Waterproofing and water retaining structures

Mapei, a global supplier of construction chemicals, speaks to Kirsten Kelly about achieving watertight concrete structures – which takes a combination of design, workmanship and selective materials.

When building concrete water-retaining structures (dams, reservoirs, irrigation channels, water and wastewater treatment plants), it is important to ensure their watertightness to prevent the flow of water from inside the structure to outside, and the infiltration of water from the surrounding soil into the structure.

High-density, good-quality concrete

“Not all water-retaining structures or water/wastewater treatment works use waterproofing products. High-density, good-quality concrete is effectively watertight with a high level of abrasion resistance. Watertightness is achieved by a low water-to-cement ratio, the proper placing and curing of concrete, as well as its compaction,” states Ross Creasey, product manager, Mapei South Africa.

Superplasticisers (like the Mapei Dynamon range) aid in creating dense concrete, as they can reduce the water-to-cement ratio (and increase strength) while maintaining workability. Crystalline admixtures (Mapei Idrokrete) also reduce the permeability of concrete and even seal small hairline cracks.

Creasey believes that with waterproofing, it is important to consider the service life of the infrastructure. “Once a waterproofing product reaches its service life, it needs to be removed and reapplied. For instance, the lining of an irrigation channel is not necessarily a costeffective decision as it will need to be waterproofed again every 15 to 20 years.

This can be an expensive and technical exercise. Furthermore, some of these irrigation channels are open at the top, so if a rock or sediment falls inside and is carried away with the flowing water, it could destroy any waterproofing coating. Often, it is less expensive to rely on quality concrete and plan for marginal water loss than to keep relining the channel.”

Waterproofing systems

There are times when a waterproofing coating is needed, particularly when there is a low concrete cover over the rebar or a serious leak in the structure.

“Cementitious waterproofing systems (like Mapelastic Smart) are popular for waterproofing water-retaining structures. There are also roll-on applied polyurethane systems (Purtop Easy DW), as well as the more technical systems like the pure polyureas that rely on a spray-based system, requiring specialist contractors,” says Creasey.

He adds that polyurea waterproofing can be applied rapidly and creates a seamless, high-performing waterproofing barrier that can withstand chemical attacks (like chlorine). While the cementitious waterproofing systems are generally more forgiving, the polyurea systems are expensive and need extensive surface preparation.

Surface preparation

Waterproofing systems will not perform adequately without decent surface preparation. Concrete surfaces need to be cleaned from all forms of scale, laitance, dust, mould, form oils, wax and curing agents, as well as any material that can impact the bonding of the waterproofing membrane to the substrate.

Cleaning can be done via mechanical means (such as compressed air, sweeping, abrasive cleaning or highpressure water jetting) or using chemicals to remove materials such as oil or dirt. “Surface preparation will differ according to the type of waterproofing system used. Normally, we would recommend scarifying the concrete surfaces for a polyurea system. The best possible bond needs to be created between the surface and the waterproofing system,” explains Creasey.

Furthermore, there should not be any honeycombing and all rebar needs to be sufficiently covered. Typically, a water tank that holds chemicals will have a greater amount of concrete cover for reinforcement. It is also critical to ensure that there are no cracks, or that existing cracks are repaired with epoxies or high-strength repair mortars.

Joints

While joints play a vital role in fortifying concrete structures, they represent the most vulnerable part of the structure from a waterproofing perspective. Without an effective joint waterproofing system, the water-retaining structure will leak.

“There are various types of joints in water-retaining structures – cold joints, expansion joints, contraction joints and construction joints. In most cases, our Idrostop range of PVC waterstops or swellable waterstops (for cold joints) are cast in situ into the concrete structure to stop the passage of water at the joints. We also offer joint sealers that can handle chemical resistance. The idea is to make the structure as seamless as possible,” explains Creasey.

With pipes and conduits, flanges are used to make the structure watertight. Engineers may also recommend a wet-to-dry epoxy that glues the concrete on to the pipe as it dries, as well as swellable waterstops.

Standards and certifications

All products should conform to either the relevant South African National Standards (SANS), British Standard (BS), European Standards (EN) or the American Society for Testing and Materials (ASTM) standards, should they exist.