Blinding concrete is a handy concrete pour which aids in the construction of pavements and footings. This quick little article will take a look at exactly what it is and what are its benefits…
Blinding concrete is a thin concrete pour, usually of lower strength grade, which is laid as base preparation beneath pavements and footings. The purpose of blinding concrete is to provide a smooth, level base for reinforcement to be chaired upon and also to prevent concrete seepage into the support soil or water being soaked up by the soil and extracted from the concrete mix. Blinding may also be used to increase a shallow footings founding depth.
Benefits of Concrete Blinding in Construction of Pavements
For structural elements which are cast directly on the ground, the use of blinding concrete has multiple benefits:
- After excavation has occurred, the final soil surface can never be fully flat. A blinding layer of concrete allows for a flat surface to be provided. This ensures that the structural depth of the footing or pavement element as well as the design cover to the reinforcement is maintained.
- For muddy soil conditions, the placement and chairing of reinforcement can be problematic. In some cases reinforcement chairs can sink into the soil under the eight of the reinforcement. This reduces reinforcement cover and therefore reduces the life-span of the pavement.
- A blinding concrete layer provides a cleaner and sturdier temporary work platform. This assist in the temporary storage of building materials while the pavement slab is being constructed. It also results in a construction site which is cleaner overall.
Benefits of Blinding Concrete in Construction of Footings
Blinding concrete has a very useful application when adopted for shallow footings…
The soil conditions across a site can vary. This variation is not always picked up in the site specific Geotechnical investigation which is carried out by the project Geotechnical Engineer. When an shallow footing is constructed, there is a chance that it can be located above a localised “soft-spot” in the soil.
A soft-spot is a localised region of soil on a site which contains a volume of soil which is of much lower bearing capacity than its surrounding soil. To remedy this, the soft-spot is usually excavated out of the ground until the desired competent soil is encountered. This usually results in a larger volume of excavation which now needs to be filled as part of the footing construction. Rather than provide costlier higher strength concrete to fill this larger volume, lower strength blinding concrete is poured to fill the void then the original intended volume of concrete for the footing is constructed above it. Here is a set of sequence sketches explaining this process…
Concrete blinding can also be used to intentionally deepen the founding depth of a shallow footing. This may be required if there is a water pipe or drainage pipe close to the footing. Or a proposed future trench to be dug close to the shallow footing.
If the new trench or existing drain pipe lie within the line of influence (or angle of repose) of the shallow footing, the footing will be undermined in the case of a trench or the drain pipe will be significantly loaded and may crack.
The introduction of blinding concrete can increase the founding depth and in some cases therefore ensure that the line of influence from the footing lies below a potential nearby water pipe. An example of this is shown in the image below…
What is the Strength Grade of Concrete Blinding
Concrete blinding is usually of lower strength grade than the pavement or footing which is above it.
This is because, the blinding layer is only loaded in compression and not required to transmit loading through bending or shear.
The blinding concrete is generally provided with a strength grade of anywhere from 15 to 25MPa (2,175 to 3,625 psi). Even at these relatively low strength grades, the compressive strength is usually much higher than the soil or rock which is beneath it.
Most concrete suppliers provide concrete at various different strength grades. Boral in Australia is one such supplier who provides concrete with strength all the way up to 100MPa (14,500 psi)
Can Compacted Sand be used as Blinding
Compacted sand can generally be used as a blinding layer for the application of pavements. It is rarely used as a blinding layer beneath footings as the stress concentration for these applications is usually too high.
If compacted sand is used as a blinding layer for pavements it is generally adopted in conjunction with a damp proof membrane. This not only prevents the water from being soaked out of the concrete mix by the sand but also prevents rising damp entering the building once it is completed.
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