Rebar placement

[KHyam]

I recently had a contractor ask to use patio blocks to hold the rebar up (3") above the bearing soil, in this circumstance I said no to it and he got pretty irritated with me. Wondering if anyone else has used patio blocks as chairs? In further thinking about it I do not see it being a big issue, but I do not like the idea of a large discontinuity in the base of a strip footing....thoughts?

 

[JAE]

We allow concrete based blocks to be used as chairs in on-grade situations like that....not clay based products.

We don't allow blocks on suspended slabs but require chairs.

 

[AELLC]

Agree w/ JAE -

Did he provide #3 or #4 short @48" to help support the cont. bars?

 

[KHyam]

Ok that is the conclusion that I came to as well.

AELLC can you clarify what you mean by short? Also I work in Canada so use the 10M rebar for continuous small loading foundations.

the rebar was 3 rows continuous with horizontals every 24". We compromised on a modified chair system (plastic) to provide the support.

 

[AELLC]

I meant the transverse bars. 20" long for a 24" wide wall or strip ftg.

 

[Ron]

Concrete bricks are fine, since their density approaches that of cast concrete. Patio blocks are very porous, so if there is a potential for the footing to be at or near the water table, I would not allow their use.

As JAE noted, "no" to clay brick.

 

[AELLC]

ps

We used 48" oc, which is far part relatively, but we justified that by reasoning that pour doesn't get stepped into and the cont. bars don't get pushed down.

 

[dik]

I've used parts of patio blocks and concrete bricks with no adverse effects... sitting on 6 mil PEVB. They also make concrete chairs with tie wires in this locale.

Clay bricks, because they swell are not used as noted by Ron and JAE.

Dik

 

[TXStructural]

CRSI is about to publish a standard on bar supports (within a week or two) which addresses this. The short answer is that cinder block is more porous than proper precast bar supports (aka, dobies), and using them is unacceptable. Where concrete is cast against soil or in contact with soil during use, ACI requires thicker cover or other corrosion prevention measures because almost all soil supplies moisture. The proper use of bar supports is important to achieving correct construction of your design.

 

[JAE]

So I guess that rules out empty beer cans too, right?

 

[Ron]

JAE....depends on who emptied them! I'll take another lager please.

 

[TXStructural]

Steel cans only, unless you account for dissimilar metals and corrosion. Most alloys of aluminum are sacrificial for steel, so there shouldn't be local corrosion.

But I question the wisdom of this unless loads are quite low. Also, consolidation around and inside the can will prove problematic.

I'll take a hoppy ale, please. Anything to help with the construction process.