---
title: "Compressive strength test; acceptance criteria"
subject: "Concrete Technology and Masonry Structure"
chapter: 5
key: "semester_5/concrete_technology_and_masonry_structure:compressive-test-procedure-acceptance"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/compressive-test-procedure-acceptance
site: BCT Engineering
---

# Compressive strength test; acceptance criteria

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

- Asked in: 2079 Bhadra, 4+2 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-test-procedure-acceptance
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5

## Question

Explain the procedures of compressive strength test of concrete. What are the acceptance criteria of test results?

## Short answer

Test procedure: sample at placing, cast three 150 mm cubes in layers, tamp/vibrate, cure at 27°C. After 28 days, wipe, load on side faces at 14 N/mm^2/min. fc = P/A, average of three; discard if any differs >15% from mean. Acceptance per IS 456: mean of four non-overlapping samples >= fck + 0.825s and >= fck+3 or +4; individual >= fck-3 N/mm^2.

## Answer

#### 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; $f_{cyl} \approx 0.8\,f_{cube}$.

#### Acceptance criteria (IS 456, Cl. 16.1, Table 11)
- Strength of a sample = mean of three specimens; discard if one differs by more than 15% from the mean (Cl. 15.4).
- The mean of every group of 4 non-overlapping consecutive results must be at least $f_{ck}+0.825\,s$ (rounded to the nearest 0.5 N/mm$^2$), and not less than $f_{ck}+3$ (M15) or $f_{ck}+4$ (M20 and above), whichever is larger.
- Each individual result $\geq f_{ck}-3$ N/mm$^2$.
- If these limits are not met, the concrete may be checked by cores, NDT and load tests (Cl. 16.3, Cl. 17).

## Check yourself

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

1. How is a 150 mm cube compacted by tamping in the compressive strength test?
   A. 3 layers, 35 strokes of a 16 mm bar per layer
   B. 2 layers, 25 strokes of a 25 mm bar per layer
   C. 5 layers, 10 strokes per layer
   D. 1 layer, 100 strokes of a 16 mm bar
   Answer: A. IS 516 specifies 3 layers with 35 strokes of a 16 mm bar each for 150 mm cubes. (Key point: Cube compaction)

2. How is the cube loaded in the compression machine?
   A. Load on the cast top face, at 140 N/mm2 per minute
   B. Load on the sides as cast, at 14 N/mm2 per minute without shock
   C. Load on the sides as cast, suddenly applied
   D. Load on the top with a capped surface at 14 N/mm2 per second
   Answer: B. The cube is turned so the load acts on cast sides, applied steadily at 14 N/mm2/min. (Key point: Loading direction and rate)

3. How is the strength of one sample obtained and when is a specimen discarded?
   A. Highest of 3 specimens; discard those below 15 N/mm2
   B. Average of 2 specimens; discard if differing by 5%
   C. Average of 3 specimens; discard one differing more than ±15% from the average
   D. Lowest of 3 specimens; discard none
   Answer: C. The test strength is the mean of three; a specimen more than ±15% from the mean is discarded. (Key point: Sample strength)

4. What is the minimum for the mean of every group of 4 consecutive non-overlapping results (M20 and above)?
   A. fck - 0.825 s, but not less than fck - 4
   B. fck + 1.65 s only
   C. fck + 3 for all grades
   D. fck + 0.825 s, but not less than fck + 4
   Answer: D. The group mean must be at least fck + 0.825s and not less than fck + 4 for M20 and above. (Key point: Acceptance: group mean)

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