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Clark
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Posts:62
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| 10/10/2009 11:52 PM |
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From my first introduction to ICF, I wondered why so much rebar is called for in the HUD, IRC and other ICF prescriptive design standards. Even ICF manufacturers recommend substantial amounts of rebar in contrast to typical construction practices for traditional residential poured concrete foundations in my neck of the woods. I think I might have figured out the reason...
Recently, I bored a 5" hole through a 7.5" ICF below-grade wall. When I extracted the concrete core from the bit, I noticed a crack in the concrete aligned with the position of one of the PVC ties. I can't be sure (without removing some EPS) that the crack extends the full height of the wall, but I'm assuming it does. The crack appeared tight and I've experienced no negative affects of such cracks (e.g. leakage.)
The more I ponder this observation, the more I'm convinced that the existence of the many vertically aligned plastic ICF ties penetrating the concrete produces weak spots in the wall, much like control joints in a concrete slab. The metal ties used in standard removable forms are far less numerous and, therefore, less likely to affect the strength of the concrete. My thought is that an ICF wall requires substantial steel reinforcement to overcome this inherent weakness causes by the plastic ties.
My question is: Has there been any study as to the affect on the strength of an ICF wall due to the numerous plastics ties that penetrate the concrete? Has anyone removed the EPS from an ICF wall that's been subject to stress (e.g. earth backfill, water pressure) to see if cracks in the concrete exist? Was #4 rebar 2' o.c. vertically and 3' o.c. horizontally not enough to prevent cracking of this kind in a 7.5" x 10' ICF foundation wall? |
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ICFconstruction
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Posts:638
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| 10/11/2009 7:52 AM |
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In many cases the engineering is the same for a poured, removable-form wall of the same thickness. I have never heard the theory of ties weakening the wall nor have I heard engineering account for it.
Although common sense would be that the concrete is weaker in-line with the ties, I am sure it is negligible. You should consider other causes. |
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Brad Kvanbek - ICFconstruction.net |
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Alton
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Posts:662
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| 10/11/2009 8:48 AM |
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Correct me if I am wrong, but rebar will not keep concrete from cracking. However, with the right schedule, it will keep the cracks tight when the concrete fails. |
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Alton C. Keown
Residential Designer and Construction Technology Consultant
Auburn, Alabama
E-mail: alton at auburn dot edu |
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ICFconstruction
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Posts:638
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| 10/11/2009 9:10 AM |
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| I believe rebar will lessen the likelihood of cracking and will greatly increase holding it together when it does. Shrinkage cracks are something else, that I understand less. |
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Brad Kvanbek - ICFconstruction.net |
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jamesmacdonald1
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Posts:70
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| 10/14/2009 8:19 AM |
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Brad is correct. Typicallly, reinforced concrete codes have a minimum requirement for steel for crack control. Assuming a wall is designed to span "one-way" vertically from floor to floor (this is typical), the vertical rebar is the rebar required for bending strength and the horizontal rebar is present to control cracking (in theory). The amount of vertical rebar is usually more than the minimum required for crack control and that is why you might see larger rebar vertically than horizontally (the horizontal is the minimum required and the vertical is based on structural design).
From a design perspective, the compressive strength is based on the concrete strength itself and the bending strength is based on a combination of the vertical steel to take the tensile bending stresses and the concrete is designed to take the compressive bending stresses.This is still assuming the typical one-way design methodoloby (straight forward, simple and conservative and is what the majority of engineers do).
I have no doubt that the aligned plastic ties do induce vertical cracking, but it doesn't matter. All concrete cracks to some degree and design of the wall is still adequate with respect to the design codes. Furthermore since most (if not all walls) are designed one-way (vertically), the vertical crack plays even less of a role in strength. The only real consideration is to not count thickness of the tie in detemining your section geometry. This is a minor consideration since the plastic tie is 1/4" every 6". This is approx 4% reduction in strength and applies primarily to the axial load capacity which is never the design limiting factor.
I sometimes have a problem with people saying that one wall is stonger than another wall. As long as it is designed by an engineer, they are alll capable of resisting the same design loads with the same factor of safety. This is the requirement of building codes in conjunction with the design codes. (The building codes will determine the loads to be designed for and the safety factor multiple on these loads, while the design codes will stipulate the methodology of calculating the resistance of the wall and the safety factor applied to it). For example, if you are building in location X, the building code will state the design wind load is Y. With the applicable safety factor you need to design for 1.5Y. The reinforced concrete code then states that calculate the strength of the wall based on certain methodologies and built in factors, etc. You then get a wall strength of R which is also factored down to 0.75 R (for example). Then you need to make sure that 0.75 R > 1.5 Y.
This is true with any system and saying that a wall without ties is stronger than a wall with ties is a bit misleading. The main reason that prescriptive methods are more conservative than analytical methods is because of what they are. Prescriptive means that it is based on experience, long standing experience without any analytical design to support it. It is the rule of thumb. Analytical methods (like ACI 318 the reinforced concrete code) is based on theory with experiimental testing to support the theory. By it's very nature, prescriptive methods are more conservative and result in larger reinforcing requirements, shorter height limitations, etc.
So my final thought is that the vertical cracks are not something to be overly worried about.
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ICFengr
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Posts:5
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| 10/16/2009 12:40 AM |
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Hello. The reason of needed rebars is that a concrete without rebars is not a structural element. Concrete mass against reinforced concrete.
When it comes to a structural wall, there are not only vertical forces creating compressive stresses (that the concrete is very good at resisting them) but also bending moments and posible lateral forces (earthquakes) that will potentialy create tensile streses (that the concrete is very weak at resisting them).
The forces, moments and basically the stress distributions on a structure is a matter of careful structural design. The "prescriptive design standards" have a limiting clause at the beginning.
The plastic or metal ties in ICFs do not and should not create cracks. At least not in our projects.
Cracks of any kind are indications of bad construction practice or bad engineering detailing and design. The crack(s) you are describing maybe related to inefficient crack control rebar details, but on the bottom line there shouldn't be any cracks period. |
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Mark Fleming
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Posts:217
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| 10/16/2009 12:48 PM |
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The IRC schedules are based on "recent" destructive testing of ICF walls, rather than the old ACI tables developed many years ago (some of which was WWII research) . There's no way to not factor in the effect of plastic ties if you're using destructive testing as your method of determining rebar schedules for a wall that has plastic ties. If the ties effect the strength of the wall, it will effect the test results and, ultimately, the recommended schedule.
That being said, as noted above, the major benefit of concrete is it's compressive strength (ignoring fire resistance, bug resistance, rot resistance, thermal mass, etc. etc.). Minor shrinkage cracks, and even most major cracks, won't effect compressive strength.
I built with an ICF listed under the IRC as a "screen grid." Because it's a grid, it basically has " 6 inch holes" alligned vertically and horizontally 15 inches on center. In theory, a condition much worse than plastic ties arranged in a similar fashion. Yet, under destructive testing, the screen grid walls test out remarkably close to solid ICF construction, even under lateral loads (i.e., seismic situations). Cracks, plastic ties, and even holes, just don't have as much effect as you might think.
The IRC ICF rebar schedules are higher than plain-jane concrete walls. Some ICF manufacturers recommend rebar schedules even beyond the IRC. I went even beyond the manufacturer's recommendation. I probably put in an extra $1,200 of rebar in my ICF walls as compared to IRC plain-jane walls. Probably $800 more than the manufacturer's schedule (I used #5 instead of #4 bar). Money well spent, considering you can spend $800 on a toilet install.
Mark
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icfcontractor
Registered Users
Basic Member
Posts:277
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| 10/16/2009 12:50 PM |
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ICFengr,
Just out of curiosity, how will the rebar ever take a load if the concrete never cracks.
We use a licensed structual engineer on our projects, not a civil engineer, PE, or any other type of engineer. The one we like to use most, has been working with concrete almost his entire career from pre-cast and tilt up, to all kinds of cast in place. What my experience has shown me is that most of the time smaller or less rebar can be used if it is placed in a proper postion in the wall, beam, or slab depending on the forces anticipated for that specific structure. Since we spend a little more on our engineering we see big reductions in our amount of rebar used, thickness of wall, and number of connections. It is my opinion that if an engineer quotes the IRC or some other prescriptive code they are not engineering the building. They are however legally plagerizing someone elses engineering used by the the IRC or prescriptive code that is conservative to an extreme.
It is funny, we currently have a client that got bids from several engineers including the SE mentioned above. He chose one of the lower bids from a PE that had a bid of about $1000.00 less than the SE with tons of concrete experience. So far all of the engineering for the concrete that I saw was pulled directly from the Logix manual. This engineer did not engineer anything, he got paid to pull the super conservitive numbers and rebar schedules out of a book I provided him. I can do that, I don't need an engineer to do that. If you are paying an engineer, make sure they are engineering your building. By the way the running total so far is about $2500 in extra rebar and connections and that is just for the basement.
Just as a side bar I am not down on PEs or Civil Engineers, I know many of them and we use them all the time on projects and I know PEs that do a better job than some SEs. There is a reason they call them "practicing" engineers kind of like "practicing" doctors.
Thats my 2 cents worth, put me in the column of Pro Value Engineering.
ICF Contractor |
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Clark
Registered Users
New Member
Posts:62
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| 10/17/2009 11:28 PM |
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| In northern Illinois, it's not uncommon to pour 8' x 8" basement concrete walls with no rebar at all. Occasionally, a crack will develop at the corner of a basement window, so some rebar around window bucks is considered good practice. Except for the plastic ties, what's different about an 8" ICF wall that it should require so much more rebar? The inconsistencies among the various "recommendations" concerning ICF rebar schedules doesn't help to clear the air. Over-engineering the wall may be the safest, but not the most economical course of action. Better to design the ICF wall to withstand the loads expected for the particular building, location, soil type, etc. But, is that really possible? Do SEs really apply the theory, or simply plug some numbers into a software program based on ACI 318? |
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ICFconstruction
Registered Users
Advanced Member
Posts:638
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| 10/18/2009 8:01 AM |
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ICFengr, Concrete is structural without rebar, look at footings, they rarely are specified to use rebar.
I also believe that our walls are usually way over-engineered. But better over than under. Other construction methods require little reinforcement. CMUs for example, why anyone would use them for a basement is beyond me? |
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Brad Kvanbek - ICFconstruction.net |
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GreginAlaska
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Posts:11
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| 10/19/2009 3:28 AM |
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Posted By Clark on 10/10/2009 11:52 PM
"...Was #4 rebar 2' o.c. vertically and 3' o.c. horizontally not enough to prevent cracking of this kind in a 7.5" x 10' ICF foundation wall?
That doesn't seem like a lot of steel to me. We just poured our walls a couple of weeks ago, I have #5 grade 60 vertical at 8 inches OC and 16 inces oc horizontal....and 4 runs of #7 horizontal on the top. heh |
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robinnc
Registered Users
Basic Member
Posts:195
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| 10/19/2009 5:43 PM |
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icfcontractor.....I thought the PE stood for 'profession engineer'...?? You have to pass your states lic test for this.
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