Concrete beam : What interval for Rebar Chairs ?

[SurinBeach]

Gents,

My continuing saga of weekly questions on Concrete/Rebar !

So building a carport/garage, in total it's 9meters long by 4.5 meters wide, with six pillars

So in effect 2 structures connected of 4.5M x 4.5M

Used lots of rebar (ie 6 x 16mm) for the beams which are 200mm x 200mm

I asked the builder to put in "chairs" under the rebar in the beams, he's put in one or two per 4,5Meter section, thats it !

Any suggestion re what interval they should be at ?

Many thanks for all responses ....

Regards

SB

[krisb]

Chairs you sit on?

Why is there rebar in beams? Beams are made from steel or timber.

Rebar, like shown in the pic below, goes in the foundation.

[SurinBeach]

Beams (in Thailand) are mainly made from concrete and rebar....or am I using the wrong term ? ie ringbeam

[Crossy]

Beams (in Thailand) are mainly made from concrete and rebar....or am I using the wrong term ? ie ringbeam

Terminology looks fine to me, a beam is a horizontal structural member, it matters not what it's made from.

Our contractor placed home made spacers every metre or so to keep the steel in the sweet zone for each beam.

In answer to the OP I suggest "often enough to correctly position the rebar" is the answer If it looks like it's sagging too close to the formwork, slip in another spacer.

[IMHO]

Beams (in Thailand) are mainly made from concrete and rebar....or am I using the wrong term ? ie ringbeam

Terminology looks fine to me, a beam is a horizontal structural member, it matters not what it's made from.

Our contractor placed home made spacers every metre or so to keep the steel in the sweet zone for each beam.

In answer to the OP I suggest "often enough to correctly position the rebar" is the answer If it looks like it's sagging too close to the formwork, slip in another spacer.

Right. The goal is simply to keep the rebar between 15mm and 25mm from the edge of the forms. No more, otherwise it will be susceptible to cracks, no less or it won't perform well in a fire, and can corrode.

[Goanna]

Generally you should have 50mm concrete cover. If you don't, water will seep through the concrete, causing the reo to rust, which will in turn, expand the reo with growing rust and it will bust out your concrete. In high rise, they call this concrete cancer.

For wet areas, like water storage, the norm would be 65mm, for the same reason.

Cheers.

[Lee4Life]

I don't have a problem with the beam word, but what is a "chair"? Do you mean permanent support posts (pillars) under the beams perhaps? Up here in Isan it is not common to use concrete or cement beams in carports or garages, but instead steel beams welded to the rebar that protrudes from the tops of the pillars or posts. In either case the deciding factor on when more support is required will be the weight of the structure being supported, in your case the roof I would suppose.

[IMHO]

Generally you should have 50mm concrete cover. If you don't, water will seep through the concrete, causing the reo to rust, which will in turn, expand the reo with growing rust and it will bust out your concrete. In high rise, they call this concrete cancer.

For wet areas, like water storage, the norm would be 65mm, for the same reason.

Cheers.

Correct for slabs, not correct for posts and beams though.

[steve187]

chair strange word for what is a spacer, but chair is the word used in Thailand, normally made from small 'washers' of concrete with a piece of wire sticking out.

[eyecatcher]

I just wonder if a chair is a Brit only term.

But i use this term referring to the plastic spacers to support reinforcing mesh is floor slabs.

As inferred above the purpose is to keep the mesh a designed distance from the concrete surfaces and the number of chairs depends on the weight and flexibility of the mesh.

But your in thailand now so next best is to make do is half a brick or an offcut of bamboo.

....that was a sarcastic comment not to be taken seriously.

[IMHO]

This is how we make these "chairs" in TH:

Once set, this is the broken up into individual cubes, which are tied to the rebar to space it from the forms/ground as needed.

[ToddinChonburi]

Pillars yes pillars.

[eastbay]

In the western USA those concrete blocks are called "dobies", don't know where that name comes from. "Chairs" refers to stands for the same purpose but made of formed wire or plastic.

They should be placed as required to maintain alignment and coverage, and also to prevent movement or displacement of the reinforcing steel (rebar) during concrete placement (pour)- they are secured in place using the little wires.

Coverage, per IBC:

[FT10toLOS]

Generally you should have 50mm concrete cover. If you don't, water will seep through the concrete, causing the reo to rust, which will in turn, expand the reo with growing rust and it will bust out your concrete. In high rise, they call this concrete cancer.

For wet areas, like water storage, the norm would be 65mm, for the same reason.

Cheers.

Correct for slabs, not correct for posts and beams though.

I and building codes back Goanna on this one.

This is especially important for structural components, ie posts and beams.

Anything less than these tolerances will leave the reobar susceptible to corrosion causing concrete spawling or "concrete cancer" thus reducing the structures integrity, ie load carrying capacity.

[hawthorne]

It appears that a lot of expats in Thailand are very knowledgable about construction. Most probably retired or older people enjoying the life. It seems appropriate to form a type of self help group for other expats building homes around Thailand. I certainly would enjoy helping others build their homes once I retire at no charge just to help others and meet people. Got off topic but it seems the question was fully covered.

[sometimewoodworker]

It appears that a lot of expats in Thailand are very knowledgable about construction. Most probably retired or older people enjoying the life. It seems appropriate to form a type of self help group for other expats building homes around Thailand. I certainly would enjoy helping others build their homes once I retire at no charge just to help others and meet people. Got off topic but it seems the question was fully covered.

It has been around for about 10 years now called CoolThaiHouse

Edited March 11, 2015 by sometimewoodworker

[IMHO]

Generally you should have 50mm concrete cover. If you don't, water will seep through the concrete, causing the reo to rust, which will in turn, expand the reo with growing rust and it will bust out your concrete. In high rise, they call this concrete cancer.

For wet areas, like water storage, the norm would be 65mm, for the same reason.

Cheers.

Correct for slabs, not correct for posts and beams though.

I and building codes back Goanna on this one.

This is especially important for structural components, ie posts and beams.

Anything less than these tolerances will leave the reobar susceptible to corrosion causing concrete spawling or "concrete cancer" thus reducing the structures integrity, ie load carrying capacity.

OK, let's take a typical, basic 20cm x 20cm post, with 4 rebars - you are suggesting that the rebars inside only make a 10cm square?

No, I didn't think so

Right, so for beams and posts it's like I said - feel free to look up any code you want from any country - if you can find one where the math adds up to anything outside of a 15-35mm range for the surface-to-rebar spacing on a 20cm post/beam, I'll be adding that country to my no-go list

[eastbay]

Look at the chart that I posted above, from the International Building Code- up to 75mm plus coverage required for grade beams (cast against and permanently exposed to earth), down to a minimum of 37mm for columns and beams not exposed to earth or in contact with the ground.

[IMHO]

Look at the chart that I posted above, from the International Building Code- up to 75mm plus coverage required for grade beams (cast against and permanently exposed to earth), down to a minimum of 37mm for columns and beams not exposed to earth or in contact with the ground.

1.5 inches = 38mm, more commonly rounded to 35mm in the building trade.

75mm would be 3"

[mango66]

he use 16 mm rebars, so they will not hang too much down,

more importint will be to put enough rings around to keep the rebars in same distance over the entire lenght of the beam;

[SurinBeach]

Thanks Gents,

Chairs or spacers etc :

Mine were made out of a 4cm blue conduit filled with concrete or was it mortar !

I'll post a pic later, school runs beckon first ...

Yes I used 16mm Rebar in the BEAM so it aint sagging much ...before we pour !

Best Regards

SB