Rebar, short for reinforcing bar, is a critical component in concrete footing for any structure. It is a steel rod or wire mesh added to concrete to enhance its strength and durability. Rebar can significantly improve the load-bearing capacity of the foundation, making it able to withstand heavy loads, natural disasters, and other external forces.
When used correctly, it can also prevent the formation of cracks and increase the overall lifespan of the structure. However, using rebar in concrete footings requires careful planning and attention to detail to ensure it is installed correctly and at the appropriate depth.
This guide will provide you with all the essential information you need to know about using rebar in concrete footings, including its benefits, types, placement, and installation procedures.
Why use rebars in concrete footings?
Using rebar for concrete footings is essential for several reasons:
1. Increased strength – Including rebar in concrete footings imparts tensile strength to the structure, enabling it to resist cracking and breakage caused by the weight of the superstructure and the settling of the ground. This extra reinforcement enhances the footings' ability to support the load and maintain stability over an extended period.
2. Improved load distribution - The use of rebar in concrete footings promotes uniform load distribution by evenly distributing the structure's weight across the footing. This even load distribution minimises the likelihood of uneven settlement and potential structural damage. This load distribution is especially crucial in regions with variable soil conditions or where the footing is under heavy or concentrated loads.
3. Crack control - Concrete is resilient in compression but vulnerable to tension. Hence, it is prone to cracking when subjected to tensile stresses such as ground movement, temperature changes, or shrinkage during curing. Incorporating rebar in the concrete reinforces it, providing tensile strength and controlling and reducing the size and frequency of cracks that may occur in the footing.
4. Enhanced structural integrity - Including rebar in footings links them to the rest of the building, such as columns, walls, or slabs, creating a harmonious and unified system that collaboratively supports the structure. This connection enhances the overall structural integrity and stability of the building.
Types of Rebars
1. Steel rebar: Steel rebar is the most commonly used type in construction. It is made of carbon steel and has excellent strength and durability. Steel rebar is available in different grades and sizes to meet construction requirements. It is suitable for all construction projects, including high-rise buildings, bridges, and highways.
2. Fibreglass rebar: Fibreglass rebar is a lightweight and corrosion-resistant alternative to steel rebar. It is made of glass fibres and a polymer resin matrix, which makes it ideal for use in environments with high moisture or chemical exposure. Fibreglass rebar is non-conductive, making it an ideal choice for structures that require electrical isolation. It is also easy to handle and install, and its lightweight properties make it easier to transport and manoeuvre.
3. Stainless steel rebar: Stainless steel rebar is a more expensive option than steel rebar but offers exceptional corrosion resistance. It is made of steel and chromium, creating a passive oxide layer that protects the rebar from corrosion. Stainless steel rebar is ideal for structures exposed to corrosive elements such as seawater, deicing salts, and acidic environments.
4. Galvanised rebar: Galvanised rebar is a steel rebar with a protective zinc coating. The zinc coating provides a barrier that resists corrosion better than uncoated steel. Galvanised rebar is ideal for use in concrete structures that are exposed to corrosive elements such as salt, chemicals, and moisture.
5. Epoxy-coated rebar: Epoxy-coated rebar features a protective epoxy coating that prevents rusting. The epoxy coating provides a barrier that resists corrosion and protects the rebar from the effects of concrete alkalinity. Epoxy-coated rebar is ideal for use in concrete structures that are exposed to corrosive elements such as salt, chemicals, and moisture.
Rebar sizes and grades
1. Size - Rebar is available in various sizes and grades to meet construction needs. The sizes of rebar range from #3 (3/8") to #18 (2 1/4") and are chosen based on the size and load capacity of the footing.
2. Length - Rebar bars commonly come in lengths of 10' and 20', although lengths from 2' to 60' can be purchased at various locations.
3. Grade - Rebar grades denote the yield strength of the steel and are typically 40, 60, and 75. The higher the grade, the stronger the rebar. The selection of the appropriate rebar size and grade is essential to ensure that the footing can support the load and remain stable over time.
Rebar size guide for footings
The size of rebar to use for footings depends on the type of footing being constructed.
For residential wall footings and deck footings, #4 or #5 rebar is suggested.
For column footings and slab footings, #5, #6, or #8 rebar is recommended.
Grade beams require slightly larger rebar, with #5, #6, or #8 being appropriate.
For retaining wall footings, #5, #6, or #8 rebar is suggested. A larger rebar is necessary when constructing bridge pier footings, with #8, #9, or #10 being suitable.
Finally, industrial footings require even larger rebar, with #6, #8, or #10 being the suggested sizes.
The appropriate rebar size ensures that the footing can support the weight of the structure above and resist the natural settling of the ground. The size and type of rebar utilised in concrete slab footings will be based on the project's specific demands, including the slab's dimensions, weight, and local building regulations.
Similarly, the amount of reinforcement used in a slab footing will vary based on factors such as the dimensions of the footing, the spacing between rebars, and the level of reinforcement required. Generally, for a 24" wide slab footing, two or three rebars are laid in a flat, horizontal position along the entire length of the footing.
Rebar in slab footings is usually arranged systematically, forming a grid-like pattern that includes horizontal and vertical bars crossing at regular intervals. This grid-like pattern is essential for ensuring balanced reinforcement throughout the slab, leading to optimal tensile strength and resistance to cracking.
The main bars or horizontal rebars run parallel to the length of the slab, while distribution bars or vertical rebars run perpendicular to the main bars. Adequate support must be provided for the rebars during the concrete pour to maintain correct positioning and ensure proper concrete cover. This support can be in chairs, spacers, or bar supports.
To ensure compliance with local building codes and regulations regarding rebar in concrete footings, it is crucial to seek guidance from a structural engineer or refer to the relevant building codes applicable to your area.
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