---
title: "Detailed specification of RCC M20 column"
subject: "Construction Management"
chapter: 12
key: "semester_8/construction_management:detailed-spec-rcc-m20-column"
source: https://www.bctengineer.com/q/semester_8/construction_management/detailed-spec-rcc-m20-column
site: BCT Engineering
---

# Detailed specification of RCC M20 column

Construction Management · Chapter 12: Specification

- Asked in: 2069 Bhadra (old course), 4 marks
- Video: [How to calculate Cement Sand and Aggregate for M20 concrete | cement sand aggregate calculation |](https://www.youtube.com/watch?v=lSIq4974l1Q) — Civil Engineers 07
- Page: https://www.bctengineer.com/q/semester_8/construction_management/detailed-spec-rcc-m20-column
- Whole chapter: https://www.bctengineer.com/md/semester_8/construction_management/chapter-12

## Question

Write a detailed specification of RCC with M20 concrete in column.

## Short answer

M20 concrete (1:1.5:3), w/c ≤0.50, slump 50–75 mm. Clear cover 40 mm, compacted by needle vibrator in layers ≤450 mm. Formwork removed after 24–48 h. Curing 7 days. Concrete measured in m³, steel in kg, formwork in m².

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does M20 mean and what is its nominal mix?
   A. 20 N/mm2 at 7 days, mix 1:2:4
   B. 20 N/mm2 characteristic strength at 28 days, nominal mix 1:1.5:3
   C. 20 kg cement per m3, mix 1:3:6
   D. 20 mm aggregate, mix 1:1:2
   Answer: B. M20 has a characteristic strength of 20 N/mm2 at 28 days and nominal mix 1:1.5:3. (Key point: M20 meaning)

2. Which values apply for columns?
   A. Cover 10 mm, slump 200 mm
   B. Cover 75 mm, slump 0
   C. Cover 5 mm, slump 150 mm
   D. Clear cover 40 mm, slump 50-75 mm
   Answer: D. Columns have 40 mm clear cover (IS 456) and slump of 50-75 mm. (Key point: Cover and slump)

3. How should concrete be placed in columns?
   A. Dropped from any height without vibration
   B. Within 30 minutes of mixing, in 300-450 mm layers, from less than 1.5 m height, compacted by needle vibrator
   C. In one layer 3 m deep
   D. After 3 hours of mixing
   Answer: B. Limiting drop height and layer thickness prevents segregation; vibration removes air voids. (Key point: Concreting practice)

4. What is the acceptance criterion for cube strength?
   A. Any value is accepted
   B. Each cube above 40 N/mm2
   C. Average 10 N/mm2
   D. Average not less than 20 N/mm2 and no individual below 16 N/mm2
   Answer: D. IS 456 requires mean strength at least 20 N/mm2 and no individual result below 16 N/mm2. (Key point: Acceptance criteria)

## Related in Construction Management

- [Types of specification: general and detailed](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aspec-types-general-detailed) — Construction Management, chapter 12, asked 7×
- [Detailed specification of brick work in 1:4 mortar](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Adetailed-spec-brick-1-4) — Construction Management, chapter 12, asked 5×
- [Detailed specification of brick work in 1:6 mortar](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Adetailed-spec-brick-1-6) — Construction Management, chapter 12, asked 5×
- [Detailed specification of cement sand mortar 1:4](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Adetailed-spec-cs-mortar-1-4) — Construction Management, chapter 12, asked 2×
- [Detailed specification of cement sand mortar 1:6](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Adetailed-spec-cs-mortar-1-6) — Construction Management, chapter 12, asked 2×
- [Detailed specification of cement plaster 1:4](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Adetailed-spec-plaster-1-4) — Construction Management, chapter 12

## Related in other subjects

- [Cost of M20 PCC for RCC column](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arate-m20-pcc-column-cost) — Estimating and Costing, chapter 4
- [RCC column with footing: concrete and steel](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arcc-column-footing-h) — Estimating and Costing, chapter 6
- [Underground RCC tank: brickwork 1:4, M20 PCC, steel](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aug-tank-e-fig1) — Estimating and Costing, chapter 6
- [Rate analysis: M20 (1:1½:3) RCC per 10 m³](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Arate-m20-rcc-10m3) — Estimating and Costing, chapter 4