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High-rises – the importance of a solid foundation

Thanks in part to the invention of the working elevator by Elisha Otis in l852 and humankind’s willingness to dream, the ability to construct buildings higher and higher became more possible. Without Otis’s innovation over a century and a half ago, the rise in popularity for tall structures may have been delayed. The innovation was inevitable, but the reality brought
about an evolution in construction.

Gary Penk, Kryton international, London, UK

According to a report by Cathy Hawker in World Property Journal, cities only occupy 2% of the earth’s land mass, yet are home to 53% of the world’s population with experts predicting that figure will increase up to 75% by the middle of this century. People aren’t moving away from the cities, but instead towards them. For example, Canada’s largest city, Toronto, has more high rises under construction than any other city in the world, 50% more than second place New York. Toronto is also located in the second largest country in the world with one of the smallest population densities. Nevertheless, building out isn’t the option once believed to be the cure of over-populated city centres. Instead the discussion around the globe, as evidence by megastructures becoming commonplace and this century‘s so called ‘space—race’, is for cities to incorporate the growth in population by building upwards.

The current tallest building in the world, Dubai’s Burj Khalifa (828m), is reported to have used 45,000m3 of concrete to construct the all-important foundation and 330,000m* of concrete overall. Concrete, being the most used man-made product on the planet, is essential in the building of a high-rise. Concrete is strong, durable and when reinforced with steel, can sustain an incredible amount of strain, both tensile and compressive. However, in reaching for sustainability we must build these structures for the longest lifespan with the least amount of associated life-cycle costs.

Durable materials
Concrete is used so much because of its ability to be easily formed; it is readily available and also a durable product. To that end, it is also a major component in building a high-rise structure. For example, as stated earlier, the Bug Khalifa was built with a great deal of concrete, amassing over 110,000 tonnes. Other high-rises will experience similar numbers and indeed it is true to say that concrete will make up a vast amount of any given building — especially in the foundation.

This means that if a sustainable and resilient building is to be achieved, one with a lifespan that exceeds expectations, a concrete foundation must be built to be as durable as possible. Unfortunately, concrete does not come without its limitations. For instance, concrete may be formable, durable and have a high-compressive strength; however, there is evidence worldwide that concrete structures are deteriorating and unable to stand up to anticipated Iifespans.

Furthermore, when we mention concrete deterioration, in most cases it is caused through the transport of fluids through the concrete. Water continues to damage or completely destroy more buildings and structures globally than war or natural disasters. Thus, a waterproofing barrier must be used to ensure the foundation of a high—rise building, with its below-grade parking design, which is usually combating water pressure from all directions, is watertight.

Ensuring a waterproof concrete foundation
Surface-applied membranes
One of the options in concrete waterproofing is the surface-applied external membrane.

These waterproofing membranes form a barrier against water penetration, installed after the hardening of concrete. This membrane is used to ensure foundations, roofs, walls and basements are free from water ingress.

Unfortunately, the limitations of the externally applied membrane are numerous:

• puncture damage is a risk
• installation of a membrane is labour—intensive
• valuable time and money will be diverted from the rest of the project
• difficult to repair because of inaccessibility and in locating the failed area.

The situations in which an external membrane will fail can be catastrophic to the entire project. External membranes are frail, need to be applied in certain conditions and require care and attention which construction teams can’t afford.So, why not make the concrete the waterproofing barrier?

Integral concrete waterproofing
Instead of applying a membrane to the outside of hardened concrete, integral concrete waterproofing (ICW) is an admixture added to the concrete at batching, making the hardened concrete the waterproofing barrier. In most cases, concrete without an admixture is expected to develop micro-cracks that allow the passage of water, which decreases a structure’s service life.

However, when an ICW admixture is added to a concrete, the passive chemicals in the product get activated within the concrete matrix once it comes in contact with moisture in the pores and micro-cracks.

When the crystalline technology is activated, it develops millions of needle-like crystals in the concrete that block the flow of water and waterborne chemicals. Integral waterproofing is a permanent solution because it becomes part of the concrete, for the lifetime of the structure. Some of the main benefits of ICW are:

• saves time by eliminating the need for an external waterproofing membrane
• requires less repair, meaning lower overall cost
• eliminates the need for harmful materials, such as petroleum-based membranes
• self-seals micro-cracks for the life of the concrete
• offers durability and lifespan improvements.

Crystalline technology transforms the concrete into an impermeable barrier with self-sealing abilities that provide a powerful defence against water damage. Crystalline technology also reduces the amount of site labour, which is an important aspect of high-rise building, by eliminating the need to install a surface-applied membrane.

Conclusion
High-rise buildings are the present and certainly will be the future of our cities. The use of concrete in the foundations of these buildings is great and therefore, should be built with the highest standards, striving for the longest lifespan possible, and it all starts with the foundation. In order to achieve the longest lifespan, use of the most durable materials is a must.

 Source: Concrete Engineer International