December 28, 2016
By Alireza Biparva, B.Sc., M.A.Sc.
Apart from mechanical damage, water ingress is the most detrimental force to concrete structures. Buildings the world over are failing far earlier than construction designers had planned. In some cases, this results in high maintenance costs unforeseen by building owners. In more extreme—but not altogether uncommon—situations, building failure results in the need for a complete replacement of the structure. This is very expensive, and can negatively affect the lives of a population, given concrete is the backbone of infrastructure construction.
More often than not, attention to detail throughout the construction process is vital to building a durable structure. This includes providing a consistent waterproofing system from beginning to end to create a completely watertight building. (This principle is explained in greater detail in the second edition of The Construction Waterproofing Handbook by Michael T. Kubal, which was published in 2008.)
According to Michael T. Kubal’s 90 per cent/one per cent principle from the Construction Waterproofing Handbook, “90 per cent of all water intrusion problems occur within one per cent of the total building or structure exterior surface.”
In other words, most water infiltration happens not within the scope of the main waterproofing solution provided, but via small, overlooked details. For instance, a pipe-penetration or tie-hole could be a pathway for water and waterborne chemicals if not taken into account. A tie-hole comprises an extremely small percentage of the structure, but nonetheless could leave a building at risk.
Kubal was also responsible for developing another important waterproofing concept: the 99 per cent principle. This suggests almost all leaks are attributable to causes other than material or system failures—meaning only one per cent of envelope leaks can accurately be attributed to the materials or systems failing. Instead, this usually occurs because of:
Kubal’s two principles suggest minute aspects of complete concrete waterproofing are the most likely gateways for water intrusion.
Correct products create correct systems
When discussing the waterproofing of a concrete structure with below-grade aspects, it is important to consider complete systems rather than individual products. For instance, using just an external membrane to waterproof a concrete structure leaves other, overlooked aspects at risk. The same can be true for other surface-applied membranes, as well as integral waterproofing products. These products are only components of concrete waterproofing—more is needed to make a completely watertight structure.
In fact, the American Concrete Institute (ACI), an authority on concrete best practices, has communicated a complete waterproofing system is needed. (For further reading, visit www.concrete.org.) The revised edition of ACI 212.3R, Reports on Chemical Admixtures for Concrete, calls for the use of a waterstop and other products to supplement the main waterproofing aspect. That is, a complete system, rather than a single product, must be used to effectively waterproof concrete structures.
ACI 350, Code Requirements for Environmental Engineering Concrete Structures, supports this point in more general terms, pointing out the correct waterproofing system will fully protect a concrete structure from water ingress when combined with the correct jointing system. Put plainly, a waterproofing system needs to be supplemented by a jointing system to have waterproofing success.
Regardless, in some circumstances, the construction joints and penetrations of concrete—which provide a path for water and waterborne chemicals—are overlooked. A construction joint is created when one slab or form of concrete ends and another begins. This forms a weak area where water can infiltrate the concrete structure.
The different kinds of joints are:
Over time, it is quite common for a construction joint to start leaking water, especially in below-grade applications where hydrostatic water pressure is present. In fact, construction joints and penetrations are the most common cause of leakages in a below-grade structure. Protecting the joints in concrete is therefore vital, especially considering the disastrous implications if ignored or when the correct product within the correct system is not used. Due to the presence of this water pressure in many below-grade concrete structures, the designer usually specifies some type of joint protection material.
To ensure a concrete structure is completely watertight, all aspects—including the construction joints—must be protected. Waterproofing systems are normally supplemented, in specifications or otherwise, through some type of waterstop. The waterstop is placed at the construction joints, with concrete then poured over it. As water infiltrates the joint, the waterstop blocks or absorbs the moisture, stopping it from proceeding through the entire joint.
There are a number of different options and materials available for waterstops, including:
For the most part, the first three technologies dominate the waterstop landscape. These products are placed between the rebar of a construction joint and either nailed or glued into place before concrete is poured. As concrete shrinkage occurs, the waterstop fills the space in the concrete joint and blocks the flow of water.
PVC waterstops are also known as ‘dumbbells’ due to their shape. These plastic sheets are placed across the joint before concrete is poured, creating a physical barrier blocking water penetration. This particular type of waterstop relies on the ribs in the concrete design to prevent water from passing through the joint.
The PVC waterstop is great for blocking water and relatively inexpensive, but has limitations. Improper compaction around the waterstop can result in a high probability of honeycombing, while concrete shrinkage can lead to the formation of small gaps between the PVC and concrete. This material does not bond well to concrete and can be tedious to install. Further, it can bend while concrete is poured, forming a tunnel where water can infiltrate.
These issues create instances where water leakage can exist. This is not to say PVC is not an effective waterstop product, but rather it does support the 99 per cent principle, given its installation is difficult and often done incorrectly. Leakage does not necessarily occur due to the fact the waterstop is not working, but instead because of an error in installation. To make matters worse, once a problem has been created, it is virtually impossible to recognize until the joint begins to leak.
Over the past 25 years, more advanced and economical waterstops and materials for joints have been developed, many of which are easier and more efficient to use than older models.
Bentonite strips have increased in popularity in the waterstop market, and traditionally work well in various situations. Bentonite is a hydrophilic clay material applied at the construction joint to stop the flow of water through concrete in vulnerable areas. Unlike the PVC waterstop, bentonite has the ability to expand several times its volume when it comes in contact with water, filling voids in concrete and forming a watertight seal.
Further, these strips are flexible, allowing for ease of application. They require no special pieces or procedure to connect, as the ends join naturally together to form a continuous waterstop. These strips have also proven effective under continuous or intermittent hydrostatic pressure. Additionally, bentonite waterstops’ swelling capabilities allow them to block the intrusion of water to a greater capacity than a PVC product, which only acts as a physical barrier. This capacity also allows bentonite to fill voids and cracks to compensate for poorly consolidated concrete.
However, there are also a few drawbacks to using bentonite strip. It can:
Synthetic rubber swelling strip
Synthetic rubber strips are also hydrophilic waterstops. They are extremely effective at keeping water out of construction joints, creating a more durable concrete structure. This product’s material and innovative design allows it to stop water not only under normal conditions, but also when high hydrostatic water pressure is involved. Such conditions can occur with structures built in or near a harsh marine environment, and in below-grade applications where water pressure is a concern.
Synthetic rubber swelling strips have a great swelling capability within the joint. Unlike bentonite or other clay-based hydrophilic strips, they have a controlled swell within the joint, only swelling to fill the voids in concrete. Like bentonite, however, if the concrete is poorly compacted, the rubber strip will fill voids and cracks, blocking water and chemical infiltration.
One advantage the rubber strip has in this area is its shape. Some synthetic rubber strips are trapezoidal, allowing for increased compaction of the concrete when poured over the waterstop. Regardless of compaction, however, the waterstop still compensates for errors by swelling to fill possible water pathways within the concrete.
One of the features setting the synthetic rubber swelling strip apart from other products is its ability to swell in extreme conditions. It swells and blocks water intrusion not only under hydrostatic conditions, but also when facing salt water or contaminated water. This is important considering the state of the moisture seen in below-grade and marine environments around the world.
Rubber strips have superior overall swelling capacity regardless of conditions. These strips have the capability of swelling to many times their original size—more than most standard products. Even with this capacity, a rubber strip can return to its original form and deal with freeze/thaw conditions, making it a durable, long-term joint and detail waterstop for concrete structures.
Rubber strips do have challenges they must overcome, like any other jointing product. They are unsuitable for expansion joints and can be dislodged if poorly installed. When placed in a structure facing high hydrostatic water pressure, the rebar in reinforced concrete can be subject to corrosion—the leading cause of concrete deterioration. However, this can be simply rectified by using a supplementary jointing product, such as a slurry treatment. This double system ensures joint protection and durable concrete structure.
A complete waterproofing system is needed to ensure concrete structures remain watertight for the duration of their lifespan. Joints and details, despite being small aspects of the overall structure, can cause extreme waterproofing problems if ignored. ACI guidelines, along with the principles discussed in this article, make it clear it is vital to use the correct products with the right systems and installation methods if a fully watertight concrete structure is to be created.
Alireza Biparva, B.Sc., M.A.Sc., LEED Green Associate, is research and development manager and concrete specialist at Kryton International. He has more than 10 years of experience in the field of concrete permeability. Biparva oversees a variety of research projects, focusing primarily on concrete permeability studies and the development of innovative products and testing methods for the concrete waterproofing and construction industries. He is an active member of the American Concrete Institute (ACI). Biparva has published several research papers in international journals and conferences on concrete permeability, waterproofing, durability, and sustainability. He can be reached by e-mail at firstname.lastname@example.org.
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