Lab Test & Procedure

Compressive Strength of Concrete Cubes – Lab Test & Procedure

The compressive strength of concrete determines a structure's overall strength, such as flexural resistance and abrasion resistance. Compressive strength of concrete is defined as the characteristic strength of 150mm concrete cubes tested at 28 days, according to Wikipedia.

Procedure

Cube Casting

  • Calculate the dry proportions of the materials (cement, sand, and coarse aggregate) according to the design specifications. There should be enough ingredients to make test cubes.
  • Thoroughly mix the dry ingredients to obtain the uniform mixture
  • Add design quantity of water to the dry proportion (water-cement ratio) and mix well to obtain uniform texture
  • Fill the concrete to the mould with the help of vibrator for thorough compaction
  • Finish the top of the concrete with a trowel and a good tapping until the cement slurry reaches the cubes.

Curing

  • After some time, cover the mould with a red gunny bag and leave it undisturbed for 24 hours at a temperature of 27 ° Celsius 2
  • After 24 hours remove the specimen from the mould.
  • Submerge the specimen in fresh water at 27 degrees Celsius. The specimen should be preserved for seven to twenty-eight days. The water should be replaced every seven days.
  • The specimen should be removed from the water 30 minutes prior to the testing.
  • The specimen should be in dry condition before conducting the testing.
  • The Cube weight should not be less than 8.1 Kgs

Why do we test at 7, 14 & 28 days?

Concrete is a macro-content that contains Sand, Cement, and Coarse Aggregate as micro-ingredients (Mix Ratio) and hardens to 100 percent strength.

Take a look at the below table.

Days after Casting Strength Gain
Day 1 16%
Day 3 40%
Day 7 65%
Day 14 90%
Day 28 99%

As you can see the concrete gains its strength rapidly til 7th & 14th Days. Then gradually increases from there. So we can’t predict the strength until the concrete comes to that stable state.

Compressive Strength of Concrete Lab Test

Objective: To find compressive strength value of concrete cubes.

Required Equipment & Apparatus

  • 150 mm Cube Moulds (with IS Mark)
  • Electronic Weighing Balance
  • G.I Sheet (For Making Concrete)
  • Vibrating Needle & other tools
  • Compressions Testing Machine

Testing Process

  • Now place the concrete cubes into the testing machine. (centrally)
  • The cubes must be properly put on the machine plate (check the circle marks on the machine). Align the specimen with the spherically seated plate with care.
  • The load will be applied to the specimen axially.
  • Now slowly apply the load at the rate of 140kg/cm2 per minute till the cube collapse.
  • A compressive load is defined as the maximum load at which the specimen breaks.

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