Basement floor and its waterproofing

Basement floor

A basement is the floor of a building that is partly or wholly below the ground level. Basements are used for parking, services, storage and sometimes for living. The basement floor is the lowest floor, usually a raft or slab resting on the ground, with a base of well-compacted hardcore and PCC.

Type of wall appropriate for basement

The wall faces earth pressure and ground water, so it must be strong and watertight. A reinforced concrete retaining wall (M25 or higher, 200 to 300 mm thick, with waterproofing admixture), cast monolithic with the raft, is the most suitable. Masonry walls (brick or stone in 1:4 cement mortar) are acceptable only for shallow, dry basements; the RCC wall can be designed for lateral pressure and hydrostatic pressure.

Waterproofing techniques

1. Integral waterproofing: Dense concrete with low water-cement ratio (below 0.45), plasticiser or crystalline admixture, and PVC water bars in construction joints.
2. Tanking (membrane waterproofing)
   • External (positive side) tanking: Membrane (bituminous coating, APP/SBS sheet, or HDPE) applied on the outside face of the wall and under the raft. This is the best way because water is stopped before it enters the wall. A protection board or brick wall guards the membrane during backfilling.
   • Internal (negative side) tanking: Used when the outside is not reachable. Waterproof cement plaster with crystalline coating, or mastic asphalt, is applied inside and held by an inner loading wall.
3. Cavity drain system: An inner skin separated by a cavity; leaking water flows to a sump and is pumped out.
4. Sub-soil drainage: A perforated pipe around the footing in graded gravel, leading to a sump.
5. Grouting: Cement or chemical injection in cracks and joints.
6. Surface treatment: Cement-sand 1:3 plaster with waterproofing compound, 20 mm thick, in two coats.

  GL ___                      ___
     |  | backfill           |  |
     |  | protection board   |  |
     |  | membrane           |  |
     |  | RCC wall           |  |
     |__|____________________|__|
     Basement floor: screed
                     membrane
                     PCC + hardcore
          sump [ ]   <- drain pipe