Concrete additives

[USAFpj]

When I was initially discussing the addition of rebar into a pole barn slab, he said that an ingredient that is in the crete mix would have an adverse effect on the rebar/ perhaps rusting?

Is this some type of sodium used to speed curing? Perhaps a densifier?

The contractor isn't nearby or I would just ask him.

 

[Streetbu]

Uh, never heard that one before! Otherwise no one would use rebar in concrete. You'll be fine. I might look into another contractor though.

 

[USAFpj]

Yeah, this was more for my knowledge. He's actually the 'project manager' for the pole barn company who I went through on my building. He's a good ol' boy, but he throws out a lot of terms I'm still learning about.

The initial slab calls for 3K psi, 4in with fiber. When I had first talked about rebar, he said it wasn't needed, and that if I went that way, his concrete supplier would then have to change the mix to accommodate the steel rebar... something about the compatibility.

 

[DougWil]

Yeah, this was more for my knowledge. He's actually the 'project manager' for the pole barn company who I went through on my building. He's a good ol' boy, but he throws out a lot of terms I'm still learning about.

The initial slab calls for 3K psi, 4in with fiber. When I had first talked about rebar, he said it wasn't needed, and that if I went that way, his concrete supplier would then have to change the mix to accommodate the steel rebar... something about the compatibility.

Your good ol' boy is either ignorant or BSing you.

Fiber is not a substitute for primary reinforcement, rebar or mesh.
It is OK as a secondary reinforcement to lessen shrinkage cracking.
Fibermesh® 650 is a specially engineered alloy polymer macro-synthetic fiber designed to provide high-performance secondary concrete reinforcement.
http://www.fibermesh.com/page.aspx?name=Fibermesh+650+Macro+Fibers

Adding too much accelerator like calcium chloride can have detrimental effects on steel reinforcement. But since you probably aren't pouring in winter it isn't a concern.
http://www.concreteconstruction.net/concrete-construction/chlorides-and-concrete.aspx

 

[USAFpj]

Doug, that's it- calcium chloride.

I was originally thinking calcium, but knew that wasn't an issue with steel, so I said sodium. Calcium chloride is indeed what he said.

At the time we discussed this, it was February, so perhaps that's why it was on his mind.

Either way, I've been educated over and over by you guys, so when I return to the States, we'll get it done right.

 

[DougWil]

Doug, that's it- calcium chloride.

I was originally thinking calcium, but knew that wasn't an issue with steel, so I said sodium. Calcium chloride is indeed what he said.

At the time we discussed this, it was February, so perhaps that's why it was on his mind.

Either way, I've been educated over and over by you guys, so when I return to the States, we'll get it done right.

Good.

With rebar, I would recommend 5" slab with #4 bars at 16" or 18" each way.
16" is for guys that can't add and 16" marks are already on the tape measure.

 

[joes169]

Doug is correct, calcium chloride has been known to aid in corrosion of steel re-enforcment for decades now. Non-chloride acccelerators have been readyinly available for decades because of this. If your contractor makes a big deal out of the small increase to switch between the two, I'd be a little leary of that as well as the rest of it........

 

[Jlbc212]

Re: Concrete additive

Proper compaction of the material under the concrete as well as getting a good pour is probably even more important than rebar or mesh. Not all concrete suppliers or contractors are reliable. Not enough cement in the mix or too much water added to the mix is a recipe for future failure. It may look good for awhile, but these shortcomings will reveal themselves over time.

 

[ms fowler]

Re: Concrete additive

Proper compaction of the material under the concrete as well as getting a good pour is probably even more important than rebar or mesh. Not all concrete suppliers or contractors are reliable. Not enough cement in the mix or too much water added to the mix is a recipe for future failure. It may look good for awhile, but these shortcomings will reveal themselves over time.

Over my 40+ year career in material inspection and testing, I have seen a lot of concrete. At one time I noticed that concrete from a certain supplier was always much stronger than specified. Sometime later, I was on a job, and the owner of that concrete company was there-actually working with his crews (at 3 am!). So I asked him about it, and he replied that the cost of a half a bag of cement, at his rate, was very cheap insurance against ever having to pay to remove a slab that was below strength. Wise man.

 

[Cyberbear]

Yup, quality concrete and plenty of steel has always been my requirement when pouring my shop monolithic slabs. My latest was a 36 yard pour in 1995 and still no cracks. I also believe in a surface sealer that retards the drying process while the cure is working, especially in hot or windy weather during that time. Extra details such as tying the slab steel into the footings, and extra reinforcement at large door openings are good practices as well.

 

[DCarr2]

we use calcium chloride to to speed the set times of plaster base coat sometimes, though not much need for it any more as the new basecoats now have accelerators added.

However It can be useful to make durabond 90 set in 10 minutes

oh, rust, ya... the little plastic spoon thats been in the can for the last 40 years broke.

I supplemented it with a spoon, in over 15 years that chinese spoon is still shiny in spots.

 

[DekeT]

3K psi concrete mix might be ok for a birdbath but I would never use that for a floor. Pay close attention to what Jlbc and DougWil are telling you. I personally would hire a new contractor that would insist on a better mix or pass on your job.

 

[DougWil]

Re: Concrete additive

Over my 40+ year career in material inspection and testing, I have seen a lot of concrete. At one time I noticed that concrete from a certain supplier was always much stronger than specified. Sometime later, I was on a job, and the owner of that concrete company was there-actually working with his crews (at 3 am!). So I asked him about it, and he replied that the cost of a half a bag of cement, at his rate, was very cheap insurance against ever having to pay to remove a slab that was below strength. Wise man.

Yep.
Most ready mix plants have their mixes down to a science and buy their portland cement by the tanker truck at probably the equivalent of $4-5 a sack.

They aren't going to risk intentionally shorting you a couple dollars of cement and having to pay for slab removal and replacement.

The problem is usually the contractor adding water at the site to make placement easier and lowering the strength of the mix.

 

[wssix99]

Your good ol' boy is either ignorant or BSing you.

Fiber is not a substitute for primary reinforcement, rebar or mesh.
It is OK as a secondary reinforcement to lessen shrinkage cracking.
Fibermesh® 650 is a specially engineered alloy polymer macro-synthetic fiber designed to provide high-performance secondary concrete reinforcement.
http://www.fibermesh.com/page.aspx?name=Fibermesh+650+Macro+Fibers

Adding too much accelerator like calcium chloride can have detrimental effects on steel reinforcement. But since you probably aren't pouring in winter it isn't a concern.
http://www.concreteconstruction.net/concrete-construction/chlorides-and-concrete.aspx

^ I vote for ignorant. The guys around here use salts to accelerate the cure, even in warmer weather. (It allows them to saw cut earlier, get their sealers on faster, etc.)

This is not something to get worked up over - the damn stuff comes to us rusted in the first place.

Road salt creates problems for rebar in roads and bridges where there is lots of salt and lots of water. A typical floor inside a garage or a house isn't going to see repeated drenchings of water AND repeated applications of salt.

Even if (a little bit) of salt added to the concrete does rust the rebar, it's going to do its thing, have its chemical reaction, create a little bit of superficial rust, and then it will be over. In order to get significant deterioration, you'd need to keep adding the things that cause more rust. (more salt and more water)

 

[pcmeiners]

"The problem is usually the contractor adding water at the site to make placement easier."
Here in NYC you can leave out "usually", contractor here always add water. Agree, about the worst things you can do to a concrete mix.

For the short term calcium chloride does not aid in rebar producing rust, as the concrete ages with small cracks, it does cause rust, as long as water seeps in the crack or moisture can migrate through it, it is a salt just like sodium chloride. On the other hand, if you use a vapor barrier under your slab, possibly a few generation will pass before effects from any salt would show. As there are alternatives why use it.

 

[wssix99]

More food for thought:

Over the years, (depending on where one lives) much more salt will drip off of cars on to the floor than will be in it from the original added salt to the concrete mix.

 

[DougWil]

As there are alternatives why use it.

Exactly.

More food for thought:

Over the years, (depending on where one lives) much more salt will drip off of cars on to the floor than will be in it from the original added salt to the concrete mix.

Yep and a good sealer, reapplied as needed can control most of that.

 

[brownbagg]

they call it calcium chloride but that chemical is not used anymore due to it will eat steel, but if you ask for it. the concrete company will smile and give you the correct chemical. They know what you talking about