Reinforcement bars, commonly known as rebars, are steel bars or meshes used in reinforced concrete and masonry structures to provide tensile strength to concrete, which is inherently strong in compression but weak in tension. Proper placement and handling of these reinforcement bars are crucial to ensuring the structural integrity, strength, and durability of any concrete construction.
General Requirements of Steel
Steel used for reinforcement must be clean and free from contaminants such as loose mill scales, dust, rust, paint coatings, oil, or any substances that can reduce the bond strength between the steel and concrete. Proper storage of steel bars is essential to prevent distortion, corrosion, and deterioration before use. Before assembling reinforcement bars, it is recommended to apply a lubricant that can help remove any rust and maintain the quality of the steel.
Assembly of Reinforcement Bars
The bars must be bent accurately and correctly, following the detailed drawings or as directed by the engineer. Whenever possible, bars of full length should be used; however, if cutting or straightening is necessary, it must be done carefully to maintain the structural requirements.
Overlapping bars, where needed, should be done according to the engineer’s instructions. The overlapping bars should ideally be placed with a clear concrete gap of at least 25 mm or 1¼ times the maximum aggregate size between them. If this is not feasible, the bars should be securely tied with annealed steel wire (0.9 to 1.6 mm thick) at intervals not exceeding twice the diameter of the bars. To maintain structural strength, overlapping or splicing must be staggered and must not exceed 50% of the cross-sectional area of a section.
Hooks, Bends, Anchorages, and Stirrups
- U-Type Hooks: For mild steel plain bars, standard U-type hooks are made by bending the ends into semicircular shapes matching the bar’s diameter. In seismic zones, the hook size should be eight times the diameter or as specified in structural drawings.
- Bends: Bends that form anchorages on mild steel bars must have an internal radius twice the bar diameter and a minimum extension beyond the bend of four times the diameter.
- Anchoring Bars in Tension: Deformed bars generally do not require end anchorages if the development length criteria are met. Plain bars should normally have hooks to ensure anchorage.
- Anchoring Bars in Compression: The anchorage length of compression bars should equal the development length. The length beyond bends and hooks also counts towards this.
- Binders, Stirrups, and Links: The straight portion extending beyond the bend must be at least eight times the nominal bar size.
- Welding of Bars: When electric arc welding facilities are available, welding may replace overlapping. The location and type of welding should be approved by the engineer.
Placing of Reinforcement Bars
Fabricated reinforcement bars must be positioned as per structural drawings or engineer’s directives. Bars crossing each other must be tied at every intersection with annealed steel wire (0.9 to 1.6 mm thick) twisted tightly to create a rigid steel framework and prevent displacement during concrete pouring. Track welding may be allowed as an alternative if approved.
To maintain the correct position and cover:
- For beams and slabs, precast cement mortar cover blocks (about 4×4 cm) of specified thickness should be used to maintain concrete cover.
- In cantilevered or doubly reinforced beams and slabs, vertical spacing between horizontal bars is maintained using chairs, spacers, or steel support bars spaced at one meter or less.
- For columns and walls, vertical bars are kept in place using timber templates with precisely cut slots or cement mortar blocks tied to the reinforcement.
- For complex RCC structures like arches, domes, and tanks, a combination of cover blocks, spacers, and templates is employed as directed by the engineer.
Tolerance in Placing Reinforcement Bars
The placement of reinforcement bars must adhere to specified tolerance limits to ensure structural safety:
- For effective depths up to 200 mm, the spacing tolerance is ±10 mm.
- For effective depths exceeding 200 mm, the spacing tolerance is ±15 mm.
- The concrete cover must not be reduced by more than one-third of the specified cover or 5 mm, whichever is less.
Bending at Construction Joints
At construction joints, if bars need to be bent aside and then bent back to their original positions, the bending radius should be at least four times the diameter for plain mild steel bars and six times for deformed bars. Care must be taken to avoid damage to the surrounding concrete when bending bars back into place.
Conclusion
The proper placement of reinforcement bars is essential for the strength and durability of reinforced concrete structures. Following the detailed guidelines for steel quality, bar assembly, hooks, anchorage, positioning, and tolerances ensures that the concrete structure performs well under tension and load conditions. Adhering to these standards, along with the supervision of engineers, leads to safer and longer-lasting construction.