What is blinding in construction?

Blinding is a thin layer of material - typically 50-75mm of weak concrete (C10 or C15 lean mix), sharp sand, or fine hoggin - laid over compacted hardcore to create a smooth, clean, level working surface. It fills the voids in the hardcore surface, prevents the DPM (damp proof membrane) from being punctured by sharp aggregate edges, provides a clean base on which reinforcement can be positioned accurately, and protects the sub-base from damage during construction activity.

Do I need blinding under a concrete slab?

Blinding is not always mandatory but is strongly recommended. Without it, the polythene DPM sheet laid over the hardcore is easily punctured by sharp stones, allowing ground moisture to pass through. Blinding also provides a clean, stable surface on which to position reinforcement mesh at the correct cover, and prevents contamination of the fresh concrete with soil or debris. Many Building Control officers expect to see blinding as part of a competent floor construction.

What mix is used for blinding concrete?

Blinding concrete is typically a C10 or C15 lean mix (also known as ST1 or ST2 mix). It contains less cement than structural concrete and is not designed to carry significant loads - its sole purpose is to provide a smooth surface. A 1:3:6 (cement:sand:aggregate) site-mixed lean concrete or a C10 ready-mix is typical. Thickness is usually 50mm. In less critical applications, 50mm of sharp sand is used as an alternative to concrete blinding.

Groundworks noun

Blinding

/ ˈblaɪndɪŋ /

Also known as: blinding layer, blinding concrete, lean mix blinding

Definition

Blinding is a thin layer of weak concrete (typically C10 or C15 lean mix), sharp sand, or fine hoggin laid over compacted hardcore. It creates a smooth, clean, and level working surface for placing the damp proof membrane (DPM) and the structural concrete slab or foundation above. Blinding fills surface voids in the hardcore that would otherwise puncture the DPM and prevents contamination of the structural concrete with soil.

In practice

The typical ground-floor slab build-up from the bottom up is: compacted sub-grade, compacted hardcore (150mm), blinding (50mm weak concrete or sand), DPM (1200 gauge polythene), insulation (if floor insulation is under-slab), reinforcement (A142 or A193 mesh), and structural concrete slab (100-150mm). The blinding layer protects the DPM from being punctured by the hardcore aggregate above, and provides a clean datum surface on which the reinforcement can be set at the correct cover.

Where blinding concrete is used (as opposed to sand blinding), it must be allowed to set before the DPM is laid. Sharp sand blinding is quicker but more vulnerable to disturbance. On foundation bases, blinding provides the clean surface on which the setting-out lines are marked and reinforcement cages are positioned with correct cover to all faces.

Building Regulations

Relevant PartPart C - Resistance to weather and ground moisture

Typical thickness50 mm weak concrete (C10/C15) or sharp sand

Mix for blinding concreteC10 (ST1) or C15 (ST2) lean mix

Approved Document C requires ground floors to resist moisture rising from the ground. The blinding layer plays an important supporting role by protecting the DPM from puncture, but the DPM itself is the critical moisture barrier. Where foundation bases are formed in aggressive ground conditions (sulfate-bearing soils), the blinding concrete specification may need to be upgraded to resist chemical attack.

Full Building Regulations guidance

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Blinding

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