Detailed specification of RCC M20 column

Detailed specification: RCC M20 concrete in columns

Scope: Providing and casting reinforced cement concrete of grade M20 (nominal mix 1 : 1.5 : 3, characteristic strength 20 N/mm² at 28 days) in columns, including formwork, reinforcement, curing and finishing, as per IS 456:2000 and the drawings.

Materials

• Cement: OPC 43 grade/PPC, fresh and in good condition; minimum cement content 300 kg/m³ (IS 456 Table 5).
• Fine aggregate: clean river sand, zone II, free of silt and organic matter.
• Coarse aggregate: crushed stone, hard and angular, graded 20 mm down; flaky particles within limits.
• Water: potable; water-cement ratio not more than 0.50 (max 0.55 for mild exposure).
• Steel: Fe 415/Fe 500 deformed bars as per drawing, free from rust and oil.

Formwork: rigid, tight, plumb steel or timber/plywood shuttering with proper bracing, oiled with release agent. Cleaned and checked before concreting.

Reinforcement: cut, bent and placed as per bar bending schedule, tied with binding wire; clear cover 40 mm for columns (IS 456 clause 26.4.2.1) using cover blocks; lap lengths and stirrup spacing as per drawing.

Concreting

• Batching by weight preferably, or by volume using gauge boxes in a mechanical mixer; mixing for at least 2 minutes.
• Slump 50–75 mm for columns.
• Concrete is placed within 30 minutes of mixing, in layers of 300–450 mm, from a height less than 1.5 m to avoid segregation, and compacted by a needle vibrator to remove air voids.
• Place columns in a continuous pour; the construction joint at 25–50 mm below the beam soffit when needed.

Stripping of forms: vertical column forms may be removed after 24–48 hours (IS 456 clause 11.3).

Curing: wet curing (gunny bags or ponding) for at least 7 days.

Testing: cube samples (150 mm) at least one set per 5 m³ per day, tested at 7 and 28 days; slump tests; average strength not less than 20 N/mm² and no individual result below 16 N/mm² per IS 456 acceptance criteria.

Tolerance: column plumb within 5 mm per 3 m; size ±5 mm.

Measurement and payment: concrete in m³ (net, no deduction for steel), reinforcement in kg (as per bar bending schedule), formwork in m². Rates include all materials, labour, vibration, curing and scaffolding.