Rebar supports in a slab

[FTG-05]

Definitly never use bricks to elevate your rebar. Typically for standard 1/2" rebar, a placement interval of about 16" is just fine for the rebar grid. Normally 6' to 8' apart for the chairs is good. Any longer and the rebar starts to sag between chairs. Make sure you have either the 2.5" chairs or 3" chairs. Also recommend after placing your rebar grid, and depending on your budget, placing some welded wire sheets directly ontop of your rebar.

Double check to make sure your base below the concrete is well compacted. Also, some like a vapor barrier at base of concrete, others ignore a vapor barrier.

Why is that?

 

[Frankenkubota]

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Any downside to adding welded wire other than expense?

I've seen one or the other but never both...

We built a new imaging center 5 years ago and all the rebar was fiberglass... could not have any ferrous material in proximity to MRI machine... it was interesting just how involved and different the build was.

That's funny. That's what I did at the end of my career. Evaluate sites for imaging locations, specifically MRI. We would search for various fields that could give us problems, magnetic and RF fields primarily. MRIs don't like metal, especially moving metal.

Quick story. The Italian engineers I worked with we're checking me out, this American with zero imaging experience!

We set up the equipment that measures magnetic fields and pointed it out the window. One of the Italians went out and drove his car past the window which caused a huge magnetic pulse. Makes sense, car moving etc etc. Then they drove the car up to the window and stopped. Same magnetic field and the car was standing still. I had no clue. Turns out it was the pistons moving, not the car. Car goes to slow.

We were at some ancient historical building that had been converted to a hospital, somewhere, Spain I think but no matter.

A train ran by the MRI room and bingo, every time! In this case, you ain't movin the train and you sure as **** ain't moving the hospital so....you wait.

Elevators in high rises and generators doing their weekly self test are also trouble makers. Always thought those generator self tests should happen after hours.

Then start talking about shielding for radioisotopes.....that's a whole new ball game.

These are the days of miracles and wonder!

 

[ultrarunner]

Yep... very interesting project as a large hole was cut in roof and the biggest crane hired to do the lift after all was prepped including bobcats excavating and concrete pour Saturday morning starting at 2 am

It's when I learned rebar did not have to be metal.

 

[user.69169]

Does lead lined room like Xray room shield MRI like it does Xray?

Rebar now available in fiberglass as well as Basalt. Fiberglass tough on hands Basalt stronger & lighter than steel by huge factor.
Fasten seatbelt & clear mouth of food when getting price quotes.
Heard number for Basalt in bridge deck instead of steel, cheaper to replace deck in 5 years than use Basalt.

 

[Streetcar]

Any downside to adding welded wire other than expense?

I've seen one or the other but never both...

We built a new imaging center 5 years ago and all the rebar was fiberglass... could not have any ferrous material in proximity to MRI machine... it was interesting just how involved and different the build was.

It is cheaper to use larger rebar rather than add wire mesh. Rebar in middle of slab is for crack control not strength.
Bricks or chucks of concrete can be used to position rebar with little or no downside

 

[Streetcar]

Any downside to adding welded wire other than expense?

I've seen one or the other but never both...

We built a new imaging center 5 years ago and all the rebar was fiberglass... could not have any ferrous material in proximity to MRI machine... it was interesting just how involved and different the build was.

It is cheaper to use larger rebar rather than add wire mesh.
Rebar in middle of slab is for crack control not strength.
Bricks or chucks of concrete can be used to position rebar with little or no downside
Trucks driving on subgrade can leave ruts

 

[Gee Ray]

What mix of concrete is he using? Rebar must be in place and the permit must be sign off here before any concrete is poured. You can not tie rebar in place correctly fast enough to keep the edge of your concrete alive. You will get what they call, "cold joints" that will crack over time. Tell the contractor to rent a pump. If not kick him off the job and find a crew that knows how to place concrete correctly.

 

[Streetcar]

Rebar for a slab is usually not continuous. There are gaps for expansion joints. The tied mat can be dropped in place immediately before placing concrete is placed. As rebar is for crack control ,most areas do not require it to be signed off for permit.

What mix of concrete is he using? Rebar must be in place and the permit must be sign off here before any concrete is poured. You can not tie rebar in place correctly fast enough to keep the edge of your concrete alive. You will get what they call, "cold joints" that will crack over time. Tell the contractor to rent a pump. If not kick him off the job and find a crew that knows how to place concrete correctly.

 

[Frankenkubota]

Does lead lined room like Xray room shield MRI like it does Xray?

Rebar now available in fiberglass as well as Basalt. Fiberglass tough on hands Basalt stronger & lighter than steel by huge factor.
Fasten seatbelt & clear mouth of food when getting price quotes.
Heard number for Basalt in bridge deck instead of steel, cheaper to replace deck in 5 years than use Basalt.

Re lead for MRI. Are you testing me? Honestly, I donk know if they can use lead to shield MRI machines. I doubt it as lead is not ferromagnetic. Our products were basically a cage built around the machine, made of aluminum, took everything to ground. Many shielding companies fill the walls with metal.

Interesting, MRI suites much be designed so that there are no clear unobstructed paths to magnet, hallways and the like. There are a number of stories about metal objects, gas tanks etc etc flying thru the air killing paitients.

Once I walked by a MRI machine with a hammer in my hand. All the sudden that hammer was stuck to the side of the machine, it took quite a pull to get it off, and the Chinese eng about wet himself. F him!

 

[ultrarunner]

Yep... magnet is powerful...

As things were wrapping up with signs everywhere the fire sprinkler guy went in to place trim rings on the heads...

Let’s just say it didn’t go well and this was after being told...

Wood Ladder flew out of his hands, the chrome over steel trim rings tore his shirt pocket as he went after his ladder and then his steel toe boots were next...

The ladder damaged a cover on the machine and it was not cheap...

Wheel Chair, Fire Extinguisher, etc... all have to be MRI suitable.