Blinding

1. Before the concrete is laid , it is usual to blind the top surface of the hardcore .
2. If hardcore is well graded with a mixture of fine and course material , when the hardcore is adequately compacted , the surface finish is relatively smooth and level , thus a blinding layer is unnecessary
3. Consists of a layer of sand 25-50mm thick or a 50-75mm layer of weak concrete (1;12 mix usually suitable )

THe functions of blinding are :

1. To prevent the wet concrete running down between the lumps of broken brick or stone .
2. TO even off the surface of hardcore if a damp-proof membrane is to be placed under the concrete bed
3. To prevent the damp-proof membrane from being punctured by the hardcore
4. To provide a true surface from which the reinforcement can be positioned .