What is compressive strength test of concrete?
Compressive strength is the capacity of concrete to withstand loads before failure. It is measured by a compression test, which involves crushing cylindrical concrete in a compression testing machine. The compressive strength test is the most important of the many tests applied to the concrete, as it gives an idea about the characteristics of the concrete.
Purpose of this Test
Assume a slab at our site is designed to cast M30 grade of concrete, but we could not define its strength in the semi-solid state.
The purpose of a compressive strength test is to determine the capacity of concrete to withstand a load and at which point it will fail. It’s also used to determine the compressive strength of other building materials, such as cement.
To determine the compressive strength of a specific concrete batch, a portion of the concrete is cast into a cube during the pouring process. This cube is then cured carefully for 28 days before being subjected to a Compression Testing Machine for evaluation.
Compressive Strength of Concrete Test
Apparatus Required for Compressive Strength of Concrete
- Steel Mould (150mm x 150mm x 150mm)
- Tamping rod (Dia-16mm,Height-600mm)
- (Tamping Bar- The tamping bar shall be a steel bar weighing
- 2 kg, 40 Cm long, and shall have a ramming face 25 mm square.)
- CTM Machine
- Trowel
- Weighing machine
Method of Testing
- After 30 minutes of removal from the curing water, wipe the cube dry completely.
- Before proceeding, weigh the specimen to determine its mass.
- For accurate testing, meticulously position the specimen between the plates, ensuring proper alignment.
- Compressive Strength of concrete cube = Maximum load(in N)/Area of the cube(mm2)
Example.
Assume that the compression load is 450 KN (1 KN =1000 N)
Cross Sectional Area – 150 x 150 = 22500 Sqmm.
Compressive Strength = (450×1000)/22500 = 20N/mm2 (Mpa)
What is the Purpose of Cube Weighing?
One way to identify the unit weight of concrete is to consider its type. Typically, the unit weight of plain cement concrete (PCC) is around 2400 kg/m3, while for reinforced cement concrete (RCC), it’s usually closer to 2500 kg/m3. However, it’s important to remember that these are just standard values, and the actual unit weight of your concrete may vary depending on the specific mix design and materials used.
How can we measure it?
- Simple. Assume, the approx. weight of one concrete cube = 8.2 kg
- Then the volume of concrete = 0.15 x 0.15 x 0.15 = 0.0033 m3; So for 1 m3 = 8.2/0.0033 = 2484.84kg/ m3. As concrete weight is factored into structural calculations, exceeding the assumed value results in a potential increase in the overall load on the structure.
What is Target Mean Strength?
Building upon the concept of “Cube Strength,” we introduce the target mean strength (fm). Defined as the design strength specifically intended for producing reinforced concrete, it is calculated using the following formula:
fm = fck + 1.65𝛔
Where, fm = Target mean strength, fck = characteristic compressive strength, 𝛔 = standard deviation
Why we use cube instead of cylinder?
The cube could carry a higher load than the cylinder (both cross-sections remained identical). At the same l/d ratio and shape (cube only), the compressive strength of foam concrete, compressive strength did not decrease as the specimen size increased, Actually we do use other dimensional parameters for finding the strength of concrete. See the main reason behind using that 15x15x15 cube is totally the standard concrete code we are following.
lets take few examples :
In India
According to IS:456 2000, the characteristic strength of concrete is determined by creating a 150mm x 150mm x 150mm cube and testing it after it has been cured for 28 days. This test is known as the Cube Strength test.
This Cube Strength procedure in also followed by some European Countries too.
In countries like the USA, France, Japan, Australia, and New Zealand
To determine the Cylinder Strength of concrete, testers create a cylindrical sample with dimensions of 300 millimeters in diameter and 150 millimeters in height.
How many concrete cube sets should cast per M3?
The Clause 15.2.2 of the IS: 456-2000 stipulates minimum frequency of sampling of concrete.
1 to 5 cubic meter – no of sample 1
6 to 15 cubic meter – no of samples 2
16 to 30 cubic meter-no of samples 3
31 to 50 cubic meter – no of samples 4
51 and above – 4 plus one additional sample for each additional 50m3 or part there of.
Download IS 456:2000
How many concrete cubes (specimen) are in 1 sample of cube for testing compressive strength of concrete?
However, according to IS 456-2000, Clause 15.3, while a set of three (3) cubes constitutes an “a-sample” for testing the average compressive strength of concrete, the individual test result is not simply the value of a single cube.