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 on concrete cylinders. Only cylindrical concrete test specimens are shaded using the appropriate method. When the ends of cylindrical test specimens are not level within 0.05 mm, it is necessary to cap those test specimens. The shaded surfaces should be at right angles to the axis of the specimens. After capping the sample, check the flatness of the cap through the straight edge and feeler gauge, making at least three measurements at different diameters. The caps are made as thin as possible and should not be damaged when the specimen is tested.


We can use any one of the following 4 methods to cap the cylindrical concrete test specimen.

  1. neat cement capping
  2. sulfur capping
  3. gypsum plaster capping
  4. cement mortar capping

1. Neat Cement Capping

  1. Prepare a hard cement paste by mixing cement and water in the desired water/cement ratio.
  2. Leave it for 2 to 4 hours to avoid the tendency of the cap to shrink.
  3. Using a scoop, place some amount of hard clean cement paste over the cylindrical test specimen.
  4. Take a glass plate whose thickness is not less than 6.5 mm and the diameter is at least 25 mm larger than the diameter of the test specimen.
  5. Using this glass plate, press the hardened cement paste by giving a rotary motion to the plate until it makes full contact with the rim of the mold. Before doing this apply a thin layer of oil or grease on the glass plate so that the paste does not stick to the glass plate.
  6. While rotating the plate, ensure that the plate remains parallel to the end surface at all times. After preparing the cap, leave it for some time so that it hardens.
  7. Repeat the same process to cap the other end of the cylindrical test specimen.

pay attention

  • This type of capping is done 4 hours after the concrete has been molded, so that the concrete solidifies completely in the mould.
  • The cement paste should have the desired strength so that the caps are not damaged during the application of the load.

2. Sulfur Capping

Capping Mold With Fix Bar
Capping Mold With Fix Bar
  1. Heat the sulfur compound (consisting of 2 to 3 parts sulfur to 1 part inert filler) to a suitable temperature until it reaches pour consistency.
  2. Take a scoop of molten sulfur and pour it into the cap mold. Before doing so, apply a thin layer of oil or grease to the cap mold to avoid adhesion of the liquid sulfur with the cap mold.
  3. Immediately thereafter, insert one end of the cylindrical test specimen into the cap mold containing the liquid sulfur and press it down. Keep it in this position until the sulfur hardens.
  4. After that, tap the cap mold down and remove the cylinder from the cap mold.
  5. Repeat the same process to cap the other end of the cylindrical test specimen.

pay attention

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

3. Gypsum plaster capping

  1. Prepare hard plaster by mixing gypsum and water in desired proportion. Mix it well while making the paste. This will help the 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 whose thickness is not less than 6.5 mm and the diameter is at least 25 mm larger than the diameter of the test specimen.
  4. Using this glass plate, press the hardened gypsum plaster by giving the plate a rotary motion until it makes full contact with the rim of the mold. Before doing this, apply a thin layer of oil or grease to the glass plate to avoid adhesion of the plaster with the glass plate.
  5. While rotating the plate, ensure that the plate remains parallel to the end surface at all times. After preparing the cap, leave it to harden for 20 to 30 minutes.
  6. Repeat the same process to cap the other end of the cylindrical test specimen.

pay attention

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

4. Cement Mortar Capping

  1. Prepare a mortar using the same cement as concrete and sand that has passed through a 300-micron sieve and retains a 150-micron sieve. The water/cement ratio of the mortar should not exceed the concrete of which the sample is made.
  2. If any free water has accumulated on the surface of the sample, remove it with a sponge, or blotting paper, or other suitable absorbent material.
  3. Prepare the cap by firmly applying the mortar to the sample and compact it with the trowel on a slightly convex surface above the sides of the mold.
  4. Then using a glass plate, press the cap thus formed in a rotary motion until it makes full contact with the rim of the mould. Apply a thin layer of oil or grease to the glass plate to avoid adhesion of the mortar to the plate.
  5. Leave the glass plate in position until the sample is removed from the mold.

pay attention

  • This type of capping is done 4 hours after the concrete has been molded, so that the concrete solidifies completely in the mould.


Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.