Making a hole in concrete slab

[downtownguy]

Hi all,

I'm installing drains for a shower above a concrete slab. All drain pipe to be run in wall above slab except under shower where it will be built up about 4" to accomodate drain. (cannot run drains through floor - other tennants below). I want to make a 3" hole in the slab (slab is 6" thick) to drop the p trap into to minimize the height of build up. l;m planning on using a concrete abrasive blade in circular saw and cross hatching the area and then chiseling it out. Any thoughts or recommendations before I start? I'm aware of dust and respiration issues.

Thanks in advance,

Bob

 

[DFBonnett]

I did mine with a bull point chisel and a hand sledge. Wear a dust mask and expect a sore arm before you're done. Once you've made a penetration, you can undermine the rest. That being said, rent an electric jackhammer.

 

[phishfood]

Obviously, I can't know what type of concrete construction your building is. But there is a type of concrete slab called a post tension slab that it would be unwise to do this to without some very careful preplanning.

Post tension slabs have steel cables with a plastic casing run through the slab. Once the concrete is cured sufficiently, the cables are tightened and locked in this tensioned state with wedges. The cable provides the reinforcement necessary for the concrete.

Cutting into one of these slabs with a concrete blade is dangerous, as the blade can cut right through the cable without you knowing. On an elevated slab, such as what you are dealing with, the possibility exists that the slab could collapse.

In my area, it is not common to build an elevated slab using post tension, but I have heard of that being done. So make sure that you know what you are dealing with before you proceed.

 

[downtownguy]

Thanks for the heads up on cables in the slab. it's a loft buiding in nyc. 7 floors. built around 1920. there are some openings cut in the slab already i.e. for boiler flues that were added about 20 years ago. Didn't see any signs of cables in that opening. I could try to just go at it with chisel and sledge but the contractor who intalled our wood floors told me that the concrete slab was really hard and that he had trouble shooting nails into it. That's why I thought to make some cuts first.

 

[phishfood]

If the slab is 1920's era, I seriously doubt that it has tension cables in it. I would carry on as you originally planned.

 

[PlumberinSC]

Just a tip. If it's applicable, I would use water when running the concrete saw. It keeps the blade cool and works on the dust.