To prepare, cast, cure, and test concrete cube specimens of size
150 × 150 × 150 mm or 100 × 100 × 100 mm
to determine compressive strength of concrete at specified ages
(usually 7 days and 28 days).
2. Apparatus Required
Cube moulds – 150 mm or 100 mm
Mixing tray and scoop
Tamping rod (16 mm diameter)
Trowel
Concrete mixer (if required)
Curing tank (27 ± 2°C)
Compression Testing Machine (CTM)
3. Preparation of Cube Moulds
Clean moulds to remove dust and hardened mortar.
Assemble moulds properly and tighten bolts.
Apply thin uniform oil layer on internal faces.
Check alignment and squareness of mould.
4. Sampling and Mixing of Concrete
Sample concrete from freshly mixed batch.
Mix thoroughly until uniform colour and consistency are achieved.
Start casting immediately to avoid loss of workability.
5. Casting of Concrete Cubes
Fill mould in three equal layers.
Distribute concrete evenly around mould.
6. Compaction of Concrete
Compact each layer by rodding or vibration.
Manual compaction: 35 strokes per layer.
Rods to penetrate into the previous layer.
Tap mould sides gently to remove air voids.
Finish top surface smoothly using trowel.
7. Identification and Initial Storage
Date of casting
Grade of concrete
Cube number / location
Store cubes undisturbed for 24 hours at 27 ± 2°C.
8. Curing of Concrete Cubes
Demould cubes after 24 ± ½ hours.
Immediately immerse in clean water.
Maintain curing temperature at 27 ± 2°C.
Continue curing till testing age.
9. Compression Testing of Cubes (IS:516)
Remove cube from curing tank (SSD condition).
Clean cube and CTM platens.
Measure dimensions (nearest 0.2 mm).
Place cube centrally on CTM platen.
Apply load gradually at ≈140 kg/cm²/min.
Record maximum load at failure.
Note: Improper centring causes eccentric loading and wrong test results.
10. Calculation of Compressive Strength
Compressive Strength (N/mm²)
= Maximum Load at Failure ÷ Loaded Area
✅ ±15% Variation Check:
All individual cube strengths fall within the permissible range of
0.85 × Average to 1.15 × Average for their respective samples.
This confirms uniformity in batching, mixing, compaction, and curing of concrete.
✅ Minimum Individual Strength Check:
Each tested cube has achieved a compressive strength
greater than or equal to fck − 2, i.e.
23 N/mm² for M25 concrete.
No cube strength is below the minimum permissible limit.
✅ Average Strength Check:
The overall average compressive strength of all samples is
29 N/mm² or higher, which satisfies the requirement of
fck + 4 for acceptance of concrete under normal sampling conditions.
✅ Quality and Compliance Confirmation:
Since variation, individual strength, and average strength criteria
are all satisfied, the concrete meets the strength acceptance
requirements prescribed under IS practice for M25 grade.
Final Decision: ✅ CONCRETE ACCEPTED
One-Line Site Memory Rules
Single sample → Average ≥ fck + 4
Multiple samples → Each cube ≥ fck − 2
±15% variation is compulsory
Final Takeaway
Concrete cube testing is the backbone of quality control.
Correct casting, curing, testing, and acceptance checks ensure
strength, durability, and compliance with IS standards.
✅ Why Concrete is ACCEPTED
Uniform Strength: All cube test results lie within ±15% of their respective sample averages, indicating proper batching, mixing, compaction, and curing.
Sufficient Individual Strength: No cube strength is below fck − 2 (23 N/mm² for M25), ensuring minimum safety at the individual specimen level.
Adequate Average Strength: The overall average compressive strength is ≥ fck + 4 (29 N/mm²), satisfying IS acceptance requirements.
Statistical Reliability: Multiple samples provide confidence that at least 95% of concrete will achieve the characteristic strength.
❌ Why Concrete is REJECTED (When Failure Occurs)
One or more cube values fall outside ±15% of the sample average.
Any individual cube strength is less than fck − 2.
Average strength of the sample or overall concrete is less than fck + 4.
High variation indicates inconsistency in material quality, batching, compaction, or curing.
Q1. Why is ±15% variation rule used in concrete testing?
The ±15% rule ensures uniformity and consistency of concrete. Even if the average strength is high, excessive variation indicates poor quality control, which can lead to unsafe structures.
Q2. What happens if one cube fails but average is OK?
If any cube falls outside the ±15% range or below the minimum permissible strength, the entire sample is rejected, regardless of average strength.
Q3. Why is fck + 4 used for acceptance?
The extra margin ensures statistical reliability, so that 95% of concrete achieves the characteristic strength, accounting for normal variations.
Q4. Can 100 mm cubes be used instead of 150 mm cubes?
Yes, 100 mm cubes may be used when approved, but correct area and strength conversion formulas must be applied.
Q5. What is the most common reason for cube failure?
Poor compaction, incorrect water–cement ratio, improper curing, and misalignment during testing are the most common causes.
