How to Space Rebar Chairs Correctly: A Practical Guide

In reinforced concrete construction, proper positioning of reinforcement is crucial for structural integrity. Bar chairs, the unsung heroes of concrete work, ensure correct placement and cover of reinforcing steel. This guide focuses on the correct spacing of bar chairs according to Australian standards and best practices.

Why Does Rebar Chair Spacing Matter?

Proper spacing of bar chairs is not merely a technical detail but a fundamental aspect of quality concrete construction. Here’s why it matters:

• Ensures correct concrete cover, protecting reinforcement from corrosion and environmental damage
• Maintains rebar position during concrete pouring operations
• Provides adequate support for the weight of reinforcement and construction personnel
• Critical for structural integrity and longevity of the concrete element
• Essential for compliance with Australian building codes and engineering specifications
• Prevents sagging or displacement of reinforcement during concrete pouring

General Rules for Spacing Rebar Chairs in Australia

Spacing Between Chairs

In Australia, specific standards govern the spacing of bar chairs. According to the National Construction Code Volume 2, the maximum spacing for bar chairs in buildings is 800mm (approximately 0.8 meters). For residential pavements, AS 3727.1:2016 requires a maximum bar chair spacing of 600mm (0.6 meters).

As a general rule, the minimum spacing ranges from 0.5m to 1.0m, depending on the concrete placing method and reinforcement grade/size. A simple calculation method is using an average of 1 chair per square meter.

For optimal support, bar chairs should be placed sufficiently close together to ensure the specified cover is maintained throughout concrete placement and to prevent potential crushing of the chairs or their penetration into the formwork.

Spacing from Formwork Edges

Bar chairs must be positioned to maintain the required concrete cover from all surfaces. This includes not only the distance from the top surface but also from side formwork. The position tolerance for surfaces controlled by cover in beams, slabs, columns and walls is -5mm to +10mm, while for slabs-on-ground it’s -10mm to +20mm.

The correct positioning from edges ensures sufficient concrete cover to protect reinforcement from environmental factors while maintaining structural performance. Long continuous linear runs of supports should be avoided, each individual length of support must be laterally offset from its adjacent support by at least 200mm to prevent potential linear cracking in the concrete.

Factors Affecting Your Specific Spacing Needs

Several factors can influence the optimal spacing of bar chairs for your project:

Reinforcement Size and Weight

The diameter and weight of the reinforcing steel significantly impact chair spacing requirements. Heavier and larger diameter bars require closer chair spacing to provide adequate support and prevent sagging.

Application Type

Different applications have different spacing requirements:

• Residential pavements: Maximum 600mm spacing
• Building construction: Maximum 800mm spacing
• Precast elements, suspended decks, walls, and columns: Spacing varies based on reinforcement type and project specifications

Chair Type and Design

Various types of bar chairs are available, each designed for specific applications:

• Fast clip bar chairs: Designed for precast elements, suspended decks, walls, and columns
• Combination bar chairs: Suitable for supporting mesh at different heights
• Panel bar chairs: Designed for precast and tilt-up panels
• Mesh chairs: Designed to clip easily to mesh intersections, providing greater strength and stability

The design and strength rating of the chair affects how many are needed and how they should be spaced.

Project Specifications and Engineering Requirements

Always defer to the project’s engineering drawings and specifications, which may stipulate specific spacing requirements based on structural calculations and load requirements. The specifications may include:

• Minimum concrete cover requirements
• Position tolerances
• Reinforcement spacing and arrangement
• Load requirements and structural performance criteria

How to Place Rebar Chairs: Step-by-Step

Follow these steps to ensure the correct placement of bar chairs in your concrete construction:

1. Review Project Specifications: Check engineering drawings and specifications for specific reinforcement positioning and cover requirements.
2. Select Appropriate Bar Chairs: Choose chairs that comply with AS/NZS 2425:2015 standards and provide the correct cover height for your application. Ensure they have an adequate strength rating for your application.
3. Prepare the Substrate: Ensure the base (substrate, formwork, or blinding layer) is clean, level, and prepared according to project specifications.
4. Mark Chair Positions: Mark out a grid for chair placement, remembering the maximum spacing requirements (600mm for residential pavements, 800mm for buildings).
5. Place Edge Chairs First: Install chairs near formwork edges, maintaining the required cover and position tolerance (-5mm to +10mm for most applications).
6. Install Field Chairs: Place the remaining chairs in a grid pattern according to the spacing requirements for your specific application and reinforcement type. As a general rule, use approximately 1 chair per square meter.
7. Check Chair Stability: Ensure all chairs are stable and properly seated on the substrate. Unstable chairs may tip or move during reinforcement placement or concrete pouring.
8. Position Reinforcement: Carefully place reinforcement on the chairs, ensuring it remains at the correct height and position.
9. Secure Reinforcement: Tie reinforcement to chairs where necessary to prevent movement during concrete placement. Not all intersections need to be tied – where bar spacing is 300mm or less, alternate intersections may be sufficient.
10. Final Inspection: Before concrete placement, conduct a thorough inspection to verify chair spacing, stability, and reinforcement positioning comply with project specifications and Australian standards.

Common Rebar Chair Spacing Mistakes

Avoid these common mistakes when spacing and placing bar chairs:

Exceeding Maximum Spacing

Placing chairs, whether that be metal chairs or plastic chairs, too far apart can result in reinforcement sagging, compromising the specified concrete cover and potentially affecting structural performance.

Insufficient Edge Support

Failing to properly support reinforcement near formwork edges can lead to insufficient concrete cover, increasing the risk of corrosion and compromising durability.

Using Non-Compliant Products

Using bar chairs that don’t comply with AS/NZS 2425:2015 standards can risk structural integrity. All plastic chairs and spacers used in reinforced concrete construction must comply with this standard.

Incorrect Chair Height Selection

Selecting chairs that don’t provide the specified concrete cover compromises reinforcement protection and potentially violates building codes. Bar chairs should be selected to provide the exact cover required – not one “close to” the requirement.

Linear Arrangement of Chairs

Placing chairs in long continuous linear runs can induce linear cracking in the concrete. Each individual length of support should be laterally offset from adjacent supports by at least 200mm.

Inadequate Chair Quantity

Using too few chairs results in inadequate support, particularly when considering the weight of reinforcement, construction equipment, and personnel that may walk on the reinforcement during installation and concrete placement.

Proper spacing of rebar chairs is essential for ensuring structural integrity, durability, and code compliance in concrete construction projects. Following Australian standards and best practices for chair spacing, generally 600mm for residential pavements and 800mm for buildings, helps maintain correct reinforcement positioning and concrete cover.

Always consult project-specific engineering specifications and consider factors such as reinforcement size, application type, and chair design when determining optimal spacing. If you have any questions reach out to the friendly team at Reozone who will be happy to assist.