Compressive Strength of Concrete Cube Test is all about its best resistivity to compressive force and to check characteristic features of concrete.
By the compressive test, we can make sure proper concreting as the compressive strength plays a major role in concrete.
The concrete mix is designed as per the requirement and serviceability of the structure for general construction 15 MPa (2200psi) to 30 MPa (4400psi) and it could be higher as per structure need.
Compressive strength is largely betting on the formation of concrete and therefore the formation of concrete includes quality of concrete material, Water-Cement quantitative relation, Admixtures, Strength of cement, proper quality control during its manufacturing.
The compressive strength of concrete is calculated by dividing the entire load at failure by the area of concrete after its 28 days of curing.
There are various properties of hardened concrete like elastic modulus, coefficient of thermal expansion, creep coefficient which we should keep in mind at the time of designing of the structure.
Compressive Strength Of Concrete
Compressive strength is defined as the capacity of any substance or structure to bear the load on its surface without resulting in any deformation and deflection.
The compressive strength is expressed in terms of the characteristic compressive strength of the concrete mix. Characteristic compressive strength is expressed as (fck).
Characteristic strength of concrete means the samples of concrete which had taken for the test should not fail more than 5% of a total number of samples is known as its characteristic strength.
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Compressive Strength is totally depending on the manufacturing of concrete as the raw materials of concrete have their certain proportion of cement, fine aggregate, coarse aggregates, admixtures but the water-cement ratio has its own significance in the compressive strength of concrete.
The water-cement ratio is inversely proportional to the compressive strength of concrete.
Minimum Quantities of Samples Taken for Testing Compressive Strength
|Quantity of Concrete (m3)||Number of Samples taken for testing|
|1 – 5||1|
|6 – 15||2|
|16 – 30||3|
|31 – 50||4|
|51 +||4 + 1cube for every 50 m3 of concrete|
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Compressive Strength For Various Grades of Concrete
Every Concrete Grade has their various compressive strength according to its design mixes. It helps to design Structures as per their usability.
Concrete is further categorized into categories like; Nominal grade concrete, Standard grade concrete, High strength grade concrete.
Nominal Grade Concrete
It includes concrete grades like M5, M7.5, M10, M15, M20 these grade of concrete are called Nominal Grade Concrete
Standard Grade Concrete
It includes concrete grades like M25, M30, M35, M40, M45 these grades of concrete are called Standard Grade Concrete.
High Strength Grade
It includes concrete grades like M50, M55, M60, M65, M70 these grades of concrete are called High Strength Grade Concrete.
|Concrete Grade||Compressive Strength Of Concrete Specimen|
Compressive Strength Formula
Compressive Strength is determined by the load at the time of failure of a specimen divided by the cross-sectional area of the concrete on which the load is applied.
Compressive Strength = Total Load At Failure / Cross-sectional Area
Compressive Strength As Per American Standards
As per American Code, the concrete specimen is taken in cylindrical mold And specimen measurement is 150 mm Diameter, and Height is 300 mm.
Loaded with uniaxial compression loading longitudinally after 28 days of curing.
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Compressive Test of Concrete Cubes
Compression Testing Machine
Concrete Cube Specimen Preparation
The Concrete Cube specimen material proportionality is the same as on site.
The specimen sizes of concrete cubes is 150 mm X 150 mm X 150 mm each.
Mixing of Concrete Specimen
The mixing of concrete can be done by mixer or by hand it depends on availability of instruments on site.
- Mixing of concrete should be on the surface which is non-absorbent
- In hand mixing, firstly coarse aggregate is used then fine aggregates and cement should be used until the mixture blends properly.
- Mix water as it needs up to its required consistency.
Sampling of Cube Test
- Cleaning and greasing of mold should be done properly
- Concrete should be poured in layers in mold.
- Compaction of each layer should be done by 16 mm dia bar with at least 35 strokes.
- Level the concrete properly by the instruments like a trowel.
Curing of Cubes
Curing of cubes should be done after 24 hours when it is removed from the mold. After that, it should be submerged into fresh water and will be taken out at the time of the test.
Precautions For Test
The temperature must be of 27 degree. Curing must be tested after every seven days.
Procedure for Concrete Cube Test
- The Concrete should be used for testing immediately after taking out from the water.
- Excess water on the surface of the cube should be wiped.
- Take measurement nearest to 0.2 m.
- Weight should note prior to testing.
- If we got a specimen dry then we should put it in the water tank for curing at least for 24 hrs.
- The base plate of the machine should be aligned with the specimen properly.
- Load should be applied gradually on the specimen without any shock at the rate of140 kg/cm2/minute
- Note the unusual feature and maximum load at the time of failure.
The Compressive strength can be calculated by dividing the total load by the cross-sectional area of concrete. We’ve to take three specimens and the average of their compressive strengths is the main compressive strength of that batch of concrete. And if the variation between the strength of specimens is more than 15% then the specimens should be rejected.
Size of Cube = 150 mm X 150 mm X 150 mm
Area of Specimen = 22500 mm2
- Test Date.
- Specimen Age should be noted.
- Manufacturing of specimen data, as well as curing conditions, should be noted.
- Cracks and creeps on the surface of concrete cubes and types of cracks should be noticed.
Concrete Compressive strength = ……………………. N/mm2.
Importance of Compressive Strength of Concrete.?
It is important to design structures and give idea for strength of concrete of that particular grade of concrete.
The best test for concrete strength
There are two tests which are used in concrete compressive strength ; cube test and cylinder test to find out the strength of concrete after 7 days, 14 days, 21 days, 28 days.
Concrete cube sizes used for concrete
Two Cube size used in testing:-
150 mm X150 mm X 150 mm.
100 mm X 100 mm X 100 mm.
Loading Rate on Compression Testing machine
A load of 140Kg/cm2/ minute gradually applied to the specimen.