Plumbing in wrong place

[Guest]

I am currently having a home built. Unfortunately, the plumber just put the toilet, tub, and sink, 8 feet off (into a future bedroom) with white pvc pipes sticking out of the slab. The construction mgr. said they will correct the problem by cutting the slab, moving the plumbing, and filling it with new concrete and a crack prevention kit. Should I be concerned so early in the construction?

[Bob NH]

Cutting a hole in a concrete floor is virtually certain to cause cracks.

They will cut with a saw and will leave sharp corners. Corners are "crack starters". You will have cracks going out from all of the corners. Even if they round the corners you will have cracks. And the joints will crack. No joint is ever as good as the original continuous concrete.

If you ever tile it, you will have massive problems without special processes. It could even be a problem with other types of flooring such as vinyl or wood.

The plumbers Errors and Omissions insurance should cover the cost of a new slab that extends to the OUTSIDE of the foundation and is tied to the foundation. The only joints should be outside the floor area and new concrete should be tied to old concrete with adequate steel reinforcing.

If they talk you into accepting the floor, then make them give you a fully prepaid, securely bonded transferable warranty, at least 20 years, that will survive their bankruptcy, to fully restore and repair any damage to the floor and consequent damage to anything else that is in any way associated with cracks and joints in the floor.

Otherwise, get them to give you a big discount in addition to repairing the floor. Then you will be able to pay to fix the problems later.

[hj]

What Ivory Castle do you live in? Concrete floors are cut and patched many times every day. NO COMPANY would pay to have the entire floor replaced. If the floor is going to crack, it will crack regardless of whether it was cut and patched or not. As long as the patch is keyed to the existing floor, so it cannot slide down and settle, it will be perfectly okay. And as for any "lifetime guarantee", if you want it, and could even get it, you would have to pay for it.

[Bob NH]

If I plan to put ceramic tile on the floor, or glue down vinyl, the cracks and joints will reflect through the tile or vinyl. Check what they say over at www.johnbridge.com about tiling over joints and cracks in the underlying concrete floor.

When I buy a new house, I am buying a whole floor with no built-in cracks caused by cutting out and replacing big chunks of concrete. If it were my responsibility, such as by a change order from me relocating a bathroom, I would live with it and deal with the defect. If it is caused by the builder, I want a floor that is fully equivalent in every respect to a newly poured floor.

[hj]

There are many ways to tile over cracks in the floors, but most of those cracks are natural ones and often regenerate themselves due to site conditions. We are talking about created cracks and if those are done, and patched, correctly they have no relationship to the ones discussed at the John Bridge site. Again, if you think anyone is going to replace an entire floor because a small area has to be repaired, you are living in an Ivory Castle, and have never dealt with the situation in the real world.

[sjsmithjr]

I would make sure, or for peace of mind have a third party like a structural engineer make sure, that the repoured area is keyed to the existing slab, prepped properly, etc., etc. As always, the key is prep and execution. It should be be fine. We don't do residential, but it works fine for our 20,000 psi repairs (yes, you can park a tank on it) without cracking or failing.

-Sam

[Bob NH]

Keying in the patch to the original concrete will probably take care of the differences in level that might occur between the patch and the original pour. But the "keyed in patch" will have no tensile bonding to the original and will do nothing to prevent cracking across the slab or along the joint.

Concrete has very low tensile strength and the bond strength between pours is virtually zero. Cracks originate at internal corners created by the removal of the original concrete because the discontinuity created when going from a full section to the reduced section at the hole creates a stress concentration at the corners. It is the same principle that applies when you notch a stick or a cast iron pipe to break it. Removing and replacing a large section of a concrete floor will cause cracks, regardless of keying. Furthermore, shrinkage of the new concrete will cause the patch to pull away from the original, effecively creating a crack all around the patch.

Cracks in the concrete or opening up of weak joints will damage ceramic tile or resilient floor coverings. Joints are treated as cracks in the tiling business (Go to the www.Johnbridge.com forum). It should not be the responsibility of the owner to take extraordinary measures to deal with cracks caused by construction errors. The concrete slab should be restored to the condition of a new slab, or the owner should be compensated enough to cover the cost, inconvenience, and uncertainty of dealing with cracks that will occur, even after a nominal one-year "warranty".

[sjsmithjr]

There are techniques to address cracking at stress load locations, such as inside corners. There are also techniques for cutting concrete that will not leave a sharp corner. I personally don't know which ones are "over the top" or appropriate for a residential repair. A local structural engineer can provide instruction as to how best to make the repair and to visit the jobsite during repair. Compared to the cost of the home, bonded transferable warranties, etc., etc., their fees will be a bargain.

As to how to successfully tile over a concrete slab, repaired or not, then the answer is to use an uncoupling membrane. Should add about $2.00 a square foot to a tiling job. I believe a lot of installers make the assumption that the slab is going crack, repaired or not.

Should the buyer be concerned that a mistake was made this early in the building process? I don't really know. If everyone is being up front about problems encountered during the building process and how to address them, then probably not.

-Sam

[Bob NH]

"There are techniques to address cracking at stress load locations, such as inside corners. "

The usual technique for preventing or limiting cracking at inside corners is to put a lot of steel around the outside of the corner. That is done before the concrete is poured.

I suspect that horizontal drilling of the original floor to bond bars into it to tie the corners together, with possible post-tensioning, is one of the "over the top" techniques that would be excessive for residential construction.

The owner should be compensated for having to deal with the cracks that will almost certainly occur as a result of the construction error. Maybe payment for the structural engineering consultation (someone not associated in any way with the contractor) and $2 per square foot of the total slab would be a starting point for negotiations.

[hj]

You are speaking from a theoretical point of view. The day to day practicality of construction is vastly different. If it is a post stressed slab, then no cutting or drilling can happen. For all others, commercial and residential, the concrete is poured back level with the existing floor and keyed/tied to it in any of several ways. My residence did not have any repairs to it, but the tile company still used a slip membrane to minimize the possibility of cracks being transmitted through the tile, and a contractor friend of mine had a "natural" crack in his floor that was perennially telegraphing through the tile. 10,000 psi concrete? That is beyond airport runway strength. What mixture do they use to get that

[sjsmithjr]

Add silca fume, a superplastisizer, and make sure you're real close to the batch plant.

-Sam

[hj]

Batch plant? For a seven foot square, at the most, and probably less than that in reality? Unless they can get a truck on its way back to the yard with a half yard left in the drum, they are not going to be calling a ready mix truck.

[sjsmithjr]

Lost me there. I thought you were asking what differentiates 20K psi mix from your basic mix. 20K is mil spec; sometimes used in specialized commercial. I was in no way suggesting that it be used to repair such a small area in a residential application.

-Sam