Rebar supports in a slab

[czechsonofagun]

i

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!

Very interesting. sir.

 

[user.69169]

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!

Nopee, just wonderin. Took a couple rides in dat contraption and it skert me some. Little dalin went down her check list on automatic, fellow come in and saked how I'd figured I collected so much metal over time. Bout 4 items down de list he sends me over to another machine to see if de magnet gonna pull me apart. Evidently lot of chips an such dissolved over time.

Seen lead walls and ceilings bein built, RF screen rooms de same, so I was wonderin.

First MRI machine I ever saw took up a pair of 48 foot drybox trailers set up so de machine could run. Sat beside hospital for few years magnitizin people while dey built expensive new MRI rooms.

 

[crashz]

I have used concrete bricks as chairs often. Nothing wrong with that and nothing to rust!

I also suggest a pump. It may cost you about $1500 extra, but as other posters mentioned, the rebar cannot be tied quickly enough to keep up with the pour. However, make sure the plant and your contractor know about the pump and order the concrete appropriately. The pump will cause the loss of any entrained air in the concrete, so more additive is typically needed. Slump needs to be adjusted with additives, not more water. Pump guy always wants a loose slump.

 

[fried1765]

I have used concrete bricks as chairs often. Nothing wrong with that and nothing to rust!

I also suggest a pump. It may cost you about $1500 extra, but as other posters mentioned, the rebar cannot be tied quickly enough to keep up with the pour. However, make sure the plant and your contractor know about the pump and order the concrete appropriately. The pump will cause the loss of any entrained air in the concrete, so more additive is typically needed. Slump needs to be adjusted with additives, not more water. Pump guy always wants a loose slump.

Yup.....I too see nothing wrong with using CONCRETE bricks.
Watch'em carefully......they will ALWAYS try to add more water!

 

[ovrszd]

If you add mesh over rebar the workers will be smashing your rebar to the bottom of the pour with every step. Not a good idea for what I want in the end. Which is the rebar suspended.

flexible chairs that recover are preferred. The rebar is going to get stepped on. Quality workers will try to avoid it. But it'll still happen.

Pumper for sure.

 

[EddieWalker]

Most people just position the rebar so it's in the middle area of the pad. For most applications, this is fine. For the best results, the rebar should be at the 1/3 point from the bottom. Think of concrete as a beam going across a span, and it wants to sag in the middle. Putting the rebar at the lower third point of that beam will give it the most support at holding it stiff and flat. Rebar holds the concrete together. It doesn't add an PSI to it.

Almost every pad is poured with too much water. The excess water will evaporate and leave a void behind where it used to take up space. You lose volume to our concrete when that excess water goes away.

Most concrete crews will add as much water as they can get away with because it makes it so much easier to spread the concrete. A good contractor will stop this, but most also want to get the job done as quick as possible, so they allow it to some degree. On commercial jobs, they test the concrete by putting some of it in a small pail and then flipping it upside down to make a sand castle. If the concrete comes out of the pail and remains in shape, you have the right amount of water. Most pails to do this are about a quart. Google "Slump Test" if you want to learn more.

To gain the most strength out of your concrete, you use the smallest amount of water. Adding Portland Cement to the mix will also increase your PSI. Making your pad thicker will increase PSI. After that, it's very hard to make noticeable gains for a reasonable amount of money.

Wire mesh works better then rebar at holding the concrete together. Unfortunately it is impossible to keep the wire mesh in the middle of the pour. Liars, I mean concrete contractors, will tell you that they lift it up while they are spreading the concrete and you are supposed to believe that it stays in the middle of the concrete while they walk on top of it. This also happens to rebar without chairs. Anybody pouring concrete without chairs is a hack and should be fired on the spot, unless it's such a small area that you can actually add the rebar or wire mesh to the concrete and never on any of it.

Most rebar is done on a 24 inch grid. Tied together and the chairs are installed close enough together that the rebar is flat. If you tie your rebar really well, and overlap each stick, you can go farther out with your chairs. I won't say what that distance is because I've never paid any attention to it. I just put the chairs under the rebar after it's all tied and decide on where they go one after the other.

I prefer rebar on a 18 inch grid. It's not a lot more rebar, and you can still walk through it easily while spreading the concrete, but I think adds enough to the pad to make it worthwhile. All concrete cracks, but if you have enough rebar in it, and you use the correct amount of water, there is no reason that you have to see those cracks!!!

 

[fried1765]

Most people just position the rebar so it's in the middle area of the pad. For most applications, this is fine. For the best results, the rebar should be at the 1/3 point from the bottom. Think of concrete as a beam going across a span, and it wants to sag in the middle. Putting the rebar at the lower third point of that beam will give it the most support at holding it stiff and flat. Rebar holds the concrete together. It doesn't add an PSI to it.

Almost every pad is poured with too much water. The excess water will evaporate and leave a void behind where it used to take up space. You lose volume to our concrete when that excess water goes away.

Most concrete crews will add as much water as they can get away with because it makes it so much easier to spread the concrete. A good contractor will stop this, but most also want to get the job done as quick as possible, so they allow it to some degree. On commercial jobs, they test the concrete by putting some of it in a small pail and then flipping it upside down to make a sand castle. If the concrete comes out of the pail and remains in shape, you have the right amount of water. Most pails to do this are about a quart. Google "Slump Test" if you want to learn more.

To gain the most strength out of your concrete, you use the smallest amount of water. Adding Portland Cement to the mix will also increase your PSI. Making your pad thicker will increase PSI. After that, it's very hard to make noticeable gains for a reasonable amount of money.

Wire mesh works better then rebar at holding the concrete together. Unfortunately it is impossible to keep the wire mesh in the middle of the pour. Liars, I mean concrete contractors, will tell you that they lift it up while they are spreading the concrete and you are supposed to believe that it stays in the middle of the concrete while they walk on top of it. This also happens to rebar without chairs. Anybody pouring concrete without chairs is a hack and should be fired on the spot, unless it's such a small area that you can actually add the rebar or wire mesh to the concrete and never on any of it.

Most rebar is done on a 24 inch grid. Tied together and the chairs are installed close enough together that the rebar is flat. If you tie your rebar really well, and overlap each stick, you can go farther out with your chairs. I won't say what that distance is because I've never paid any attention to it. I just put the chairs under the rebar after it's all tied and decide on where they go one after the other.

I prefer rebar on a 18 inch grid. It's not a lot more rebar, and you can still walk through it easily while spreading the concrete, but I think adds enough to the pad to make it worthwhile. All concrete cracks, but if you have enough rebar in it, and you use the correct amount of water, there is no reason that you have to see those cracks!!!

The proper way to test concrete slump is with a 12 inch "slump cone".

 

[Macinnis]

Was reading an interesting article the other day that basically said that while going from a 4” slab to a 5” slab, while increasing your costs by 20%, adds 50% strength.

 

[/pine]

I think the bigger problem with wire mesh is not that it gets walked on and pushed down...it's that it does not get pulled up in the first place...it just gets forgotten in what can be a hectic job for the time it takes to make a pour...

If you take a section of wire mesh and cover it with gravel a couple of inches thick (something like crusher run)...then pull the mesh up through the gravel...you can't push it back down under the gravel even by stomping on it...sometimes the mesh will get pulled up too high and the concrete has to be dug out under it to get it below where a finishing machine will hit it...

As for a slump test look for the 'ASTM' testing method...it is what is required by most engineers etc...

 

[rademamj1]

Texas is expanding I35. They have 36" compacted fill, followed by 5" asphalt layer, then 14" thick concrete slab with two layers of 1" rebar at 18" grid. The chairs are stackable, holding rebar at 5" and 9" within the 14" thick slab.