What Is Blinding In Construction? Benefits Of Blinding Concrete In Construction
What Is Blinding In Construction?
Blinding in construction refers to a layer of concrete, sand, or mortar that is laid over a surface prior to installing permanent fixtures or flooring. The purpose of blinding is to create a smooth and even surface for subsequent layers, to improve the overall strength of the structure, and to provide a stable base for other materials.
Blinding can be used in flooring and paving work, as well as in foundations, retaining walls, and other structures.
Blinding is an important aspect of construction that involves laying down a thin layer of weak concrete or sand over the hardcore. This layer, usually around 50 mm thick, provides a clean, level, and dry surface for construction work.
The purpose of the blinding is to create a stable and level surface for further construction work, whether it’s a pavement, concrete floor, footing, raft foundation, or reinforcement. Additionally, it serves as a protective layer over the hardcore, preventing the concrete from seeping into it and reducing moisture penetration into the concrete.
What Is Blinding Used For?
Blinding is particularly useful when a damp-proof membrane (DPM) is to be placed over it. The blinding helps to prevent the DPM from being damaged by the hardcore. In cases where a DPM is not being laid directly over the blinding layer, the blinding layer still provides a sturdy surface on which construction can take place.
The blinding also helps to create a cleaner and more even working surface on construction sites, making it easier to store materials and carry out construction work. If necessary, steel mats can be used to reinforce the blinding layer or specific areas of high loading, ensuring the stability and strength of the surface.
Benefits Of Blinding Concrete In Construction
Blinding concrete is a material with a practical application for shallow footings. The soil in a construction site may vary, which may not be detected in the site-specific geotechnical investigation conducted by the project geotechnical engineer.
This can result in a shallow footing being built on top of a “soft-spot,” an area of soil with lower bearing capacity compared to the surrounding soil. To address this, the soft-spot is typically excavated and filled with lower-strength blinding concrete, instead of using more expensive high-strength concrete.
The original intended volume of concrete for the footing is then built on top of the blinding concrete. This process is explained through a set of sequence sketches.
Additionally, concrete blinding can also be used to intentionally deepen the foundation depth of a shallow footing for example, if there is a water pipe or drainage pipe close to the footing or if a future trench is planned near the shallow footing. This is necessary to prevent the footing from being undermined or the pipe from cracking under the additional load.
Can Compacted Sand Be Used As Blinding
Yes, compacted sand is a suitable option for use as a blinding layer in pavements. However, it is not common to use it as a blinding layer under footings because the stress concentration in such applications is usually too high.
If used for paving, it is often combined with a damp proof membrane to prevent water from being absorbed by the sand and rising damp from entering the building once it is finished.
What Is The Strength Grade Of Blinding?
Concrete used for blinding is typically of a lower strength grade compared to the pavement or footing it supports. This is because it’s only subjected to compression and doesn’t need to withstand bending or shear.
Concrete blinding strength ranges from 15-25 MPa (2,175-3,625 psi). Although these grades are low, the compressive strength is typically greater than that of the soil or rock below. Concrete suppliers offer various strength grades, with some like Boral in Australia offering up to 100 MPa (14,500 psi).