Concrete vibrator?

[shanesmith80]

Am pouring a small (1700lx 1600w x 180d) slab for a pizza oven in the backyard.

I've read that you should vibrate the concrete slab to help remove air pockets. How much does it need to be vibrated? I've cobbled together a rough vibrator like I read on line of a piece of rebar attached to a drill and inserted in a piece of pipe. It certainly vibrates...
Would this be sufficient?
I'll be using it for both the slab and for core filing the besser block stand.

Cheers
Shane

[BobL]

Does it have any reinforcing?
If yes, then for a pizza oven, I wouldn't bother with a vibrator
If not, I would just poke the concrete up and down with a spade.

[NCArcher]

I wouldn't bother with the slab but it might be useful for the core fill.

[aldav]

I wouldn't bother with the slab but it might be useful for the core fill.


He's asking about vibrating wet concrete not consolidating loose material with a vibrating plate. Unless the ground has been recently filled you shouldn't need the vibrating plate for your purposes and since you've made a concrete vibrator of sorts you might as well use it. As stated by others vibrating the concrete to consolidate it shouldn't be critical. If you're going to have an exposed concrete edge consolidation in those areas would make the finish more visually appealing.

[chambezio]

Vibrating concrete in the core of the blocks can, if you get over zealous could easily break the blocks. I would be using a spare bit of re-bar to poke up and down to consolidate. It is very easy to over vibrate concrete and in so doing can segregate the gravel from the fines

[FenceFurniture]

I wouldn't worry about vibrating. For the core, just use a piece of reo to stir it up. You could then just leave the reo in the core if you want it cyclone proof :D

That's not quite as silly as it sounds because I reckon the masonry blocks may well crack with the heat, and the reo would help hold it all together in that case (rather than pieces potentially splitting off). Back in the day, I used proper firebricks in a fireplace I had built, and even they cracked.

You only have to look at a masonry block severely and it will crack.

[ian]

just to clarify

Vibrating concrete is mostly about removing air bubbles from the mix. Poorly vibrated concrete is typically evidenced by voids where the concrete was in contact with the form work.
For a domestic on ground slab, vibrating is really optional. Especially where the edge of the slab will be back filled and hence hidden.

Although in theory it is possible to over vibrate concrete, it's a really hard thing to do, especially where the mix contains a reasonable amount of cement and fly ash. When we were building Pennant Hills Road, we could vibrate the slabs to the extent that you could walk on the fresh concrete (30 mins out of the mixer) and hardly leave any foot prints.

[NCArcher]

He's asking about vibrating wet concrete not consolidating loose material with a vibrating plate. Unless the ground has been recently filled you shouldn't need the vibrating plate for your purposes and since you've made a concrete vibrator of sorts you might as well use it.
Who said anything about a vibrating plate? The slab is 180mm thick so doesn't need vibrating as air pockets are not likely to develop. Pouring concrete into besser blocks has the potential to develop air pockets that won't be filled. It's not however supporting any great weight and won't be very high so probably not a problem but when core filling besser block walls, for instance in substation constructions that I have managed, the concrete is vibrated to remove air pockets. I am assuming Shane is hand mixing(or holding a GTG and getting us to hand mix :D ) so there will be a limited amount of concrete available at one time. Air pockets will most likely not be a concern. I am looking forward to a pizza out of the new oven. (hint hint :U )

[aldav]

Yep, I got the wrong end of the stick there. :B Now Dave, read and understand, if you think you understand you're probably mistaken! :doh:
If you're in any doubt NCArcher, yes that is an apology.

[Bushmiller]

Shane

We are talking about a very small slab here with relatively little weight above it so I agree with the comments that vibrating the concrete is probably unnecessary. It is only the edges that will appear "pock marked" and as others have mentioned the edge will probably be ultimately covered by earth or some other fill.

180mm deep is very thick for any slab let alone for a pizza oven. A house slab has edge thickening: Say 300mm deep and 200mm wide. This varies depending on soil tests and which state you are in....I don't mean sober or inebriated :rolleyes:

For your purposes a 100mm thick slab would be ample, but any slab like this needs some rio. Concrete has excellent compressive strength, but poor tensile strength: This is where the rio plays it's part.

To make the edges "pretty" your improvised concrete vibrator will probably work well enough. The most simple method of all is to tap the formwork with a large claw or ball pein hammer. Lots of moderate taps all the way around are best. Another easy way is to use an old electric sander: The rectangular type not the the ROS. Find the worst one you can (frequently the cheapest if you haven't already got one and one that makes your hand go numb after a few minutes) and sand the outside of the formwork after pouring the concrete. We used to use this method when making large pavers. We had the pavers up on a platform with the sander fixed by screws through the body of the sander underneath.

All the concreting advice is to not overdo the vibrating and certainly with the dedicated concrete vibrators about five seconds in each spot about a meter apart is ample. I doubt you can overdo it with your improvised tool, a hammer or the electric sander. For the pavers we used to do about ten seconds with the sander in just the middle.

I have always thought a pizza oven would be a very good project. I hope you will be posting the results here as I am most interested to see how you do it and the outcome.

Regards
Paul

[ian]

Just to confuse the issue, thick lightly loaded slabs smaller than about 3 m x 3 m don't really need reo.
Proper curing and keeping the water:cement ratio low (<0.5) are the key. A low shrinkage cement helps, but is not as critical as minimising the quantity of water in the mix.
I don't remember the whole spec, but the mix we used for the unreinforced slabs on Pennant Hills Road was something like 300 kg low shrinkage cement, 150 kg fly ash and 140 litres of water per cubic metre.

[shanesmith80]

Hmmm I typed up a reply to this last night but seems I forgot to hit submit. Dumb*#s. So here goes again.

Thanks all all for the advice. Will decide on the day whether or not to vibrate. Depend on energy levels.

Reinforcment eh. You could say it's got reinforcements, or or could say over engineered.

My basic plan is coming from way to much research on the matter. Which is why the slab is as thick as it is, that and I dug a little to deep. My goal was for a 150mm slab but it's about 180mm, close enough.

Next question..
My plan was to just stack the blocks, maybe with a little adhesive of some sorts and core fill. Providing my slab turns out level is this acceptable or should I put a bit of mortar between. I see the mortar way leading to the possibility of unevenness with my lack of proven bricklaying skill.

And another?
With core filling, was looking at just doing every second(ish) with concrete, and yes they will have rebar as well. And the others with the huge amount of road base that I have left over.

My my computer busted at the moment so can't get photos of camera, but will try post some progress picks. And definitely plan on a pizza GTG one day. Will rely on FF for some fresh produce.

Cheers
shane

[FenceFurniture]

My plan was to just stack the blocks, maybe with a little adhesive of some sorts and core fill.

With core filling, was looking at just doing every second(ish) with concrete, and yes they will have rebar as well. And the others with the huge amount of road base that I have left over. Dunno about glue/adhesive and roadbase - anything you use has to be able to tolerate the heat and not give off fumes. Doesn't roadbase have some bitumen or similar in it (I'm not sure)?

They'd probably be ok without mortar but I'd be inclined to core fill all of them, rather than every second one. If you are staggering the blocks then they should be locked together enough via the corefill.

It is, after all, only an oven. Did you take my Cyclone proofing to heart? :D

[NCArcher]

I think the core filled blocks are only supporting the oven. I'm picturing a slab on the ground, core filled blocks supporting the oven base and oven constructed from solid/fire bricks on that. For glued, core filled blocks check out Aerohydro's thread.
http://www.woodworkforums.com/f245/concrete-block-shed-217206

[Chris Parks]

Shane, I can't think of any reason to core fill the blocks, that is only required for structural integrity in buildings, retaining walls etc.

[shanesmith80]

After talking to a plumber mate, I've been advised best to go with some mortar to hold together. Then probably not necessary to core fill, which will help save a few bucks.

NcArcher is right. Concrete slab with besser block base. Then a suspended slab. Then tiles/ bricks for oven base, then dome oven.

Form work and reo in ready for concrete turning up tomorrow. Hope the wheelbarrow holds up, had to do some last minute maintenance today. Wheel not spinning real well. 🤞

[rwbuild]

Put a couple of chairs under the mesh in the middle so the concrete does't push it to the bottom. There being no plastic under the slab will make it go off very quickly and will cause shrinkage cracks and you will also have a sweaty slab inside when its closed up. Put plastic under the slab before you pour and if you don't get the driver to add a touch more water than normal as well as wetting the ground (not sloppy, just wet to about 30mm penetration) and you will have to definitely vibrate it which will help with making it more waterproof but more importantly will help with reaching the correct design strength when cured. Steel trowel finish.

[Bushmiller]

My memory is that concrete achieves 75% of it's strength in 28 days but continues to increase in strength for a long time after that. The trick is to slow down the curing time of the concrete. This is achieved by wetting down the concrete once the initial troweling has been finished (starting the next morning). Keep it wet and if possible covered too. Just a sheet of plastic or a tarp (for a slab this size) will do. The slower it cures, the greater the ultimate strength. In this manner you will be able to achieve far in excess of the nominal 20MPa you ordered.

Regards
Paul

[rwbuild]

How did your concrete pour go today?

[shanesmith80]

The pour went well. Managed to offload 12 barrow loads in 10 minutes. Thanks rwbuild for the tips. I ended up putting a few more chairs in, after all I've got about 100. I considered putting plastic down, but in the end didn't. not sure how the ad hoc vibrator went, only time will tell. Kept it well hydrated yesterday and is covered up with some black plastic today. Probably leave it like that and in the form till Tuesday/Wednesday next week. I do have a photo but it's on the camera and the computer is busted and takes too long on the iPad. I of course did put our names in it.

Thanks again ain for all the tips. You have all earns a pizza.
Cheers
shane