Reinforced Concrete Design

[oz93666]

I am thinking of making some reinforced concrete posts for use as fencing , but also frame elements for a geodesic dome , the faces will be mosquito net .

 

Since I have a lot of 4 mm high tensile steel already,   I plan to use this. Although only 4 mm dia  it is spring like ,  sold in a 1 meter diameter coil , when released it springs out strait , and is much stronger than normal re bar ... thin walled low quality rigid plastic water pipe is very cheap so I plan to use this ... "2 inch" (in reality it has a 55mm internal diameter ) ... 

 

I will have 16 lengths of the 4mm steel wire  in each drainpipe tight to the circumference on the inside of the pipe and arranged symmetrically all around the circumference , the plastic pipe will not be removed after concrete sets and this will protect from frost /water damage , this allows me to keep the reinforcement tight to the surface for maximum strength in finished posts .. I will vibrate well , and apply vacuum  to the top of pipe after concrete is poured , to get air bubbles out.

 

I have two questions ...

 

1 ... I understand it is customary to have  hoops of wire going around the 16 lengths of steel and tied to them , I think this maybe mainly to hold these in the correct alignment, but will also give a strength component preventing failure of the finished post ... Do I really need these for strength ? I can secure the steel wire by other means , if I do need them , how many , how frequently ...

 

2... It maybe possible to pre stress  the 16 wires , what is the approximate % increase in strength of finished product from doing this ???

 

Thanks in advance ...oz

[Stewpot]

You don't say much about the geo.dome - is it for residential purposes? How big is it? Does it have to pass any building control regulations?

 

I haven't really much to contribute, but I thought geo.domes were intended to be made with lightweight structural elements - heavy frame elements at the apex of the dome and it might require some sort of additional vertical support. I would guess that your idea is going to end up with some pretty heavy frame elements, particularly if they are long enough to need steel reinforcement. If the frame elements are short, then I would have thought the plastic pipe would have added significantly to the integrity of the concrete.

 

If it was me, I think I would be looking at alternative fillers - maybe PU foam - to create lightweight frame elements with your drainpipes. Some trial pieces first would be a good idea, testing them to destruction.

[oz93666]

10 hours ago, Stewpot said:

You don't say much about the geo.dome - is it for residential purposes? How big is it? Does it have to pass any building control regulations?

 

I haven't really much to contribute, but I thought geo.domes were intended to be made with lightweight structural elements - heavy frame elements at the apex of the dome and it might require some sort of additional vertical support. I would guess that your idea is going to end up with some pretty heavy frame elements, particularly if they are long enough to need steel reinforcement. If the frame elements are short, then I would have thought the plastic pipe would have added significantly to the integrity of the concrete.

 

If it was me, I think I would be looking at alternative fillers - maybe PU foam - to create lightweight frame elements with your drainpipes. Some trial pieces first would be a good idea, testing them to destruction.

 

No building regs where I am Stewpot ... I'm a Brit ex-pat living in Thailand ... 

 

For the dome the struts would be about 2.5 meters length ... this design  , an outside hang out /growing area free from mosquitoes and veggie eating bugs 

 

This plan for the struts seemed best , I have lots of this wire , just getting rusty , concrete costs nothing , and the plastic pipe will make a nice surface to attach the mosquito net to.

Edited December 15, 2019 by oz93666

[Russell griffiths]

You need the reinforcement to be completely covered in concrete, fixing them to the smooth plastic will not allow concrete to in case the steel, so rendering its reinforcement properties to nil. 

The steel should be spaced away from the sides to allow at least 15mm of concrete cover. 

You might be better of winding this steel together to create a wire rope, and then positioning this central in the tube. 

[Stewpot]

I'm trying to work out the forces in the frame elements of a geo.dome. The spokes of the topmost hexagon will all be in compression. The rim of that hexagon will be in tension, constraining the compression forces of the above. But my brain can't get further than that at this time on a Sunday morning.

 

Intuitively, I feel that your main problems are going to be as a result of the weight of frame elements that are made of concrete. Consider the apex: the weight of all that concrete is trying to move vertically downwards; the angle of the apex elements is small (to the horizontal), so the tension in the restraining rim of that hexagon will be high, and you will have very little scope for movement (free play in the joints, lack of rigidity, error in manufacture, etc.) before the apex collapses.

 

Have you considered that the rainwater pipe alone may be adequate for the job? Things would be so much lighter that problems like that due to weight would be practically eliminated. Why not build the top hexagon out of empty pipe, and see how it is? If it seems strong enough, complete the build. If it's not, you can still try out your concrete plan.

[Temp]

I don't think I've ever seen reinforced concrete at such a small diameter. What size aggregate do you propose to use? Think it would need to be pretty  a small to fit between the bars?

 

Even if you can get the elements made how do you propose to join them? Weld the ends of the exposed rods? 

 

I think I would just use wood like the image you posted.

 

 

[oz93666]

13 hours ago, Russell griffiths said:

You need the reinforcement to be completely covered in concrete, fixing them to the smooth plastic will not allow concrete to in case the steel, so rendering its reinforcement properties to nil. 

The steel should be spaced away from the sides to allow at least 15mm of concrete cover. 

 

 

I beg to differ on this ...My understanding is this 15mm is to protect from damp and frost ... I cannot see what strong forces would be acting to push the wires out of alignment...

 

If you think it would fail because of not enough cover in what way?? 

 

Consider bamboo , it's a similar arrangement , all the fibers are aligned along it's length , the way it can be split so easily shows there's nothing going around the circumference , there's nothing much keeping the long fibers in place , if it had a solid core it would be much stronger ....

 

I have experimented with 40mm tube and 6 wires the result was very strong  

 

13 hours ago, Stewpot said:

I'm trying to work out the forces in the frame elements of a geo.dome. The spokes of the topmost hexagon will all be in compression. The rim of that hexagon will be in tension, constraining the compression forces of the above. But my brain can't get further than that at this time on a Sunday morning.

 

Intuitively, I feel that your main problems are going to be as a result of the weight of frame elements that are made of concrete.

Have you considered that the rainwater pipe alone

 

Those Triangles composing the dome are the ultimate structural element , I don't think they can go anywhere ....

 

When talking concrete we do tend to imagine large weight , but remember the strut are only 55mm dia , that's cross sectional area of 24 sq cm ... density of reinforced concrete is 2.5 ... that makes less than 6Kg per meter ... a 2.5 meter strut would weigh 15Kg ... very light ( I will probably have a small cental hole running along  the center for feeding lighting wires .This will reduce the weight to 12 kg 

 

I have experimented with pipe alone , but it sags and degrades in sunlight , then sags even more ..

 

12 hours ago, Temp said:

I don't think I've ever seen reinforced concrete at such a small diameter. What size aggregate do you propose to use? Think it would need to be pretty  a small to fit between the bars?

 

Even if you can get the elements made how do you propose to join them? Weld the ends of the exposed rods? 

 

I think I would just use wood like the image you posted.

 

 

I have a supply of perfectly sized granite chips  , Sharp but about the size of a pea .... Sand is a big problem here ... to make perfect concrete I understand sand should be carefully graded .... 

Yes ...how to make joints ???  I was thinking of making a shutter around the joint , concrete  around the wires which would come out 10cm from the end of each strut ..

 

Wood has many advantages , could staple on the mosquito net  ... termites and rot and protection from sun an issue 

 

It may all be too labor intensive ... I'll experiment with making fence posts first and see how quickly I can make them 

 

Thanks for your input everyone ... Keep it coming 

[Mr Punter]

14 hours ago, oz93666 said:

If you think it would fail because of not enough cover in what way?? 

 

 

Corrosion of the wires would be an issue.  Also it is very difficult to achieve a concrete mix and pour and compact to a a satisfactory standard in something so slender.

 

Your bamboo would probably be a better bet with superb strength to weight ratio and good for the member sizes you propose.