Few building materials can match the versatility of concrete. It is hard and durable, yet can readily be manufactured or poured on-site in almost any shape or configuration. This allows architects to create daring, innovative designs that would be difficult or impossible to execute with any other material. However, concrete also has limitations. Whether you’re an architect or a homeowner contemplating a DIY project, it’s important to know those limitations.
About Concrete
Concrete begins with cement, a combination of limestone and silicon dioxide mixed with water. It’s been used for thousands of years, primarily for mortar to hold bricks or stones together. By the late 19th century, builders had learned to combine gravel or sand, referred to as the "aggregate" to cement, to produce modern concrete. The aggregate contributes greatly to the strength of concrete, and high-performance concrete uses custom aggregates for that reason.
Tensile and Compressive Strength
Whether it’s used for a simple poured slab for a driveway or for a complex, towering office building, it’s important to understand the strength of concrete. Concrete has a very high degree of compressive strength, the ability to resist crushing force. That’s how concrete pillars can support the weight of large buildings. However, concrete’s tensile strength is relatively low. Tensile strength is the opposite, the ability of a material to withstand stretching and pulling. As a rough rule, concrete’s tensile strength is approximately one-eighth to one-tenth of its compressive strength.
Strength of Ordinary Concrete
Ordinary concrete, the kind used in everyday building projects, has a compressive strength of 3,000 to 6,000 pounds per square inch, or psi. However, its tensile strength is only 300 to 700 psi. Since there are 144 square inches per square foot, that equates to a range of 43,200 to 100,000 pounds per square foot. This is more than adequate for small-scale home DIY projects but a significant limitation for heavy construction.
Strengthening Strategies
Builders use a variety of strategies to strengthen concrete, making it more versatile and reliable. The most widespread technique is to reinforce it with steel mesh or ribbed bars, referred to generically as "rebar." Steel has excellent tensile strength, compensating for the concrete’s shortcomings. Concrete can also be made stronger if the amount of water in the mixture is reduced, though it becomes harder to work with. Extra-strong concrete uses aggregate of carefully controlled size and shape.
