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What Is Compressive Strength?
Compressive strength can be defined as the ability of material or structure to carry the loads on it without any crack or deflection. A material under compressive load tends to reduce the size, while in tension, size elongates.
The compressive strength of concrete can be calculated by dividing the load applied on the concrete cube at the point of failure by the cross-section area of the cube (15x15x15 cm) on which load was applied.
The concrete compressive strength for normal construction work varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and more in commercial and industrial structures.
The strength of concrete depends on factors such as water-cement ratio, the strength of cement use, quality of concrete materials, quality control during production of concrete, etc.
A concrete cube test is performed to check the compressive strength of concrete. There are various standard codes that recommend concrete cylinder or concrete cube as the standard specimen for the test.
The American Society for Testing Materials of construction ASTM C39/C39M provides Standard Test Method for Compressive Strength of cube and Cylindrical Concrete Specimens.
Compressive Strength of Concrete Formula
Compressive Strength = Load at failure / Cross-sectional Area of element
Cube Test of Concrete
For cube test of concrete two types of specimens either cubes of 15cm X 15cm X 15cm or 10cm X 10cm x 10cm depending upon the size of aggregate is used for concrete making. For most of the concrete works cubical moulds of size, 15cm x 15cm x 15cm are commonly used.
This concrete is filled in the mould and tempered properly so as to minimise any air voids available in concrete.
After 24 hours these moulds are opened and test specimens are soaked in water for curing.
The upper surface of these specimen should be made even and smooth. This is done by spreading cement paste on the whole area of the specimen.
These cubes are tested by a compression testing machine after 7 days curing or 28 days curing.
The Load on the cube should be applied gradually at the rate of 140 kg/cm2 per minute till the specimen fails. Load at the failure of a cube divided by area of specimen gives the compressive strength of concrete.
Apparatus for Concrete Cube Test
- Standard Compression testing machine
- Cube mould of standard size 15 cm x 15 cm x 15 cm
Preparation of Concrete Cube Specimen
The proportion and material for making these test specimens are should be taken from the same concrete used in the field.
Specimen
Minimum 9 cubes of 15 cm size Max. M15 or above
Mixing of Concrete for Cube Test
Mix the concrete can be done by hand or in a laboratory batch mixer
Hand Mixing
Mix the coarse aggregate, cement, and fine aggregate on a water platform until the mixture is thoroughly blended and is of uniform color.
After that add water and mix it until the concrete appears to be homogeneous and of the desired consistency.
Pouring Concrete In Cubes
Clean the mounds of concrete and apply oil. Fill the concrete in the molds in 3 layers. Compact each layer of mix with not less than 35 strokes per layer using a tamping rod (steel rod 16mm diameter and 60cm long, bullet-pointed at lower end) The top level of concrete cube and smoothen it with a trowel.
Curing of Cubes
The cube test specimens are stored in a moist air atmosphere for 24 hours and after this period the specimens are given marking and removed from the molds and kept submerged in freshwater until taken out for testing.
Precautions for Tests
The water used for curing should be tested every 7 days and the temperature of water must be at 27+-20 C.
Concrete Cube Test Procedure
- Remove the concrete cube from water after specified curing time and clear out excess water from the surface.
- Measure the dimension of the specimen to the nearest 2mm.
- Clean the test surface of the testing machine.
- Place the cube sample in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
- Place the specimen centrally on the base plate of the machine.
- Rotate the machine’s movable portion gently by hand so that it touches the top surface of the specimen.
- Apply the load gradually to the cube without shock and continuously at the rate of 140 kg/cm2/minute till the specimen fails.
- Record the breaking load and note any unusual features in the type of failure.
Calculations of Compressive Strength
Check Compressive Strength of Concrete
Size of the concrete cube = 15 cm x 15 cm x 15 cm
Area of the cube sample =225 cm2 (22500 mm2)
The same calculation should be done for 28 days compressive strength
The maximum load applied or load at failure of cube = 400 KN (400×1000 N)
Compressive strength = (Load in N/ Area in mm2)=400×1000/22500 N/mm2
= 17.77 N/mm2
Percentage of Strength Gain by Cubes In Days
The following table shows the percentage gain of compressive strength by concrete cube after curing of 3 days, 7 days, 14 days and 28 days.
| Days | % Gain Strength |
| 1 Day | 16 % |
| 3 Day | 30 % |
| 7 Day | 65 % |
| 14 Day | 90 % |
| 28 Day | 99 % |
Compressive Strength of Different Grade of Concrete
The following table shows the compressive strength of various concrete grades at different ages.
| Grade of Concrete | 1 Day (16%) | 3 Day (30%) | 7 Day (65%) | 14 Day (90%) | 28 Day (28%) |
| M 15 | 2.4 | 4.50 | 9.75 | 13.50 | 14.85 |
| M 20 | 3.20 | 6.00 | 13.00 | 18.00 | 19.80 |
| M 25 | 4.00 | 7.50 | 16.25 | 22.50 | 24.75 |
| M 30 | 4.80 | 9.00 | 19.50 | 27.00 | 29.70 |
| M 35 | 5.60 | 10.50 | 22.75 | 31.50 | 34.65 |
| M 40 | 6.40 | 12.00 | 26 | 36.00 | 39.60 |
| M 45 | 7.20 | 13.50 | 29.25 | 40.50 | 44.50 |
FAQs
How do you measure the compressive strength of concrete?
A concrete Cube Test is performed to know the Characteristics compressive Strength of the concrete. Test for compressive strength on concrete is can be performed by either using cube or cylinder. According to the standards, there are various codes that are recommended as the standard specimen for the test.
What is compressive strength test of concrete?
The Compressive strength of concrete is its ability to resist failure in the form of cracks and the fissure when a specific amount of load is acted upon it. Hence higher the value of Compressive Strength higher the value of concrete to resist failure due to acting load.
How do you measure the strength of concrete?
Various tests are performed to know the strength of concrete. The strength of concrete can also be correlated by knowing its toughness value, durability, abrasion value, shear value, etc. For example, Cube Test is performed to know the Compressive strength of concrete by applying pressure on concrete in a UTM machine.
Why do we need to test the concrete compressive strength at 7, 14, and 28 days?
Concrete has the ability to gains 16 percent strength of its total strength in one day, 40 percent in 3 days, 65% in 7 days, 90% in 14 days, and around 99% strength in 28 days. These values of concrete percentage with respect to the days are considered to be standard, as per the standard codes. Also, it is much easier to examine concrete with respect to these standards. Hence we test the concrete compressive strength at 7, 14, and 28 days.
What is the minimum compressive strength of concrete?
Minimum Compressive Strength of Concrete: 17300 KN/m2
As per the standards, worldwide minimum compressive strength of concrete varies with its grade as well as the nature of construction. However, in a general sense minimum Compressive Strength required is 17300KN/m2.
Cube Test
Compressive strength of concrete can be defined as the ability of material or structure to carry the loads on it without any crack or deflection. A material under compressive load tends to reduce the size, while in tension, size elongates. It is also known as the Cube test of Concrete in the field.
Compressive Strength of Concrete at 7 Days
Concrete has the ability to gains 16 percent strength of its total strength in one day, 40 percent in 3 days, 65% in 7 days, 90% in 14 days, and around 99% strength in 28 days. So, for M20 grade of concrete compressive strength of concrete at day will be 13 N/mm2.
Cube Strength of Concrete
Compressive strength of concrete can be defined as the ability of material or structure to carry the loads on it without any crack or deflection. A material under compressive load tends to reduce the size, while in tension, size elongates. It is also known as the Cube test of Concrete in the field.
Watch Video: Concrte Cube Test Procedure & Calculation
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Compressive Strength of Concrete