Construction defects: waterproofing, part 2

Construction defects: underground waterproofing, planning versus execution, part 2

By: engineer Ron Arad, Arad Bedek Bayit

This continues the previous article, which covered the main principles of underground waterproofing. Here we look at waterproofing systems themselves, along with the common failure points that create construction defects and turn up again and again in the engineering inspections home inspection engineers carry out.

Common waterproofing systems

There is a wide range of waterproofing systems in use. The common ones are:

Watertight concrete (discussed in detail below).
Bituminous sheets, used on roofs and large surfaces.
PVC sheets, for example in tunnel waterproofing.
Bentonite sheets.
Sprayed or trowel-applied materials, for small areas or walls.
HDPE sheets, common in Europe and the US.

Is concrete watertight?

In theory, yes. In practice, on site, not really.
Watertight concrete is possible only under carefully controlled conditions: the right cement-to-water ratio, the right additives, poured to the right height with continuous vibration.

The construction joints have to be made perfectly, and you have to hope the concrete mix arrives on time from the plant, mixed exactly as planned. It then needs to be smoothed and properly cured.
All of this is hard to pull off under field conditions, and it rarely goes perfectly. That is why most concrete poured on site is not actually watertight.

Failure points leading to construction defects and underground waterproofing defects

1. Clusters of pipes penetrating a wall.
Sealing pipes that are bundled closely together as they pass through a wall or ceiling is not really possible.
Project managers often overlook this, since the plumbing contractor, unaware of the waterproofing implications, simply preps the concrete pour for several closely spaced pipes.

Sealing a single pipe penetration in a wall or a ceiling/floor is already a non-trivial waterproofing detail. With several pipes bundled together, working around each one individually becomes impossible.

2. The floor-wall connection
On paper, the waterproofing detail at the floor-wall connection looks perfect: the rebar sits exactly where it should, the concrete lines are straight, everything checks out.

In practice, the rebar in the wall often ends up angled down while the mesh in the floor angles up, creating a dense cluster of bars right at the floor-wall joint. During the pour, concrete cannot fully work its way through that dense mesh, causing segregation in the concrete and opening a path for water to get in.

3. Relying too heavily on the concrete itself
Sometimes, for reasons of cost or simply limited experience, a designer treats the concrete itself as the waterproofing. We covered why that is a problem above.

Many designers know watertight concrete is hard to achieve, and plan to fix any damp spots later with concrete injections from the inside. In our experience, injections do not always solve the problem, and they do not come cheap.