Concrete Floor Cut & Replace Advice

[Red Goat]

I have a tenant in a commercial building that wants to install 2 very large milling machines. Each machine requires a 12' x 15' pad that is 12" thick.

The existing slab in the building is 5" thick and was poured back in the late 60's. The building is 10,000 SF and the existing slab was saw cut (dashed lines) in sections of 25' x 40'. Because the sections are so large, there are several cracks in the floor.

I have attached a picture below of the location where they want to cut out and replace the slab (red lines). I am concerned about the existing slab cracking more once they do this work. Any of you concrete experts have any ideas as to what I should require? Should I worry about the 90 degree cuts in the middle of the existing slab sections?

Thanks...

 

[ConCretin]

I wouldn't be too concerned about additional cracking in the the existing slab. Most cracks result from shrinkage during the initial curing period, which is obviously ancient history at this point.

I would be sure to isolate the new equipment pads from the existing slab but otherwise I think you'll be fine.

Congrats on having a prosperous tenant.

 

[Red Goat]

Thanks LL. I think I understand the need to isolate the equipment pads from the existing slab.

I was going to have them drill into the existing slab every two feet or so and install a rebar dowel so that the two slabs would remain at the same grade with forklifts and such rolling over them. Would you recommend against this?

What would be the best material to use in between the slabs to isolate them from each other?

 

[cbracer]

I would recommend not doweling into the existing slab. They make a foam insolation strip that is used to isolate the slab from the other slab, preventing any additional cracks from the thicker concrete banging / mashing into the weaker slab. They are available in different widths, normally 4/5/6 inches not sure about 12 inches, but you just need to isolate the 5" you have now and make sure none of the pour goes below the older section. Attach it to the old concrete and pour. You might leave it 1/4" down from the surface so you can fill it with flexible sealant.

 

[ConCretin]

Red Goat, the existing 5" slab would likely to crack if loads are transferred to it from the new pads via dowels. Put another way, if the larger pad wants to move or settle, the 5" slab won't be able to stop it. That's why I'd isolate the two using a foam expansion strip as cbracer suggested.

 

[frantylouis]

If your concrete is in extremely bad condition, it may not be a suitable candidate for restoration and level the patch with a straight-edged board at least a foot longer than the width of the hole.

 

[JakeKohl]

If your concrete is in extremely bad condition, it may not be a suitable candidate for restoration and level the patch with a straight-edged board at least a foot longer than the width of the hole.

That awkward feeling when you realize you just walked into the wrong room.....

 

[joes169]

We install a lot of "machine foundations" on a regular basis. As mentioned by others, it should absolutely be isolated from any surrounding floor, so no dowels. Typically, we use asphaltic felt between the two. The main reason to keep them isolated is because you don't want any other harmonics/movement in the building to transfer to the extremely precisely operating mill......

 

[Red Goat]

Thanks for all the reply's! I met with their contractor today and his recommendations aligned with yours. Your input makes me feel more comfortable with what they want to do.

Cheers...