Basement waterproofing
Basement waterproofing design
For many years, membranes have been used for basement waterproofing, but that has changed significantly lately because there have been many problems with membranes shearing, puncturing, and allowing water into the building.
They are still very common, but there are other forms of waterproofing that are growing at much quicker rates than membranes.
Below-ground structures are the area of a building that most often has leaks. It is also commonly known that leaks in basements are much harder to fix than leaks above the ground in any structure.
Designing leak-free basements
It is important to think carefully about the best way to waterproof belowground structures such as basements. It is not only waterproofing the concrete itself that is important. Good design demands the use of good technique to prevent the water from trying to come through the concrete in the first place. This means the installation of good drainage and sometimes sump pumps.
Of course this is more important in areas where there is a lot of groundwater, such as high water tables.
As discussed above, with good drainage, much of the water can be diverted away from the concrete in the first place. This is beneficial because when the concrete structure is surrounded by water, it will always be looking for the point of least resistance. So, if water does leak through the concrete into the basement and that leak is patched from the inside, the water will then be looking for the next point of weakness in the concrete, whereas if it has been drained away, that issue is not so relevant. So creating a leak-free structure depends not only on the choice of product but also on having a good plan to protect the building.
The development of hydrophobic concrete
To counter these problems, hydrophobic concrete was developed in Australia in the mid-20th century. Since then, hydrophobic concrete has been used around the world.
“Hydrophobic” is a term that means repelling water. Photographs of water beading on a leaf are typical of a hydrophobic surface. These forms of waterproofing are typically used as surface applications and have little ability to penetrate the concrete. However, even if there is a surface coating, the pores of the concrete remain open; water can still be forced in under pressure. Thus, hydrophobic concrete proved more suitable to masonry surfaces than reinforced concrete due to its inability to withstand ponding water.
Concrete Admixtures for basement waterproofing
A 50+ year history
Originally crystal growth products (now mostly referred to as crystalline or capillary waterproofing) were applied as a surface coating to the concrete, but for basement waterproofing, they are typically used as an admixture.
Basement waterproofing with crystal growth
Under a microscope, concrete can be seen to have millions of voids, capillaries, and pores. It could be said that it is similar to a sponge.
Crystal growth products are a specific form of integral waterproofing. There are two or three different types of waterproofing admixtures added into concrete, but crystal growth has been by far the most successful and is now used in virtually every country in the world.
Crack sealing
Concrete itself is porous, and of course low-MPA concrete is more porous than high-MPA concrete. Although some claim that very high MPA concrete is waterproof by itself, that does not consider the fact that nobody can stop concrete cracking. So even if the concrete is, say, 60 MPA, if it develops cracking, it is undeniable that water can penetrate to the rebar, regardless of the concrete’s strength.
This is one of the advantages of capillary waterproofing. Crystal growth products react with free lime and moisture within the concrete to form crystals. These can seal cracks that may develop in the future and remain reactive for the life of the structure.
The crystals actually grow towards moisture and remain reactive in the concrete for the life of the structure. Thus, if leaks develop in the future due to cracks, the admixture will immediately react and automatically seal them.
Benefits of waterproofing admixtures for basements
Because they are normally added to the concrete at the batch plant, waterproofing admixtures require no manual on-site labour.
Very fast application because they are usually added to the concrete at the time of pouring. This saves delays that occur with membranes waiting for the concrete to cure before application of the membrane.
They continue to work for the life of the building and never need replacing.
Require no future maintenance.
Cannot be damaged by trades during application, and unlike membranes, cannot be punctured and do not debond or tear.
Because they dispersed throughout the concrete matrix, the result is equally effective on the positive and negative side.
They allow building right to the edge of the construction site without having to allow a perimeter for outside application to the building, thus maximizing the building space and the client’s investment.
Cannot be damaged during backfilling operations. Often during backfilling membranes are damaged and will never work effectively, but this cannot happen with crystalline admixtures.
Most crystal-growth products contain no VOCs (volatile organic chemicals) and are safe with potable water.
Where are waterproofing admixtures used in basements?
Basement waterproofing
Elevator pits
Foundation walls and footings
Below-ground car parks
Ground slabs
Retaining walls
Drinking water tanks
Crystal growth—for basements and areas of low thermal stress
Crystal growth products are composed of millions of tiny crystals. These crystals have no flexibility and are therefore better suited for areas of low thermal stress, with less contraction and expansion, such as water tanks, basements, and tunnels.
On the other hand, crystal growth should not be used on concrete areas where movement can be expected. This includes suspended slabs to some extent but definitely applies to rooftops that will have movement as a result of expansion and contraction caused by fluctuating thermal stress.
As a result, for rooftop applications where only one form of waterproofing is being used, a product such as CWS100, which displays flexibility and high resistance to thermal stress, is a far more suitable product than any form of crystal growth admixture.
Crystal growth and the effect on concrete
The first question most specifiers ask for using admixtures for basement waterproofing is “What is its effect on concrete”?
What is the effect on concrete strength?
Generally, crystal growth admixtures have minimal effect on concrete strength. In the case of CWS Admix, with which we are most familiar, repeated testing has shown little or no effect on concrete strength
Does it increase slump?
Most crystal growth products have some plasticizing effect, which is why a trial mix is always recommended.
Do admixtures slow concrete setting times?
For most products in this category, normally the answer is yes. Most will retard concrete setting times, and sometimes this retardation can span 1 to 2 hours. Of course, this can be offset by using an accelerator.
Hydrophobic versus Hydrophilic
Above we referred to hydrophobic waterproofing and hydrophilic waterproofing.
One of the differences between hydrophobic and hydrophilic is that, while hydrophobic waterproofing repels water (such as water beading onto a leaf), it will not withstand water pressure. Hydrophilic waterproofing is classified by the American Concrete Institute as a PRAH (Permeability Reducing Agent for Hydrostatic pressure), whereas hydrophobic waterproofing is a PRAN (Permeability Reducing Agent for No Hydrostatic pressure).
The effects of hydrostatic pressure
When designing below ground structures, obviously hydrostatic pressure is an important factor. The deeper basements go into the ground, the more this can be a concern. Consultants need to be certain that the waterproofing will withstand the hydrostatic pressure that will be encountered below the ground.
The most common hydrophobic products, such as silanes, siloxanes, and stearates, are essentially fats that react with calcium hydroxide and line the walls of the pores. None of these can withstand hydrostatic pressure or actually seal cracks. Hence, they are good mostly for vertical applications such as columns but are not intended for deep basements or constant submersion under water.
Crystal growth waterproofing has an excellent ability to withstand hydrostatic pressure. For example, CWS Admix has outstanding testing to 14 bars of hydrostatic pressure. This is the equivalent of 142 meters of waterhead—approximately the height of a 40-story building. This compares to most other products, which are only tested to 5 bars of pressure.
CWS Admix basement waterproofing
CWS Admix was developed nearly 50 years ago and is one of the foremost products of its type in the world. CWS Admix is arguably the best product of its type in the world. It has several advantages over the competitors: Firstly, it will resist hydrostatic pressure to a massive 14 bars of pressure. This compares to the majority of products, which will only withstand five bars of pressure. For test results, please email us.
Additionally, CWSAdmix has a constant dosage of 0.8 kg/m³ regardless of cement content. This makes it effortless for dosing and avoids the mistakes of others whereby constantly changing cement contents in the mix design result in mistakes of the product content going into the mix.
To see some of the projects done with CWS Admix around the world, please see our project page.
For more information on CWS Admix, please go to our product page with many more details of the product or email us directly.