---
title: "Rationale for reinforcing concrete with steel"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:rationale-steel-reinforcement"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/rationale-steel-reinforcement
site: BCT Engineering
---

# Rationale for reinforcing concrete with steel

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2066 Jestha (old course), 4 marks
- Video: [why do we provide steel in concrete? | why is concrete weak in tension and strong in compression?](https://www.youtube.com/watch?v=2SrX_ErYP2g) — Civil Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/rationale-steel-reinforcement
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

Write the rationale of the use of steel bars for reinforcing concrete.

## Short answer

C oncrete is strong in compression but weak in tension. S teel reinforcement provides tensile strength, bonds well with concrete, has similar thermal expansion, is protected from corrosion by alkaline paste, and adds ductility.

## Answer

Concrete is strong in compression (20-60 MPa) but weak in tension (only about 8-12 per cent of its compressive strength, $f_{cr} = 0.7\sqrt{f_{ck}}$ as per IS 456 cl. 6.2.2). When a beam or slab bends, the tensile zone cracks at a low load. Steel bars are placed to carry the tension. The reasons are:

1. **High tensile strength of steel**: Fe 415/500 has a yield strength of 415/500 MPa, so it takes all the tension after cracking while concrete takes the compression.
2. **Good bond**: concrete grips the steel (deformed bars give greater bond), so the two act together without slip; bond is developed over the development length ($L_d$, IS 456 cl. 26.2).
3. **Nearly equal coefficients of thermal expansion** (concrete 10-14, steel 12 $\times 10^{-6}$/°C), so no large internal stress from temperature change.
4. **Corrosion protection**: the alkaline nature of concrete (pH about 12.5) forms a passive film on steel; the cover (IS 456 cl. 26.4) protects the bars from corrosion and fire.
5. **Ductility**: reinforcement gives warning before failure; under-reinforced sections fail gradually.
6. Steel also resists shear (stirrups), controls shrinkage and temperature cracking, and confines columns (ties).

Hence reinforced concrete combines the economy and durability of concrete with the tensile strength of steel.

## Check yourself

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

1. Concrete's tensile strength is about:
   A. 50%
   B. Equal to compressive
   C. 8-12% of compressive
   D. 200%
   Answer: C. So tension zones crack at low load. (Key point: Weak tension)

2. Steel acts with concrete because of:
   A. Smooth surfaces
   B. Chemical melting
   C. Different expansion
   D. Good bond (deformed bars improve it)
   Answer: D. Bond develops over development length. (Key point: Bond)

3. Why does temperature not break the bond?
   A. Nearly equal thermal expansion coefficients
   B. Steel does not expand
   C. Concrete does not expand
   D. Steel is insulated
   Answer: A. No large internal stresses. (Key point: Thermal)

4. Corrosion protection arises from:
   A. Acidic concrete
   B. Alkaline concrete (pH about 12.5) and cover
   C. Paint
   D. Zinc coating
   Answer: B. A passive film forms on steel. (Key point: Corrosion)

## Related in Concrete Technology and Masonry Structure

- [Concrete ingredients; concrete versus steel](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aconcrete-ingredients-vs-steel) — Concrete Technology and Masonry Structure, chapter 1
- [Bond between steel and concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Abond-steel-concrete) — Concrete Technology and Masonry Structure, chapter 5
- [Concrete as a structural material](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aconcrete-structural-material) — Concrete Technology and Masonry Structure, chapter 1, asked 3×
- [Causes of deterioration; effects of steel corrosion](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acauses-deterioration-corrosion-effects) — Concrete Technology and Masonry Structure, chapter 6, asked 2×

## Related in other subjects

- [Characteristics of reinforcing steel bars](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Acharacteristics-of-reinforcing-steel-bars) — Design of RCC Structure, chapter 3
- [Reinforced concrete column stresses in steel and concrete](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arc-column-8-bars) — Strength of Materials, chapter 3
- [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
- [Check column 600x400 with 8-28 mm bars](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Acheck-column-600x400-with-8-28) — Design of RCC Structure, chapter 10