Metalfree Concrete Slabs 1

[StructuralEd]

Any suggestions for designing a metal-free concrete slab/foundation for an MRI support slab?
I'm sure we could just design the concrete section, but it would be good if we could try to have some resistance to cracking.

 

[csd72]

Use some fibre reinforced plastic (google it).

 

[StructuralEd]

From what I read, durability is not known or thought to be less than desirable on the FRP.

 

[Qshake]

StructuralEd -

I don't know where your sources are from, but the transportation market is using FRP reinforcing in limited applications and in state or provinces where de-icing is typical.

That said, when you look at ACI 440 for the necessary strength formulas, you'll also see a environmental factor that is applied based on exterior, interior and or exterior aggressive environment. These factors are presently used to ensure the bars/slab will not fail due to degradation any more than uncoated steel reinforcing would in a corrosive environment.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[StructuralEd]

http://www.azom.com/details.asp?ArticleID=1074

 

[hokie66]

StructuralEd,

How big is this MRI? I would think as long as it is not too big and you can prevent edge restraint, no reinforcing should be required to prevent shrinkage cracking. Just use isolation joints around the slab and make sure the base is flat, frictionless, and impervious as possible. And cure it well.

 

[Boiler8019]

FRP is experimental and isnt widely used. I wouldnt try it.

The slab needs flexural reinforcement...how's that going to be addressed??

 

[Qshake]

There is much research and laboratory work on FRP reinforcing....see Benmokrane for more articles in bridges.

ACI has prepared 440 to deal with the engineered performance of concrete reinforced with FRP reinforcing. Including FRP used as flexural, shear and temperature reinforcing. In many cases, carbon is the material of choice for flexural reinforcing or is augmented with glass to offset the cost of carbon.

At present there are numerous bridges whose decks are reinforced with FRP. So far the performance is exactly as specified (very low stress/strain) in the bars based on the reinforcing guide lines in ACI 440. If this is true in bridges, I don't see what the problem of using this in a localized building area (other than first costs).

I don't believe that a hospital MRI floor slab is loaded any more heavily or directly exposed to the environment such as bridges decks and would think it's very safe to use FRP if it isn't already common place.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[lkjh345]

StructuralEd:

Consult with the MRI supplier about amount of steel their unit can tolerate.

We have done 16" thick MRI footings with bars in the bottom without messing up the machine.

We have retrofitted MRI machines onto elevated floor slabs of composite metal deck and steel beams without a problem.

Some MRI machines don't have as big of problem with proximate steel as widely believed.

 

[StructuralEd]

The documentation on the equipment is forthcoming.
Thanks all.

 

[samdamon]

Ed-

Consider using stainless steel rebar. It is nonmagnetic and available in common smaller diameters (Although costly). You are going to need at least some flexural resistance in the pad.

MRI manufacturers are sometimes limit not only about the total density of ferromagnetic material below the magnet but also are concerned if it is uniformly dispersed. These limits also vary from manufacturer to manufacturer, and even between product lines. Read all the fine print in the manufacturer's installation data.

 

[mbullism]

I've only done one of these, a slab on grade pad for a portable/trailer MRI unit that was at the hospital 2 or 3 days a week. As I recall, the manufacturer would allow 3-4 PSF of steel rebar.

We decided to design the slab as an unreinforced pad and use the FRP bars per ACI 440 for crack control, and paid special attention to the mix design/aggregate size, joint details (stainless dowels) and curing, preparation of subgrade, etc. The rational was that 1)no one wanted to use the 4 psf of rebar and find out afterward that it did affect the unit somewhat, or they started getting a different unit in the future with different specs. (It's awful hard to get the rebar back out)... and 2)"failure" of the FRP was not going to result in anything catastrophic.

 

[StructuralEd]

Mbullism,
You bring up an interesting point.
I don't imagine the truck chassis of that vehicle was non-metallic.

 

[hokie66]

Not all of these machines are located on the ground floor. When on a suspended slab, I think they use some type of floor isolation system. They worry about nails and screws in the partitions around the machine, but not the supporting structure.

 

[mbullism]

"I don't imagine the truck chassis of that vehicle was non-metallic."

I have to admit we didn't concern ourselves with the structure of the trailer, but I hadn't thought of that... the point is that the unit was tolerant of some steel, in the right places. I think you could be closer to the sides than the top and bottom for this particular unit... and anything moving was a concern (dumbwaiters, steel cabinet doors, etc.) though that wasn't our issue.

We also had to design a loading dock of sorts, with an airseal for the truck to park adjacent to... I DO remember we had to pull our TS columns back, and be mindfull of any rebar in the platform foundation and slab as well.

"They worry about nails and screws in the partitions around the machine, but not the supporting structure."

I don't think the concern was that the magnet would rip anything out of the wall, but rather the interference in it's magnetic field and the effect on the scan/image quality...