---
title: "Acceptance criteria of concrete strength"
subject: "Concrete Technology and Masonry Structure"
chapter: 5
key: "semester_5/concrete_technology_and_masonry_structure:acceptance-criteria-concrete"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/acceptance-criteria-concrete
site: BCT Engineering
---

# Acceptance criteria of concrete strength

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

- Asked in: 2082 Baisakh, 2 marks; 2078 Kartik, 6 marks; 2076 Chaitra, 4 marks; 2070 Chaitra, 4 marks; 2064 Jestha (old course) (asked 5 times)
- Video: [Acceptance Criteria of Concrete as per Indian Standard Code IS 456 - 2000](https://www.youtube.com/watch?v=0VaH1_nhItY) — Nadeem Qureshi
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/acceptance-criteria-concrete
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5

## Question

What are the acceptance (compliance) criteria while testing concrete? Write down the acceptance criteria of compressive and flexural strength according to IS 456.

## Short answer

As per IS 456: for compressive strength, the mean of four consecutive non-overlapping results shall be ≥ fck+0.825s (min fck+4 MPa for M20+) and any individual result ≥ fck−3 MPa. For flexural strength, mean of four results ≥ fck+0.3 MPa and each ≥ fck−0.3 MPa.

## Answer

The acceptance (compliance) criteria decide whether the placed concrete meets the specified characteristic strength $f_{ck}$. IS 456 (Cl. 16) deals with them; the strength for each sample is the **average of three specimens** (Cl. 15.4), a specimen differing by more than 15% from that average being rejected.

#### Compressive strength (IS 456, Cl. 16.1 and Table 11)
Concrete is accepted if both conditions hold:

| Grade | Mean of group of 4 non-overlapping consecutive results | Any individual result |
|---|---|---|
| M15 | $\geq f_{ck} + 0.825\times s$ (min. $f_{ck}+3$) | $\geq f_{ck}-3$ |
| M20 and above | $\geq f_{ck} + 0.825\times s$ (min. $f_{ck}+4$) | $\geq f_{ck}-3$ |

Here $s$ is the established standard deviation (Table 8 values may be used) and the value is rounded off to the nearest 0.5 N/mm$^2$. The larger of the two limits is taken.

#### Flexural strength (IS 456, Cl. 16.2)
Concrete is accepted if
- the mean of any group of four consecutive test results exceeds the specified characteristic flexural strength by at least 0.3 N/mm$^2$, and
- every individual result is not less than the specified characteristic strength minus 0.3 N/mm$^2$.

#### If the criteria fail (Cl. 16.3 and 17)
The concrete may be accepted at a lower rate by the engineer, or NDT, drilled cores (average of cores at least 85% of $f_{ck}$ and no core below 75%), or load tests on the structure may be done.

## Check yourself

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

1. The strength of one sample is:
   A. The highest of three
   B. The average of three specimens
   C. The lowest of three
   D. The median of five
   Answer: B. Cl. 15.4: average of three, rejecting a specimen differing by more than 15%. (Key point: Sample strength)

2. For M20 and above, mean of 4 consecutive results must be at least:
   A. f_ck + 0.825 s (minimum f_ck + 4)
   B. f_ck + 0.825 s (minimum f_ck + 3)
   C. f_ck - 3
   D. f_ck + 0.3
   Answer: A. M15 uses minimum f_ck+3; M20 and above uses f_ck+4. (Key point: Compressive criteria M20+)

3. Flexural acceptance: mean of four exceeds f_cr by at least, and no individual below f_cr minus:
   A. 3 N/mm^2 and 3 N/mm^2
   B. 0.825 N/mm^2 and 0.3 N/mm^2
   C. 0.3 N/mm^2 and 0.3 N/mm^2
   D. 4 N/mm^2 and 3 N/mm^2
   Answer: C. IS 456 Cl. 16.2 uses 0.3 N/mm^2 for both. (Key point: Flexural criteria)

4. If criteria fail, accepted drilled cores must average at least:
   A. 100% of f_ck
   B. 85% of f_ck with no core below 75%
   C. 50% of f_ck
   D. 75% average with none below 85%
   Answer: B. Core average >= 85% f_ck, no core < 75%. (Key point: If criteria fail)

## Related in Concrete Technology and Masonry Structure

- [Compressive strength test; acceptance criteria](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-test-procedure-acceptance) — Concrete Technology and Masonry Structure, chapter 5
- [Compressive strength test of brick wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-test-masonry-wall) — Concrete Technology and Masonry Structure, chapter 10, asked 4×
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- [Importance and method of compressive strength test](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acompressive-strength-importance-method) — Concrete Technology and Masonry Structure, chapter 5
- [Characteristic strength of concrete cubes](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acharacteristic-strength-cubes-26) — Concrete Technology and Masonry Structure, chapter 5

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