---
title: "Importance and method of compressive strength test"
subject: "Concrete Technology and Masonry Structure"
chapter: 5
key: "semester_5/concrete_technology_and_masonry_structure:compressive-strength-importance-method"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/compressive-strength-importance-method
site: BCT Engineering
---

# Importance and method of compressive strength test

Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control

- Asked in: 2065 Shrawan (old course), 4+6 marks
- Video: [Compressive Strength of Concrete (Cube Test) | Procedure, Calculation, Acceptance Criteria & Example](https://www.youtube.com/watch?v=9xOUBt7d6wU) — Always Civil Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/compressive-strength-importance-method
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5

## Question

Write the importance of the compressive strength of concrete in the design of reinforced concrete structures. Explain the method of determining compressive strength of concrete.

## Short answer

Compressive strength fck is the basis of RC design: it defines the design stress block (0.446 fck), elastic modulus (5000 sqrt(fck)), flexural strength (0.7 sqrt(fck)), shear and bond. Test: cast and cure 150 mm cubes, at 28 days load on side faces at 14 N/mm^2/min, fc = P/A, average three cubes. Acceptance: group mean >= fck+0.825s, individual >= fck-3 N/mm^2.

## Answer

#### Importance in RC design
- Compressive strength is the **basis of structural design**: the grade (M20, M25 ...) gives the characteristic strength $f_{ck}$, which sets the design stress of $0.446 f_{ck}$ in the limit-state stress block (IS 456, Cl. 38.1) and the permissible bending compressive stress $\sigma_{cbc}$ of about $0.33 f_{ck}$ in working stress design (IS 456, Table 21).
- Other properties are derived from it: $E_c = 5000\sqrt{f_{ck}}$ (Cl. 6.2.3.1), flexural strength $0.7\sqrt{f_{ck}}$ (Cl. 6.2.2), shear $\tau_c$ and bond $\tau_{bd}$ (Cl. 26.2.1.1).
- It is the principal index of the quality of concrete, since it also reflects density, durability and permeability.
- It is the easiest to test and is the criterion of mix design (IS 10262), quality control and acceptance (IS 456 Cl. 16).
- Concrete resists compression in columns, the compression zone of beams and slabs, footings and walls, which is its main role in RC members.

#### Compressive strength test (IS 516)
1. **Sampling and casting:** take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
2. **Curing:** keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
3. **Testing:** take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on two opposite faces of the cube as cast (not on its top and bottom as cast). Apply the load without shock at 14 N/mm$^2$ per minute (about 140 kg/cm$^2$/min), until the specimen fails. Record the maximum load $P$.
4. **Result:** $f_{c} = P/A$, where $A = 150\times150 = 22500$ mm$^2$. The test strength is the **average of three specimens**; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; $f_{cyl} \approx 0.8\,f_{cube}$.

## Check yourself

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

1. How is compressive strength used in RC design?
   A. The grade gives fck, from which design stress, modulus and other properties are derived
   B. It is used only for mix design
   C. It is used only for deflection checks
   D. It is unrelated to modulus and shear
   Answer: A. fck is the basis for the stress block and for deriving Ec, flexural, shear and bond values. (Key point: Basis of design)

2. Which expression for the modulus of elasticity of concrete does IS 456 give?
   A. Ec = 0.7 sqrt(fck)
   B. Ec = 5000 sqrt(fck)
   C. Ec = 0.446 fck
   D. Ec = 5000 fck
   Answer: B. Ec = 5000 sqrt(fck) N/mm2; 0.7 sqrt(fck) is flexural strength. (Key point: Modulus of elasticity)

3. How is the cube placed and loaded in the IS 516 test?
   A. Cast top against platens, load suddenly
   B. Cast sides against platens, load at 1.4 N/mm2 per minute
   C. Cast sides against platens, load at 14 N/mm2 per minute to failure
   D. Lying on its edge, load at 140 N/mm2 per minute
   Answer: C. The load acts on the sides as cast, steadily at 14 N/mm2/min. (Key point: Test loading)

4. How is the test strength calculated from the cube result?
   A. fc = P x A, best of three
   B. fc = P/A, take only one specimen
   C. fc = 2P/(pi L d), average of three
   D. fc = P/A, average of three specimens, A = 22500 mm2
   Answer: D. Strength is the maximum load over area 150 x 150 mm, averaged over three specimens. (Key point: Result)

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