Blinding 2

[99491]

Dear All,

A very simple question to you all.

What is the need to put blinding at the footing of a structure. Why can the beam not just sit directly on the compacted ground?

 

[Ron]

Unless you are in an area that has this as a code requirement, as long as the bottom of the excavation is stable, relatively dry, non-reactive to concrete, and compactible, then there is no reason you would have to use a blinding layer. Provided you have adequate cover on the rebar, concrete may be cast against earth.

 

[hokie66]

Ron is certainly correct, but there are practical reasons for using a blinding layer. It helps a lot in fickle weather conditions to create a stable, hard base to work off rather than chancing that the bottom of your footing will turn to mud.

Sometimes where the required bearing pressure is achieved at varying levels, it is more practical to replace the soft material with low strength concrete than to lower the actual reinforced pad. This material in the overexcavated area is often called blinding concrete.

Another use for blinding concrete is under a continuous slab or mat foundation which is required to be waterproofed. The waterproofing is done on a blinding layer, then the mat is built.

 

[Ron]

hokie66...excellent points.

 

[BigH]

Agree that blinding concrete (or what I call a mudmat) is a good idea many times for ease in setting formwork and steel, to prevent damage of the founding soil in case of rain or just construction (workmen) damage. You could use a good crushed stone layer as well . . . I would say that when I was in Canada, we put the footing directly on the soil in 90% of the cases. Of course, we had good soil (clayey tills for the most part).
In India, it was interesting in that they specified 6 inches of sand below the mudmat - why? It got damaged during placement of the mudmat - with the workers disturbing it and all. Stupid - but I've also seen this in Indonesia as well . . . Go figure . . . (wished I didn't have to).

 

[kslee1000]

Provide a controlled, level, solid surface for ease of reinforcing steel assemblage, and provide stable support for steel cages.

We call it mud-slab.