What is Blinding Concrete and How Thick Should It Be?

What is Blinding Concrete?

Binding concrete is a 50-100mm layer of unreinforced concrete poured over a hardcore fill to create a level working surface. It also prevents sharp stones and gravel in the hardcore from coming into contact with the damp proof course and tearing the membrane.

Hardcore infills provide a stable and robust base for foundations, roads, driveways, and other heavy-load bearing areas. However, without a concrete blinding, they would not be as effective as subsequent work would not have a reliable base.

In addition to creating a level and smooth surface, the concrete stops moisture from rising and finding its way into the structural elements of the building.

In some construction projects sand alone is suitable for blinding. However, concrete will be used in addition when there is a risk that the sand will fall through the gaps in the aggregates below, which could create an uneven surface.

It is called blinding concrete as it covers the hardcore layer and prevents it from being seen.

What Type of Concrete is Used For Blinding?

Blinding concrete can be made from a variety of standardised prescribed concretes and general purpose concrete. These types of concrete are not particularly strong and are often unreinforced. 

The mixtures in the table below are suitable for blinding according to cement manufacturer, Hanson.

Grade of concreteEstimated strength after 28 days

While these grades are often used, you should check what type of concrete has been specified by the project’s architect.

What is Blinding Concrete Made Of?

Blinding concrete is made of portland cement, aggregates, sharp sand, and water. ST1 concrete, which is regularly used as a concrete blinding, is made using a mix ratio of 1 part cement, 3 parts sharp sand, and 6 parts aggregate.

You can adjust this ratio to make it stronger, however, blinding concrete is a lean mix and is not specified for its strength.

Do Houses Need a Concrete Blinding?

Digger removing soil ready for concrete foundations

Concrete blinding is not required as standard. However, the National House Building Council (NHBC) recommends using blinding concrete to raise the ground level when foundations have been overdug.

It should also be used to fill soft spots that have been removed and to protect the ground when there has been a delay in pouring the foundation.

If builders have dug too deep or soft spots have been removed, you cannot simply replace the soil, as it would eventually compact and become uneven.

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