During concrete works, one detail that is often overlooked by non-engineers is the rebar chair. These small supports hold reinforcement bars at the required height before and during concrete placement.
To many workers, rebar chairs may appear insignificant, but to engineers, they are essential for maintaining structural strength, durability, and accurate reinforcement positioning. Without adequate chairs, reinforcement can move during concreting, changing the designed concrete cover and reducing the performance of the structural element.
Rebar chairs are not optional accessories. They form part of the reinforcement system and directly affect the performance and service life of slabs, beams, footings, and columns. This article explains why rebar chairs are important and how they help concrete elements perform as designed.
1. Rebar Chairs Maintain the Correct Concrete Cover
Concrete cover is the protective thickness of concrete surrounding the reinforcement. It protects the steel against corrosion, fire, and environmental exposure.
During concrete placement, reinforcement cages may sink, rise, or move if they are not properly supported. Chairs hold the reinforcement at the required height to maintain:
- Bottom cover
- Top cover
- Side cover
Without chairs, the concrete cover may become inconsistent. Even a small reduction in cover can expose reinforcement to moisture and air, eventually leading to corrosion, cracking, and spalling.
2. Chairs Prevent Rebar Movement During Pouring
Concrete pouring involves vibration, workers moving across the reinforcement, heavy equipment, and pressure from fresh concrete. Reinforcement can easily shift if it is not properly fixed and supported.
Rebar chairs help to:
- Prevent bars from sinking under workers’ weight
- Stop reinforcement from lifting during concrete placement
- Maintain spacing when vibrators come into contact with the bars
- Keep the reinforcement aligned despite the pressure and movement of fresh concrete
Once the reinforcement cage moves after concrete placement has started, correcting its position becomes extremely difficult. Properly installed chairs significantly reduce this risk.
3. Chairs Help Maintain Structural Depth
Structural depth is the distance between the top and bottom reinforcement layers. This depth affects:
- Load-carrying capacity
- Flexural strength
- Crack control
- Deflection limits
If the reinforcement sinks during pouring, the effective depth of the slab or beam is reduced. This may lower its strength and affect its structural behaviour.
Rebar chairs maintain both reinforcement layers in their designed positions, helping preserve the intended structural performance.
4. Chairs Improve Concrete Consolidation
Correct chair spacing keeps the reinforcement elevated and allows fresh concrete to flow around and beneath the bars.
When reinforcement sits directly on the formwork without proper chairs:
- Concrete may not fully fill the space beneath the bars
- Honeycombing may occur
- Voids may form along the bottom surface
Proper concrete consolidation around the reinforcement improves bond, durability, and overall structural performance.
5. Chairs Ensure Accurate Spacing Between Rebars
Reinforcement bars require sufficient clear spacing to allow concrete to flow properly and to achieve the intended structural behaviour.
Chairs, spacers, and supports used within reinforcement cages help maintain:
- Clear spacing between bars
- Uniform load transfer
- Proper anchorage and lap development
Incorrect spacing may result in reinforcement congestion or excessive gaps. Both conditions can negatively affect concrete placement and structural integrity.
6. Chairs Improve Quality Control on Site
Rebar chairs make it easier for engineers and inspectors to verify the position of reinforcement before concrete placement.
Without proper supports, it can be difficult to confirm whether the bars are installed at the correct height and alignment. Chairs provide:
- A consistent reinforcement support system
- Clear visual confirmation of bar positions
- Better control of the reinforcement layout
This improves inspection results and helps ensure that construction complies with the drawings and code requirements.
7. Chairs Reduce Long-Term Corrosion Risk
Insufficient concrete cover is one of the main causes of reinforcement corrosion. When reinforcement touches the formwork or surrounding surfaces, moisture, chemicals, and air can reach the steel more easily.
Rebar chairs prevent steel from coming into direct contact with:
- Shuttering or formwork
- Soil
- Masonry
- Plastic sheets or membranes
Maintaining the required concrete cover slows down corrosion and extends the service life of the structure.
8. Chairs Support Worker Movement Over the Rebar Cage
During reinforcement fixing and concrete placement, workers often step on reinforcement mats. Without sufficient chairs, the reinforcement cage may bend, sink, or become displaced.
Properly arranged chairs:
- Transfer workers’ weight toward the formwork
- Reduce reinforcement displacement under foot pressure
- Help provide a more stable reinforcement mat during site activities
They help keep the reinforcement in its intended position throughout construction.
Different Types of Chairs Serve Different Functions
Rebar chairs are available in different materials, sizes, and shapes, including:
- Steel chairs for heavy loads and large reinforcement mats
- PVC chairs for slabs where rust staining should be avoided
- Circular spacers for columns and walls
- Concrete cover blocks for footings and vertical reinforcement
The correct chair or spacer should be selected based on the structural element, required cover, reinforcement weight, and exposure conditions.
Chairs Help Meet Code and Design Requirements
Building codes and structural drawings specify minimum concrete cover requirements. Failure to maintain the required cover may result in:
- Rejection during inspection
- Reduced structural performance
- Lower fire resistance
- Increased maintenance and repair costs
Rebar chairs help achieve these requirements consistently and reliably across the structural element.
Why Chairs Are Essential Even in Small Structures
Small residential slabs, beams, columns, and footings may appear simple, but they are still vulnerable to reinforcement movement during concrete placement.
Rebar chairs help ensure:
- Predictable structural behaviour
- Better crack control
- Correct reinforcement positioning
- Improved long-term durability
Even small structures can suffer from corrosion, excessive cracking, and deflection when reinforcement chairs are ignored.
FAQs
1. Are rebar chairs really necessary for small slabs?
Yes. Even small slabs require the correct concrete cover and stable reinforcement. Chairs prevent bars from sinking or moving during concrete placement.
2. Can bricks or stones be used instead of rebar chairs?
No. Bricks and stones do not provide accurate or consistent cover. They may absorb water, break during construction, or create weak and uneven areas within the concrete. Only approved chairs, spacers, or concrete cover blocks should be used.
3. What happens if reinforcement touches the formwork during concreting?
The reinforcement will not have the required concrete cover. This increases the risk of corrosion, cracking, spalling, and long-term durability problems. Properly installed rebar chairs prevent direct contact between the reinforcement and the formwork.
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