Hello everyone,
this is not directly related to house construction but seems to have a similar structural approach.
I am building a pool in the garden and have two utility shafts here that I want to cover with a concrete slab, including an opening to later climb down with a ladder.
The walls are made from 24cm (9.5 inches) concrete formwork blocks.
Dimensions of shaft 1 (pool equipment)
1.5m x 2m (5 feet x 6.5 feet)
Dimensions of shaft 2 (water tank)
2m x 2m (6.5 feet x 6.5 feet)
For the access opening, I was thinking of making it the size of a manhole cover, so I can later use a suitable cover for it.
Now the question is, what is the best way to create a concrete slab on top? I have already searched online but only found information about how slabs for houses must be built or references to precast elements.
On the slab of the first shaft, a terrace made of wooden decking will be built. On the slab of the second shaft, soil will be placed later to fully plant the surface.
My specific questions are:
- How should the formwork for this be constructed?
- What kind of reinforcement is required? (For example, is a simple steel reinforcement mesh sufficient?)
- What is the minimum thickness for the slab?
I look forward to your help and advice.
Thank you.
this is not directly related to house construction but seems to have a similar structural approach.
I am building a pool in the garden and have two utility shafts here that I want to cover with a concrete slab, including an opening to later climb down with a ladder.
The walls are made from 24cm (9.5 inches) concrete formwork blocks.
Dimensions of shaft 1 (pool equipment)
1.5m x 2m (5 feet x 6.5 feet)
Dimensions of shaft 2 (water tank)
2m x 2m (6.5 feet x 6.5 feet)
For the access opening, I was thinking of making it the size of a manhole cover, so I can later use a suitable cover for it.
Now the question is, what is the best way to create a concrete slab on top? I have already searched online but only found information about how slabs for houses must be built or references to precast elements.
On the slab of the first shaft, a terrace made of wooden decking will be built. On the slab of the second shaft, soil will be placed later to fully plant the surface.
My specific questions are:
- How should the formwork for this be constructed?
- What kind of reinforcement is required? (For example, is a simple steel reinforcement mesh sufficient?)
- What is the minimum thickness for the slab?
I look forward to your help and advice.
Thank you.
Hello abc
I would do it like this: Insert vertical #12 rebar every 30cm (12 inches) into the concrete blocks, 15 to 20cm (6 to 8 inches) deep, with 10cm (4 inches) protruding above. About 7 to 8cm (3 inches) above the blocks, place a #12 rebar connecting all the vertical bars along the entire length. Formwork should be installed below and on the sides. Then, place #12 rebar crosswise every 20 to 30cm (8 to 12 inches) along the entire length. Use spacers on top and lay a cut-to-size steel reinforcement mesh over it. Make sure everything is well tied together. (If electrical wiring needs to be installed, now is the right time.)
Then, with 2 or 3 friends, mix concrete (1:3.5) and pour at least a 20cm (8 inch) thick slab. That should hold.
As I said, that’s how I would do it (and have done it before). But with work like this, everyone is responsible for themselves.
Structural engineers do the same thing, just in a more precise way.
And of course, the concrete mix should be well prepared. Don’t switch between 1:4 and 1:3 ratios. The mixer really needs to concentrate.
Steven
I would do it like this: Insert vertical #12 rebar every 30cm (12 inches) into the concrete blocks, 15 to 20cm (6 to 8 inches) deep, with 10cm (4 inches) protruding above. About 7 to 8cm (3 inches) above the blocks, place a #12 rebar connecting all the vertical bars along the entire length. Formwork should be installed below and on the sides. Then, place #12 rebar crosswise every 20 to 30cm (8 to 12 inches) along the entire length. Use spacers on top and lay a cut-to-size steel reinforcement mesh over it. Make sure everything is well tied together. (If electrical wiring needs to be installed, now is the right time.)
Then, with 2 or 3 friends, mix concrete (1:3.5) and pour at least a 20cm (8 inch) thick slab. That should hold.
As I said, that’s how I would do it (and have done it before). But with work like this, everyone is responsible for themselves.
Structural engineers do the same thing, just in a more precise way.
And of course, the concrete mix should be well prepared. Don’t switch between 1:4 and 1:3 ratios. The mixer really needs to concentrate.
Steven
Hello
Sorry, I just noticed that the ceiling I specified is not quite right.
So: the first area is 2 x 1.5 meters (6.6 x 4.9 feet).
The second is 2 x 2 meters (6.6 x 6.6 feet).
And each needs an opening for a drain cover?
The reinforcing steel should be bent, 5 cm (2 inches) from the edge of the drain cover. Otherwise: more is better. Put in good reinforcement and possibly 30 cm (12 inches) of concrete. Make the concrete layer nice and thin and compact it thoroughly.
Steven
Sorry, I just noticed that the ceiling I specified is not quite right.
So: the first area is 2 x 1.5 meters (6.6 x 4.9 feet).
The second is 2 x 2 meters (6.6 x 6.6 feet).
And each needs an opening for a drain cover?
The reinforcing steel should be bent, 5 cm (2 inches) from the edge of the drain cover. Otherwise: more is better. Put in good reinforcement and possibly 30 cm (12 inches) of concrete. Make the concrete layer nice and thin and compact it thoroughly.
Steven
Hello Steven,
thanks for your response. Since the concrete formwork blocks will be fully concreted, I can do everything in one go. I have 6-meter (20-foot) long 14mm (0.55 inch) reinforcing bars here. I will bend them accordingly and then twist everything directly into the formwork blocks.
I will have the concrete for filling the formwork blocks delivered ready-mixed, meaning it only needs to be placed on site. There are no differences in the mixing ratio then. For vibrating the ceiling, I will get a needle vibrator to ensure all the air is properly removed.
In this way, the ceiling and the blocks are essentially cast together... this should provide even more stability since it forms a composite structure.
Thank you very much for your tips.
thanks for your response. Since the concrete formwork blocks will be fully concreted, I can do everything in one go. I have 6-meter (20-foot) long 14mm (0.55 inch) reinforcing bars here. I will bend them accordingly and then twist everything directly into the formwork blocks.
I will have the concrete for filling the formwork blocks delivered ready-mixed, meaning it only needs to be placed on site. There are no differences in the mixing ratio then. For vibrating the ceiling, I will get a needle vibrator to ensure all the air is properly removed.
In this way, the ceiling and the blocks are essentially cast together... this should provide even more stability since it forms a composite structure.
Thank you very much for your tips.
abc12345 schrieb:
can I do all of this in one go then. Hello abc
Don’t imagine it to be that simple.
How high will the concrete blocks be stacked? The cavities inside the blocks are somewhat complex due to the cross bridges. And there is a lot of rebar inside as well. You need to make sure the concrete reaches all the way down. Air pockets would be really bad.
A plasticizer in the ready-mix concrete is essential.
How will the concrete be delivered? Pumped directly in? That works. But parking somewhere and then transporting it with a wheelbarrow—that I don’t see working.
How many cubic meters of concrete will you need? If it’s no more than 2 or 3 m³ (2.6 or 3.9 yd³), you can do it yourself with 2 or 3 friends on a Saturday.
You also need to secure the concrete blocks well. They are only laid horizontally. They tend to shift while pouring, which is not good.
If you mix the concrete yourself, I would suggest building the walls and the ceiling on two separate days. All the formwork and reinforcing is very complex. Once the walls are stable, casting the ceiling is easier.
Steven
I already have the #14 rebar here for reinforcing the walls. I bend it using a vise. Thanks to the physics of leverage, it works quite well.
The formwork blocks will reach a final height of 2m (6.6 ft), but they are poured with concrete at a height of 1m (3.3 ft) as an intermediate step to prevent shifting. Unfortunately, you can’t use a vibration needle inside the blocks, so it’s necessary to poke and tamp the concrete manually. The blocks are filled by hand, not with a pump. This means ordering ready-mix concrete with a retardant and then shoveling it in with a few friends on a Saturday. One person is always by the blocks, poking and tamping the concrete inside. In total, about 11m³ (390 ft³) of concrete is needed for the blocks, which means roughly 5.5m³ (195 ft³) per pour. Hand mixing that much concrete is out of the question for me—I don’t want to put myself or my friends through that. This way, the quality remains consistent.
I used a pump for the slab with 14m³ (494 ft³) of concrete, which cost me €450... two deliveries would be €900, money I’d rather spend elsewhere.
The formwork blocks will reach a final height of 2m (6.6 ft), but they are poured with concrete at a height of 1m (3.3 ft) as an intermediate step to prevent shifting. Unfortunately, you can’t use a vibration needle inside the blocks, so it’s necessary to poke and tamp the concrete manually. The blocks are filled by hand, not with a pump. This means ordering ready-mix concrete with a retardant and then shoveling it in with a few friends on a Saturday. One person is always by the blocks, poking and tamping the concrete inside. In total, about 11m³ (390 ft³) of concrete is needed for the blocks, which means roughly 5.5m³ (195 ft³) per pour. Hand mixing that much concrete is out of the question for me—I don’t want to put myself or my friends through that. This way, the quality remains consistent.
I used a pump for the slab with 14m³ (494 ft³) of concrete, which cost me €450... two deliveries would be €900, money I’d rather spend elsewhere.
Similar topics