Reason concrete cube samples 150mm and Procedure

In concrete compression test,  normally 150mmx150mmx150mm concrete cube samples is used for testing.Why isn’t  100mmx100mmx100mm  concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples?

Basically,the force supplied by a concrete compression machine is a definite value.For normal concrete strength application,say below    50MPa, the stress produced by  a 150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample.

However,if the designed concrete strength is 100MPa,Under the same force(about2,000kN) supplied by the machine,the stress under a 150mmx150mmx150mm cube Is not sufficient to crush  the concrete cube.Therefore,100mmx100mmx100mm concrete cubes are used instead.To increase the applied stress to crush the concrete cubes.For normal concrete strength,the cube size of 150mmx150mmx150mm is already sufficient for the crushing strength of the machine.

Cube Test

Instrument And Material.

• Concrete cube mould of size 150mm or 100mm. It is used for aggregate size of not greater than 40mm and 25mm. Cube mould for test must be made from steel or cast iron with smooth inner surface. Every mould must have steel plate to support and to prevent leakage.
• Compacting steel rod with 16mm diameter and 600mm long.
• Compression test machine.

Procedures

• Mould and base plate must be clean and applied with oil to prevent concrete from sticking to the side of the cube. Base plate is attached to the mould by using bolt and nut.
• Fill the cube with concrete in three layers.
• Every layer must be compacted for 25 times. This process is carried out systematically and compaction is done uniformly to all surfaces of the concrete. Compaction can also be conducted using machine.
• The surface of concrete must be smoothen so it will has the same level with the upper side of the mould.
• Cubes which are made at construction site must be covered with plastic cover for a period of 24 hours before the moulds can be dismantled.
• After remoulded, the concrete cubes are submerged in water for curing.
• Compression strength test must be carried out for concrete at age 7, 14, and 28 days by using compression test machine.

Result

The Strength value of each cube must be recorded and compared with the targeted strength value. The purpose of carrying out concrete test on 7 th day and the 14 th day is to predict whether the concrete could archive the targeted 28 th day strength.In general, concrete would have acquired 70% strength on the 7 th day.