Guideline to use the plastic bar chair for steel rebar in reinforced concrete structures
1/ Selection of Plastic bar chair:
Before using the plastic bar chairs, the users must make sure the requirements of the thickness of concrete cover regulated in the design. In case, there is no instruction in the design, the user must refer to the current regulations. The selection of plastic spacer should be implemented in the following sequence:
Identify the type of structure: In general, most of all concrete spacer can be used in various structures and installation position. However, some spacers are specially designed with certain advantages for the particular type of structure. Consequently, a selection of right spacer will bring the advantage in technical aspect, the convenience in construction and reduce the cost for the project as well. Some type of plastic spacer are designed for bottom mesh, top mesh, steel work of columns/wall, the beam ends…
Plastic bar chair for slab mesh
Plastic bar chair for beam
Plastic chair for column/wall rebars
Identify the surface supporting the bar chairs: The load bearing capacity of the foundation or formwork is decisive to the selection of bar chair. In general, if the surfaces supporting the bar chairs are firm formwork or bedding concrete, the bar chairs will contact their supporting surfaces on the minimum areas. This is the common design for plastic bar chair of the type “ A shaped”, ”H shapes”, circular shape, rectangular shaped…
In case where the supporting surface are not firm, such as soil, stone base, we must use the bar chairs with their base areas large enough to distribute the load during the construction onto the ground and ensure the bar chair as well as the rebar they are supporting, will not be settled.
Plastic bar chairs with large base
Identify the layer of mesh: For supporting the bottom mesh layer, we can use any kind of plastic bar chair with the suitable height. However, if using the bar chair for upper mesh layer, we should consider to use the bar chair with the large base, such as cone shaped bar chair, which will avoid their turning over during the concreting work.
Plastic bar chair for top mesh
Some bar chair products are designed for both bottom and top mesh.
Plastic bar chair for both mesh layers
Determine the binding requirement with steel rebar
The plastic spacers directly bind the steel rebar with the elastic clip details. Depend on the requirement, the user will make decision whether to use the plastic spacer with clip or not.
To the reinforcement cage of footings, beams, slab are placed on the bar chairs and shall not be moved during the construction, we do not need to use the bar chair with clips. However, in case that the reinforcement cage of structures are removable, such as bore pile, precast structures…or they can be displaced during the construction by the equipment, workers…the user should select the spacer with clip to ensure them will not be lost during the construction.
Plastic bar chair wihout clip
To the rebars installed on the horizontal and vertical direction but there is no gravity forces to hold the bar chair, the bar chair with clip must be selected.
Spacer with clip to bind the steel rebar
To determine the concrete cover thickness: This is the main parameter of the bar chair product. The height of bar chairs are commonly designed with the interval of 5 or 10mm, depend on the type of product.
The plastic bar chair with various heights
To identify the rebar diameter: The bar chair are generally designed for a range of diameter from small to large. The user must select the right diameter of the bar chair, especially with the clip-on bar chair. The bar chairs with wrong diameter of the clip will not able to fit the steel bars during installation.
Hole of wheel chair will fit for some diameter of steel rebar only
2/ Installation of plastic bar chair
Check the steel rebars have been correctly installed and completed.
Check the right bar chair products for each structure.
To the reinforcement of beam or slab: Raise the steel cages for a gap between steel and formwork then place the bar chairs underneath; Adjust the position bar chairs in ensuring the good contact with the supporting surface.
To the rebar of column or wall: Determine the height, spacing suitable for installation of plastic spacer; adjust the wheels by turning them to the right position to obtain a good contact with the formwork.
Install wheel chair for the column steel cage
Install plastic wheel chair for wall steel cage
If using the clip-on or wheel bar chairs, the user pushes the bar chairs into the steel rebar or vice versa until the steel rebar reach to the inside face of the bar chairs.
Installation of steel rebar using plastic bar chairs
The distance of bar chairs will comply with the designer's guidance. According to the standards of some countries in Europe, the distance between the spacers/bar chair is 50 times the diameter of the reinforcement.