Reinforcing rods are usually of round section mild steel 10 or 12 mm diameter for lintels up to 1.8 m span. The ends of the rods should be bent up at 900 or hooked as illustrated in Fig. 91.
The purpose of bending up the ends is to ensure that when the lintel does bend the rods do not lose their adhesion to the concrete around them. After being bent or hooked at the ends the rods should be some 50 or 75 mm shorter than the lintel at either end. An empirical rule for determining the number of 12mm rods required for lintels of up to, say, 1.8 m span is to allow one 12mm rod for each half brick thickness of wall which the lintel supports.
Fig. 91 Ends of reinforcing rods.
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.