How Strong is Concrete?

The Strength of Concrete

Concrete is known to be one of the strongest and most durable building materials available. Combined with its low cost and the plentiful nature of its primary components, this makes it extremely popular for a wide range of projects and purposes.

The question is, why is concrete so strong? What makes this material such a titan of construction? Also, what does that strength mean? The strength of concrete relies on many factors. The reason for concrete’s strength and its application are extremely important pieces of knowledge for concrete contractors and residential users.

The Chemistry

Concrete is comprised of three primary ingredients: portland cement, aggregates such as stone and sand, and water. Other admixtures may be added for decorative or structural purposes, but these generally comprise a very small portion of the overall mixture and do not affect the concrete formation. In fact, concrete can be formed with just portland cement and water. The aggregates provide strength and form to the mixture but are not absolutely necessary.

When cement mixes with water, the molecules of the two ingredients combine in an exothermic reaction. Heat is produced, known as the heat of hydration. The newly combined molecules form crystals throughout the concrete. Think of Superman’s Fortress of Solitude, just on a much smaller scale. The crystals latch and interlock throughout the concrete, creating a tight inner structure that provides stability. The crystals in essence cement themselves together, filling in cracks and gaps throughout the concrete. As the crystals continue to grow and interlock, the concrete becomes stiffer and harder to work with until it sets up completely.

Concrete may appear hard with a few hours or days of pouring. However, the crystallization process continues as long as the concrete retains moisture. Ideally, the concrete should retain enough water to cure for as long as a month, or even longer. The crystals will continue to interlock and provide structure to the concrete. After about a week, the concrete will have 50 – 75% of its final strength. About a month or longer is required for the structure to reach 100% strength and provide the maximum amount of support to the surrounding construction.

The Physics

It may appear that the reason for reinforcing concrete is simple. Adding another layer of structure with steel rebar or a similar reinforcement method just provides extra support, right?  Well, the answer is both yes and no.

The reason that steel is placed vertically through a concrete wall is that steel has a very high tensile strength that concrete lacks. The strength of concrete is in compression. Think about attempting to compress or squeeze a piece of concrete, even a small piece. It takes a huge amount to pressure to compromise concrete with compression. However, if concrete is pulled or tried to be bent, it can crack fairly easily. This is why concrete becomes more stable as more weight is placed on top of it, in contrast to pressed against it.

Steel, on the other hand, has low compression strength, but a very high tensile strength. It is very difficult to bend steel rods or columns. By combining the tensile strength of steel with the compression strength of concrete, it is possible to build a much more durable and structurally sound wall than with either material on its own.