Repouring basement bathroom. Can I reuse old concrete?

First, never ever use a masonry blade in a circular saw. They are nearly useless. I knew I should have gotten that diamond one and then I could have scored properly, which would have meant this situation wouldn't have happened (a huge amount of concrete came out because it started cracking up to the walls).

Anyway, the main area here including the big chunk with the huge crack is about 5X5. I need to pour 3" thick to get back to how it was originally but I may go thicker if I can.



All of the existing and the new concrete will sit on a vapor barrier. Thus this huge chunk is "floating" on gravel. I deliberately moved it to the left expanding this crack to a thickness of one inch. Can I reuse this chunk? If so, I have another couple I'd like to reuse:



I want to reuse them because then I don't have to dump them but more because I feel it could be a lot easier dealing with fewer bags of concrete and troweling/floating it would be a great deal easier. I'm more interested in doing this right than easy, but keep in mind I've never done it before and it's a one-shot deal, plus I really think of why I shouldn't re-use these (they are not cracked underneath, visibly)

This is a "floating slab" with the 1/2" gap between the wall, which I'll maintain with new forms in the corner.

So can I fill that one inch crack with quikrete or sakrete? Should I instead make it a little wider? I know I can force all the concrete down in that gap.

I was not going to bother putting in mesh to toughen up this concrete because it just may not be the right thing to do.

For curing a guy at Lowes who appeared very knowledgeable said I should mist it every hour for the first several hours, then go down to twice/day. Since I have a ton of left over 6 mil poly would it be better to cover it with that and just leave it for a week? This guy also said use a paint brush to help rub up/paint the new concrete up against the existing edges to help the bond (plus spraying it with water). Seemed like a good idea.

Is the cheap sankrete or quikrete ok or should I splurge on the 5000 psi? I don't need the strength obviously; I just want to avoid cracking. I'm using so few bags the money isn't a huge concern.

 

I'll put my vote in for removing the old pieces, installing some wire, and floating a new uniform slab. The savings for floating over the old pieces is minimal compared to the aggravation of problems later. I will admit to burying some stones and odd pieces of concrete when the pour was running a few cubic feet short to complete the job, but they were where it would never matter and I don't live there to know they are down there.

You have a lot of effort invested, don't stop short now. There is always pride in knowing a job was done right.

Now we will see if one of the pros will comment, please.

Bud

 

Not a concrete "pro", but I've done my share, and concur with Bud on not using the old concrete slabs back. Small pieces here and there in footers is ok, but those you have won't ever work. the floor will crack for sure. Remove them, install re-fence and make a solid pour. You will be glad you did.

 

This chunk is pretty large, heavy, and secure, though. It only budges when I really put my weight behind it. If the new concrete is expected to "mate" to the rest of the basement slab anyway, is there any reason to think it couldn't mate to this and then the two new married would be fine together?

If I completely remove it I need to figure out an approach to pour about 20 bags worth of quikrete in a manner that won't have it start to set before I can smooth it out.

The guy at lowes said if I use that approach I could cordon the area off and fill it in sections, then after I've filled one section and finished it off at the top with a float I could remove the fence/form and then pour the next section. This way I am smoothing out the first quarter worth of cement before it sets too much and the new section is filled before the first dries (so that it's all in one pouring session). I presume once the first section's forms are removed it would be firm enough that it wouldn't sag to the side as its wall is taken away. Is this a good approach? It would save me the hassle of screwing elevated boards on the foundation walls so that I can have a scaffolding to crawl over.

Of course this is all taking place directly on the vapor barrier, so if I'm staking these sections of board into the ground I'd probably tape up the holes after removing each section.

 

Rent a concrete mixer, and do many bags at a time and be done with it. We poured a few footings on Friday and it called for 12 bags of mix. Borderline on renting a mixer. I didn't, but my helpers mixed it all by hand, so it worked for me. Any large amount, I would rent a mixer.

 

I'm not a concrete pro but I've done the job that you're looking at. I rented a mixer, built a chute (couldn't get the mixer in the basement) to pour the mix and used a wheelbarrow in the basement. I did about 30 s/f in a couple of hours by myself.

I didn't use a vapor barrier because none of the rest of the slab had it. I used remesh for strength. I busted up the chunks of the old slab. I wouldn't worry much about misting it since it's inside. After it's floated just cover it with some plastic and check it for a couple of days.

 

I had a thought about the plastic, it seems that since it's sitting on plastic already, if I have plastic on top it would have a really hard time drying out, mightn't it?

If I can't keep that existing block I will end up renting a mixer.

 

I'm going to have plywood up against the wall to create my 1/2" gap into the drainage gravel below. Should I paint it first with motor oil? I'll remove it 24 hours later.

I ran the numbers again and in fact would need about 13 bags if I went with 80 lbers and did the entire area (30 square feet).

What if I put a divider in place and did half of this at a time; is it going to cause an issue if I repour a few days or a week later after the first has dried?

 

There are a ton of reasons to take out all the old concrete and pour new. The main one being that the new concrete will shrink a bit during the cure. Since it can't shrink around a solid object such as an old chunk of concrete, the new concrete will crack around it. If you don't mind a lot of cracks, go ahead and do it, but it's not an accepted way of doing things and it will actually be more difficult to get a flat slab.
In addition, why do you want to leave a 1/2 inch gap between the slab and wall? If you want them to be separated, place expansion joint material between the wall and the slab. This is compressible material that stays in place after pouring, you don't remove it. They sell it at Lowe's, etc. by the concrete.
As to the misting....do it or don't. For a tiny interior slab it's not going to make a difference. If you're planning on leaving the big chunk of concrete (which is wrong), then why worry about something like curing?
Finally, when you mix the concrete, keep in mind that the wetter it is, the more it will ultimately shrink when it's dry. Mix it in strict accordance with the directions on the bag. If you mix it wet enough to actually "pour", then it is too wet. If you pour really wet concrete on plastic sheeting, it compounds the problem because the excess mix water has nowhere to go but up. This will cause excessive bleedwater to accumulate on top of the slab and make finishing difficult. You should wait until all bleedwater has dissipated before any finishing operations (trowelling or floating) begin, or you will weaken the slab surface and probably have a problem with "dusting" of the surface after you're done.
Good luck.

 

This is how the rest of the basement is so that any water in the walls can drain through this gap into the gravel below and ultimately make its way to the sump.

If I remove the chunk and do the concrete properly am I pushing my luck hoping for no cracking away from the existing slab anyway? Is it worth buying the quikrete crack resistant mix?

 

two comments on the plastic below. The bleedwater that Pecos is describing will run off somewhere if your floor is tilted just the slightest. Be prepared to protect other areas from a minor flood.

And, if you place plastic under the new slab, crown it, so current water and future water can't find a home to collect. Saw that on a large pour where water from the roof would find the edge of the slab and flow in under the concrete, above the vapor barrier. It would then perc up through the floor in the middle of the room.

In a confined area, you will actually have trouble getting the concrete to dry, so misting would be going the wrong way in the beginning. Also, concrete doesn't dry, it cures, and can do so even when under water. It is just the quality of the finish surface that is at risk.

Pecos is the pro, good advice.

Bud

 

I've been pouring and finishing concrete, including basements for over 23 years and have never heard of or seen anyone intentionally leaving a 1/2 inch gap for drainage. I'm just guessing here, but I would almost bet that the gap you see is actually where the poured concrete shrunk away from the walls during curing. If the concrete was poured wet (as most basements are), it could certainly shrink almost that much. On the other hand, they may have used expansion joint material initially, then removed it later to create the gap after discovering water problems. If you try to block it out with 1/2 inch plywood and then remove it later, I don't think you'll be able to do it. If you really want to leave a gap, use styrofoam or something that will be easy to dig out after the concrete sets for about a week. Plywood, even greased with something, will be difficult to remove.
The new concrete, even if brushed against the existing concrete, will still shrink enough to leave a visible crack within about a week or so.
Regular cheap sakrete/quikrete will work for your application instead of opting for the 5,000 psi. However, the 5,000 psi will likely be creamier and easier to finish due to the extra cement added to the mix.
Finally, even with a diamond blade your circular saw would still have you in the same situation. It simply doesn't cut deep enough to avoid cracking up the edges when breaking it out. You really need to cut almost all the way through to get a clean edge, and that's typically 3.5 to 4 inches. A circular saw will only cut 2 inches deep. However, the diamond blade would have been much faster.

 

This gap is so consistent along the entire slab/wall that it was deliberate. That is a good idea about the foam, though, and in fact I think that's what they used; it explains why I found a small piece of white foam in the gap at one section as I was pulling this up. I will definitely go that route instead of ply.

It's too bad there's no way around the crack between old and new cement. I suppose it wouldn't really matter if there is one from a functional standpoint (?). For what it's worth I do intend on framing in the near future over this (hopefully in November if I can pour this in the next week) but don't intend on tiling over it until next year.

 

Is it worth applying any concrete bonding adhesive to the old concrete before I put this on? It's mainly meant for self-leveling stuff, but would this be of help in this application:

QUIKRETE® - Concrete Bonding Adhesive

 

No, it would have no effect. The cracks you will get are from the concrete shrinking, and the bonding agent will not keep that from happeneing. It's meant to prime the surface for horizontal applications like overlays. It won't hold slabs together.

 

Makes sense. I won't bother with a bonder.

The chunk is now gone. The 12 lb made very short work of it. Now I know how bad arse it is it doesn't surprise me at all I cracked the floor to hell without score lines in place.

In the end, though, if I can get this poured and have it cure ok I'll be glad for having done this. At some point in my life I had to play with concrete.

 

The gap is an expansion joint used to isolate the slab from the frost wall. You can rip strips of built-rite or buy expansion material to use and just leave it in place. I do not think the gab was designed as a de-watering strategy.

As for curing, I don't think you need to be concerned with misting or curing agents. As someone else said it will not be a problem. Hot temperatures and arid conditions are when you need to take extra precautions to prevent rapid cure rates.

 

Still plumbing, hoping to pour this this weekend still, though

I see some talk of putting in rebar or an expansion anchor with a lag screw sticking out of it, should I do this to help the new/old connection, too? My concrete is only 3" thick so I cannot use an expansion anchor as it will split the concrete. I doubt 1/2" rebar is worth it either, for a similar reason. However, a 4" lag screw drilled half way in would give some sort of an attachment point for the new mix if done every foot?

Carriage bolts are pretty cheap and also galvanized, perhaps some of those plugged in with cement would be good? I'd only have an inch above them. Not sure if the concrete would crack over them.

 

Inspection passed and pouring the concrete tomorrow. I have 3/8 rebar purchased and a concrete anchoring compound to hook it into the old slab. I had thought of, every 18", having this rebar sick out of the old 8" or so to connect the old with new but otherwise no reinforcement (because I'm using fiber-concrete). Should I do this or is it actually preferable to not do it so that any cracking will happen between the old and new concrete?

 

Rebar dowels will help protect against settling, but not against cracking of any sort. If/when concrete cracks, rebar will hold the pieces together, but rebar will not prevent shrinkage cracking. It can't hurt to put it in, but in your situation it probably won't really help all that much either.

 

Thanks for the response! If it was your basement, would you bother?

 

I would go ahead and put them in, that way you won't always be wondering "what if". As I said, it certainly can't hurt.
In addition, if you're planning on leaving the 1/2 inch gap against the wall, the concrete could conceivably move away from the other slab without the dowels. If you were pouring all the way against the wall, the concrete would have nowhere to move to. For the life of me, I still can't fathom anyone intentionally leaving that gap.

 

The job is done!

I think the main mistake I made was waiting too long for the final finishing. The post-pour floating went ok but had some ridges left in the concrete as expected. I believe I waited too long before doing the final troweling, though. I had to press hard and not long after white splotches started to appear, so now come morning the slab has white crystal looking things over much of it. Hopefully this is only an aesthetic issue at the surface and will become moot once I cover it with flooring or tile.

 

Link

I found this from a premier inspection company locally about my particular concrete slab.

Modern homes are typically equipped with “Floating Slab” basement floors. These types of drain systems have been in use for 35+ years in the Rochester area. The basic principle of the floating slab is that ground water and water that seeps through the foundation will be channeled to drain tiles (perforated pipes) beneath the perimeter of the basement floor. These pipes carry water to a crock and sump pump. The pump will then automatically pump the water to the exterior or to separate storm sewers.