Blinding

[edit] What is blinding in construction?

In construction, 'blinding' typically refers to a base layer of weak concrete or sand that is laid above a layer of hardcore to provide a clean, level and dry working surface. A thin layer, usually around 50 mm (2 inches) thick, of blinding is poured over the hardcore, sealing in the underlying material and levelling off the surface.

The concrete used for blinding is weak compared with structural concrete, and can withstand lower loading. If necessary, steel mats can be used to reinforce either the entire blinding layer or the specific areas of high loading.

[edit] What is blinding for?

If a damp-proof membrane (DPM) is to be placed above it, the blinding will help prevent it from becoming damaged by the hardcore.

If a damp-proof membrane is not being laid directly above the blinding layer, it provides a sturdy surface on which a pavement, concrete floor, footing, raft foundation or reinforcement can be positioned, and it prevents the concrete from seeping down into the hardcore. It also reduces the amount of moisture penetrating up into the concrete.

A blinding layer can also provide a more useable working surface on construction sites, it creates a cleaner and more even site and provides a suitable ground for the storage of materials.

[edit] Alternative definitions of blinding

NB Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019, gives an alternative definition of blinding as: ‘A temporary accumulation of impermeable debris on the lower part of a screen that creates a ‘weir’ effect, reducing the flow area and increasing upstream water levels. Preferred term is variable blockage.’ Where a screen is: ‘An arrangement of bars, grids or poles placed in a watercourse.’