Skyscraper Concrete and Foundations

What is concrete?

Concrete is the building block of our modern society.  This extremely useful material is often taken for granted or ignored completely.  We all walk over it, and use it in some form everyday.  Whether this driving on the highway or working in a skyscraper, we all interact with this material everyday of our lives.

What is concrete composed of?  Concrete is a composite mixture of aggregate and cement binder.  The most common form of concrete consists of Portland cement, aggregate, and water.  Aggregate is basically sand and gravel.  When concrete hardens, it does not actually 'dry'.  The concrete absorbs the water, in a process known as hydration which binds the other ingredients together, creating the well known concrete product we see every day.  As of 2005, six billion cubic meters are produced annually.

What about asphalt?

Asphalt is a form of concrete as well, with bituminous materials replacing Portland cement as the binder.

Asphalt is much cheaper than concrete, but does not last nearly as long.  One major advantage to using asphalt is its’ flexibility in moving terrain. 

The strength of concrete

Concrete has a high compressive strength, but a significantly low tensile strength.  In fact, concrete's tensile strength is only about 10% of its compressive strength.  Tensile strength is the ability to be pulled apart.  Here is a simple diagram showing what tensile strength is:

Compressive strength refers to just the opposite, the ability to withstand a compression of compaction force.

Pre-tensed concrete and rebar

For concrete to be truly useful, its’ tensile strength needs to be greatly improved to meet the modern demands of construction.  To do this, steel or 'rebar' becomes the skeleton for which concrete can be poured.  A metal cage of steel is constructed before the concrete is laid.  This steel goes through a process to 'pre-stress' it, greatly increasing its tensile strength.  When the steel is stressed before the concrete's casting, it is called pre-tensioning concrete; and when performed after the concrete has cured, it is referred to as post-tensioning concrete.

Concrete foundations in Skyscrapers

Due to the great height of skyscrapers, huge foundations are needed to support these structures.  First, a large hole is dug into the ground to reach a point of stable soil (often bedrock).  After some stability is reached, large steal footings are placed, and from these, vertical steal beams are placed along with a network of rebar.  This is an image in actual proportion; the foundation of the CN Tower is 50 ft (15m) deep, to support the 1,815 ft (553m) structure.  The foundation itself is almost five floors deep!  Most house foundations are only one to two feet deep.  Compare that to a skyscraper!

Original Illustration by Stefan

But this is nothing compared to the worlds deepest foundation!  The Petronas Towers has a foundation that is 394 ft (120 m) deep!

The unusually deep foundation needed for the Petronas Towers was because of the areas bedrock.  The bedrock in Kuala Lumpur, Malaysia is quite deep, causing many headaches for construction companies.

Learn more about Skyscraper Construction here.