Last edited 19 Jun 2020

Blinding

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.

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, the blinding instead provides a sturdy surface on which reinforcement for a concrete floor or raft foundation can be positioned, it prevents the concrete from seeping down into the hardcore, and it reduces the amount of moisture penetrating up into the concrete.

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.

NB Culvert, screen and outfall manual, (CIRIA C786) published by CIRIA in 2019, defines 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.’

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