Concrete Chairs

[jmc]

Having rebar 1/3 up in the slab maximizes its ability to provide tension strength (bending strength) to the concrete to keep it from fracturing. Since it is cast into the concrete, stress riser effects are minimal, if and only if, there is enough concrete below it. That's why "lifting rebar" is so detrimental to the strength of the slab. From a practical perspective, nobody is going to get the rebar at the right height consistently by lifting it, resulting in a weaker slab. My motto is use enough chairs that walking around on the rebar doesn't bend the rebar, or shift it.

I get it that rebar, below the neutral plane, adds more tensile strength- my point was that the chairs themselves leave defects (stress risers) in the tension side of the neutral plane.

 

[paulsharvey]

Just saw this thread, and didn't read everything yet; but I will comment; per FDOT standard plans; sidewalks, curbs, and driveways get no rebar or wire. Now, of coarse inlet tops, structures, gravity walls, get an incredible amount of rebar. Try smashing up RCP pipe, and the amount of heavy gauge wire (#2) is incredible.

For residential, and some commercial, I've seen Adobe (I think it's called), or a solid concrete brick, with tie wire embedded in it, and tie the rebar to that to keep it spaced off the ground.

IMO, property installed wire beats fiber hands down; BUT fiber beats wire in the dirt...

 

[paulsharvey]

With concrete, as all things, Subgrade is where it starts. I see people pour driveways on sloppy mud, no compaction over pipes, and then adding 30 gals of water; and then wondering why it cracks.

 

[paulsharvey]

What's your opinion on post tension slabs?
I did not even know it was a thing until a few years ago.
My initial thought was it seems like a good technique with the only downside being the equipment required for the tensioning.

I've been on many large post tension slabs when doing apartments. Very little rebar needed, other than for the thickened edge; no wire. It works great, but requires engineering.

When I was maybe 20, I was an assistant superintendent (think salary laborer, and doing the crap the superintendent didn't want to); and standing on the slab, watching them tension. Super came up, and said, "do you like your nuts; if so, don't stand over the cable when the tension it". I never had one break, but its bad when they do...

 

[Streetcar]

I’ve watched a lot of concrete poured and tested quite a bit of it. In the engineering world there is often a trend of not using rebar for flat work. Parking lots and highways are often that way and if done properly will last. Often on highways the pavement will not have continuous reinforcement but have joints that are reinforced. The concrete is also very thick.

For residential I agree, mesh almost always ends up at the bottom and rebar needs to be on chairs.

Rebar cost money. unless slab is heavily loaded, it is cheaper to gain strength with increase thickness of concrete.

fyi concrete will slowly gain strength for years if kept moist

 

[ponytug]

I get it that rebar, below the neutral plane, adds more tensile strength- my point was that the chairs themselves leave defects (stress risers) in the tension side of the neutral plane.

I see your point; do you have any data/personal experience for chairs causing a stress riser issue? I don't do lots of concrete work, so I'm no expert, but I haven't happen to me or on a job that I was working on. (So, that doesn't mean much, but I haven't ever read that it is an issue, either.) If it is an issue, I wonder whether concrete chairs or plastic ones are better? IIRC: the concrete institute recommends both, but doesn't call out a stress riser issue, but my memory isn't that great...

All the best,

Peter

 

[paulsharvey]

One thing, and don't take this as disrespect, but Many concrete guys either don't understand or don't care how concrete actually works; beyond "turns gray, gets hard, and cracks". Water cement ratio is a real thing. I'm not a concrete guy, and have Personally only poured and finished a 120 yards or so; a lot of guys seem to think you can't work with a 5" slump; that's BS, you don't need to pour an 8" slump on flat work, and dumping the water to it is bad. Now drilled shafts, ect, yes, you need a 8-9" slump, but that mud is designed to reach strength at those slumps.

As an interesting note; we had a mast arm drilled shaft get hit in a wreck, and the 7/8" bolts got damaged. Anyways, the engineered repair design involved chipping it down 60", adding a 9000 psi epox, and recasting the top 60" with new bars and bolts. Got some of that epoxy on a boot, and it pulled the boot apart trying to pick it off...

 

[paulsharvey]

So, on chairs or rebar tied to Solid brick, 100% should be used. On WWF on chairs; my only concern has always been how much of a void is left in a 4" house slab at that chair? Never seen any evidence of it; But maybe a nagging doubt.

 

[Streetcar]

Ultra runner fiberglass is significantly more expensive than steel. Only used in highly corrosive environments or where magnetic properties are issue

 

[Streetcar]

One thing, and don't take this as disrespect, but Many concrete guys either don't understand or don't care how concrete actually works; beyond "turns gray, gets hard, and cracks". Water cement ratio is a real thing. I'm not a concrete guy, and have Personally only poured and finished a 120 yards or so; a lot of guys seem to think you can't work with a 5" slump; that's BS, you don't need to pour an 8" slump on flat work, and dumping the water to it is bad. Now drilled shafts, ect, yes, you need a 8-9" slump, but that mud is designed to reach strength at those slumps.

As an interesting note; we had a mast arm drilled shaft get hit in a wreck, and the 7/8" bolts got damaged. Anyways, the engineered repair design involved chipping it down 60", adding a 9000 psi epox, and recasting the top 60" with new bars and bolts. Got some of that epoxy on a boot, and it pulled the boot apart trying to pick it off...

For structural concrete high slump is obtained by use of plasticizers.water cement ratio is kept low. Structurally all concrete below pour of rebar is ignored except to provide protection to rebar.