Slab Repair After Plumbing Work

In order to do some drain relocation I have cut open several big holes in my concrete slab. The slab is 4" thick and sitting on compacted fill which is mostly sand down here in Miami, FL.

I have done this in several bathrooms and the holes vary in sizes, the larges one 4'x7' and the smallest one 2'x5'.

After the drain relocation was done I put the sand back in, compacted it and used a garden hose to soak the soil, let dry, recompacted and soaked with water again.

Then I contact several concrete contractors to repair the slab. The trouble I am having is they are giving me very different opinions.

One says I should drill holes every 18" into the existing concrete, insert 3/8" rebars into them so the old and new concrete will tie together, or else the new concrete would most likely settle over time.

The other says nonsense, drilling large diameters into existing concrete will more than likely crack that concrete and create another distress line, that I should just pour new concrete and level it with the existing concrete.

Then the first one says he would order concrete delivered by a truck and that way the concrete is very consistent and strong. He says if he does it with a hand mixer on site it he would not be able to pour in all the needed concrete in the large holes and it may end up with multiple batches of concrete with varying densities and consistencies.

The other one says concrete from a truck vs hand mix are the same strength, that he prefers hand mix because they could set their pace and work on one bathroom at a time. With a concrete truck you have to do it real fast and wheel barrow it in and it will make a mess on my tiles and probably bang up your narrow hallway.

They both seem to make sense.

Any thoughts?

 

Everyone has his own way of doing things. I have done exactly what you're describing both ways. Neither one is more right than another, it's just a different means to the same end.
If the holes have jagged sides or edges, no rebar will be needed because the shape alone will keep it from settling. If you sawed the holes with a concrete saw and they have smooth sides, then in my opinion rebar would be a good idea.
I prefer a truck delivered mix as long as it can be wheelbarrowed without causing damage. I only prefer it because it's a lot easier, and yes, more consistent. To me, mixing concrete is simply a pain in the butt that I prefer to avoid whenever possible. However, if someone is used to mixing it himself and can keep it consistent, there's nothing wrong wiith that either. For a bathroom floor repair, either concrete will perform just as well as the other. I would go with whoever you trust more.

 

Are the slab edges sawcut? In the commercial work I design, we require all cutouts to be sawcut. That means the edge is smooth and there's nothiing for the new concrete to key into. So our specs always require 3/8" rebar @ 18" forcefit into the center of the existing slab edge. Seeing as how my office tends toward the conservative side, I would do the rebar if it were mine; that way I would know for sure it would not move. Whatever you decide to do, make sure you don't get too wet of a mix. The more water in the concrete, the more it will shrink away from the edges.

 

Yes they were saw cut. There is a little bit in some areas that after the initial saw cut I needed additional notches so I used a rotary hammer to drill holes every few inches and them break with a sledge hammer even in those locations its pretty straight and smooth. Here are some pictures (I have six to seven areas like such to repair).





How about the concern of drilling sideways will crack and weaken the existing concrete?

 

One more question I forgot to ask. I have one bathroom where the opened area is surrounded by three concrete walls and the existing slab. In other words, I only have existing slab on one side. See picture below.



This makes rebar difficult because I don't think the concrete block wall that are hollow is a good candidate to insert rebars into, right?

To make matter more complicated, I want the entire area that is opened now to be a shower without a raised curb at the edge., in other words, the new concrete slab will need to be lowered by two inches or so, such that when I build up a slope and shower pan later the edge of that shower will be flushed with the rest of the bathroom floor.

How would I do the concrete preparation in that case in regards to rebars?

I am thinking on the side that it joins with the existing concrete, instead of rebar I can dig the soil under the existing to create an overhang for two to three inches, and pour the slab so that the new concrete will go under the old concrete a little to create a "lip".

But that will not stop the new concrete from settling, and still the other three walls are issues right?

 

Nope, no worries about weakening the slab with 3/8" holes. Just stay approximately in the center of the slab. If I remember correctly, we embed 4" to 6" into the existing slab and the same amount into the new concrete. So 8" to 12" long dowels.

 

Thanks.

But how do I put rebars into concrete hollow block walls when three sides of this are walls that goes deep and rest on footings? See the picture above.

Thanks in advance,

 

you can drill into the walls the same as into the slab. To get it more secure, drill into the mortar between the blocks instead of the blocks themselves.
The areas you sawed would be considered smooth. The areas you drilled and then broke off are considered rough. If concrete were poured against the rough parts, it would conform to the contours of the sides and it would be physically impossible for it to move up or down. Against the smooth sides it could do either, so doweling is the way to go there.

 

OK thanks I think I got it now. I have a few more questions as I plan this.

Let's say my hole is 4'x6'. I drill every 18" or so a hole 4" deep. I then paint on some epoxy and pound in rebars 8" long. That leaves 4" of the rebar out. All along the interior of the slab cut I will have 4" of rebar sticking out, correct?

or should I get LONG rebars like 24" long, so the rebars from opposite sides of the slab cut will overlap each other?

should I add wire mesh into the hole, sitting on the rebar?

 

one final question. If the holes are small, like a 2'x2' hole, should I still bother to use dowels?

 

In this case, the rebar is only to keep the concrete from settling, so it doesn't need to overlap, nor do you need wire mesh. Use it on the 2x2 sections as well to keep them from settling.

 

When it comes to plumbing retrofits in basements, the concrete jobs you have talked about are happening all the time. It sounds like you have done a good job wetting and compacting the base. Make sure just before pouriing concrete that you wet the base so that the dry sand does not wick the water out of the freshly poured batch. You can drill a few 3/8" holes in existing edges if you want and drive in some dowels. That won't hurt a thing but where you are, you don't ever need to worry about frost heaving. And look, the heaviest thing this floor has to endure in is probably going to amount to a 225 lb man.

Pour the concrete flush with existing edge and trowel soon to get course stuff settled in the freshly poured concrete while you can and keep seem clean & distinct. You will be looking to feather thhose edges nicely together. When it's all said and done it's undoubtedly going to be covered with tile anyway.

I would pour the shower base flat for now with a larger form around the stubbed out drian pipe. Use a preformed shower base or let the tile guy get the pitch to drain with his thinset and a shower curb across the door.

Good luck with your project.

bs5

 

I wet and compacted the sandy soil, then I sprayed the opening area with a 10 year termicide. Then I laid down a 6 mil vapor barrier.

After that I epoxied in the rebars. I end up using #3 rebars at 16" intervals. Where I can I overlapped rebars from opposing sides and tied them together. When the edges are not existing slabs but walls, I end up drilling into the seams as suggested, in some cases I drilled into the hollowed cells, but I punched a small hole right above it, poured in some concrete to fill the cell below so the rebars are completely embedded in solid concrete.

A bit of an overkill, but I am ready to pour concrete now. Tired...

 

how am I doing?

 

Good enough to land a 747 on! Just kidding of course, but it is quite the overkill. Like Bullshooter said, this floor doesn't have to withstand much of a load; probably just a man's weight. The only other thing I'll say is that the cardboard box will not hold it's shape when the concrete is placed against it. The cardboard will saturate and bow. 1.5 or 2 inch thick styrofoam insulation (not the compressed white beads, but the pink or blue) would work much better as a form, and would be easy to remove afterwards. Just break it out. Your finishng will be easier and the floor flatter if you place the top of the form at grade, instead of sticking up above the concrete.
Finally, expect the new concrete to shrink slightly as it cures. There will ultimately be a gap of about 1/16 of an inch around the entire perimeter of your pour. Don't have a cow when it happens. It's normal and there's nothing you can do to stop it from happening. The wetter the concrete is mixed, the more shrinkage you will have. The new concrete will also likely crack from the corners of the box, or any other re-entrant corner (a formed corner pointing into the new concrete). The rebar will not stop this from happening, but could help to minimize the width of the crack.
Good luck!

 

OK I will remove the cardboard and replace with something sturdy. I don't have any foam, but may be scrap plywood cut in four pieces nailed together.

I have a hard time really getting the vapor barrier in. Around the edges to the existing there is no way to really seal completely, the old visqueen was torn out by the demo crew, and the edges are irregular, all I could do is to take it to the edge of the concrete and fold back a little.

Thanks for all the help! I hope to make more progress today.

 

If you use plywood, make certain to cut all 4 pieces the same length and stagger each corner. In other words, lap one end of each board and butt the other end. Do this on all four corners. If you make two sides longer and two shorter, you will have a bear of a time removing the forms once the concrete is hard because the longer boards will be wedged in. To make removal easier, use smaller finish nails to join the boards.

 

It's looking very good and your copper is sleeved against the concrete and it's future effects with copper. Good job and the pictures are fantastic. Just as Pecos said, it would be nice to make an improvement on your 2" trap form and it would be very easy to set a little box form in there level with the floor on either side so your screed can ride on the existing floor on either side. Perhaps you might use 1/2" or even 3/8" plywood tacked at the corners. It would be easy to work the thin edges of that form down in that sand of yours so that it is level. A few shovels of freshly mixed concrete around the outside to hold it in place and you are ready to screed. When the concrete sets and it's time to take it out you can just slip a flat bar in and bust the thin sides to remove the form before any filling.

I don't know what you have planned for your shower but it looks like perhaps it will be custom built and likely tile. If so, you might want to look into a fabric kit carried by HomeDepot and Lowes. I wish I could remember the company but it is available in a kit with a premolded base designed with the floor pitched to the drain. The tile guy just applies his thinset and fabric and then thinset againt and tile. In order to use the pre molded base the trap and stub out has to be placed strategically. If you think you will use something like that make sure you place the shower drain precisely so the tile layout and base with the distance from the wall and size of shower works out. Perhaps the trap form could be a little bit smaller then or trap installed and area filled when you do the pour using the base as a template. It always nice to have grout line centered on the floor drain. The fabric permanently prevents any future leaks and rotten wood floor plates. Just one of several different ways to do a shower.

I just recalled the company name and website where you can view the system on a video and consider using something like that. Keep up the good work.

Schluter Systems - Homepage - Schluter-Systems

bs5

 

After all the prep work in getting all the trenches ready I hired a concrete contractor to come backfill the concrete into the trenches. After that we covered it with some plastic and sprayed some water on it each day.











It has been 4 days and so far no cracks between the old and new concrete yet.

How long do I wait before I pull off the plastics? I can walk on it now but I have to wait a little before I roll a wheel barrow full of dirt over it right?

Do I need to scraper to keep scraping the new concrete to make it smoother?

 

You'll get different answers from different people...here's mine.
Since you've begun the water curing process, keep it up until the end of 7 days. Then you can remove the plastic.
The concrete should have been set up enough to support wheelbarrow weight after a day, but go ahead and wait the same 7 days. In that time, the concrete should have gained about 75% of its final strength. (90% after 28 days, and the remaining 10% years from now). I've seen them begin framing on slabs that are a day old with no breakage, and that's a lot of weight on un-reinforced (in most cases) concrete!
The concrete should have been finished to the smoothness you wanted prior to ever covering it with plastic. There would be no need to scrape it to make it smooth. I've never heard of that in my 25 years of doing concrete.
It's nice to hear you have no crack between the differently aged concrete. I still think it will happen though, and sooner rather than later. By wet curing, you've delayed the inevitable shrinkage and possibly even minimized it.
As this is a bathroom floor that I assume will be covered with something later, I'd have just poured it, finished it, and let it air cure. In any case, wait the full 28 days before any floor coating or covering is applied.
In my opinion, some things are made much more difficult than need be by overthinking or over-engineering them. However, it's your project and I'm sure you want it to last so I salute you for taking the time and energy. Good job, and I'm sure your project will last much longer than you own the house.

 

Thanks for the advice.

I have no urgent need to remove the plastics, nothing much is going on during the holidays so they are just sitting there idle.

As far as the smoothness, when the concrete was poured some of the concrete dripped on the floor, forming some shallow lumps, and as they finish the surface some of the excess also went on the edges. Not too bad, I just scraped them off. The surface seem to be smooth enough overall, but as I tried scraping them it seem I can make it smoother. Around drain and supply pipes the edges are not smooth, they sort of "ramp" up the pipes. The supply lines will be inside walls so as long as they are not in the way of the bottom plates, I am not worried about them, the larger 4" toilet pipes, I will have to smooth out the edges so it does not create tiling problems or flange seating problems later.

All I can say is, the concrete pour after it was done, it was like a war zone, what a mess, clean up took me a while.

 

One thing I forgot to mention.

As I worked on prepping the rebar and moisture barrier I did something that I thought was a "smart" thing to do but in retrospect I am not so sure.

On all my slabs I drilled and epoxied #3 rebars, overlapped them by at least 8 to 10 inches and tied them together. However there is one room where I have existing walls on three sides so there is no existing slab to bond to. The existing walls are all hollow block walls resting on footings several feet deep. Here is what I am talking about.



I know the recommendation here was to find the joints between the blocks and drill into that because that is the strongest and most solid support for the rebars. However, I was not able to get the rebars from both ends to overlap because opposing walls do not have joints at the same locations.

I still end up drilling into the blocks but it being hollow in some locations do not give me much confidence. While I was struggling to find a solution I decided on one.

What I did was where I had drilled into a hollow block, I used a rotary hammer and chiseled out the block cell, which means some of the rebars are no longer pushed through the little drilled holes in the blocks, but they are just sitting inside a large hole instead. I did that for about 10 holes. I then used small pieces of the concrete block I chiseled out so the rebars sit at the correct height, the rebars are still sitting inside the block cells. I then had them pour the concrete into the floor and at the same time pushed the new concrete into the hollow cells and filled them solid. So now, I have rebars embedded in solid concrete, only they are new, not existing, but they are sitting inside the existing block cells.

To me it means the new slab are no longer rectangular, but have "several pockets" that extended and sat inside existing block walls, with rebars embedded inside.

I am still debating with myself if this was a dumb thing to do, or a smart thing to do.