Re-bar

[Big Neil]

So I think it's probably accepted that one doesn't want metal inside of a structure, particularly within a concrete part of it such as foundations or walls in icf, to rust, correct?

 

well in that case why is it that whenever i see a picture of rebar, be it panels or cage, it has a ayer of rust on it, and this is what goes in the concrete. Will this not rust inside the concrete? Also, those little plastic stands one puts underneath to keep the bar away from the insulation/ground underneath. Does this not potentially create a path for external moisture to get in and rust the bar?

 

I realise this is a ridiculous set of questions.

[Moonshine]

13 minutes ago, Big Neil said:

So I think it's probably accepted that one doesn't want metal inside of a structure, particularly within a concrete part of it such as foundations or walls in icf, to rust, correct?

 

well in that case why is it that whenever i see a picture of rebar, be it panels or cage, it has a ayer of rust on it, and this is what goes in the concrete. Will this not rust inside the concrete?

 

 

You do want metal in there to provide extra strength to the concrete. steel will rust when exposed to air (oxygen) and water, hence having a layer of rust on it when it goes into the concrete (but this is only minor surface rust).

 

However to continue to rust (to a level that would undermine the metal), it need continued exposure to air (oxygen) and water, which it won't get as it is encased in concrete (unless there is a significant problem with the construction).

Edited November 26, 2018 by Moonshine

[Ian]

20 minutes ago, Big Neil said:

So I think it's probably accepted that one doesn't want metal inside of a structure, particularly within a concrete part of it such as foundations or walls in icf, to rust, correct?

 

well in that case why is it that whenever i see a picture of rebar, be it panels or cage, it has a ayer of rust on it, and this is what goes in the concrete. Will this not rust inside the concrete? Also, those little plastic stands one puts underneath to keep the bar away from the insulation/ground underneath. Does this not potentially create a path for external moisture to get in and rust the bar?

 

I realise this is a ridiculous set of questions.

It's the extreme alkalinity of concrete (ph of 12 to 13) that prevents the embedded steel from corroding.

[Big Neil]

Right - so no more oxygen - no more rust (broadly speaking). Correct?

 

I realise it has a much higher cost, but has anyone used basalt as an alternative to steel, on the basis of it's green credentials?

[Onoff]

Concrete is, for years, post pour an alkaline medium. There's actually a protective, passive layer formed around the steel and even if it's rusty this layer reduced further corrosion to a minimum.

 

The problem occurs when over time the alkaline medium turns acid. From memory this is to do with CO2 penetrating over time ( I think). I've an article somewhere detailing why some of the early art deco buildings failed along with that block of flats that came down.

 

Best bet is use rebar that's as rust free as possible or stainless even.

 

There are additives you can add to the initial mix and even paint on solutions designed to arrest further corrosion.

[Ed Davies]

38 minutes ago, Big Neil said:

Also, those little plastic stands one puts underneath to keep the bar away from the insulation/ground underneath. Does this not potentially create a path for external moisture to get in and rust the bar?

 

This is something I've wondered about, too. Anybody?

[Onoff]

6 minutes ago, Ed Davies said:

 

This is something I've wondered about, too. Anybody?

 

 

47 minutes ago, Big Neil said:

Also, those little plastic stands one puts underneath to keep the bar away from the insulation/ground underneath. Does this not potentially create a path for external moisture to get in and rust the bar?

 

Your concrete should be above the DPM layer so not an issue.

[Simplysimon]

it's quite beneficial for the reinforcing to have a surface coating of rust, this allows better adhesion to the concrete

[Onoff]

1 hour ago, Simplysimon said:

it's quite beneficial for the reinforcing to have a surface coating of rust, this allows better adhesion to the concrete

 

The rough guide is that if you take a hand wire brush to your rusty rebar, when it's off the rebar should remain within it's given tolerance. 

 

It's the ribs along the rebar by design that provide "grip" in the concrete not the rust. Stainless rebar is of course available.

[scottishjohn]

7 minutes ago, Onoff said:

 

The rough guide is that if you take a hand wire brush to your rusty rebar, when it's off the rebar should remain within it's given tolerance. 

 

It's the ribs along the rebar by design that provide "grip" in the concrete not the rust. Stainless rebar is of course available.

or GRP--4times the price though 

used in radar bunkers etc -as steel would mess up the radar signal -if it worked at all

 

 

[Nickfromwales]

1 hour ago, Simplysimon said:

it's quite beneficial for the reinforcing to have a surface coating of rust, this allows better adhesion to the concrete

Ahhhhh, so it was YOU I saw in Lidl sucking the brandy out of all the little Xmas puds.........?

[Simplysimon]

1 minute ago, Nickfromwales said:

Ahhhhh, so it was YOU I saw in Lidl sucking the brandy out of all the little Xmas puds.........?

there's brandy in them? i'll need to stop believing what i'm told. theresa got a good deal is wrong then?

[kxi]

I was concerned about the long-term durability of standard steel rebar, so looked into alternatives a bit. My layperson's summary below.

 

However, I'm now just going to use standard steel in our own Reinforced Concrete parts. While steel corrosion seems a major problem for structures like bridges, dams, etc, my SE has assured me any RC with standard rebar kept protected from; the elements, unusual chemicals, and cyclical loading should be fine for over 150 years. I.e. ours will be fine, and I'd guess anything inside ICF also fine. As below, all the alternatives have some disadvantages.

 

But if you are building something with exposed RC and care about it lasting more than 50 years, the lifecycle costs seem to suggest everything is more cost-effective than using standard steel.

 

Some evidence of standard steel / iron reinforcement durability:

• The first RC house ever built 1852 is still standing https://en.wikipedia.org/wiki/François_Coignet
• The first one in the US 1876 is apparently fine https://en.wikipedia.org/wiki/William_E._Ward_House
• The first RC skyscraper 1903 is also apparently fine https://en.wikipedia.org/wiki/Ingalls_Building

 

Alternatives:

(Some comparison here https://www.usbr.gov/research/projects/download_product.cfm?id=2493 )

 

Stainless steel

A fairly straightforward replacement for normal rebar (though apparently, you need to use a bit more to get the stiffness) and used for many years. Main issue is the cost, which might be 3 or more times the cost of normal rebar. I didn't check the specific price.

 

Glass Fibre Reinforced Polymer GFRP

Proposed as a corrosion-free alternative to steel and much lighter, with higher tensile strength. Has been well studied and increasingly in use in some structures worldwide, and has building codes that govern its safe use. Unfortunately, it's not without problems:

• Fire - while the glass withstands very high temperature, it's in resin that softens at over 150 C. For this reason, the Institution of Structural Engineers do not recommend it for situations in which fire is a concern.
• Unknown long term behaviour in concrete - glass doesn't like alkali i.e. concrete, or moisture, and as per one of the suppliers' own descriptions "At this time, there is no consensus as to what would be an accurate service life prediction model for the use of GFRP bars". http://www.aslanfrp.com/media/aslan100.pdf
• Creep - This same supplier also notes that GFRP has creep rupture with sustained high utilization. Though these are AFAIK accounted for in the building codes that apply to GFRP. My SE also noted its long term creep is not good.
• Elasticity - much more elastic than steel and so requires more reinforcement or more concrete depth to compensate.

Perhaps of note Owens Corning recently bought the manufacturer of Aslan GFRP, so presumably they think it has long term potential.

 

Carbon Fibre Reinforced Polymer CFRP

Really good, really expensive, doesn't like fire. 

 

Basalt Fibre Reinforced Polymer BFRP

A new contender that has only been in use for a few years, and the subject of much current research especially in China. Similar properties to GFRP (light, high tensile strength) but may address some of the issues with GFRP and appears to be able to be produced more cheaply e.g. in the US you can get rolls at $6.60 a metre, and Gatorbar claim cost parity with standard steel. https://www.neuvokascorp.com/sites/all/themes/theme923/pdf/GatorBar_data_20160907_HIGH.pdf

 

BUT it shares GFRP's disadvantages in that

• The resin doesn't like fire
• Still elastic

The major drawback is that it's so new, people seem nervous about doing anything structural with it:

• There's almost no example of it being used in house construction other than floors & shallow foundations
• Gatorbar currently limit use to slab on grade and low walls/foundations (confirmed this with them)
• UK suppliers of Galen's Rockbar (Magmatech) said it hadn't been used structurally and any structural use of it would have to be approved by an SE

 

Various BFRP references:

http://www.thestructuralengineer.info/onlinelibrary/pdfs/SustainabilityMasterBuilder_Sep10.pdf 
https://www.ripublication.com/ijaer18/ijaerv13n8_37.pdf
https://livrepository.liverpool.ac.uk/16333/4/SalhLuna_Feb2014_16333.pdf
http://www.carnationconstruction.com/Materials/01-Materials-Rebar.html
https://www.sciencedirect.com/science/article/pii/S187661021730022X
https://www.monolithic.org/link-to/basalt-fiber-rebar
https://smarter-building-systems.com/smarter-building-basalt-faqs/
https://www.researchgate.net/publication/315642749_Investigation_of_Structural_Members_with_Basalt_Rebar_Reinforcement_as_an_Effective_Alternative_of_Standard_Steel_Rebar
https://pure.qub.ac.uk/portal/files/154263432/180620_polymers_10_00678_paper_accepted_in_J_of_Polymer.pdf

 

BFRP durability:
https://www.sciencedirect.com/science/article/pii/S0264127518308724 
https://www.researchgate.net/publication/319943787_A_refined_prediction_method_for_the_long-term_performance_of_BFRP_bars_serviced_in_field_environments
https://ascelibrary.org/doi/pdf/10.1061/(ASCE)CC.1943-5614.0000497

 

Some BFRP suppliers:
https://www.neuvokascorp.com/sites/all/themes/theme923/pdf/GatorBar_data_20160907_HIGH.pdf 
http://magmatech.co.uk/products/rockbar/   
http://orlimex.co.uk/composites/
https://www.monolithicmarketplace.com/collections/basalt-rebar-products/products/rebar-12mm-diameter
http://galencomposite.com/products/composite-reinforcement-rockbar/