Questions about ICF Walls and Footers
Last Post 06 Dec 2011 01:51 PM by Alton. 26 Replies.
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AltonUser is Offline
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02 Dec 2011 10:12 AM
Does anyone see anything wrong with using a keyway and rebar dowels in footers for below-grade, highly backfilled ICF walls that have 8" or more of concrete?
 
Do ICF installers typically use a water stop in the cold joint between the footer and a below-grade ICF wall?
 
How thick should the concrete core be for a 21' tall wall backfilled 10'?

Is it costly to switch over from placing a separate footer and wall to a monopour?  In other words, are there items that need to be purchased for monopouring footers/walls and can some of those items be reused on the next several projects?

Your thoughts will be appreciated.  I will not rely upon your responses without the advice of professional engineers.
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
Chris JohnsonUser is Online
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02 Dec 2011 08:51 PM
I don't see any issue with both a keyway and dowels, it is common practice amongst engineers in California that I dealt with

I use waterstop in areas that are known for having water issues, dense clay, etc.

Need more detail for you wall, i.e. length of wall, intermediate connections if any, etc.

When ever possible pour the footing and than start the wall, trying to set up for a monopour I find slower, mind you, we don't do it very often, so that may answer your question as well, also if you have an issue it may be harder to get it back fixed with a wet footing and wet concrete in the first couple of courses at the same time. Monopour are not for rookies and are really site specific
Chris Johnson - Pro ICF
North of 49
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03 Dec 2011 04:04 AM
Depending on how many corners your house has will have a big impact on how high much pressure is on the walls when backfilling. The longer the wall the more strength it needs to hold back the dirt. So you are saying that you are backfilling 10' up on the wall? So is this 10' 10" w/ the footing? Is the floor going to be poured concrete? I see very often engineers design walls that use the floor as a support to hold the wall from tipping in. The wall is not allowed to be back filled more than 4' until the first deck is poured. This allows for a much cheaper design for the wall. If you pour the basement floor prior to backfilling this again puts less forces on the wall, and a need for less reinforcing. If your 1st floor is out of concrete I would say a 8" wall w/ #4 rebar verticals every 2' OC and horizontals every 18" is way more than enough almost a bit of overkill. If the wall needs to retain the dirt on its own (no concrete 1st floor) I would suggest you pour a 4' wide footing and 10" wall. The wall would not sit center on the footing. Set 2' 9" of the footing to the outside of the house where the back fill will be. Same rebar detail will be more than enough just make sure the footing has dowels, that are actually bent, with the tails towards the outside of the house.
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03 Dec 2011 10:47 PM
Posted By Chris Johnson on 02 Dec 2011 08:51 PM

Need more detail for you wall, i.e. length of wall, intermediate connections if any, etc.



My concern is the connection between the footer and wall and how dense and strong the wall is.

I am asking for the wall to have an 8" concrete core placed with 4000 PSI mix since it is below grade. Upper remaining part of wall above grade to be 6". ICF contractor prosposes wall from footer to roof be 6" with #4 (#13 metric) rebar 16" horizontal and 24" vertical placed with 3000 PSI mix with 6" slump vibrated with pencil vibrator.

Footer: 8" x 24" x 26' formed in friable sandstone trench.  So the footer is not likely to ever move.
21' tall wall backed filled 10' above footer.
Straight Wall: length = 24'    No pilasters or offsets in 24' long wall but at each end will be an ICF wall which creates a 90° corner.  Side walls above grade to be with 6" core.
 
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
Chris JohnsonUser is Online
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03 Dec 2011 10:58 PM
In my humble opinion....No and the thing that is throwing it off is the 10' backfill. I don't have any tables handy, but I'm thinking 8" minimum and possibly 10" core, 15M bar @ 8" o.c. verts. Factors for consideration are seismic zone, backfill materials, etc.

If I get a chance tomorrow I will look it up, but if I was estimating without a stamped engineers spec in front of me it would be 10".

I am currently doing similar right now in downtown Toronto, our walls were 34' long, 13'6 high and was 12" core with double mat 15m 8" o.c. verts on the inside and 10m verts 16" o.c. exterior side, backfill height 13' with 2" stone compacted and we are holding back a hill with no pilasters, the no pilasters I have an issue with but it's stamped so I cannot argue other than give my $0.02 worth
Chris Johnson - Pro ICF
North of 49
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03 Dec 2011 11:47 PM
Is there a floor system attached to that wall? If so, how is it constructed and how high above the footing? What is the backfill material? Is there a slab that will be poured inside the wall above the footing? If so, how thick?

Off hand, I would say use a minimum 8" thick core wall with #5 (5/8") GR 60 vertical rebar at 16" OC for the below grade wall with 10' of backfill. The vertical rebar is more important than the horizontal. If you are going to try to save money on materials, it is the horizontal rebar rather than the verticals that would be placed at the greater OC spacing or reduced size. Insure that horizontal rebar is placed near the inside face of the concrete for greatest effect.
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04 Dec 2011 07:08 AM
The walkout basement will have a concrete floor but the concrete floor will not be resting on top of the footer since the house will be on bedrock.  In other words, 10" of clean gravel will be compacted on top of the bedrock and footer and then the concrete basement floor will be placed.  This 10" of gravel will give the plumber space to install the waste lines without having to remove any of the bedrock.  At this point in time we plan for the concrete slab to be placed up against the ICF wall without having any of the 2.5" of EPS foam removed from inside the wall.  Since we plan to slope the bedrock surface so that water will drain through footing drains to grade, there is not much concern about rising water under the slab entering the basement between the edge of the basement concrete slab and ICF wall.  The plan is that any water that gets under the slab will drain through the clean, tamped gravel to the walkout area before rising high enough to enter the basement by the slab edges.

Since the slab will be up against the EPS foam instead of the concrete wall, the slab will not brace the tall wall.  That is the main reason that I would prefer a thicker wall, keyway and rebar dowels to lock the wall into place.  I also prefer a thicker wall than 6" with 4000 PSI concrete with fly ash added to the mix to make the wall denser to resist hydrostatic pressure from forcing water through the wall.  A thicker wall than 6"will allow the rebar to be placed in the wall on the tensile side (inside) to resist the backfill and hydrostatic pressure. 

The call for vibrating is to reduce the chance of voids by consolidating the concrete, especially the concrete below grade.

The support for the Main Level wood floor will be installed at about 9' above the 4" thick basement slab.  This will result in approximately 10' of backfill against the wall - especially where the garage connects to the house.  Waterproofing of the below-grade wall is to be done with footer drains and a 3D type waterproofing membrane such as Delta MS.  The Main Level floor will be installed before backfilling the walls with clean, tamped gravel.

If the basement slab needs to brace the tall wall that is highly backfilled, then the EPS foam can be removed from the below-grade wall before the basement slab is placed.  Dowels could also be specified to connect the basement slab to the wall if the EPS foam is removed.

In similar situations in the past with non-ICF walls, we used a 12" thick concrete core with closely spaced rebar.  Why should an ICF wall require less?  Everyone's thoughts are welcomed before we take this application to a structural engineer.  The more I know, the easier it will be to support my client.  We are not only concerned about strength of the wall but also resistance to water penetration since the house will be built on the side of a hill.

My understanding of the ICF installer's plan is that the horizontal rebar will be every 16" so that it can be easily placed in the middle of the top plastic tie of the ICF block before the next course of block is installed.  I was told that the rebar placed in a 6" concrete core had to be in the middle to get proper concrete coverage.    My client and I have been assured that the ICF installer is willing to build a better wall if there really is a need for it.

Email:  Alton at Auburn dot Edu
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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04 Dec 2011 11:44 AM
Posted By Alton on 04 Dec 2011 07:08 AM
...I was told that the rebar placed in a 6" concrete core had to be in the middle to get proper concrete coverage.   
That is incorrect for walls.  3/4" is the minimum allowable coverage for rebar in walls in this type of application.  See previous discussion here:

http://www.greenbuildingtalk.com/Fo...fault.aspx

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04 Dec 2011 12:12 PM
Arkie,

Thanks.  I remember reading this posting.  My ICF contractor probably know this too but he is somewhat worried that even with vibrating with a pencil vibrator that the 3/8" fractured gravel in the mix will not consolidate around a #4 bar that close to the foam. 

Regardless of wall thickness I would prefer the rebar be placed to act in tension, that is towards the inside for a backfilled wall.

Question:  Now for the 6" concrete wall above the backfill, should the rebar be placed in the middle of the wall?  Is there a tensile side to favor when the wall is above grade. 

I have heard during lectures from PCA at trade shows that the vertical rebar in above-grade walls is primarily for temperature control.  Does the vertical rebar also add strength to the wall, especially for a 21' wall above grade?  Does it offer wind resistance also?
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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04 Dec 2011 12:18 PM
Alton, will the slab be poured before or after the walls are constructed? If the slab will be poured after the walls are built, how will the ICF contractor anchor his wall bracing above that bedrock and 10" of gravel. Driving a metal stake in 10" of gravel won't hold much.

I would consider a rigid connection between the slab and wall. One way to do this is to remove 3" or so diameter cut outs every 16" or so in the foam at the level of the slab. If the slab will be poured later, you can cover the cut-outs with pieces of OSB screwed to the plastic ties to hold the concrete in. Be sure to vibrate this area sufficiently during the first pour to insure the concrete flows out to the OSB. If vibrating internally, I would still hit these OSB covers sharply with a hammer a few times to make sure no voids exist in this area. This will allow the slab to float if need while still providing lateral resistance.
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04 Dec 2011 12:49 PM
Posted By Alton on 04 Dec 2011 12:12 PM
Arkie,

Thanks.  I remember reading this posting.  My ICF contractor probably know this too but he is somewhat worried that even with vibrating with a pencil vibrator that the 3/8" fractured gravel in the mix will not consolidate around a #4 bar that close to the foam. 

Regardless of wall thickness I would prefer the rebar be placed to act in tension, that is towards the inside for a backfilled wall.

Question:  Now for the 6" concrete wall above the backfill, should the rebar be placed in the middle of the wall?  Is there a tensile side to favor when the wall is above grade. 

I have heard during lectures from PCA at trade shows that the vertical rebar in above-grade walls is primarily for temperature control.  Does the vertical rebar also add strength to the wall, especially for a 21' wall above grade?  Does it offer wind resistance also?

Hello Alton,  Some thoughts: 
 
Your contractor should not have any trouble consolidating around the #4 bar.   This is the minimum coverage because as you said, you generally want to favor tension side of the wall.  However, if you get too close to the edge of the tension side (where the greatest tension exists) you wont get the bar properly surrounded.
 
Most engineers are going to go for the middle of the wall for above grade (single mat).   You can review predominant wind(s) for your area and find a tension side to favor.  But since your construction is so strong already I'm not sure if its worth it.    That said sometimes the tension side is both if you get strong winds from both sides depending on time of year or hurricanes, tornadoes, etc.   If this is known to be the case they will sometimes double mat to handle the problem.

You might want to go back an check with PCA but I think verticals are for lateral loads and horizontals are for temperature control (and more importantly lintels).   Usually, at least around here, when an engineer gets carried away (usually for a good reason);)  - it's with the veriticals.   E.g.  #6 bar 8" O.C.  etc. while horizonatals are #4 or #5 on 18" or 16" O.C.  This varies of course but more engineers I work with are more concerned with the verticals due to shear at the slab and lateral loads due to wind and etc.   Of course the horizontals are crititical for all lintels openings and etc.  As you know you have the same situation here with an opening -- you have the bottom of the lintel predominately in tension so most of the steel (e.g. bottom bar size) will go there.  Regards.


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04 Dec 2011 01:20 PM
Thanks to everyone.

I may have mis-undestood what I was hearing at the trade show or the person may have mis-spoke about the role that vertical steel plays.

I think it will be interesting to see what the structural engineer will design for this wall.  I think this wall is beyond the code enough that a structural engineer will be required.
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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04 Dec 2011 02:15 PM

I agree that driving stakes into 10" or more of gravel will not hold well.  The bedrock underneath would prevent further depth.

General practice in my area is for the ICF contractor to brace from inside the basement area and then later brace off of the wood floor for the main level walls.  However, in this case, we are asking for brick ledges to be on the inside of the building which will probably require the bracing to be from the outside for the basement walls.  The brick ledges will be only on the long walls that are above grade.  Now if steel channels are used instead of TrimJoists to support the main level floor, then we may not need brick ledges.  Regardless, we would prefer that the rough-in plumbing and basement slab be installed after the basement walls are up so that the plumbing will not be disturbed and the slab will not have to be formed.

The intermittent rigid connection you mentioned between the slab and wall is not to serve as a ledge like a footer would, but to let the slab periodically brace the wall about 10" above the footer (slab edge).  Right?

Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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04 Dec 2011 02:29 PM
Posted By Alton on 04 Dec 2011 02:15 PM

The intermittent rigid connection you mentioned between the slab and wall is not to serve as a ledge like a footer would, but to let the slab periodically brace the wall about 10" above the footer (slab edge).  Right?


Yes.  This would effectively reduce the unsupported wall height.
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04 Dec 2011 02:38 PM
If you go with an 8" or 10" wall for the below grade wall with a 6" wall for the above grade wall, you might consider a taper top form for the thicker wall to provide a ledge to support the floor system. The advantage of this is that there is no "bump out" of the wall like a brick ledge which makes it easier to brace and finish. It is also stronger with fewer if any rebar stirrups required for the ledge. Brick ledge typically requires a stirrup every 8" to tie the inside and outside horizontal rebar at the brick ledge.
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04 Dec 2011 05:41 PM
Arkie6,

Yes, I have considered using a double taper top block but in this case we need the brick ledges to run East/West but the tall backfilled wall that might need to be thicker is running North/South.  Since the East/West walls are above grade, then we can not use a taper block to create much of a ledge on top of a 6" thick concrete wall.

We have considered using Simpson Strong-Tie clips in the ICF wall but prefer a ledge more so for 4x2 floor trusses or TrimJoists.  The Simpson method requires three clips on each end of a truss.

In order to avoid having a brick ledge to the inside, we are considering Dietrich Trade Ready Joists and Steelform joists.  I do not know which steel system will be the least expensive until we get quotes.  At least with the steel joists, all we have to do is attach a steel channel to the East/West walls and then screw the steel joists to the channel.  A brick ledge would not be needed.

If we go with steel joists instead of wood floor trusses, then the question becomes whether to use bolts or ICF clips to hold the c-channel to the side of the wall.  If we use bolts, then it would probably be Red Head Large Diamter Tapcons installed after the concrete has set.
 
What would be your preference?

We are also considering using Tapcon screws or bolts to attach the top plate to the cured wall regardless whether the top plate is wood or steel.  I think we will get better alignment if we attach the top plate after the concrete has set.  In other words, do not bury anchor bolts in the wet concrete.  I say that because I have never seen a residential contractor stretch a string when installing anchor bolts in fresh concrete.

By the way, we plan for the tops of all of the walls to be sloped at a 2 to 12 pitch.  This would include the long East/West walls to have both sides of the foam cut at the same height.  The North and South walls would have to have the foam cut lower on the North side as opposed to the South side 11" away for 6" concrete core walls.  If the top of the walls are level, then the top plate would have to be sliced to meet the 2 to 12 slope.

My idea for accomplishing this would be to attach a screed board with screws to the plastic ties to establish the proper angle.  Then the concrete could be trowelled to the screed boards on each side of the wall.  The screed boards would be removed after the concrete has set.  I would like to set the thin screed boards on the inside of the foam but my guess is that the plastic ties will be in the way.  If so, and the screed boards are set to the outside then the concrete will be 5" wider at the top unless we compensate for the thickness of the foam on each side.  It is critical that the wall heights are correct since we plan to use 42" wide roof SIPS.  Writing this just made me think how we can use screed boards without making the wall thicker. 

Thoughts and suggestions appreciated.
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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05 Dec 2011 07:51 AM
Every job that I'm involved with we do monopour it's the only way to pour especially with tall walls. No keyway or dowels to worry about. F ooting and walls in one pour, I've never had a problem with backfilling even though I've had excavators insist on the walls be braced. One thing you can do if you are unsure about backfilling is to install the floor system before backfilling. Framers don't like it but so what.
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05 Dec 2011 07:59 AM
Alton, you asked a question about cost for a system and the one that I use cost about $2.50 per lin ft for 20" footing which is my most common width.
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06 Dec 2011 12:08 AM
Posted By Alton on 04 Dec 2011 07:08 AM
The walkout basement will have a concrete floor but the concrete floor will not be resting on top of the footer since the house will be on bedrock. 

Email:  Alton at Auburn dot Edu


Alton - if its a true walkout basement, I believe that will add another complexity to the equation. The only restraint at the top is now the diaphragm action of the subfloor as the subfloor cannot push against the backfill on the other side. Realistically I thing you are building a high retaining wall and hanging a floor off it. Just a thought. Bob
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06 Dec 2011 12:12 AM
Posted By Chris Johnson on 03 Dec 2011 10:58 PM
In my humble opinion....No and the thing that is throwing it off is the 10' backfill. I don't have any tables handy, but I'm thinking 8" minimum and possibly 10" core, 15M bar @ 8" o.c. verts. Factors for consideration are seismic zone, backfill materials, etc.

If I get a chance tomorrow I will look it up, but if I was estimating without a stamped engineers spec in front of me it would be 10".

I am currently doing similar right now in downtown Toronto, our walls were 34' long, 13'6 high and was 12" core with double mat 15m 8" o.c. verts on the inside and 10m verts 16" o.c. exterior side, backfill height 13' with 2" stone compacted and we are holding back a hill with no pilasters, the no pilasters I have an issue with but it's stamped so I cannot argue other than give my $0.02 worth


I would agree that 8" with tight verts is minimal. On the other hand I don't think (if I remember right) that Alton has to deal with frost expansion of clay soils ;-) Bob
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