Blinding concrete: dosage, composition and usage

Blinding concrete is the thin layer of lean concrete cast at the bottom of an excavation, before the structural foundation concrete. Its role is essentially practical: it cleans up and prepares the working zone. It provides a clean, flat surface, keeps the rebar out of direct contact with the soil (mud, clay), and lets the foundation be cast on a regular base at the exact level. It is not a load-bearing element: it is dosed "lean", with far less cement than structural concrete.

A deliberately lean dosage

Where standard reinforced concrete needs about 350 kg of cement per cubic metre, blinding concrete makes do with 150 to 250 kg/m³ depending on the cement class: roughly 150-200 kg/m³ with CPJ 45 (class 42.5), up to 200-250 kg/m³ with CPJ 35 (class 32.5), the most economical cement of the range, whose availability varies with production campaigns. CPJ 45, stocked continuously, remains the standard choice for this use. The frugality is logical: the layer takes no part in the foundation's strength calculation. Enriching it needlessly spends cement for no structural gain; going too low weakens the layer, which must still withstand site traffic and carry the rebar without cracking.

Thickness and placing

Usual thickness sits between 5 and 10 cm, cast on a cleaned and compacted excavation bottom. No fine finish needed: a simple screed to the rule suffices, the layer being destined to be covered. On damp soils, a polythene sheet can be laid within the blinding to cut capillary rise while the foundation is cast. Blinding is generally unreinforced; some sites still lay light mesh over large areas to limit cracking.

Computing the quantity

The calculation follows the same logic as any concrete: volume × dosage. For a 40 m² excavation covered to 8 cm: 40 × 0.08 = 3.2 m³ of concrete. At 175 kg/m³ (mid-range for CPJ 45), that is about 560 kg of cement — 11 to 12 50-kg bags — plus a 5-10% margin for losses. The site's cement calculator performs those conversions directly, including rounding to available packaging (bags, big bags, or bulk from 30 tonnes).

The real gain: protecting what matters

By isolating the rebar from the soil, the blinding layer prevents steel corrosion at the foot of foundations — a critical durability point, especially in aggressive environments. It also fixes the casting level and stops structural concrete being "consumed" by irregularities in the excavation bottom. A few hundred kilos of cement in the right place protect tonnes of reinforced concrete: that is the whole spirit of this modest but strategic layer.