Rebar Placement in Foundation

[Leviticusgauge]

We are going to be building our foundation on our 18x24 foot cabin soon using cinder blocks. The footings are already poured, but I'm wondering if we need to add rebar to the equation. This highest point of the block walls will be 4 foot. Do we even need rebar for something that short? If so, what is the best method for installing rebar with the cinderblocks? Do we need to drill the rebar into the footings or can we cement them into the first course of block? How far apart should the rebar be?

[rick91351]

To be correct your verticals should have been tied to the horizontal rebar in the footing.  Rebar needs to be held up off the soil by either tie wire or chairs.  Verticals should never be 'wet set' or shoved down in to wet cement least they have contact with the soil.  (Though when I started working concrete many years ago it was accepted and everyone did it.)  But then inspectors were non extant now it has to be in and wired at footing inspection.       



Vertical rebar out of a footing



However does not sound as if you installed and poured rebar into the footings.       So what to do     Four foot is hardly considered short.  Two foot - 24 inches is normal.  However my expertise or experience in this matter is concrete not block laying or engineering.  I would be concerned about hammer-drilling in rebar for your verticals.  This would most likely require a Hilty or Bosh hammer drill which are usually available for rent at a good rental store.  Bits for them are sort of expensive and sold there or in the big box stores.  Half inch or five eights of an inch would accommodate #4 rebar but you might have to hammer it in using a half inch bit.  If it were mine I would drill at least 6 inches and take the compressor and blow out the dust in the hole.  I would also caution you to use a dust mask for yourself and others that might be helping drilling and blowing out the holes.  Hope that helps --- hope others might chime in as they might have a better solution.  Especially the block-layers......  As far as how far apart in the house I am building the engineering called for verticals 18 inches on center of #4 if I remember correctly.  He also called for a vast amount of #5........  Rebar is the cheapest investment in adding longevity and strength to buildings.                     

[Don_P]

It is best practice to connect things together so hooking vertical rebar under the horizontal rebar in the footing would be best in an uplift or overturning situation. Drilling into the footing and setting rebar  would certainly help in a lateral shear situation, where for instance unbalanced fill outside is pushing on the wall. The code minimums for height, thickness and reinforcement are in chapter 4 of the IRC, it sounds like you should be looking at table R404.1.1(1), plain masonry is unreinforced, no rebar... but as one fellow I worked for said, if it's worth concrete and mortar, it's worth reinforcing steel.

From Rick's comments, yes we used to "wet stab" reinforcement and anchors into fresh concrete. There are a couple of problems with that. first is that you push the aggregate out of the way and the steel is then covered by just the cream, often with a void over it, with few or no rocks to really hook it down, it's easier to pull an anchor back out. Second is it is about impossible to maintain the min 3" of cover from the outside of the concrete so the steel is much more subject to corrosion from moisture and oxygen. Rust is much bigger than the original steel. Steel rusting and swelling is every bit as powerful as wood swelling, it'll bust concrete as it rusts.

[Patrick]

I dont think it was mentioned but there is a specific epxoy that is used if you are anchoring rebar to concrete,And anywhere you place the verticals you will want to fill with concrete or grout and In my opinion if you are doing verticals they should then be tied to rebar  in your final course using a bond beam which is also filled with concrete and it is best to have your anchor bolts in where the verticals have rebar and everything gets tied together basically tying the anchor bolts to the footing itself.

[MountainDon]

Rust is much bigger than the original steel. Steel rusting and swelling is every bit as powerful as wood swelling, it'll bust concrete as it rusts.

This is the main cause of failures in concrete structures. Many bridge and building failures can be traced to the rusting of reinforcing steel. There have been several cases recorded where this has been noticable in as few as four years.   It may be of interest to note that "oxide jacking" is a big problem we face all round the world with historical structures. 

[Squirl]

It depends.

How much soil will you have unbalanced?

What is your frost line?

What kind of soil do you have?

How thick will the blocks be?

Are you in a severe weather zone? (earthquakes, tornadoes, hurricanes)

[rick91351]

This is the main cause of failures in concrete structures. Many bridge and building failures can be traced to the rusting of reinforcing steel. There have been several cases recorded where this has been noticable in as few as four years.   It may be of interest to note that "oxide jacking" is a big problem we face all round the world with historical structures. 

100% right on..... Many structures now well aged are failing yet were built to the exacting codes and building regulations of their day.  Many of those codes are now rust covered in the vaults that hold them.

Another reason I am some what suspect on the four foot wall really being proper without reinforcing yet the IRC allowable    Might be right by code but is it?  Same with long spans with todays lumber and more efficient modern day grading..........