5.2 Types of reinforcements
Reinforcement is a term used in the construction industry to refer to materials, mainly steel, that are embedded in concrete to improve its tensile strength. This includes deformed and plain steel bars (without surface deformations), welded wire mesh, and other steel products. Since concrete is strong in compression but weak in tension, reinforcement is essential to prevent cracking and structural failure, ensuring concrete structures’ durability and load-bearing capacity. Figure 5.1 below illustrates the stress distribution across a beam’s cross-section under bending. The upper portion of the beam is subjected to compressive stresses, while the lower portion experiences tensile stresses. Therefore, steel reinforcement should be placed in the tension zone to enhance the beam’s strength and prevent failure.
A reinforcement bar is a steel product manufactured with dimensional tolerances to ensure consistency in construction applications. While it is typically supplied in straight lengths, it is also available in coiled form for specific uses (Figure 5.2).
Reinforcing bars are identified based on both the quantity and diameter of the bars. The designation follows a standardized format, where the first number represents the total number of bars, and the subsequent number indicates the diameter of each bar in millimetres. For instance, a specification such as 6N12 denotes a set of six reinforcing bars, each with a 12 mm diameter. This notation helps in accurately specifying reinforcement requirements for structural design and construction.
Reinforcing mesh is a prefabricated steel reinforcement product used in concrete construction to enhance tensile strength. It is available in two primary forms: flat sheets and rolls. Flat sheets are manufactured with reinforcing bars up to 12 mm in diameter, while rolls are typically made with bars up to 5 mm in diameter. The typical standard sheet size for reinforcing mesh is 6 metres in length by 2.4 metres in width. The mesh is composed of reinforcing steel bars welded together in either a square or rectangular grid pattern. Depending on the structural requirements, reinforcing mesh can be supplied in two main types (Figure 5.3): square mesh and rectangular mesh. Square mesh is designated using “SL” notation, such as SL72, SL82, SL92, and SL102, while rectangular mesh is identified with “RL” notation, such as RL1218, RL1018, and RL918. The designation of these meshes is based on two factors namely (i) the diameter of each reinforcing bar and (ii) the spacing between consecutive bars. For example, SL62 is a square mesh consisting of 6 mm diameter bars arranged at 200 mm intervals in both directions, and RL1218 is a rectangular mesh consisting of 12 mm diameter bars spaced at 100 mm intervals in one direction and 8 mm diameter bars spaced at 200 mm intervals in the perpendicular direction.
Trench mesh (Figure 5.4 a) is a type of reinforcing mesh specifically designed for concrete footings and beams, typically manufactured in long, narrow strips to suit these applications. It is commonly used in strip footings, edge beams, and other structural elements requiring continuous reinforcement. The designation of trench mesh is based on the number of longitudinal bars and their diameter. For example, 3-L11TM consists of three longitudinal bars, each with a diameter of 11 mm, while 4-L12TM consists of four longitudinal bars, each with a diameter of 12 mm.
In addition to the longitudinal bars, ligatures (fitments) are often incorporated to separate the top and bottom layers of trench mesh in residential slabs or to assist in the fabrication of beam and column cages. These ligatures (Figure 5.4 b) help maintain the correct positioning of reinforcement within the concrete, ensuring structural integrity and compliance with engineering specifications.
