Blinding in construction refers to the process of closing off, sealing, or covering openings, voids, and exposed surfaces in masonry or concrete structures to restore structural continuity, weather resistance, or fire safety.
How it works
Blinding in masonry
When applied to brickwork or blockwork, blinding means filling in openings that are no longer required. This could involve bricking up a disused doorway, sealing pipe chases after services have been installed, or closing cavities that have been left exposed. The goal is to return the wall to a continuous, unbroken state that meets its original performance requirements for strength, fire resistance, and thermal or acoustic insulation.
The term is closely related to the use of blocking bricks, but blinding can also involve applying a render coat or parge coat over an area to seal it, rather than filling it course by course with individual bricks.
Blinding in concrete work
In a different but equally common usage, blinding refers to a thin layer of lean-mix concrete (typically 50-75 mm thick) poured over an excavated surface before the main structural concrete is placed. This blinding layer serves several purposes:
- Clean working surface — it covers loose soil, mud, and gravel so that reinforcement can be positioned accurately.
- Moisture barrier — it prevents ground moisture from contaminating the fresh structural concrete.
- Protection of membranes — waterproof membranes and damp-proof courses are laid on the smooth blinding surface, which protects them from being punctured by sharp stones.
- Setting out — engineers can mark precise lines and levels on the flat blinding surface for formwork and steel placement.
Material and thickness
Blinding concrete is a low-strength mix, typically C7.5 to C10 (7.5-10 N/mm2 compressive strength), because it is not designed to carry structural loads. Its thickness depends on ground conditions: 50 mm is common on firm, level subgrades, while 75 mm or more may be needed on soft or uneven ground.
Importance
Skipping the blinding layer is a false economy. Without it, reinforcement may sink into the soil, cover distances cannot be maintained, and waterproof membranes risk damage — all of which compromise the long-term durability of the foundation.
Related terms
- Blocking brick
- Lean-mix concrete
- Damp-proof membrane
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