Last reviewed 16 Sept 2026 · 7 min read
Why RCC
Plain concrete is strong in compression but weak in tension (tensile strength roughly a tenth of its compressive strength) and fails suddenly. Steel is strong in tension. Reinforced cement concrete (RCC) places steel bars where tension occurs, so the composite section carries bending, shear and axial load efficiently. The two work together because:
- they bond well (especially deformed bars),
- their coefficients of thermal expansion are almost equal (about –/°C),
- concrete's alkalinity protects steel from corrosion when cover is adequate.
Grades of concrete
A grade such as M25 means concrete whose characteristic compressive strength on 150 mm cubes at 28 days is 25 N/mm².
| Group | Grades |
|---|---|
| Ordinary concrete | M10, M15, M20 |
| Standard concrete | M25 to M55 |
| High-strength concrete | M60 to M80 |
Characteristic strength is the strength below which not more than 5% of test results are expected to fall. With standard deviation :
This target mean strength is used in mix design (IS 10262).
Properties of concrete used in design
- Modulus of elasticity (short-term static): N/mm²
- Flexural strength (modulus of rupture): N/mm²
- Modular ratio in working stress design: (accounts for creep)
- Poisson's ratio for elastic analysis: about 0.2
- Coefficient of thermal expansion: depends on aggregate, typically –/°C
For M20: N/mm² and N/mm². For M25: N/mm², N/mm².
Creep
Creep is the slow increase of strain under sustained load. It increases long-term deflection and causes loss of prestress. IS 456 gives the creep coefficient (ultimate creep strain ÷ elastic strain) according to the age at loading:
| Age at loading | Creep coefficient |
|---|---|
| 7 days | 2.2 |
| 28 days | 1.6 |
| 1 year | 1.1 |
Creep is higher for younger concrete, higher w/c ratio, lower humidity and higher stress.
Shrinkage
Shrinkage is the reduction in volume as concrete dries, independent of load. In the absence of test data IS 456 permits an approximate total shrinkage strain of 0.0003. It causes cracking where restrained; it is reduced by lower water content, proper curing and adequate reinforcement.
Workability
| Placing conditions | Degree of workability | Slump (mm) |
|---|---|---|
| Blinding concrete, shallow sections, pavements with pavers | Very low | (compacting factor 0.75–0.80) |
| Mass concrete, lightly reinforced slabs, beams, walls, columns; floors; hand-placed pavements; canal lining; strip footings | Low | 25–75 |
| Heavily reinforced slabs, beams, walls, columns | Medium | 50–100 |
| Slipform work, pumped concrete | Medium | 75–100 |
| Trench fill, in-situ piling | High | 100–150 |
| Tremie concrete | Very high | (flow test) |
Durability — exposure, grade, cement content, cover
IS 456 links durability to the environment:
| Exposure | Example environment | Min. cement content (kg/m³) | Max. free w/c ratio | Min. grade | Nominal cover (mm) |
|---|---|---|---|---|---|
| Mild | Protected against weather, except coastal areas | 300 | 0.55 | M20 | 20 |
| Moderate | Sheltered from severe rain; buried in non-aggressive soil; condensation and rain | 300 | 0.50 | M25 | 30 |
| Severe | Alternate wetting and drying; severe rain; coastal environment; saturated salt air | 320 | 0.45 | M30 | 45 |
| Very severe | Sea-water spray; aggressive sub-soil or ground water | 340 | 0.45 | M35 | 50 |
| Extreme | Tidal zone; members in direct contact with aggressive chemicals | 360 | 0.40 | M40 | 75 |
Additional cover rules:
- For mild exposure, 15 mm cover is allowed for main bars of diameter up to 12 mm. For severe and very severe exposure, a reduction of 5 mm is allowed where concrete of grade M35 and above is used.
- Longitudinal bars in columns: nominal cover not less than 40 mm nor less than the bar diameter; for columns of minimum dimension 200 mm or under with bars not over 12 mm, 25 mm may be used.
- Footings: minimum cover 50 mm.
- Cover must not be less than the diameter of the bar.
Reinforcing steel
| Type | Designation | Yield / 0.2% proof stress | Remarks |
|---|---|---|---|
| Mild steel (plain round) | Fe 250 (IS 432) | 250 N/mm² | Ductile, weaker bond, used for ties and stirrups in some work |
| High-yield strength deformed (HYSD) bars | Fe 415, Fe 500, Fe 550 (IS 1786) | 415, 500, 550 N/mm² | Ribs improve bond; most common main reinforcement |
| Thermo-mechanically treated (TMT) bars | Fe 415/500/550 D grades | as grade | "D" grades have higher ductility |
| Hard-drawn steel wire fabric | IS 1566 | — | Slabs, shells |
- Modulus of elasticity of steel N/mm² (all grades).
- HYSD bars have no definite yield point; their characteristic strength is the 0.2% proof stress.
- Standard bar diameters: 6, 8, 10, 12, 16, 20, 25, 32, 36, 40 mm.
- Mass of a bar per metre kg/m ( in mm), e.g. 16 mm → 1.58 kg/m.