HOW TO CAP CONCRETE CYLINDERS? (4 METHODS OF CAPPING)

Capping of Concrete Cylinders

Capping is required to give a smooth surface for applying compressive load to concrete cylinders. Only cylindrical concrete test specimens are capped using appropriate method. When the ends of cylindrical test specimens are not plane within 0.05 mm, it is required to cap those test specimens. The capped surfaces should be at right angles to the axis of the specimens. After capping the specimen, check the plainness of cap by means of a straight edge and feeler gauge, making a minimum of three measurements on different diameters. Caps are made as thin as practicable and it should not get damaged when the specimen is tested.

We can use any one of the following 4 methods for capping the cylindrical concrete test specimen.

1. Neat cement capping
2. Sulphur capping
3. Gypsum plaster capping
4. Cement mortar capping

1. Neat Cement Capping

1. Prepare a stiff cement paste by mixing cement & water at a desired water/cement ratio.
2. Leave it for 2 to 4 hours in order to avoid the tendency of the cap to shrink.
3. Using a scoop, place some amount of stiff neat cement paste on top of the cylindrical test specimen.
4. Take a glass plate having thickness not less than 6.5 mm and diameter at least 25 mm larger than the diameter of the test specimen.
5. Using this glass plate, press down the stiff cement paste by giving the plate a rotary motion until it makes complete contact with the rim of the mould. Before doing this, apply a thin layer of oil or grease on the glass plate to avoid adhesion of the paste with the glass plate.
6. While giving a rotary motion to the plate, make sure that the plate remains parallel to the end surface at all times. After preparing the cap leave it for some time to become hard.
7. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after 4 hours of molding the concrete, so that the concrete completely settle down in the mould.
• Cement paste should have the desired strength so that the caps will not get damaged while applying load.

2. Sulphur Capping

1. Heat the sulphur compound (consisting of 2 to 3 parts of sulphur to 1 part of inert filler) at an appropriate temperature, until it reaches a pouring consistency.
2. Take a scoop of melted sulphur and pour it in the cap mould. Before doing this, apply a thin layer of oil or grease on the cap mould to avoid adhesion of the liquid sulphur with the cap mould.
3. Immediately after that, put one end of the cylindrical test specimen into the cap mould containing liquid sulphur and press it down. Hold it in this position until the sulphur becomes hard.
4. After that, tap the cap mould downward and remove the cylinder from the cap mould.
5. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after removing the concrete cylinder from the mould.

3. Gypsum Plaster Capping

1. Prepare a stiff plaster by mixing gypsum & water at a desired ratio. While preparing the paste mix it well enough. This will help gypsum to gain strength.
2. Using a scoop, place some amount of gypsum plaster on top of the cylindrical test specimen.
3. Take a glass plate having thickness not less than 6.5 mm and diameter at least 25 mm larger than the diameter of the test specimen.
4. Using this glass plate, press down the stiff gypsum plaster by giving the plate a rotary motion until it makes complete contact with the rim of the mould. Before doing this, apply a thin layer of oil or grease on the glass plate to avoid adhesion of the plaster with the glass plate.
5. While giving a rotary motion to the plate, make sure that the plate remains parallel to the end surface at all times. After preparing the cap leave it for 20 to 30 minutes to become hard.
6. Repeat the same procedure to cap the other end of the cylindrical test specimen.

Note

• This type of capping is done after removing the concrete cylinder from the mould.

4. Cement Mortar Capping

1. Prepare a mortar using cement similar to that used in the concrete and sand which passes 300 micron sieve and retained 150 micron sieve. The mortar should have a water/cement ratio not higher than that of the concrete of which the specimen is made.
2. If any free water has collected on the surface of the specimen, then remove it with a sponge, or blotting paper or other suitable absorbent material.
3. Prepare the cap by applying mortar firmly on the specimen and compact it with a trowel to a slightly convex surface above the edges of mould.
4. Then using a glass plate, press down the cap so formed, with a rotary motion until it makes complete contact with the rim of the mould. Apply a thin layer of oil or grease on the glass plate to avoid adhesion of mortar with the plate.
5. Left the glass plate in position until the specimen is removed from the mould.

Note

• This type of capping is done after 4 hours of molding the concrete, so that the concrete completely settle down in the mould.