Rebar supports in a slab

[EddieWalker]

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

Is there a difference between a "12 inch slump cone" and a 1 quart pail, for a home owner to use when checking their concrete?

 

[fried1765]

Is there a difference between a "12 inch slump cone" and a 1 quart pail, for a home owner to use when checking their concrete?

Might not matter for homeowner use, but the approved method of checking concrete slump is by using a slump cone.
Do you cut the bottom out of the very small 1 qt. pail, fasten it inverted to something flat, fill it, and then lift it up?
I do not quite understand how using a 1 qt. pail could provide accurate results.

 

[EddieWalker]

Might not matter for homeowner use, but the approved method of checking concrete slump is by using a slump cone.
Do you cut the bottom out of the very small 1 qt. pail, fasten it inverted to something flat, fill it, and then lift it up?
I do not quite understand how using a 1 qt. pail could provide accurate results.

If accurate results where required, then using a pail would be wrong. If you are new to concrete, and you do not know how much water should be added to the mix, or if they are lying to you when they say that they need more water, then scooping it up with a one quart pail and seeing if it will stand up is a very good method to shut a contractor up and stop them from adding more water. Granted, this wont work on commercial jobs, or even residential if somebody is inspecting the concrete, but hopefully if that's happening, they will bring their own pail.

 

[goeduck]

I wish Eddie did business out West. Professional fer sure.

 

[rademamj1]

If accurate results where required, then using a pail would be wrong. If you are new to concrete, and you do not know how much water should be added to the mix, or if they are lying to you when they say that they need more water, then scooping it up with a one quart pail and seeing if it will stand up is a very good method to shut a contractor up and stop them from adding more water. Granted, this wont work on commercial jobs, or even residential if somebody is inspecting the concrete, but hopefully if that's happening, they will bring their own pail.

I can see the pail method working just fine, when in a hurry to evaluate water content of the mix. From my experience, most concrete contractors don't do any slump checks at all, and just accept the load from their favorite mix supplier regardless of issues. This good Ole boy network prevales for mainly residential customers and small businesses. Larger businesses and government offices, always demand slump checks and demand that proper forms documenting the slump be filled out on location.


https://www.amazon.com/dp/B00C0OGZAS/ref=cm_sw_r_cp_apa_fabc_B9XF1WW8HXF0ND6VTRDN

 

[Gee Ray]

A slump test is a simple thing to do but there is a prosses to follow. Where in the load to take the samples. How to mix the samples together. How to load the sample into the cone, in three lifts. How many times to "rod" each lift. How many seconds it takes to lift the cone. Then measure the "fall" of the cone. The cone is 12 inches high and you measure how much the cone falls gives you the slump. It is not throw a shovel full of mud into a bucket and turn it over.

In the company I worked for I was often called to a job site to test the slump before it was placed. The contractor wants a high slump to make the placement easier but the owner wants a low slump to make the concrete stronger.

You also need to check on the strength of the concrete mix. The plants that I worked in had several different mixes for example there were at least 4 different mixes for 3000 psi concrete. One was for residential use, one for commercial use, one for a pump mix and one for the DOT road mix. The mixes all made the 3000 psi but some took 18 days, 4-5 days, and some in hours. There were also job specific mixes where the contractor wanted their own formula for the concrete.

 

[jcbGM]

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.

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!!!

This can not be overstated. Putting the rebar or wire in the middle of the slab has MINIMAL effect on slab strength. The only reason steel is incorporated into concrete is to add tensile strength. Concrete has tremendous compressive strength. It's primary characteristic is compressive strength. Therefore it's classification. A 3000 psi mix will withstand 3000 psi of compressive force. Tensile strength of concrete is negligible. It's easy to pull a test sample of concrete apart.

Steel on the other hand has great compressive and tensile strength with tensile being the lower of the two. It's classified by it's yield (tested with tension) strength. A36 steel can withstand 36,000 pounds of tensile force before it yields.

As Eddie says, think of the slab as a beam, or a pencil with a load in the middle causing sag or trying to break the pencil. The stress profile of the beam at the center would show the top of the beam under compressive load while the bottom surface is under tensile load. The stress profile between the tow points is linear with the center of the beam under no stress! The center is balanced between the compressed top surface and the stretched (tensile) bottom surface. The steel wouldn't add significant tensile strength to the slab until after the bottom surface has cracked and failed under tensile load shifting then stress profile up through the slab. The bottom 1/3 is a sweet spot. Too low in the slab and adhesion between the concrete and steel becomes an issue.

There's a reason why bridges and structural steel beams are "I" beams. That profile provides the most strength with the lowest weight per foot. The wide top flange handles the compressive forces, the wide bottom flange handles the tensile forces, and the ultra skinny web that connects the two is compatible neutral with regards to tension and compression. Box sections are stronger, but not to the order of magnitude that requires additional design to support the significant weight increase of a box over and "I" and "H"

PLEASE stop putting steel in the middle of a slab. 2 chairs are for 6 slabs. 2.5 chairs are for 8 slabs. Use wire for 4 sidewalks.

As for cracks, you can't stop them. Saw cuts will let you determine where they form instead of nature deciding.

 

[Frankenkubota]

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!!!

yes yes yes regarding stepping on the wire mesh. i saw my guys pulling up the wire with rakes while standing on the wire at the same time. seemed like a waste of time and money.

 

[fried1765]

yes yes yes regarding stepping on the wire mesh. i saw my guys pulling up the wire with rakes while standing on the wire at the same time. seemed like a waste of time and money.

Actually, if the wire is pulled up frequently, across the pour, wire pulling is generally an effective procedure.
The wire should be in the bottom 1/3 of the pour depth, and properly sized aggregate in the concrete will prevent the wire from being forced to the bottom.
It is best to have one man singularly focused on pulling the wire up.
Most concrete contractors are not willing to pay to dedicate a man for that!
Using shovels with notches cut in, are useful tools for pulling the wire, as are simple home made rebar hooks.

 

[ovrszd]

It is impossible to reach out far enough to pull mesh up while you are standing on it effectively. Maybe with a long tool. But then, if you reach out 6-8 feet from where you are standing to lift the steel can you effectively lift it? I doubt it. To prove this, unroll some mesh and then stand on it. See how far away you have to reach and lift to get the wire to even make a small attempt at staying elevated.....

Any pour that's too large to do without standing in it should not use mesh.