How to Make Concrete Cube for Compression Test?

Concrete's compressive-strength mostly depends on the mix design. But it is affected by several other factors. Such as mixing of concrete, placing of concrete, curing of concrete as well as quality of concrete ingredients. So we can't be assured that if we produce concrete as per mix-design we will get desired compressive strength.

However, we should know the concrete compressive strength for every part of concrete-structure.

For knowing compressive-strength of concrete, we generally test concrete-cube or concrete-cylinder at laboratory. These cubes/cylinders are made on construction site. In this article I'll discuss the procedures of making concrete cube.

Apparatus:

• Cube Mould (150x150x150 mm or 100x100x100 mm)
• Tamping bar (16 mm diameter and bull-nosed
• Steel Float/Trowel

Procedures of Making Concrete Cube:

Making concrete-cube specimen is simple and it is done in three simple steps.

1. Cleaning & Fixing mould,
2. Placing, Compacting & Finishing concrete, and
3. Curing.

1. Cleaning & Fixing mould

• Clean the cube-mould properly and apply oil on inner surface of mould. But no oil should be visible on surface.

• Fix the cube mould with base plate tightly. No gap should be left in joints so that cement-slurry doesn't penetrate.

• Place the mould on levelled surface.

2. Placing, Compacting & Finishing concrete

• Take concrete from three or four random mixes.

• Place concrete into mould in three layers. Compact each layer by giving 35 blows of tamping bar.

• Remove excess concrete from the top of mould and finish concrete surface with trowel. Make the top surface of concrete cube even and smooth.

• Left the mould completely undisturbed for first four hours after casting.

• After ending undisturbed period, put down casting date and item name on the top of concrete specimen with permanent marker.

3. Curing

• After 8 to 10 hours of casting, wrap the cube mould with wetted hessian cloth. Cover the mould's top portion with a polythene sheet so that water doesn't fall on concrete surface.

• Uncover and remove the cube specimens from mould after 24±½ hours of casting. For removing specimen from mould, first loosen all nut-bolts and carefully remove specimen because concrete is still weak and can be broken.

• Immediatly after removing, put the specimen into a tank of clean water for curing. Make sure cube specimen is fully submerged in water.

• After 28 days of curing take out specimens from water tank and send to laboratory for testing.

Generally three cube specimens are tested separately at laboratory and the average result is counted as concrete compressive-strength. So we make three specimens on site at a time.