Steel reinforcing bar, or rebar, is used to reinforce concrete in building work. It can be a complicated area of construction to navigate if you’re unfamiliar with it. Rebar sizes vary greatly and choosing the right ones for the job requires a certain amount of knowledge and skill.
As well as the size of rebar, the quality is an important consideration too. Fortunately, global standards are in place that govern the physical, mechanical, and chemical properties of rebar, regardless of the source. This has gone some way to ensuring safety and consistency in construction projects across the world.
Before taking an in-depth look at rebar sizes we’ll briefly discuss how to check whether the rebar conforms to national and international standards.
Quality of the steel
To confirm authenticity and to assess the quality of the rebar being produced, proper mechanical testing is needed. This determines if the rebar being used meets published specifications, ensuring the quality of the product. There are various factors to be tested that fall into the following categories:
- Tensile strength
- Bending / malleability
Why do we need Different Rebar Sizes?
Rebar in concrete provides a uniform structural integrity, even though concrete material is very strong in compression, it is almost completely devoid of tensile strength. Without reinforcement, this inherent weakness of concrete becomes obvious in the material’s behaviour as it will bend and easily fracture.
Therefore, choosing the correct rebar size and weight is critical to safe building practices.
What rebar sizes should be used?
For small scale and domestic projects, rebar with diameters of 6mm, 8mm and 10mm will usually suffice.
For instance, the common practice for driveways and patios is to use the lightest 6mm rebar. Bear in mind that this is for smaller projects which will be bearing minimal loads and therefore creating low levels of tensile stress.
Whereas, for construction of walls, piers or columns, it’s recommended that an 8mm or greater rebar should be used. Similarly, for building footings, foundations etc. a good choice would be 10mm rebar. 10mm diameter or greater rebar is best for footers and foundations to minimize settling.
For large scale, civil engineering projects such as bridge or tunnel construction, the higher diameter rebar would be used, particularly where long spans are needed.
The installation of the right rebar size is critical for the success of the overall project. The placement of rebar should be even throughout the project. When a flat slab is under consideration, such as a driveway, you would typically use 6mm rebar in approximately 450mm grid spacing. For a patio you would most likely use 6mm diameter rebar with a grid spacing of approximately 600mm.
For further information on the specification of steel rebar in the UK please refer to BS 4449: 2005 Steel for the reinforcement of concrete weldable reinforcing steel, bar, coil and decoiled product. This is the national standard that has replaced the European standard for reinforcement of concrete.
Yield strength or tensile strength is an important consideration when designing rebar. This measurement indicates the overall strength of the steel. High yield steel is best used for heavy duty rebar and has grade 500 reinforcement has a yield strength of 500 Mpa (or N/mm2).
The maximum permitted yield strength under BS 4449:2005 is 650 MPa.
Please note that adjusting the rebar sizes by increasing the diameter does not make it twice as strong. There is a definite increase in yield strength, but the grade of the steel affects it more.
Accuracy in placement and size of rebar
Extra care needs to be taken when placing the rebar before pouring the concrete. The yield strength of the concrete can be severely compromised if placement is not correct. Precise measurements are crucial, mistakes of just a cm or two in rebar spacing can reduce the tensile strength of the concrete by up to 20%.
Also, make sure you refer to the BS 4449:2005 table when choosing the size. Placing the wrong size rebar can result in 35% less reinforcement than is needed for structural strength and integrity.