Concrete beam : What interval for Rebar Chairs - ASEAN NOW

Gents,

You will get efficient and thoughtful service from Trico.

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

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.

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.

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.

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.

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.

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.

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:

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.

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.

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.

Are you interested in learning more about Concrete Reinforcing Rebar Support Chairs? Contact us today to secure an expert consultation!

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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

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.

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;

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

When constructing with concrete, rebar chairs are critical in maintaining structural integrity by properly supporting and spacing reinforcement bars. This article outlines their types, functions, and installation, offering clear insights into the optimal use of rebar sizes in your construction projects.

Rebar Chairs: Key Takeaways

• Chairs for rebar are critical components in construction for accurately positioning and securing steel reinforcement bars within the concrete, coming in various materials such as metal and plastic, each catering to specific project needs.

• Key factors such as the project type, rebar size, and load demands dictate the installation and spacing of rebar chairs, which is instrumental in maintaining the stability and structural integrity of concrete structures.
• Utilizing chairs for rebar in construction offers significant benefits including enhancing the strength and longevity of structures by supporting rebar, distributing weight evenly, and protecting against corrosion, especially in crucial applications such as bridge building.

Types and Functions

Rebar chairs are engineered to elevate and secure reinforcing steel bars during concrete pouring, ensuring optimal positioning for structural integrity. They come in various shapes and sizes, with the most common being metal and plastic chairs for rebar. Plastic are often preferred over metal due to their quick and convenient implementation, low cost per unit, high strength, stackability, adjustability, continuous secure footing, and suitability for deeper concrete slabs. Each type has a specific role in a construction project and is selected based on the specific requirements and demands of the project.

Metal

Constructed from steel, metal chairs for rebar are frequently employed in robust construction projects such as landscaping, retaining walls, and commercial structures that use structural steel. These serve as vital supports, ensuring precise positioning and alignment during the concrete pouring phase.

Their strength and durability make them a favored choice in construction projects that demand high structural integrity.

Plastic

Contrarily, plastic chairs, celebrated for their corrosion resistance, lightweight nature, and affordability, are a practical choice for various construction tasks. They are available in many shapes and sizes to accommodate various project requirements and rebar sizes.

Reinforcing Steel Bars and Rebar Stands

Rebar stands, including chairs for rebar, serve the purpose of supporting and preserving the placement of reinforcing steel bars within a concrete structure. They are frequently manufactured from materials like steel, polypropylene, and plastic because of their robustness, strength, and resistance to corrosion. These stands are designed to hold rebar effectively, ensuring proper positioning and stability.

Their utilization in construction ensures the proper spacing of chairs, preserving material integrity and uniformity of the concrete.

Installation and Spacing Guidelines

The effectiveness of rebar chairs in stabilizing a concrete structure relies on correct installation and suitable spacing. The type of concrete project, whether slab, wall, or column, influences rebar chair placement. Project specifications, rebar chair size, and load requirements also affect placement. Additionally, the number of rebar chairs needed depends on these factors, highlighting the importance of proper spacing and choosing the right type for different applications.

Proper Placement

The stability of reinforcing steel bars is heavily dependent on the strategic placement of rebar supports and chairs. Factors such as the chemical composition of the concrete, the size of the rebar, and the required placement of the rebar mat and reinforcing bars all come into play to ensure efficient distribution of the support.

The correct placement ensures effective support during the pouring concrete process and maintains the correct flow of pouring concrete all around the rebar.

Determining Spacing

In construction, space chairs about 2 to 3 feet apart along the rebar’s length. Determine the optimal chair spacing by considering factors such as the distance between the outer surface of the finished concrete and the reinforcement inside, as well as the size of the reinforcing bar being supported.

Such careful consideration of rebar chair spacing contributes to the overall strength and durability of a structure.

Advantages of Using in Construction

Using rebar chairs in construction offers many benefits. They enhance the strength and durability of concrete structures by reinforcing the concrete and preventing cracking. The rebar alleviates tensile stress, improving overall structural strength.

Additionally, rebar chairs help absorb and distribute the load across the structure, further increasing its strength and durability.

Enhancing Concrete Strength

By providing additional support against internal tension, chairs significantly enhance the strength of concrete structures, improve their resistance to tensile stress, and increase overall structural strength. Proper placement of rebar chairs is essential for maintaining the structural integrity and strength of the concrete.

Improving Project Longevity

Preventing cracks from spreading through the material, plays a significant role in ensuring long-term durability and maintaining the stability and integrity of construction projects. Additionally, rebar enhances the strength, durability, and resistance to failure of reinforced concrete structures, so, the correct positioning using it is crucial.

Rebar Chair Accessories and Alternatives

While rebar chairs play a crucial role in concrete reinforcement projects, the market provides full access to a variety of rebar support products, including accessories and alternatives, to cater to unique needs and applications requiring rebar chairs. These include rebar spacer wheels, Unibar centralizers, and steel-plated rebar caps.

These accessories and alternatives offer improved strength-to-weight ratios, enhanced durability, corrosion resistance, and a focus on environmental sustainability while providing better access to these best practice benefits.

Rebar Spacer Wheels

In construction, rebar spacer wheels aid in maintaining the spacing of rebar in sidewalls and columns. Rebar chairs ensure appropriate concrete cover, mitigate rebar corrosion, and provide steadfast support during concrete pouring. They offer advantages such as stability, lightweight handling, and easier transportation compared to traditional steel spacers.

Steel Plated Rebar Caps

Workers use steel-plated rebar caps to meet safety requirements and local construction regulations. Placing safety caps on vertical rebar and exposed areas protects against injuries. This measure ensures worker safety at the construction site.

The installation process involves placing the cap over the rebar end, wire tying it securely, and applying firm pressure until it is securely in place.

Unibar Centralizer

Specifically designed for concrete or steel reinforcement projects, a Unibar Centralizer is a construction device used to align support cages. They play a crucial role in maintaining the reinforcing steel cage in the correct vertical position and ensuring it has the appropriate concrete cover within bored or drilled shafts. These centralizers provide numerous benefits such as:

• Lightweight and durable design
• Non-corrosive plastic construction
• Centering the reinforcement in the borehole
• Allowing minimum grout or concrete coverage
• Not obstructing grout flow

Bridge Decks and Chairs

In bridge construction, the role of chairs for rebar is vital. They:

• Support and position reinforcing steel bars (rebar) within the concrete structure
• Ensure stability and proper alignment during the concrete pouring process
• Ensure corrosion resistance and long-lasting durability of bridges

Their importance is particularly significant in bridge construction, where only laborers play a crucial role in the process.

Importance of Chairs in Bridge Construction

In bridge construction, this is indispensable. Rebar chairs provide reinforcement and increase the strength and load-bearing capacity of bridge decks. Omitting them can compromise the bridge’s structural integrity and increase the likelihood of cracks and failure.

Corrosion Resistance and Bridge Longevity

Typically, corrosion-resistant these are crafted from materials such as GTI composite, industrial-grade polypropylene, and epoxy-coated steel. These materials offer superior corrosion resistance. They work with concrete’s protective attributes to safeguard the reinforcing steel in bridge structures. This promotes long-term durability.

Regular inspections and maintenance on corrosion-resistant chairs in bridge structures are advisable to ensure maximum longevity and safety.

Summary of Rebar Chairs

In conclusion, rebar chairs, though small, play a significant role in enhancing the strength, stability, and longevity of concrete structures. They maintain the structural integrity of bridge decks and ensure the durability of commercial buildings. The correct use of rebar chairs and their alternatives, with proper installation and spacing, ensures construction projects stand the test of time.

Frequently Asked Questions about Rebar Chairs

What are chairs in rebar?

Rebar chairs support rebars in a crisscross grid pattern before concrete is poured. This ensures the reinforcement of newly poured concrete. They are a patented polypropylene support system ideal for various concrete applications.

Should you use chairs for rebar?

Yes, you should use chairs. They provide stability, ensure optimum concrete flow,, and reduce job time. This contributes to the structural integrity and longevity of the concrete structure.

Do plastic chairs stay in the concrete?

Yes, plastic chairs keep the rebar at the correct height within the concrete. They ensure the rebar stays in the desired position during pouring and setting.

What are the typical materials used in the manufacturing of chairs for rebar?

Typical materials used in the manufacturing of rebar chairs are steel or plastic, designed to accommodate various sizes.

What are the general steps for installing chairs?

To install chairs, start by placing them away from the edges. Support them at the desired height and intervals. Secure them by tying wire to the chairs. This ensures proper positioning and support.

Related Blogs for Rebar Chairs

Rebar Chair: Choosing the Perfect Chair for Construction

If you want to learn more, please visit our website Galvanized Steel Cone.