The word ‘precast’ indicates that a concrete lintel has been cast inside a mould, and has been allowed time to set and harden before it is built into the wall.
The words ‘situ-cast’ indicate that a lintel is cast in position inside a timber mould fixed over the opening in walls. Whether the lintel is precast or situ-cast will not affect the finished result and which method is used will depend on which is most convenient.
It is common practice to precast lintels for most normal door and window openings, the advantage being that immediately the lintel is placed in position over the opening, brickwork can be raised on it, whereas the concrete in a situ-cast lintel requires a timber mould or formwork and must be allowed to harden before brickwork can be raised on it.
Lintels are cast in situ, that is in position over openings, if a precast lintel would have been too heavy or cumbersome to have been easily hoisted and bedded in position.
Precast lintels must be clearly marked to make certain that they are bedded with the steel reinforcement in its correct place, at the bottom of the lintel. Usually the letter “T or the word ‘Top’ is cut into the top of the concrete lintel whilst it is still wet.
Prestressed concrete lintels
Prestressed, precast concrete lintels are used particularly over internal openings. A prestressed lintel is made by casting concrete around high tensile, stretched wires which are anchored to the concrete so that the concrete is compressed by the stress in the wires. (See also Volume 4.) Under load the compression of concrete, due to the stressed wires, has to be overcome before the lintel will bend.
Two types of prestressed concrete lintel are made, composite lintels and non-composite lintels.