Compressive strength of concrete is the ability of the concrete to withstand loads without cracking or deformation. Compressive Strength of concrete is defined as the Characteristic strength of 150mm size concrete cubes @28 days. Marginal variations in water to cement ratio, ingredient proportioning, increase in a slump, etc impacts the desired concrete strength which in turn affects the strength and stability of structures.
Quality tests are to be conducted on concrete at various stages starting from the production stage to the hardened stage, and on structures. Quality tests play an important role in ensuring the quality of a particular construction. This article covers the types of tests conducted on concrete at various stages.
Also read : Compaction factor test for workability of concrete
Quality tests on concrete
Quality tests are done on different stages as listed below,
Production stage quality tests ( On fresh concrete before placing)
Hardened stage quality tests ( hardened concrete specimens)
On structures ( tests done on the structures )
Quality tests on Fresh concrete
Quality tests on hardened concrete
- Compressive strength
- Tensile strength – Split tensile strength
- Flexural strength test
- Water absorption test
Non destructive tests on concrete
- Rebound hammer
- Penetration resistance test
- Ultrasonic pulse velocity test
Compressive strength test – significance
As a construction material, concrete should be able to withstand heavy loads. The concrete material beneath compression tends to reduce its size. Concrete gains its strength over time. The compressive strength depends on cement strength, water-cement ratio, concrete quality etc. The test gives an idea of the overall strength and above-mentioned factors. Through conducting this test, one can easily judge the concrete strength and quality of concrete produced. We can calculate the compressive strength from the failure load and the cross-sectional area of the specimen.
Concrete compressive strength for general construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and more than that in case of commercial, industrial structures and special structures.
The compressive strength helps in determining
- Quality control
- Acceptance of concrete
- Evaluation of curing
- For determining the in-place concrete strength.
- Age of concrete, etc.
Formula for compressive strength
Compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face of the concrete specimen (cube or cylinder) on which load was applied.
Compressive Strength = Load / Cross-sectional Area
Relevant IS code
IS: 516-1959
Apparatus used
- Compression testing machine or Universal testing machine
- Cube or cylinder specimen
- Weighing machine
- Vibrating machine
- Trowel
Compressive strength test of concrete
The specimen for this test can be cubic or cylindrical. The dimension of the specimens is as follows.
- Cube = 150mm x 150mm x 150mm
- Cylinder = 150mm diameter, 300mm height
The mould is made up of cast iron or steel. Cubical moulds are preferred for most works.
Compression testing of cubes – Procedure
Mixing of materials.
- The first step is to prepare the test specimen by mixing the ingredients.
- Mix the cement, coarse aggregate and fine aggregate in dry condition uniformly.
- Mixing of the materials is by using batch mixer or by hand mixing.
- Ingredients shall be bought to room temperature, before commencing the test.
Preparation of Test specimen
- Clean the cube mould and apply the lubricant inside the mould.
- Make at least three specimens from each batch.
- After mixing the concrete, fill the specimen in the mould in three-layer at 50 mm thickness.
- After that stroke 35 times and compact the concrete using a vibrating machine.
- Then remove the excess concrete using a trowel.
- After that mark the date, grade of concrete etc and immerse the specimen in freshwater.
- Then store the specimen at a temperature of 27-degree Celsius for 24 hours.
Procedure for testing
- The specimens should be tested using the compression testing machine at 3, 7 and 28 days.
- Before starting the test take the specimen from water and remove the mould.
- The test should be conducted in the wet condition of the specimen.
- Record the weight and dimension.
- Then place the specimen in the testing machine.
- Gradually apply load on the specimen at a rate of 140 kg/ cm2 per minutes.
- Note down the load at which the specimen breaks.
Calculation for Compressive strength test of concrete
The compressive strength of the concrete = Load at which the concrete breaks / Cross-sectional area of the specimen.
The compressive strength is expressed in N/mm2. The cube specimen is tested at 7, 14 & 28 days.
Calculation of compressive strength
Size of cube = 15cm x 15cm x 15cm
Area of specimen = 225 cm2
Expected maximum load = fck x area x f.s
Calculation shall be repeated for 7, and 28 days.
Results shall be furnished as average compressive strength of cube = …………… N/mm2 (7 days and 28 days)
Conclusion
The compressive strength of concrete cube gives and idea about the characteristics of concrete. With this single test we can judge the concrete quality and hence preferred for major construction works.
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