Patching Structural Slabs

[LPPE]

Lets say you have a structural slab supported on grade beams and piles. After time, the soil immediately under the slab has settled some feet. A new owner takes the building, and wants to trench the slab to run new plumbing.
Initially, the plumbing was installed, then hangers were cast into the slab to hold up the plumbing.
How do you hang new plumbing from the existing slab and then recast the trenched piece?

 

[JAE]

If it is a "structural slab" actually spanning from beam to beam I would be careful about "trenching" through it.

You should first try to get access to the space below - this can be via excavations if necessary. The new hangers could be attached with expansion bolts, expansion-threaded inserts, power driven fasteners or concrete screws (like Hilti's Kwik-Con screw). Other access could be created by cutting a small square opening in the slab at an advantageous point and excavating fromt there to get the pipes in. The opening may have to be reinforced as you have cut through primary reinforcing.

Otherwise, cutting through a structural slab must be carefully laid out by the engineer to minimize loss of strength. For a one-way slab, this could be a longitudinal slot parallel to the direction of the span of the slab. But I would avoid this as much as possible.

 

[LPPE]

Ah, JAE, you know as well as I do that the cuts will ALWAYS be perpendicular to the reinforcement!!!

Lets say you can cut a hole and get underneath to install your new plumbing hangers. How would you then patch that hole? How do you form your bottom part of the slab when you have no access to underneath the slab? Maybe a piece of plywood underneath hanging from a couple vert. steel rods which are supported by a 2x4 across the opening? Then grind the rods down after the concrete has cured??

 

[JAE]

Yes, that would work...

How about this:

When they cut the opening, have them shave all four of the edges of the opening back at an angle, perhaps 30 degrees from the vertical...so your new concrete fill would be set in the opening like a keystone.

Then, have them bolt or screw a couple of angles across the opening - the angles would be sized to take the vertical load of the patch. The top of the angles would be flush with the bottom of the slab and the angles could extend past the opening edges far enough so that the bolts/screws would be secured to competent concrete (away from the opening edges. The angles could be located parallel to two edges of the opening such that a workman could still get access between the angles to make the bolt/screw connection to the bottom of the slab underneath (a little gymnastics but can be done)

Then set a steel plate across the angles. The plate would span from angle to angle. Add WWF if desired and place the new concrete. The new concrete will be supported by a combination of the angles/plate and the tapered edges around the perimeter.

Otherwise, can't you get on your knees, fold your hands, and plead, "Please, please please don't cut my precious rebar!!!!"

 

[Ingenuity]

pylko,

Your client may also be best served if you make the opening permanent so that future plumbing or other access requirments can be easily accomplished.

You can usually find a location that minimizes the structural and architectual impacts - like under stairs, locker rooms etc. This would require some structural strengthening, and if a single access opening is all that is need to undertake the require plumbing upgrade, the strengthening may be a small cost of the overall scope of work.

Just a thought.