NUDURA rebar steel cost

[Ben100]

Hi,

 

I'm looking at the NUDURA ICF blocks and have a quote from a reseller near me. I'm trying to get a total cost estimate and need to price up rebar steels, but I'm not sure how at this stage. Can anyone who's used NUDURA ICF give me an estimate on the cost of the steels compared to the ICF blocks? For example are the steels the same cost as the ICF blocks, or 50%, 20%?

 

Cheers,

Ben

[Alexphd1]

Depends on your design ie big lintels for quantity . Rebar cost was only a fraction of icf cost for a normal designed house but watch supplier big price differences. 

[Alex]

I spent just over £1k on rebar on my nudura build, with my blocks coming in at £22k. This is for a 170m2 footprint chalet bungalow and 65m2 garage/workshop. 

For me biggest lintel was 5m wide using lots of 20mm bar plus joining stirrups, but most was 15mm bar. I can't remember exactly without digging out invoices but I think I used about 450m of bar. 

 

[Russell griffiths]

Nudura £22,000 steel cost, £1,500

i have lots of large openings and a number of large concrete beams holding up the roof

steel cost is minimal compared to the block and concrete. 

55 cubic metres of concrete in mine. 

Find a good structural engineer that knows icf and you will save a few quid. 

[Ben100]

Cheers guys, that's good to know!

[Dutch]

I am having issues finding the correct quantity of rebar to use and the size, structural engineer just said it a system he is not familiar with.

 

[Russell griffiths]

14 minutes ago, Dutch said:

I am having issues finding the correct quantity of rebar to use and the size, structural engineer just said it a system he is not familiar with.

 

Find another structural engineer. 
if they don’t know it they will just over engineer everywhere and cost you a fortune. 
ask your Icf supplier for an engineer. 
if they cannot tell you a good one then I would choose another Icf supplier. 
simple really. 

[Dutch]

He not done any calculations on the Nudura.

I have emailed Nudura so hopefully they will help.

[Gus Potter]

On 19/11/2018 at 11:32, Ben100 said:

Hi,

 

I'm looking at the NUDURA ICF blocks and have a quote from a reseller near me. I'm trying to get a total cost estimate and need to price up rebar steels, but I'm not sure how at this stage. Can anyone who's used NUDURA ICF give me an estimate on the cost of the steels compared to the ICF blocks? For example are the steels the same cost as the ICF blocks, or 50%, 20%?

 

Cheers,

Ben

Hope this gives you some pointers.

 

Lets start with the abbreviation.. ICF.. insulated concrete formwork. For walls the insulating blocks act as a shutter (formwork) to hold the wet concrete in place while it sets and then cures. The blocks themselves for walls make no contribution to the structural stength of the concrete wall. The choice of insulating wall medium has no bearing on the strength of the wall to resist the vertical an horizontal loads. The choice of block has no real bearing on the amount of rebar you need in the walls. Not quite, but let's keep it simple for general costing purposes for now. Some complex behavoir can happen at the bottom of the wall, depends on how and if it (the wall) is positively fixed to the floor slab.

 

However it when it comes to the floor slab things change a lot. If your insulating blocks go down the walls and continue under the structural floor slab then the building is supported by the insulation, which rests on the ground below.

What SE's do to design this is to look first at the ground.. how squashy is it, one property they look at is it's elastic modulus... a property of squashyness!

 

Next step is to look at the insulating blocks.. how squashy are they. Hopefully you as an SE find the insulation is more squashy than the ground below. If so plane sailing. You design for the most squashy layer which is the insulation, there are other methods but this is the most common and practical for domestic design.

 

Thus if you have an ICF block rated at 300 kPA at 10% compression (30 tonnes per square meter at 10% compression) you first say how thick is the insulation? Say 300mm thick so 10% of 300 = 30mm settlement to get your 300 kPa out of it. Now 30mm settlement is too much to tolerate normally. Also, you may have insulation that creeps a bit over 25 -50 years.. bit of a worry. Lets go for a figure of 2% compression and see what load we can put on the ICF blocks.

Thus 300 * 2% / 10% = 60 kPa = ~ 6 tonnes per square metre all nicely spread out. Closer but we need to watch out for point loading and so on.

 

Now there are many ways of carrying out the next steps to determine the shape and form of the supporting slab that sits on the insulation. But basically you strike a balance between the thickness of the floor slab and the amount of rebar you chuck at it. The easy way to get your head around this is to think about a bending a ruler. If you do so the top is in compression, the bottom in tension. Thus the thicker the ruler the less force on the tension side. Now concrete is good in compression but poor in tension so we use steel bars which are bonded into the concrete to resist the tension force. Thus for your ICF floor slab if you make it a bit thicker you'll often need a bit less rebar. But there is no free lunch as you'll need to pay for a bit more concrete. But the secret is often in reducing labour steel fixing cost,  reducing complexity and avoiding on site errors.

 

Lastly designing a ground bearing slab on ICF is not the same as designing a concrete slab that you may see in a multi storey carpark. The slab interacts with the ICF and ground below and thus the design approach is different.

 

In summary @Ben100  if you choose a higher stength insulation under your floor slab you'll maybe use a bit less rebar and concrete, but you'll lose some of the insulation properties so will then need to make it (insulation) thicker to achieve the same u value.. and making it thicker will result in more movement.. who said self building was not fun!

 

The best advice I can give is to seek out the most practical and buildable solution for you. Always start with the simple stupid. See how much that costs and then if you have spare cash explore further. Your simple stupid option is your bench mark.

 

The weight of steel you need will much depend on your slab and another of other factors. To get the best out of this you'll need to post some pretty comprehensive drawings so folk can see what you are proposing.

 

All the best.