Will Concrete Vibrator Settle Down Slab to Level?

[zeuspaul]

I'll be using a pump mix so I assume it will be on the wet side. The concrete will be pumped for a 16 ft diameter x 12 inch thick slab for the foundation of a masonry water tank.

Will a vibrator settle the concrete to near level ( +/- one inch)? Or will I have to screed it to get an even/level floor?

Thanks

Zeuspaul

 

[Tim Stuart]

If it was me doing it I would screed it off. You need to be very careful using a concrete vibrator as it will bust your forms causing a total failure or blow out. If that happens you get to spend a little time on a jack hammer .

 

[zeuspaul]

I am trying to avoid forms. Three quarters of the excavation is into *rock solid dirt*. It is dug into the side of a slope. The concrete will be against a very hard DG which had to be dug with a jack hammer.

One quarter will have to be formed as it is backed by *softer dirt*. I may form this with concrete block. Thanks for the heads up. I'll have to make sure any form work is heavy duty.

Zeuspaul

 

[davitk]

Even if pumped the concrete will have some "slump", in other words the ability to hold a shape on it's own, and will not likely flow out to level. When using a floor leveling compound the mix consistency is that of a very melted milkshake; if concrete is that wet it will be weak when cured and may even blow out the forms. Build the forms level and screed it off, the mason building on your perfectly level foundation will thank you.

 

[firedog]

Definitely screed it. The only time I remember a vibrator being used was down in footings & bell bottom piers to help move the concrete into the pockets.

Haven't heard that word in a while (slump). Used to have a friend that was in inspection & testing. I always called him a "slump chump".

 

[Egon]

The answerer lies in the experience level of the crew. if the fellow dispensing the concrete is experienced the vibrator and floats will work well. If he isn't screeding may be advisable.

In any case do not over vibrate in an attempt to move concrete around. This just makes for segregation and weak spots.

 

[EddieWalker]

The vibrator is to remove air pockets in the concrete. To get it level, the forms are the most important part. I know you don't want to use them, but your only other option is to use pins, which is even harder to work with. Pound a pin into the ground so the top of it is at grade, then screed from the top of the pin.

The wetter you make the concrete, the less strength it will have. The dryer the mixture, the stronger it will be. If you order a 4,000 psi mix, but add a bunch of water to it to get it to flow better, you can easily cut the strength of the mix to half or less.

If you do a bad job of leveling it, you can always build it up with other materials. It's more important to have a strong mix and a solid base then anything else. I've had to use self leveling cement on allot of homes to put in tile. It's amazing how many have low spots, but what really kills me is the homes with high spots that I have to chisel down, and then smooth out with the self leveling cement.

Good luck,
Eddie

 

[dmccarty]

In any case do not over vibrate in an attempt to move concrete around. This just makes for segregation and weak spots.

Egon said what I was going to say but I'll say it a bit different. Egon used a big word.

Don't over vibrate. You want the aggregate aka stones distributed evenly through out the concrete. If you over vibrate the aggregate will move down i.e., that segregation word Egon used.

I think you will have to scree the concrete. Even then I would be surprised if it really is level. The masons did pretty good on our slab but there are some low and high spots. I watched them work and they did a good job but there are some very slight bumps/dips towards the middle of the slab. I'm sure most people would not see it but I do. Not a big deal though. They did a good job.

Later,
Dan

 

[HappyCPE]

Ditto on everyone else. Flatwork doesn't get vibrated; ony big footings and walls do. There's no substitute for backbreaking work in the concrete business!

 

[zeuspaul]

Screeding it is. I still want to avoid forms along the hard edge. I would like the extra width and like the idea of having the concrete butt up against the dirt. Perhaps I could build a screeding surface. I am thinking about mounting a 2x4 on top of an eight inch brick standing on end. Probably one every 16 inches. I could set the brick in a mortar base and then place the 2x4 on top of it. I could pre drill the 2x4 and then nail it into the hard DG and leave it there when I am done. The current excavation gives me about six inches outside the concrete block. I think I need that much beyond the block to help distribute the weight of the wall.

I don't know how to construct a screeding surface towards the center of the slab. I could drive a few pins (rebar?) as Eddie indicated towards the center so I am only dealing with an eight foot swath. It could be set slightly lower so there is drainage towards the center. I am a little concerned that a pin would eventually rust away and the floor would leak water. Perhaps I could pull the pins after I use them.

Thanks for the input

Zeuspaul