They do not do the same job
Blinding concrete is poured under a foundation to create a level working surface and to keep the reinforcement off the soil. It is not a structural element, and it is certainly not waterproofing. A drainage board is a voided layer on the outer face of a retaining wall that lets water run downward.
What blinding concrete does
- Provides a clean, level base for the foundation reinforcement
- Makes formwork installation easier
- Cuts off some of the contamination and moisture coming from the soil
- Helps maintain the concrete cover
Blinding concrete is not impermeable. It has capillary voids and cannot stop pressurised water. Unless waterproofing is applied over it, it provides no protection.
What a drainage board does
An HDPE dimpled board creates a continuous air and water cavity between the retaining wall and the soil. Water moving through the soil descends through this cavity and reaches the drainage pipe at foundation level. The hydrostatic pressure bearing against the wall is thereby removed.
Choosing the right compressive strength class matters. In the Turkish Ministry of Environment and Urbanisation unit price schedule, HDPE drainage and protection boards are separated by compressive strength; classes between 200 and 250 kN/m2 and between 250 and 350 kN/m2 are defined under different items. Deep basements and high backfill levels require the higher strength class.
The correct sequence
Outside the retaining wall the layer order is: primer, waterproofing membrane, drainage board, filter geotextile, gravel backfill. The drainage board also protects the membrane from physical impact, which is why it is also called a protection board.
Is the board enough on its own?
No. A drainage board directs water, it does not make anything impermeable. If there is no membrane beneath it, the water the board collects still reaches the wall. Equally, if there is a membrane but no drainage, the membrane stays under constant pressure and fails at detail points.
Installing the board the wrong way round
One of the most common site mistakes is fitting the board with the dimples facing the soil. In that case no water cavity forms and the system does not work. The dimples must face the retaining wall.
They do not do the same job
These two elements are often discussed on site as if they were alternatives, yet their functions differ and one does not replace the other. The confusion arises because both protect the waterproofing, but protection alone is not a sufficient description.
Blinding concrete is a clean, even working surface cast beneath the foundation before the waterproofing is laid. Its job is to level the ground, create a smooth base the waterproofing can be laid on, and keep the reinforcement out of contact with soil. It works in the horizontal plane.
The drainage board is a dimpled sheet applied to the outer face of the retaining wall over the waterproofing layer. It has two jobs: protecting the waterproofing from mechanical damage during backfill, and allowing water to flow freely downwards to the drainage pipe through the void formed by the dimples. It works in the vertical plane.
A correct section contains both
Reading a foundation section from the bottom up the sequence is: ground, blinding concrete, primer, horizontal waterproofing, protective screed, raft concrete. Moving to the wall: wall concrete, primer, vertical waterproofing, drainage and protection board, backfill.
As can be seen, blinding is horizontal and the drainage board vertical. Both appear in the same section and neither can be substituted for the other. Blinding is not cast on a wall face, and a drainage board is not laid beneath a foundation.
Two common mistakes on site
Fitting the drainage board the wrong way round. The dimpled face must point towards the wall. The dimples form the void through which water flows between wall and board. Fitted the other way round, that void sits on the soil side, fills quickly and the board becomes nothing more than a protective layer, losing its drainage function.
Treating the board as waterproofing. A drainage board is not waterproofing. It is a dimpled plastic sheet and does not stop water on its own. There must always be a waterproofing layer beneath it. Moisture transfer continues in a wall where only the board has been applied.
What to check when selecting a board
Compressive strength. As backfill load and depth increase the board must perform without being crushed. A higher strength type is chosen for deep basements.
Whether it comes with a filter fleece. Some boards come with a geotextile filter bonded to them. This stops fine soil particles entering the dimple voids and clogging the system. Where an unfiltered board is used, a separate geotextile layer is needed.
Top termination. The top edge of the board must be closed with a profile. Soil and debris fill the dimple voids through an open top edge.
Bottom connection. The bottom edge of the board must reach the gravel fill containing the drainage pipe. A board ending in mid air cannot deliver the water it collects to the pipe.
In summary
Blinding concrete forms a working surface beneath the foundation, the drainage board protects the waterproofing on the wall face and directs water. They sit in different planes with different jobs. If a quotation includes blinding but no drainage board, the waterproofing on the wall face is being left unprotected during backfill.