Definitions
Waterproofing applied to the surface the water pushes against is positive side waterproofing. A membrane applied to the outer face of a retaining wall is the typical example. The water presses the waterproofing layer towards the structure and the layer seats itself.
Negative side waterproofing is applied from the opposite direction to the water, meaning from the inner face. This time the water tries to lift the waterproofing off the surface. For that reason negative side materials must form a chemical bond with the concrete or penetrate into it.
Why is positive side preferred?
- Water never enters the structure and the reinforced concrete stays dry
- The risk of reinforcement corrosion is eliminated
- Mechanically advantageous, since water pressure presses the material against the surface
- Working together with a drainage system, its service life is very long
Its drawback is that it requires excavation. In terraced buildings, on narrow plots, or where existing landscaping must be preserved, it may not be feasible.
When does negative side come in?
In existing buildings where excavation is not possible, internal treatment is often the only option. The main methods used:
- Crystalline waterproofing: a cement based material penetrates the capillary voids of the concrete, reacts with water to form a crystalline structure and blocks the voids. Because it makes the concrete itself impermeable, it also works on the negative side.
- Polyurethane injection: injected through packers into actively leaking cracks, it expands on contact with water and fills the void.
- Cementitious coatings: surface applied systems suitable for low pressure situations.
The limits of negative side waterproofing
Negative side waterproofing leaves the water inside the wall. The concrete stays saturated and the risk of reinforcement corrosion persists. For that reason it is not a solution but a concession accepted where external treatment is impossible. It does not deliver the expected performance under high water pressure or across large surfaces.
Decision table
- New build, excavation possible: positive side waterproofing plus drainage
- Existing building, excavation possible: positive side waterproofing plus drainage, the most durable solution
- Existing building, excavation impossible, low pressure: crystalline waterproofing
- Existing building, localised active leak: polyurethane injection
- Existing building, high water pressure, excavation impossible: internal drainage channel plus sump and pump, combined with waterproofing