Reinforcement Detailing of Reinforced Concrete Slabs

Reinforcement Detailing of Reinforced Concrete Structures

Reinforcement Detailing or Rebar detailing is the discipline of preparing ‘shop/placing’ drawings or shop drawings of steel reinforcement for construction. Architect/Engineers(A/E) prepare ‘Design Drawings’ that develop required strengths by applying rebar size, spacing, location, and lap of steel.

Standard Hooks:

The term “standard hook” as used in this Code shall mean one of the following:

1. 180o bend plus an extension of at least 4 bar diameters, but not less than 65 mm at the free end of the bar.
2. 90o bend plus an extension of at least 12 bar diameters at the free end of the bar.
3. For stirrup and tie anchorage
   For 16 mm φ bar and smaller, a 90o bend plus an extension of at least 6 bar diameters at the free end of the bar,
   • For 19 mm to 25 mm φ bars, a 90o bend plus an extension of at least 12 bar diameters at the free end of the bar,
   • For 25 mm φ bar and smaller, a 135o bend plus an extension of at least 6 bar diameters at the free end of the bar,
   • For closed ties and continuously wound ties, a 135o bend plus an extension of at least 6 bar diameters, but not less than 75 mm.

Minimum Bend Diameters:

The minimum diameter of bend measured on the inside of the bar, for standard hooks other than for stirrups and ties in sizes 10 mm φ through 16 mm φ, shall not be less than the values shown in Table below

Minimum Diameter of Bends | Reinforcement Detailing of RCC Structures

For stirrups and tie hooks, inside diameter of bend shall not be less than 4 bar diameters for 16 mm φ bar and smaller. For bars larger than 16 mm φ, diameter of bend shall be in accordance with Table above.

Inside diameter of bend in welded wire reinforcement for stirrups and ties shall not be less than 4 bar diameters for deformed wire larger than MD40 and 2 bar diameters for all other wires. Bends with inside diameter of less than 8 bar diameters shall not be less than 4 bar diameters from nearest welded intersection.

Bending:

Unless otherwise permitted by the engineer, all reinforcement shall be bent cold.

Reinforcement partially embedded in concrete shall not be bent in place, except as permitted by the engineer or as shown in the design drawings.

Surface Conditions of Reinforcement:

When concrete is placed, metal reinforcement shall be free from mud, oil, or other nonmetallic coatings that decrease bond. Epoxy‐coating of steel reinforcement in accordance with standards referenced in this code shall be permitted.

Metal reinforcement with rust, mill scale, or a combination of both, shall be considered satisfactory, provided the minimum dimensions (including height of deformations) and weight of a hand‐wirebrushed test specimen are not less than applicable ASTM specification requirements.

Placing of Reinforcement:

1. Reinforcement shall be accurately placed and adequately supported before concrete is placed, and shall be secured against displacement within tolerances permitted in table below,

• Reinforcement shall be placed within the following tolerances unless otherwise specified by the engineer:
• Tolerances for depth d, and minimum concrete cover in flexural members, walls and compression members shall be as set forth in below table

• Notwithstanding the provision of (a) above, tolerance for the clear distance to formed soffits shall be minus 6 mm and tolerance for cover shall not exceed minus 1/3 the minimum concrete cover specified in the design drawings or specifications.

1. Tolerance for longitudinal location of bends and ends of reinforcement shall be ± 50 mm, except at discontinuous ends of brackets and corbels, where tolerance shall be ± 13 mm and at discontinuous ends of other members, where tolerance shall be ±25 mm. The tolerance for concrete cover of shall also apply at discontinuous ends of members.
2. Welded wire reinforcement (with wire size not greater than MW30 or MD30) used in slabs not exceeding 3 m in span shall be permitted to be curved from a point near the top of slab over the support to a point near the bottom of slab at midspan, provided such reinforcement is either continuous over, or securely anchored at support.
3. Welding of crossing bars shall not be permitted for assembly of reinforcement unless authorized by the engineer.

Spacing of Reinforcement:

The minimum clear spacing between parallel bars in a layer shall be equal to one bar diameter, but not less than 25 mm, or 4/3 the maximum nominal size of coarse aggregate, whichever is larger.

Where parallel reinforcement is placed in two or more layers, bars in the upper layers shall be placed directly above those in the bottom layer with clear distance between layers not less than 25 mm.

For compression members, the clear distance between longitudinal bars shall be not less than 1.5 bar diameters nor 40 mm nor 4/3 the maximum nominal size of coarse aggregate.

Clear distance limitation between bars shall apply also to the clear distance between a contact lap splice and adjacent splices or bars.

In walls and one‐way slabs the maximum bar spacing shall not be more than three times the wall or slab thickness h nor 450 mm.

For two‐way slabs, maximum spacing of bars shall not exceed two times the slab thickness h nor 450 mm.

For temperature steel only, maximum spacing shall not exceed five times the slab thickness h nor 450 mm.

Bundled Bars 

(a) Groups of parallel reinforcing bars bundled in contact to act as a unit shall be limited to four in any one bundle.
(b) Bundled bars shall be enclosed within stirrups or ties.
(c) Bars larger than 32 mm φ shall not be bundled in beams.
(d) Individual bars within a bundle terminated within the span of flexural members shall terminate at different points with at least 40db stagger.
(e) Where spacing limitations and minimum concrete cover are based on bar diameter db, a unit of bundled bars shall be treated as a single bar of a diameter derived from the equivalent total area.

Reinforcement Detailing of Reinforced Concrete Slabs

Reinforcement detailing of a slab is done based on its support conditions. Slab may be supported on walls or beams or columns. Slab supported directly by columns are called flat slab.

Slab supported on two sides and bending takes place predominantly in one direction only is called One Way Slab. On the other hand, when slab is supported on all four sides and bending take place in two directions are said to be Two Way Slab.

The slabs having ratio of longer length to its shorter length (Ly/Lx) greater than 2 is called one way slab otherwise as two way slab. In one way slab main reinforcement is parallel to shorter direction and the reinforcement parallel to longer direction is called distribution steel. In two way slab main reinforcement is provided along both direction.

Slabs could be simply supported, continuous or cantilever. In two way slab the corners may be held down by restraints or may be allowed to lift up. Additional torsion reinforcement is required at corners when it is restrained against uplifting as shown in Fig.1.

One Way Slab
Thickness of the slab is decided based on span to depth ratio specified in IS456-2000. Minimum reinforcement is 0.12% for HYSD bars and 0.15% for mild steel bars. The diameter of bar generally used in slabs are: 6 mm, 8 mm, 10 mm, 12mm and 16mm.

The maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab. Maximum spacing of main bar is restricted to 3 times effective depth or 300 mm whichever is less. For distribution bars the maximum spacing is specified as 5 times the effective depth or 450 mm whichever is less.

Minimum clear cover to reinforcements in slab depends on the durability criteria and this is specified in IS 456-200. Generally 15mm to 20mm cover is provided for the main reinforcements. Alternate main bars can be cranked near support or could be bent at 1800 at the edge and then extended at the top inside the slab as shown in Fig.1. Curtailment and cranking of bars and is shown in Fig. 2.

Torsion reinforcement shall be provided at any corner where the slab is simply supported on both edges meeting at that corner and is prevented from lifting unless the consequences of cracking are negligible. It shall consist of top and bottom reinforcement, each with layer of bars placed parallel to the sides of the slab and extending from the edges a minimum distance of one fifth of the shorter span.

The area of reinforcement per unit width in each of these four layers shall be three quarters of the area required for the maximum midspan moment per unit width in the slab.

Torsion reinforcement equal to half that described above shall be provided at a corner contained by edges over only one of which the slab is continuous. Torsion reinforcement to be provided is shown in Fig. 3 below.

The drawing showing the detailing of reinforcement has a plan showing typical reinforcement in both direction and sectional elevations. Typical detailing of slab is shown in Fig.4 and 5.