Need advice on pouring concrete--should I rebar?

[StageLeft]

I have excavated part of my basement for bathroom plumbing and now, in one corner, have a 6X6' square that I'm going to pour tomorrow. The existing basement slab is only about 3". This is going on top of vapor barrier with a few inches of gravel beneath it. I'm using quikrete's crack-resistant fiber-enforced concrete.

I have a hammer drill and 3/8 rebar. I was going to, every 18" or so, connect 1' long pieces of rebar to the existing slab by drilling a 1/2" hole and using a concrete anchoring compound specifically for this purpose. I had thought about going around the entire edge with a long piece of rebar, too.

Finally, I also have some concrete bonder. I am hoping the old and new slab lines will not crack but my understanding is that's going to be hard to avoid.

1) Should I bother with rebar? If I'm going to have a crack perhaps it would be better to allow it between the old and new slab instead of rebar preventing that and then a crack occuring somewhere else (remember, rebar only at edges)?

2) Is the bonder a waste of time? I think it's mainly for use before pouring new concrete onto old as opposed to what I'm trying to use it for.

Here is the hole prior to plumbing, except that I have removed that chunk at the top of the picture with the huge crack between it and the old slab.

http://i897.photobucket.com/albums/ac177/Skoorb100/patch1.jpg

 

[Freejack2]

Can't help you with that, but don't forget your 3/4 HP Heavy Duty Concrete Vibrator. Have to be able to level out your concrete and remove all those air bubbles...

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41053

 

[StageLeft]

I had planed on just screeding and floating and all that stuff, I think a vibrator has to be overkill for something like this (?). I've not seen many talk about it even for larger slabs than this.

I have finally found some threads on google and I am leaning away from rebar now. With a slab this thin I could in fact cause problems. Wire mesh is a decent idea but I doubt I will bother with that since the concrete itself is a little strong and honestly my existing slab is completely unreinforced anyway. This 6X6 is a fairly decent square and if it cracks it should stay together, so it would just crack along the new/old line.

 

[Atheus]

We used to put steel cages in the deeper foundations only - made of d12 reinforcing steel - what you call rebar - and a thick mesh in the slab. Probably about 5mm steel wire in 4" squares, tied to the cages with thinner wire. I reckon if the existing slab has no mesh then you don't need it either. You certainly don't need any big arse steel bars. What you're doing will only be plugging a hole anyway it won't be structurally strong.

 

[911paramedic]

We used to put steel cages in the deeper foundations only - made of d12 reinforcing steel - what you call rebar - and a thick mesh in the slab. Probably about 5mm steel wire in 4" squares, tied to the cages with thinner wire. I reckon if the existing slab has no mesh then you don't need it either. You certainly don't need any big arse steel bars. What you're doing will only be plugging a hole anyway it won't be structurally strong.

This.

 

[StageLeft]

Yes this will hold only a bathroom. The existing slab is down as little as 2.75 in spots and up to 4" in others, a truly quality job done on the sub-grade

I am concerned with the rebar actually even around the edges that the thin concrete could cause an issue with the rebar, from either cracking around it or the thin concrete letting in too much moisture and the rebar deteriorating inside (it's not painted or in any other way protected).

I have to say I cannot think of a negative of fattening up the edges where they abut the main wall. These have to hold the new walls I'll frame in, should I make them an extra inch or so deep?

http://www.askmehelpdesk.com/construction/connecting-new-concrete-slab-old-slab-addition-220091.html

Damn, finally a hit with one of my queries

BTW I HAVE asked this in other concrete forums but ATOT has its certain own level of expertise and pimpacity, so questions even as strange as this can reward one with information.

 

[bctbct]

Dowel every 24", everything else you list is overkill, including the fiber mesh.

 

[drnickriviera]

It's not load bearing, so I wouldn't worry with it. The walls you are putting up are just for drywall right? I'd just make sure the base is compacted good and drop it in. I'd use some 6x6 steel mesh, but I don't think it's necessary for that application.

What are you going to put on top of the concrete? tile? I'd consider using a decoupling membrane like ditra

 

[StageLeft]

It's not load bearing, so I wouldn't worry with it. The walls you are putting up are just for drywall right? I'd just make sure the base is compacted good and drop it in. I'd use some 6x6 steel mesh, but I don't think it's necessary for that application.

What are you going to put on top of the concrete? tile? I'd consider using a decoupling membrane like ditra

Tile but I am not sure about subfloor situation, still trying to decide between dricore or something else. In any case, it will be at least a few months before I tile the floor and hopefully any cracks will have come through by then.

Dowel every 24", everything else you list is overkill, including the fiber mesh.

At this point I have gone to Lowes/Home Depot far too many times, I cannot spend anymore money on this! I have it all filled in right now save another few spade fuls. I can still dowel it. I believe it's actually going to crack between new/old regardless of rebar dowels or not, so at the least they would help any possible vertical separation, although that is unlikely (but they are fairly cheap). The basement is dry and I don't expect much movement of the concrete. But then I am an amateur The existing thin slab has no cracks in it, though!

 

[daw123]

I wouldn't bother with the mesh since the concrete is fibre reinforced.

I wouldn't use the re-bar. You are just introducing more problems than its worth (most defects in concrete are due to the steel reinforcement corroding). Plus you wouldn't be getting the correct depth of cover of 50mm on all sides since your slab is only 75mm thick.

If its going to crack, it will likely crack at the juncture between old and new. Does it really matter if it cracks a little if you are going to be covering it anyway with a sub floor?

 

[Greenman]

Rebar won't prevent cracking in any slab, but it will prevent differential movement at the crack.

 

[DrPizza]

Just looked at your picture. No need for rebar - you'd just be wasting money. Since you didn't cut the concrete & it doesn't have a smooth edge, when you pour the new concrete, it should lock against the other concrete, preventing lateral or vertical slipping between the new and old.

You may end up with a crack at the seam, but rebar isn't going to prevent that either.

 

[BoomerD]

Gotta agree with Doc Pizza. I LIKE fiber mesh concrete. It's what MOST of the residential concrete guys here use for sidewalks and driveways. It holds together better than rebar or mesh, doesn't cause the concrete to crack like rebar might in a slab that thin, and for 3 inches thick...rebar would make a weaker patch rather than stronger. If you absolutely HAD to put reinforcement in other than the fiber mesh, then some thin wire mesh...but IMO, you don't need it for that little pour.

 

[StageLeft]

Thanks, guys.

BTW I read the heck out of everything I do that I don't understand like this so that I can do it right. The reason this hole is such as mess is because I read several times to use a circular saw with a masonry blade to pre-score the lines. Clearly nobody has ever actually done that. The masonry blades wear out very quickly and are useless (1/4" deep about 4' long out of each blade), so I found them quite unnecessary (score lines) anyway once I got a 12 lb sledge going. Until I came within 2 feet of a wall and then on what is that it's a harline crack. I ended up busting all this concrete to get back to clean slab. Vast amount of wasted time because I had never read of people cracking the foundation like this. Obviously most people are renting the jackhammers

Now, I also read before this I could use a jackhammer or a proper wet saw or even a diamond blade in my circular, but I'm normally more interested in saving a few bucks even if it takes longer. If I ever have to do this again I will score the hell out of those lines first Obviously no money saved here, I have $100 in upgraded concrete ready to go among other things.

Shall I skip the bonding agent? I doubt it will hold this slab in any meaningful way and I could return it. The sledge did make ugly (good) lines on the old slab and in many parts I went in after and undercut them a bit further with the chisel and small sledge.

 

[iGas]

IMO, no need for rebar in a small repair job, and it it not going to take so much weight to worry.

If you must, then throw a heavy gauge mesh (4"x4" or 6"x6") and lift it off the gravel/sand about 1" with blocks before the pour.

<--- plumber

[add]
What pumbing did you move to have such a large hole?

 

[bctbct]

The only thing you should be concerned with is the new concrete offseting from the old sometime in the future. The only thing a concrete company would do is install the dowels.

Bonding agents typically are if you are patching concrete i.e. applying 1" of concrete onto 3" of concrete.

Mesh is to prevent concrete from separating if it cracks through....typically this is an exterior problem

Fibermesh was invented to replace mesh...less cost and $0 labor

Like I said before, with my 20 years of experience I would use a concrete quicky saw (you can do that next time ) to remove the concrete, dowel, and pour new...nothing else.

 

[Iron Woode]

Since you are using fiber concrete you don't need the rebar. That's why they offer that type of concrete. Mike Holmes recommends it so it must be good.

rebar is used for serious structural integrity and a basement floor doesn't really qualify.

 

[drnickriviera]

AFAIK Fiber reinforced concrete is not a replacement for steel reinforcment. I think these guys are doing it to avoid the extra labor.

My brother worked in a concrete testing lab when he was in Ga Tech. They did testing for the Ga DOT. He said the fibers were for impact resistance and hairline cracks.

Heck even the wiki article says it's not a replacement for steel

 

[bctbct]

AFAIK Fiber reinforced concrete is not a replacement for steel reinforcment. I think these guys are doing it to avoid the extra labor.

My brother worked in a concrete testing lab when he was in Ga Tech. They did testing for the Ga DOT. He said the fibers were for impact resistance and hairline cracks.

Heck even the wiki article says it's not a replacement for steel

An Easier, Safer Way to Reinforce Slabs.


No installation required; simply toss the water-soluble Fibermesh bags directly into the truck.


Compared to welded wire fabric, no fabricating or forming required; no cuts or scrapes.


Lower labor and in-place costs and reduced liability.


Saves construction time-quicker pouring schedule.


Finishes easily and smoothly with common tools; any fibers which might appear on the surface are quickly worn away by foot or vehicle traffic.


Virtually invisible Fibermesh multifilament fibers are also available for aesthetically demanding applications.


IDEAL FOR THESE APPLICATIONS AND OTHERS:


Slab-on-ground


Garage floors


Patios


Driveways


Pool decks


Stamped, patterned and colored concrete


Sidewalks


Curbs


Parking areas


Overlays & toppings

http://www.fibermesh.com/application.aspx?id=2173

 

[StageLeft]

[add]
What pumbing did you move to have such a large hole?

Basement bathroom improperly scored. Also due to unfortunate layout of the basement the drains are not all in one side but rather it's squared off, so this represents the square layout of the bathroom.

I absolutely will not rebar anything now except for the dowel situation. Is it possible that they could rust internally an aggrivate a few years down the line? As drpizza said, the edges are pretty darn roughed up, so perhaps they would assist in this vertical movement sufficiently?

BTW I was watching Holmes on Homes last night and they were using fiber concrete but the stuff they are using has stiff inch long pieces of plastic. The guy who brough the concrete said they used to use the fiberglass strands and still do a little but mainly now go with the plastic so it may be that the plastic-infused is more of a replacement for steel reinforced. I assume that on a truly large job like a skyscraper they'd still be using rebar.

 

[drnickriviera]

http://tinyurl.com/ya4h9zp
If anyone has an impartial source saying otherwise i'd like to read it.

I don't think your concrete is going anywhere skoorb, as long as the base was compacted.
You may get a small crack at the edges, which is why I suggested the decoupling membrane to keep the crack from telegraphing up to your grout. I think even lowes or depot sells it now.

 

[BoomerD]

AFAIK Fiber reinforced concrete is not a replacement for steel reinforcment. I think these guys are doing it to avoid the extra labor.

My brother worked in a concrete testing lab when he was in Ga Tech. They did testing for the Ga DOT. He said the fibers were for impact resistance and hairline cracks.

Heck even the wiki article says it's not a replacement for steel

Fiber mesh is NOT a replacement for rebar...that's for structural strength...and to prevent it from falling apart when it does crack. The fiber is to help prevent minor cracking Skoob's little pour doesn't need the structural strength of concrete and is too thin to use rebar properly. Even #3 bar is too thick for a 3 inch slab. (not enough coverage top/bottom) Odds are, the concrete will crack where the rebar is.

There's no way of totally avoiding cracks in concrete. Concrete cracks whether you want it to or not. It's the nature of the beast. In a small pour like this, it shouldn't crack, although it MAY not bond well to the existing concrete. A bonding agent MIGHT help...maybe...if the planets are aligned with Jupiter and Mars, and all the various moons are in a proper position with each other, creating a special gravitational field that...........well, the bonding agent MIGHT help...but it might not.

 

[MikeyIs4Dcats]

you could put rebar in a 3" slab if you wanted, but here it is unnecessary. The coverages everyone is talking about are the recommended coverages for the reinforcing steel. Typically one would want a minimum 1.5" coverage on formed concrete faces, and 3" on unformed. This is for corrosion protection mostly as steel placed shallower has a higher tendency to rust and that can spall the concrete as well as cause structural degradation.

 

[iGas]

Basement bathroom improperly scored. Also due to unfortunate layout of the basement the drains are not all in one side but rather it's squared off, so this represents the square layout of the bathroom.

I absolutely will not rebar anything now except for the dowel situation. Is it possible that they could rust internally an aggrivate a few years down the line? As drpizza said, the edges are pretty darn roughed up, so perhaps they would assist in this vertical movement sufficiently?

BTW I was watching Holmes on Homes last night and they were using fiber concrete but the stuff they are using has stiff inch long pieces of plastic. The guy who brough the concrete said they used to use the fiberglass strands and still do a little but mainly now go with the plastic so it may be that the plastic-infused is more of a replacement for steel reinforced. I assume that on a truly large job like a skyscraper they'd still be using rebar.

I'm not sure what it is like in the US but here in Canada code still call for rebar in commercial/industrial construction. Residential construction can use fiber concrete but it seems as if it is not nearly as popular as mesh.

Recently a house I help built use fiber concrete in their double car garage and it had 2 hair line cracks right across the slab. The builder said that the next time he would add the proven steel mesh or rebar to minimize cracking if it does exist.

If my understanding is correct, you are adding a bathroom to the basement.

If so, have you got a plumber to calculate the drainage load on the existing drainage that you are connecting into?

If the load isn't calculated properly and pipe size isn't increase or venting is improper install could give you grieves when the bathroom drainage backup.

 

[dartworth]

christ on a cracker...get a bag or two of quikete 5000 fill it up and be done with it...