rebar spacing
Last Post 12 Apr 2010 12:28 AM by Jinal. 11 Replies.
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oldsamdogUser is Offline
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30 Mar 2010 10:59 PM

Hello, I've been trying to figure out my horizontal and vertical rebar spacing.  Consulted a polysteel manual and talked with a couple concrete guys, one does icf and the other conventional forms.  I was thinking of the following: basement wall 8 inches thick  10 feet high  backfilled up to 7 to 8 feet.  Vertical rebar every 16 inches.  Horizontal rebar starting at 12 inches and at 18 inch intervals after to the top.  Rebar being #4 grade 60.  Second level 6 inches thick 9 feet tall.  I was thinking of going with the same spacing.    As far as concrete I'm not sure 3500 or 4000 psi.  What does everyone think. Does  my spacing sound somewhat right for top and bottom?  I can find charts for the vertical spacing but not as much on the horizontal.

JasonSchneiderUser is Offline
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31 Mar 2010 09:40 AM
I don't know where you are contemplating purchasing the forms, but if you would look at one of the major forms from a knowledgeable distributor, they could provide you with engineered charts for multiple scenarios. This is only one of the many value-added services a distributor will provide, and helps to explain why most block are sold through distribution, and not factory direct. That being said, if you have forms, but no technical support follow this link for the prescriptive path to designing with ICF. http://www.huduser.org/portal/publications/destech/icf.html I would strongly suggest that you get an ICF professional involved in your job. Even if you are familiar with other types of construction, there are a lot of things about ICFs that make it difficult to learn on the go. If you would like to do the majority of the work yourself, at the least get an ICF pro who can check your work and help you pour the wall. My two cents. Take it for what it's worth.
TexasICFUser is Offline
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31 Mar 2010 09:55 AM
Can't disagree with you Jason. Based on what i've seen out there the best installations include local support. A direct sale job may or may not be selling to a trained installer.

Oldsamdog, if you like 18" horizontals (as i do) considered placing one extra bar on the slab and drawing it up into the web -- it will be about 3 inches off the slab. Start your next horizontal an the top web of that block and then every 18" thereafter. Show it to an engineer. Regards.
arkie6User is Offline
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31 Mar 2010 02:20 PM
#4 rebar at 16" to 24" on-center (oc) vertical and horizontal is pretty common around here (central Arkansas). But this can vary depending on many factors such as backfill height, wind loading, seismic, etc.

For my walkout basement with ~7' of backfill on the uphill side, I'm going with #5 GR 60 rebar at 16" oc vertical and #4 @ 16" oc horizontal. On the walkout side I will use #4 verticals. This all lines up with the #5 dowels I have set in my footing. On the main floor I will use #4 GR 60 @ 16" oc vertical and horizontal. 16" oc works well with the LiteForm ICF which have ties @ 8" oc. Based on the math, I could easily get away with 24" oc everywhere except on the uphill side where the backfill is highest. But I'm on the fringe of the New Madrid seismic zone and if it goes off I would feel just a bit more comfortable having that extra rebar in the wall.

The bigger concern I have is for the rebar in the lintels. My tendency is to over design here by putting in more rebar and stirrups, but then you can have a hard time getting the concrete to flow and consolidate properly. The HUD Prescriptive Method for ICF has some good discussion on lintel design and tables showing rebar requirements. The distributer/installer that I'm buying my ICF from recommends floating in a 4' #4 rebar @ 45 deg over the corner of each opening to minimize cracking at these corners. I've seen this detail shown in a few ICF online manuals also.
Concrete ProducerUser is Offline
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31 Mar 2010 07:35 PM
In our area, you can't get a permit without an engineer's stamp, so we let the Structural Engineers figure all that out! Regarding your concrete, we usually supply a pea gravel mix for a 6 inch form and a 3/4 inch rock mix for an 8 or10" form, depending on rebar schedules. In our siesmic zone, the 4" forms require so much steel that are not economical, so we don't see them. With our materials, we run about 425 lbs. cement and 150 lbs. fly ash and get well over 5,000 psi at 28 days. This is way overkill for strength, but the mixes flow well and do not use mid-range or high range water reducers, so they are economical. Our contractors usually place just over a 6 inch slump for ICF and the good ones all use the pencil vibrator for removing entrapped air and consolidation. Be sure your concrete producer knows you are pouring an ICF wall. Some people order "Block Fill" and this is a grout that is used for CMU block. Not even close to the ICF mixes.

I would advise a quick call to the sales department at your local ready mix producer to see what they recommend for the particular block and size you are using. Concrete is made from natural materials and those materials can vary throughout the various regions in the country.
arkie6User is Offline
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01 Apr 2010 06:02 AM
Where I live, if you aren't building within the city limits, no building permits of any kind are required. No permits or inspections of any kind are required for my homes location. This even includes plumbing and electrical.
arkie6User is Offline
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01 Apr 2010 06:08 AM
Concrete Producer, are there any issues with using pea gravel (3/8" smooth aggregate) in 8" basement walls other than the additional cost for the aggregate?
TexasICFUser is Offline
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01 Apr 2010 11:22 AM
Arkie6, Don't mean to jump in but besides being less expensive regular aggregate is also stronger than pea gravel. Some of your better installers will use it in 6" as well. However, a little extra vibration is required to ensure proper consolidation in 6" - typical vibration in 8". REgards.
Concrete ProducerUser is Offline
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01 Apr 2010 12:31 PM
In regard to sizing coarse aggregates in concrete mixes, the principle is that the smaller aggregate size you use, the cement factor has to increase to coat the additional surface area of the smaller aggregates. Thus, the pea gravel mixes tend to cost a few more dollars per cubic yard, but if you have very tight rebar in an 8" ICF wall, it may be money well spent. In the siesmic zones, many of the columns in our high rises use pea gravel mixes because of the tight rebar. Often, these are 10 to 15,000 psi mixes and we can make that with pea gravel, but these mixes also have a lot of chemicals in them and are spendy, but when you are pumping them 600 feet in the air, that is what it takes. High strength concrete allow designers to make columns thinner, adding leaseable square footage, something building owners are interested in!
For ICF jobs, from what I have seen of strength specifications and in conversations with engineers, they are only looking for 3,000 psi. Remember, this is a whole wall assembly, not a structural column. ICF offers a great curing environment, so as long as proper consolidation takes place, strength is not a big issue with ICF. Proper consolidation and encapsulation of the rebar is of primary importance. Quality ready mix suppliers tend to overbuild mixes somewhat, knowing that they take some abuse (water additions, too much time on the truck, etc.) in the field. Your producer should be able to supply you with a "Mix History" where he has multiple strength tests for the mix you are considering.
Another suggestion for an 8 inch wall with tighter rebar is a good 50/50 mix, where we blend pea gravel with 3/4 inch rock. That is a popular mix with some contractors for exposed aggregate jobs and for some decorative jobs for stamping or texture mats.
In any construction project, there are always tradeoffs. In our area, sometimes the customer will want to stick with a 6" wall for economics, or to keep the wall sizes all the same, but we work with the local Structural Engineers who may require stepping up to a larger rebar in the 6" wall vs. staying with #4 bar in an 8" wall. The last six story ICF we did, they used the 10 inch forms on the bottom two stories, and then the 8" forms on the upper floors. In that job, we used 3/4 inch rock mixes all the way up, because the larger forms offer a thicker concrete section, dictating less steel. We also were involved with a four story that used the 6" system all the way up and the rebar was so tight, that in hindsight, the whole building would have been more economical using the 8" forms, saving steel and labour.
Jerry D. Coombs, PEUser is Offline
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02 Apr 2010 03:36 PM
Concrete Producer is once again thorough. Where do you hail from? I like the way they do things.
Keep in mind that there are limitations under which you're allowed to use those tables without an engineer's design and seal. Little known, and less often enforced. Check the tables of the manufacturer, then the ICF prescriptive design book. Then check out ACI 332, Residential Concrete. I hope to someday get ICFs in there, but there is good information in there about reinforcing that is in contrast with ACI 318, but limited to residential use.
Most places in the country (finally even Texas!) have building code requirements. Just because the county doesn't have the funds to administer and enforce the code, it doesn't absolve anyone from following it. Check with the state administration. The legislation adopts (usually) a minimum code standard.

When asked, I usually say max 1/2" in ICF walls, especially Polysteel waffle wall. But then, I'm not the one placing it
Jerry D. Coombs, P.E.
Coombs Engineering, P.C.

You can have with quality; You can have it fast; You can have it cheap. Pick any two.
Concrete ProducerUser is Offline
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02 Apr 2010 04:10 PM
Jerry, we supply concrete in Nevada, CA, AZ, OR, and WA. We have cement plants in CA and AZ and supply cement to Alaska, BC, Alberta, Idaho, and all the places we produce concrete and aggregates. Here in the Northwest, I work with engineers, architects, contractors and owners on the best ways to use concrete in their projects. I find projects go the smoothest and can be built more economically when we get invited to the table early to discuss options. We have lots of materials available to us and we can combine various materials, agg gradations, and admixes, etc. in ways that many people are not aware of. Making a positive contribution to these projects is the thing that makes being in the industry rewarding.
JinalUser is Offline
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12 Apr 2010 12:28 AM
Spacing can also be determined by the size of the bar specified.A smaller bar usually is tighter spacing,a larger bar increased spacing.Best to have the site situation reviewed by an engineer for your specific application.
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