Cube Testing Report
As per IS 456:2000 & IS 516
Formula: Strength = Load (kN) ÷ 22.50
| Cube No. | Load (kN) (Dial Reading) |
Strength (N/mm²) | Average Strength |
|---|---|---|---|
| 1 | - |
0.00
N/mm²
Select Grade
|
|
| 2 | - | ||
| 3 | - |
Technical Guide: Concrete Compressive Strength Test
The **Compressive Strength Test** is the most important quality control test for concrete. It determines whether the concrete batch has achieved the target strength (Characteristic Strength, $f_{ck}$) required for the structure's safety. This test is mandatory for all RCC works including columns, beams, slabs, and canal linings.
1. Standard Test Procedure (IS 516)
To get accurate results, the following procedure must be strictly followed on site:
Casting
Clean the 150mm moulds and oil them. Fill concrete in 3 layers (50mm each). Tamp each layer 35 times using a 16mm tamping rod.
Curing
Keep cubes in moulds for 24 hours under moist gunny bags. Demould and submerge in clean fresh water until the time of testing (7 or 28 days).
Testing
Remove cube, wipe surface dry. Place in CTM machine. Apply load gradually at a rate of 140 kg/cm² per minute until failure.
2. Sampling Frequency (IS 456:2000 Table 11)
How many cubes should you cast? It depends on the volume of concrete poured in a single day.
| Concrete Volume (M³) | Minimum Samples (Set of 3 Cubes) |
|---|---|
| 1 - 5 M³ | 1 Set (3 Cubes) |
| 6 - 15 M³ | 2 Sets (6 Cubes) |
| 16 - 30 M³ | 3 Sets (9 Cubes) |
| 31 - 50 M³ | 4 Sets (12 Cubes) |
| 51+ M³ | 4 + 1 for every additional 50 M³ |
3. The Math: Why divide by 22.5?
Many site engineers ask why we divide the CTM Dial Reading (in kN) by 22.5. Here is the simple math:
- Standard Cube Size = $150 mm \times 150 mm$
- Surface Area = $22,500 mm^2$
- Formula: $Strength (N/mm^2) = \frac{Load (N)}{Area (mm^2)}$
- If your machine shows Load in kN (KiloNewtons), we convert it:
- $Strength = \frac{Load (kN) \times 1000}{22500}$
- $Strength = \frac{Load (kN)}{22.5}$
4. Acceptance Criteria & Tolerance
According to IS 456:2000 Clause 15.4, a sample is accepted if:
- The Average Strength of 3 cubes is $\ge f_{ck}$ (for 28 days).
- Individual Variation: The strength of any individual cube should not vary by more than $\pm 15\%$ of the average. If it does, the sample is invalid.
Frequently Asked Questions (FAQ)
If the lab result fails, IS 456 recommends performing a Non-Destructive Test (NDT) like the Rebound Hammer or extracting a Concrete Core Sample (IS 516 Part 4) from the actual structure to verify strength.
