Why shear key is provided in column: As per my knowledge, shear key is to be provided for bonding between construction joints at column. After completion of 1st lift, shear key is to be provided. Means simply one runner piece can be embedded and it should be removed after final setting time of concrete. (approx 8 hours later).
During lateral movement, if construction joint in concreted structural members are not made correctly, it may cause to failure due to slippage.
To ensure bonding between lower and upper lift, the construction joint can be made by;
- Removing laitance from concrete surface and making aggregate surface exposed.
- Making shear key in concrete surface with wooden batten to depth of 50mm or more.
- In case concrete is over vibrated, slurry and fines accumulate at top which can become weaker layer in concrete and do cause failure of member under compression. In such case top layer of such fine material should be removed by chipping it at slanted angle.
- You can put 20mm aggregate half way in concrete and half way protruding to form a key, once initial setting of concrete is started.
Most dangerous place in any structure is Top portion of concreted member, It need to be perfect to avoid failures.