Block Foundation

[Squirl]

A full depth concrete foundation it is. (so far, but it is not set in stone (pun intended)) That leaves 2 options a concrete block foundation or a poured solid foundation.  I will address the concrete block first.  There are two methods of concrete block Drystack or Mortar. For a 6 ft high wall at $1.25 a block and 100 ft of wall (20x30) I would need around 533 blocks with delivery for $760 dollars.  If I did dry stack, I would need  24 bags of surface bonding cement at $15 a bag for $360.  $1100 looks like a good deal, except for the rebar.  Every book on block foundations seems all over the place on rebar requirements and core filling.  The code is pretty cryptic on this.  I assume the section on this is R404.1.1 Design of masonry foundation walls covers this. The link can be found here : http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec004_par001.htm

My best reading of it is TABLE R404.1.1(1)  states that you can do a 6 ft high wall with four feet of unbalanced backfill (I will have a maximum of 1 ft of unbalanced back fill) can be built with 8 inch hollow masonry ungrouted.  Only when you get to the taller walls does it refer to foot note E. "Wall construction shall be in accordance with either Table R404.1.1(2), Table R404.1.1(3), Table R404.1.1(4), or a design shall be provided."

TABLE R404.1.1(2) 8-INCH MASONRY FOUNDATION WALLS WITH REINFORCING WHERE d > 5 INCHESa, c  calls for #4 Rebar every 48".  What does d>5 mean?  I also see no mention of horizontal rebar.  Is none needed?   My understanding of drystack is a "bond beam" is needed?  Can anyone give a definitive answer of what is required?  The cost and labor of rebar and a full core fill, might change the direction of this option.

[h0rizon]

The superscript letters on the charts seem to indicate the sub-notes section, so I believe that it should be footnote C:

Vertical reinforcement shall be Grade 60 minimum. The distance, d, from the face of the soil side of the wall to the center of vertical reinforcement shall be at least 5 inches.

If I am interpreting this correctly, this is essentially saying that you need larger blocks - such as a 8x10x16, perhaps?

I've looked around at drystack, and in every instance I have seen a poured bond beam with horizontal rebar that I believe is tied to the vertical rebar.   Additionally there is horizontal rebar at any openings such as windows. Then there is Surface bond cement applied to the faces of the blocks once the wall is complete for additional strength and waterproofing (although an additional water proofing may need to be applied).

Good example:  http://www.drystacked.com/windows.html

There is a good post from SansPlans a while back on his experience with drystack.  Seems the process is easy save for the bond beam, which is a pain to install because of making the top forms and pouring the concrete.  Here is his post: http://countryplans.com/smf/index.php?topic=5653.0

Personally I'm still up in the air as to what I want to do.  Considering the foundation is so important, I'm leaning towards the extra cost solid poured concrete if I can afford it.  Plus I am looking to do a finished basement and thus it needs to be water tight.

[Squirl]

I interpreted that footnote to mean that the rebar had to be placed 5 inches from the front of the soil side.  So in an 8" thick block wall, it would be placed 3 inches from the back and 5 inches from the front. Which I don't know how that applies if it is dry stack because foot note a. states "Mortar shall be Type M or S and masonry shall be laid in running bond." Which rules out drystack because it is mortar less, unlike footnote a. from table R404.1.1(1) "Mortar shall be Type M or S and masonry shall be laid in running bond. Ungrouted hollow masonry units are permitted except where otherwise indicated. Hence my confusion.

I read that website, but didn't find it very useful, it didn't state it's applicability to the ICC. I am not putting in windows and doors or am going to be in hurricane weather. Most of it was geared towards a complete concrete building in the state of Florida. I have not been able to find any of what was referred to on that site for foundations.  Probably because Florida doesn't have a frost depth. Then again, why would he give the information away about rebar requirements when he can sell it? 

I have read the experiences of others, on the forum, but not many have mentioned code requirements.  Much of the information seems to have come from random internet advisories.  I was looking for something more detailed or authoritative, such as someone who has experience with ICC code requirements.  The codes seem to not differentiate between requirements for drystack and mortared block in table (1).  I would even consider mortared if I could just figure out the rebar requirements.

[Squirl]

Through more research I think I have another idea.  I was wondering why it would have rebar requirements in table(2) for walls under 8ft with less than 4 ft unbalanced backfill if table (1) footnotes said it was unnecessary, and I found this: R404.1.4 Seismic Design Category D0, D1 or D2.
"Foundation walls in buildings assigned to Seismic Design Category D0, D1 or D2, as established in Table R301.2(1), supporting more than 4 feet (1219 mm) of unbalanced backfill or exceeding 8 feet (2438 mm) in height shall be constructed in accordance with Table R404.1.1(2), R404.1.1(3) or R404.1.1(4). Masonry foundation walls shall have two No. 4 (No. 13) horizontal bars located in the upper 12 inches (305 mm) of the wall."
http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec004_par027.htm

Which most of the U.S. and all of NY are category C.


So far it looks like that there is no rebar requirement in short wall crawlspaces with balanced fill.  Any thoughts?

[davidj]

For a 4ft crawl space in CA in a medium seismic area (cannot remember the actual zone), I could do 4ft (I think) unbalanced back fill with 8" blocks, rebar on 32" x 16" centers and full grout fill.  Personally I wouldn't want a house sitting on completely unfilled blocks (but then I've always lived in CA, and things are wobbly here). 

Comparing a poured foundation with a fully-grouted block foundation, my feeling is the fully-grouted block is the easier way to go for an amateur if you don't have a lot of doors and windows.  Laying block is hard work but it's not that difficult, especially if you use speed blocks (with open ends).  Around my way, the cost of blocks vs. poured concrete was about the same per cubic ft, so poured cost more if you can't reuse all of the forms.  And there's less chance of a nasty blowout with block.  However, if you're paying for labor, I'm guessing block costs more.

[h0rizon]

You are correct, it says 8-INCH MASONRY FOUNDATION WALLS WITH REINFORCING WHERE d > 5 INCHES so the rebar needs to be off-center to properly support the backfill loads.  I now recall reading this before.

By definition drystack needs vertical rebar support.  Thus I would say that R404.1.1(2) is applicable even for sub-8 foot walls.

Open to interpretation though.

[Squirl]

By definition drystack needs vertical rebar support.  

 I'm not disagreeing, but by what reference.  I had seen the definitions that all poured foundations needed rebar, but they clearly do not. It all depends on height, weight, fill, size, etc...
http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_4_sec004_par003.htm

Rebar may be good practice, but it isn't a requirement for all situations.  That is one of the reasons I generally have the interpretation it does not require it.  It is not needed in a thinner solid wall of the same height and balance.  Table (1) also has that 6" solid blocks could be used.  There is no ability to even put rebar in these. It has the footnote d. "Solid grouted hollow units or solid masonry units."  I took that to mean 6" units had to grouted vs. the footnote a. for the 8" "Ungrouted hollow masonry units are permitted except where otherwise indicated." Since there was no mention of rebar and one option can't even have rebar and both options are in the same box, I assume it is not a requirement, but I don't have much experience with building code interpretation.  I also have limited experience in masonry nomenclature.

[h0rizon]

Yes, I am probably speaking out of turn here.  I am just start out with everything so I have a lot to learn myself.

1.  I also interpret footnote d to mean that 6" blocks have to be grouted.
2.  I've read through the Surfacing bonding cement (Quikcrete) data sheets and note that they have table 2, "structural properties of unreinforced wall assemblies", so they are advertising their product work with non-rebar applications for the specific wall heights listed.
3.  Table 404.1.1 certainly does indicate the use of ungrouted blocks and the below tables are only referenced with higher walls/higher backfill loads.

That all said;

1.  In my research I have not yet seen any drystack method that didn't call for vertical rebar in their instructions
2.  I've run across multiple engineer reports on drystack (such as http://www.vobb.com/Engineers-reports/Stork-engineer-report.pdf) which are stamped for said product in compliance with the IBC.  These reports utilizes vertical rebar hooks into the foundation with 4' spacing.  Not all of them clear on how high the rebar goes, and as such it may only be used in the first few rows of blocks.
3.  Section 4 of the IBC also references that the foundations must comply with applicable loads, and thus rebar may be required regardless of the minimum specifications in this section.

It is my gut feeling that not using rebar may be OK for certain applications, perhaps even a small cabin where there is little snow load or wind sheers.  And this is probably where the variation in rebar/core filling comes in.

Personally, I would sleep better at night if I spent the extra money for the rebar   

[Squirl]

You have been very helpful.  I didn't even realize about the specs at quickrete.  The SBC wall has almost triple the flexural strength for a vertical span over a mortared wall.  I agree rebar is good by policy.  It is not the just rebar it is the concrete infill.  Concrete is what adds the compressive strength.  It also adds the large amount of extra work, money, and time.  It makes it less appealing over other options.

[nysono]

I just finished my walk out basement.  Inspector did not require verticle rebar in the blocks (I used 8 x 10 x 18's) but I did put it in when core filling.  We core filled every corner (2 blocks) then every 5' with a double fill on centers.  Inspector was very happy...just and fyi block, mortar, redimix for footers and floor and cores (20 yrds total) was about $5500 for 24 x 32.

[Squirl]

Thanks you for the insight. Great job too.  I am not going to have a walk out basement.  I am simply going to have a crawl space with no more than 1' of unbalanced backfill.  I probably be submitting my plans this week for it without rebar.  I will  put rebar in if I make fast progress and the holes line up well, if the inspector rejects the plan without rebar, I will probably go back to the poured foundation idea (cheaper,stronger,faster).  I still have a few days to kick the idea around.  I wish I could do a pwf.

[h0rizon]

Thanks for the insight nysono.  I was excited to see you do block - but i thought you did masonry instead of drystack?   

[nysono]

Thanks for the insight nysono.  I was excited to see you do block - but i thought you did masonry instead of drystack?   

Yes it is masonry.

[Squirl]

I just thought I would leave an update of this.  I called the building inspector.  I figured my interpretation of the code doesn't amount to a hill of beans to compared to his.  He said I had read the code correctly and that I can build a hollow core block foundation with no rebar, mortar, or infill, with surface bonding cement.  He told me he only allows it for foundations with very little unbalanced fill, definitely not more than 2 ft.  I explained my intention was to have 1 foot of unbalanced fill and 2 ft of concrete block wall above grade.  It would give a 3 ft crawl space and only 1 ft of unbalanced fill.  This would also cut down on my excavation and time.  He said that would be fine so I submitted my building application for a drystack block foundation with no rebar or infill.  I have not heard back yet.

I did some further research based on the spec sheets horizon posted from quikrete.  It appears drystack has a compressive strength of 450 psi rather than 600 psi of mortared block.  This is because the mortar better distributes the weight through the block.  It appears from the sheets that the drystack wall would have greater shear strength than the mortared wall.  My building design has the weight fairly evenly distributed through the outside walls with the joists 12" O/C and all the weight clear spanned on the outside walls.  With it being a 20x30 single story I calculated that even if I loaded it to 50 psf roof and floor, that would give me 60,000 lbs.  If that were evenly distributed to the outside walls of 60 ft at 8 inches wide, that would be 10 psi on the concrete block.  That is pretty far below the 450 psi it is rated for.  I also can see why there is no mention of core filling blocks, just rebar requirements.  It is more about shear strength from the unbalanced fill than the compression from the building.