A reinforcement bar is a steel bar or mesh of steel wires used in reinforced concrete and masonry structures to strengthen and aid the concrete under tension.
Proper placement of reinforcement bars, as per the specified drawings is vital for the structure's performance. Additionally, anchoring, welding, forming bonds and hooks, and binding of reinforcement bars in the concrete contribute to the overall strength and durability of the structure.
In this article, we discuss the standard procedure of placing reinforcement bars in concrete.
1. General Requirement of Steel
Steel used for reinforcement shall be clean and free from loose mill scales, dust, loose rust, coats of paints, oil, or other coatings which may destroy or reduce the bond strength. It must be stored in a way to avoid distortion, deterioration, and corrosion. Prior to the assembly of reinforcement bars, a lubricant shall be applied for removing the rust.
2. Assembly of Reinforcement
- The bars shall be bent correctly and accurately to the size and shape, as shown in the detailed drawing or as directed by the engineer.
- Preferably bars of full length shall be used. Necessary cutting and straightening is done if required.
- Overlapping of bars, where necessary, shall be done as directed by the engineer.
- The overlapping bars shall touch each other, and these shall be kept apart with concrete between them by 25 mm or 1 ¼ times the maximum size of the coarse aggregate, whichever is greater.
- But when not possible, the overlapping bars shall be bound together at intervals not exceeding twice the diameter of such bars with two strands annealed steel wire of 0.90 mm to 1.6 mm twisted tight.
- The overlaps/splices shall be staggered as per the directions of the engineer.
- But in no case, the overlapping shall be more than 50% of the cross-sectional area in one section.
3. Hooks, Bends, Anchorages, and Stirrups
A. U-Type Hook
In mild steel plain bars, standard U-type hook shall be provided by bending ends of rod into semicircular hooks having a clear diameter of the bar. In seismic zones, the size of hooks at the end of the rod shall be eight times the diameter of the bar or as given in the structural drawing.
B. Bends
The bend forming anchorage to an M.S. plain bar shall be bent with an internal radius equal to two times the diameter of the bar with a minimum length beyond the bend equal to four times the diameter of the bar.
C. Anchoring Bars in Tension
The deformed bars may be used without end anchorages provided, development length requirement is satisfied. Hooks should normally be provided for plain bars in tension.
D. Anchoring Bars in Compression
The anchorage length of the straight bar, which is in compression, shall be equal to the ‘development length’ of bars is compression. If the bar is in compression, the projected length of hooks, bends and straight lengths beyond the bend shall be considered for development length.
E. Binders, Stirrups, and Links
In case of binders, stirrups, links, etc. the straight portion beyond the curve at the end shall be not less than eight times the nominal size of the bar.
F. Welding of Bars
Whenever a facility for electric arc welding is available, welding of bars shall be done instead of an overlap. The engineer shall approve the location and type of welding.
4. Placing of Reinforcement Bar
- The fabricated reinforcement bars shall be placed in position as shown in the drawings or as directed by the engineer.
- The bars crossing each other must be tied together at every intersection with two strands of annealed steel wire of 0.9 to 1.6 mm thickness twisted tight to make the skeleton of the steelwork rigid so that the reinforcement does not get displaced during deposition of concrete.
- Track welding in crossing bars shall also be permitted instead of bending with steel wire if approved by the engineer.
- The bars shall be kept in the correct position by the following methods-
- In the case of beam and slab construction, precast cover blocks of cement mortar 1:2 about 4x4 cm section and of thickness equal to the specified cover shall be placed between the bars and shutters, so as to secure and maintain the requisite cover of concrete over reinforcement.
- In the case of cantilevered and doubly reinforced beams or slabs, the vertical distance between the horizontal bars shall be maintained by introducing chairs, spacers, or support bars of steel at 1.0 meter or at a shorter spacing to avoid sagging.
- In the case of columns and walls, the vertical bars shall be kept in position by means of timber templates with slots accurately cut in them; or with a block of cement mortar 1:2 of required size suitably tied to the reinforcement to ensure that they are incorrect position during concerning.
- In the case of R.C.C. structure such as arches, domes, shells, storage tanks, etc. a combination of cover blocks, spaces, and templates shall be used as directed by the engineer.
5. Tolerance in Placing of Reinforcement Bars
The reinforcement bars in the concrete shall be placed within the following tolerance limits-
| Tolerance in spacing | ||
| a) | For effective depth- 200 mm or less | ± 10 |
| b) | For effective depth- More than 200 mm | ± 15 |
The cover shall not be reduced by more than one-third of the specified cover or 5 mm, whichever is less.
6. Bending at Construction Joints
In the place where the reinforcement bars are bent aside at construction joints and afterward bent back into the original position, care shall be taken to ensure that the radius of the bend is less than four times the bar diameters for plain mild steel or six times the bar diameters for deformed bars.
Care shall also be taken when bending back bars to ensure that the concrete around the bars in not damaged.
FAQs on Placing of Reinforcement Bars in Concrete
The steel used for reinforcement shall be clean and free from loose mill scales, dust, loose rust, coats of paints, oil, or other coatings which may destroy or reduce bond. It must be stored in such a way to avoid distortion and to prevent deterioration and corrosion. Prior to assembly of reinforcement on no account, an oily substance shall be used for removing the rust.
The annealed steel wire of thickness 0.90 mm to 1.6 mm is used to tie reinforcement bars.
The tolerance limit in spacing of reinforcement bars is ± 10 for effective depth of the member is 200 mm or less and ± 15 for more than 200mm.
Read More:
1. How to Calculate Lap Length for Reinforcement in Concrete (R.C.C)?
2. What is Rebar? Types and Grades of Steel Reinforcement
3. Reinforcement Treatment Methods for Corrosion Protection