Rebar Supports Concrete Chairs For Reinforced Concrete Slabs

The statement that concrete chairs are the most commonly used support for the bottom steel in a reinforced concrete slab resting directly on the ground is TRUE. This article aims to provide a comprehensive understanding of rebar supports, with a particular focus on concrete chairs and their crucial role in reinforced concrete construction. We'll delve into the various types of rebar supports, the advantages of using concrete chairs, and the importance of proper rebar placement for the structural integrity of concrete slabs.

The Importance of Rebar in Concrete Structures

Reinforced concrete is a composite material that combines the compressive strength of concrete with the tensile strength of steel. Concrete, while strong under compression, is weak in tension and prone to cracking. Steel reinforcing bars, or rebars, are embedded within the concrete to provide the necessary tensile strength, making the structure capable of withstanding bending and shear stresses. The rebars act as the skeleton of the concrete structure, carrying tensile loads and distributing stresses effectively.

The proper placement of rebar is paramount to the structural integrity of any reinforced concrete element, including slabs, beams, columns, and walls. Rebars must be positioned accurately within the concrete matrix to ensure they can effectively resist tensile forces. The required position of rebars is determined by the structural design, taking into account load calculations, span lengths, and other factors. If rebars are not placed correctly, the concrete structure's load-bearing capacity can be severely compromised, leading to cracking, deformation, or even structural failure. This is where rebar supports come into play, ensuring the rebars remain in their designated positions during the concrete pouring and curing process.

Types of Rebar Supports

Rebar supports, also known as bar supports or chairs, are devices used to hold reinforcing steel in place within concrete forms before and during the concrete pour. They maintain the required concrete cover, which is the distance between the rebar and the surface of the concrete. Adequate concrete cover is essential for several reasons: it protects the steel from corrosion, provides fire resistance, and ensures proper bonding between the steel and the concrete.

Rebar supports are manufactured from various materials, each with its own advantages and disadvantages. The most common types of rebar supports include:

• Concrete Chairs: These supports are made from concrete, often the same mix design as the structural concrete. They are durable, non-corrosive, and provide excellent support for heavy rebar. Concrete chairs are particularly suitable for slabs on grade and other applications where they can rest directly on the ground or a prepared surface.
• Metal Chairs: Metal chairs are typically made from steel wire or pre-fabricated steel. They are strong and can support heavy loads. However, metal chairs are susceptible to corrosion if not properly coated or protected. They may also create thermal bridges, potentially leading to surface staining or cracking in the concrete.
• Plastic Chairs: Plastic chairs are lightweight, non-corrosive, and relatively inexpensive. They are suitable for lighter rebar applications and offer good resistance to chemical attack. However, plastic chairs may not be as strong as concrete or metal chairs and may deform under heavy loads.
• Combination Chairs: Some rebar supports combine different materials, such as a plastic-coated metal chair or a concrete chair with a plastic cap. These hybrid supports aim to leverage the benefits of each material while mitigating their drawbacks.

Concrete Chairs The Preferred Choice for Slabs on Grade

Concrete chairs are indeed the most commonly used support for the bottom steel in reinforced concrete slabs resting directly on the ground, and for good reason. Their inherent properties make them ideally suited for this application. Let's explore the key advantages of using concrete chairs for slabs on grade:

• Compatibility: Concrete chairs are made from the same material as the surrounding concrete, ensuring excellent compatibility. This eliminates the risk of chemical reactions or differential expansion and contraction, which can occur when using dissimilar materials. The homogenous nature of concrete chairs ensures a strong and durable bond with the surrounding concrete, contributing to the overall structural integrity of the slab.
• Corrosion Resistance: Unlike metal supports, concrete chairs are inherently resistant to corrosion. This is particularly important in slabs on grade, which are often exposed to moisture and soil conditions that can accelerate corrosion. By using concrete chairs, the risk of steel corrosion is minimized, extending the lifespan of the reinforced concrete slab. This durability translates to long-term cost savings and reduced maintenance requirements.
• Strength and Stability: Concrete chairs provide excellent support for heavy rebar mats, ensuring the rebars remain in their correct position during the concrete pour. Their robust construction can withstand the weight of the steel and the wet concrete without deforming or collapsing. This stability is crucial for maintaining the design concrete cover and ensuring the rebars are properly embedded within the concrete matrix. The strength and stability of concrete chairs contribute to the consistent performance and structural integrity of the slab.
• Load Distribution: Concrete chairs distribute the load from the rebar evenly across the supporting surface, preventing localized stress concentrations. This is especially important in slabs on grade, where the chairs rest directly on the ground or a compacted base. The uniform load distribution minimizes the risk of settlement or displacement of the chairs, ensuring the rebar mat remains level and properly positioned throughout the concrete pouring and curing process.
• Cost-Effectiveness: Concrete chairs are often more cost-effective than other types of rebar supports, particularly for large slab projects. They can be manufactured on-site or purchased precast, offering flexibility in terms of supply and logistics. The relatively low cost of concrete chairs, combined with their durability and long lifespan, makes them an economical choice for supporting rebar in slabs on grade. This cost-effectiveness contributes to the overall affordability of reinforced concrete construction.

Proper Rebar Placement with Concrete Chairs The Key to Structural Integrity

While concrete chairs offer numerous advantages, their effectiveness hinges on proper installation and adherence to best practices. Incorrect placement of concrete chairs can compromise the structural integrity of the slab, negating the benefits of using this type of support. Here are some key considerations for proper rebar placement with concrete chairs:

• Spacing: The spacing of concrete chairs should be determined based on the rebar size, spacing, and the load-bearing requirements of the slab. The structural design drawings will typically specify the required chair spacing. Adequate spacing is crucial to ensure the rebar mat is uniformly supported and the design concrete cover is maintained throughout the slab. Insufficient chair spacing can lead to sagging or displacement of the rebar, while excessive spacing can result in unnecessary material costs.
• Placement on a Stable Surface: Concrete chairs should be placed on a stable and level surface to prevent them from sinking or tilting during the concrete pour. For slabs on grade, this typically involves compacting the subgrade or placing a layer of gravel or crushed stone to provide a firm base. Unstable surfaces can cause the chairs to shift, leading to uneven rebar placement and reduced concrete cover. Proper preparation of the supporting surface is essential for ensuring the stability and effectiveness of the concrete chairs.
• Overlapping Chairs: When using multiple concrete chairs, they should be overlapped or interlocked to create a continuous support system. This prevents individual chairs from tipping or shifting under load. Overlapping the chairs distributes the load more evenly and ensures the rebar mat remains stable throughout the concrete pouring and curing process. The overlapping technique enhances the overall stability and performance of the rebar support system.
• Chair Height: The height of the concrete chairs should be selected to provide the required concrete cover as specified in the structural design drawings. Using chairs that are too short will result in insufficient cover, while using chairs that are too tall will compromise the position of the rebar within the slab. The correct chair height is crucial for achieving the design concrete cover and ensuring the rebars are properly embedded within the concrete matrix.
• Inspection: Before pouring concrete, the placement of concrete chairs should be inspected to ensure they are correctly positioned and spaced. Any chairs that are damaged, displaced, or improperly installed should be replaced or adjusted. A thorough inspection is essential for identifying and correcting any issues before the concrete pour, preventing costly rework and ensuring the structural integrity of the slab.

Conclusion

In conclusion, the statement that concrete chairs are the most commonly used support for the bottom steel in a reinforced concrete slab resting directly on the ground is indeed TRUE. Concrete chairs offer a compelling combination of compatibility, corrosion resistance, strength, load distribution, and cost-effectiveness, making them the preferred choice for slabs on grade. However, proper rebar placement with concrete chairs is crucial for maximizing their benefits and ensuring the structural integrity of the concrete slab. By adhering to best practices for spacing, placement, and inspection, engineers and contractors can leverage the advantages of concrete chairs to create durable and reliable reinforced concrete structures.