Concrete rebar

[Steve]

How much rebar does suspended concrete need? I'm building a pizza oven platform. It has double brick walls supporting it on 3 sides in a U shape. I've built formwork so that the concrete will be in contact with the top of the brick walls and the formwork will break away from the bottom and sides. The outer edge of the walls is a metre wide, leaving about 60cm of unsupported concrete between the two side walls. The concrete will be 150mm thick.

I've got hold of 5 pieces of 8mm rebar 1m long. I was planning to place these 4 across and one in the middle front to back. But I've laid them out and it doesn't feel like enough. Do I need more? The pizza oven weighs about 600kg.

Also approx how many bags of all in ballast should I get for 0.15m³? I've got 6 so far, going to b&q tomorrow.

Cheers.

 

[i_am_fubar]

So is the base of the Pizza oven, or is this a platform for a free-standing one to sit on? how big will the load be dimensionally?

If it's 600kg point load in the centre, I would be wary of needing a bit more rebar. if 90% of the load will be directly above the walls that the concrete sits on, you could are probably going over-kill (which I always approve of if money isn't an issue).

do you have a sketch of the final thing?

Your main weak spot will be the un-supported edge span, you'd probably be better off doubling up rebar on that edge than having the one at 90 degrees. then add some 50mm mesh in there to prevent splitting between the bars.

with regard to quantities, this should be a good indicator:
https://www.calculator.net/concrete...t=centimeter&slabquantity=1&slabcal=Calculate

 

[Steve]

So is the base of the Pizza oven, or is this a platform for a free-standing one to sit on? how big will the load be dimensionally?

If it's 600kg point load in the centre, I would be wary of needing a bit more rebar. if 90% of the load will be directly above the walls that the concrete sits on, you could are probably going over-kill (which I always approve of if money isn't an issue).

do you have a sketch of the final thing?

Your main weak spot will be the un-supported edge span, you'd probably be better off doubling up rebar on that edge than having the one at 90 degrees. then add some 50mm mesh in there to prevent splitting between the bars.

with regard to quantities, this should be a good indicator:
https://www.calculator.net/concrete...t=centimeter&slabquantity=1&slabcal=Calculate

Thank you for your reply. I have since ordered more rebar and bought more ballast. B&q has no cement though.

I've attached a picture of the formwork I've made. This is just a plinth for the oven to sit on. The oven itself is coming in one piece and will have a bed of mortar under it so it'll be a spread load, nearly the full area of the plinth. I understand the oven base to be insulated so heat isn't an issue.

I have sealed the gaps between the brickwork and the formwork.

 

[i_am_fubar]

Oh, right, yeaaaaaaa. You'll be fine. In fact, I imagine it's last a damn long time with just a piece at the front. By the time you've built that up to 150mm and load has spread out at 45 degrees, you'll only have a 30cm unsupported span.

But if you're actually going for the secret entrance to a bunker.... then bend the ends and thread through the bricks a little. Also, don't forget to wire the rebar together.