Time to Drill Cores through Concrete Slabs

[IsaacStructural]

Can anyone give a rough estimate of the time to drill 3" or 4" DIA. vertical holes through 4.5" of LW Concrete? I'm looking to reinforce an existing composite slab by drilling cores, welding on new studs and then filling with non shrink grout, and I'm looking for a rough estimate of the time involved to drill the cores.

One Architect I know thought it would take an hour per core, which seems absurdly high, I'm thinking more like 10 minutes each core, but that is just a gut feeling. I'm curious to know if someone with experience doing this could give a more accurate answer.

Thanks!

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[steellion]

You may be better served instead to cut a few pockets, 2-3' long, load those pockets up with several shear studs, then fill with grout. I've used this detail a few times before (for structural concrete slabs to which I was adding steel beams for reinforcement) and it seems to work well.

 

[ztengguy]

good drill and operator, no more than 5 mins a hole.

 

[jgailla]

Usually 5 to 10 minutes, plus travel time, plus set up time. If you've got just a few it would still be at least a half day's cost. If you've got more than a hundred you could estimate 40 per day.

 

[Ingenuity]

The drilling will be less than 15 mins per hole, but you may need to factor in the occupancy below and how much time to handle water and slurry control for wet coring - with a profile composite deck the core will penetrate the existing deck and water and slurry will enter the space below. Often the time (cost) taken to provide such provisions is more than the time to core.

And if the occupancy below is an attorney, the cost to do the coring will be higher than other occupancies - based upon my direct experience of having "trashed" an attorney's office space on one occasion :-(

 

[Ron]

Why not drill and epoxy the anchors? That would be even faster and you wouldn't have to depend on the bond of a "non-shrink" grout to the smooth sides of a core hole (usually not so great!).

Drill the holes and epoxy rebar into the hole to use as an anchoring stud.

If you are concerned about fire resistance, you can use a high temp structural epoxy; however, the epoxy will be insulated by the new concrete as well.

You could also consider Dywidag expansion or resin anchors as well as other expansion anchors.

 

[CELinOttawa]

I've done essentially what Ron is suggesting with matched-size Hilti HSL anchors (Not sure if these are available where you are; They are a Hilti International product).

 

[IsaacStructural]

I'm not sure I follow, what is the connection between the anchor and the steel? Are you drilling from below, through the flange?

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[Ingenuity]

Not sure I follow Ron's "drill and epoxy anchors" either.

IsaacStructural: you have existing steel beams, supporting an existing concrete slab (on metal deck?) that you are trying to make composite via new shear studs, or equal, to the top flange, right?

 

[IsaacStructural]

Ingenuity, that is correct, though the slab is already partially composite, I'm trying to add studs. I've got some 45 foot long beams with only 28 studs, so there is quite a bit of room for additional studs (and thus additional live load capacity).

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[Ron]

Isaac....I should have read your post more carefully. I was assuming you were adding flexural reinforcement, not shear studs. Your method will work just fine. I would suggest that you use automatic welding for this rather than typical SMAW process, since your access will be limited. Shear studs can be welded either way, but if you use a dedicated "gun" to weld the studs, you will not have an access issue in the hole...in fact, you can probably use a smaller diameter hole.

You might consider using stud rails in a fashion similar to the approach steellion noted.

Sorry for the confusion.

 

[CELinOttawa]

Yes, drilling through the flange into the concrete.... There is a UK road authority design guide doing this with plates to create a composite section as well.