when I have to break up a mesh slab... the hammer/breaker seams to merely bounce off the work.... and the radiatiting damage is minimal... a fibermesh slab break up job is a pita? is this strength more likely coming from the mix or fiber mesh?

typical slabs are 4" pool decking... more often I find zero steel in them..
I had assumed it was the mesh slowing me down..
ty
ray

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......Less with the jaw & More with the paw.....