Difficult Concrete Slab Pour

[Gopher13]

All,

We are building a new concrete arms vault inside an existing building. There are existing walls on the north and south side of the vault that we will be pouring up against. There is only about 7.5 inches from the bottom of the existing joist to the top of the new concrete vault roof slab. How do you foresee the contractor pouring the roof slab? Is it possible to pour the slab and set the steel reinforcing without removing the roof deck? Please see the attached drawings. Thanks in advance.

 

[Ron]

This is difficult!

It will likely be less costly and you'll get a better job if you remove the roof decking. Otherwise, you will not be able to check any of the concrete placement across the 15 feet each way. Further, getting the rebar in place and tied will be almost impossible without the deck removal.

 

[hokie66]

Agree with Ron.

The other thing of concern is the fluid concrete pressure against the existing walls.

 

[icundiff]

What about gunite?

 

[Brad805]

Getting the rebar in the roof slab will be a challenge, and the side wall will be tricky, but avoiding the main floor removal is a significant savings in both time and money. I would investigate this with a contractor. I doubt your owner will be too particular about the finish quality of the roof slab, so you might be able to super plasticize the concrete to simplify the finishing aspect.

 

[oldestguy]

Could you pre-cast the roof inside the room, say at a convenient height, maybe 5 feet, then jack it up to position and later do the walls?

 

[Gopher13]

Thanks for the input. I like the idea of a precast roof. I will have to verify it is acceptable as the National Guard has some strict requirements for the construction of arms vaults.

A side question on designing fluid containing structures (or in this case checking the existing walls that they will be pouring against); does a fluid in a structure that is infinitely wide exert the same lateral pressure as a fluid in a structure that is only say one foot wide (infinitely skinny)? The height and density of the fluid being constant. Seems to me at some point the width should have an effect.

 

[gwynn]

Width has no effect. For a fluid pressure is dependent on the amount of head only.

Typical practice for wall forms against existing walls, from what I see here, would be to Hilti some coil rod into the existing walls, add B1 coil ties or similar and attach a wood form on the open side of the new wall. The contractor will not want to pour this as a one sided form with the existing wall taking the full unbalanced pressure. The existing wall then only needs to be able to span between the embedded rods, which will be in something like a 2' by 2' grid. If you really need to, you can restrict pour rates and temperatures per the ACI concrete pressure equations to reduce the concrete pressure.

The roof slab is the more difficult item.

 

[oldestguy]

I was thinking of just how to jackup the roof and some farm jacks I have can lift several tons each, maybe 4 or 5. Placing a pipe over the center piece would work or they even work upside down. Maye a group of 4 or 5 at each wall. With some thought, I bet a safe dependable way would work. This is one of the lower cost places and still good quality stuff. Later on sell them for a profit.

http://www.harborfreight.com/catalogsearch/result?q=farm+jacks

 

[jgailla]

The Army Reserves has used precast vaults in some locations. The slabs are connected with steel angles bolted to the precast. I've only seen this in areas where there were overhead doors, but it could be cheap to temporarily knock out and brace a section of wall.

 

[BUGGAR]

Concrete can be pumped from below. They bottom-pumped two story columns in office buildings in Seattle thirty years ago.

 

[azcats]

A crafty contractor could suspend the roof rebar mat from the existing floor joists then build the form below that. Seems like the would be enough space to pump & vibrate the concrete working parallel to or through the webs of the joists. Would a screeded finish across the top be adequate?

 

[oldestguy]

I second the ascats thoughts. With some time considering all aspects of the systems, it sounds like the job can be built without removing the roof or side walls. I'd start with the base slab to have a good working surface.

 

[bridgebuster]

Here's a thought: Instead of a concrete slab put in W8's say at 3' -6" use SIP forms and pump concrete in between the beams. The beams would be light enough that they could be slid over the walls. NYC subway roof slab construction.

 

[CELinOttawa]

Sounds like a job for self consolidating concrete, or even a pressure form and Sika 212 extended with pea gravel.