Shear strength (6,000 to 17,000 MPa [870,226 to 2,465,642 psi])
Concrete’s resistance to shearing stress is very high. This makes it fairly strong in applications with high-shear stresses. The shear strength of concrete is calculated as the force that tends to produce a sliding failure along a plane that is parallel to the direction of the force. Concrete is hundreds of times stronger in shear than in compression, and thousands of times stronger in shear than in tension and flexure.
Of the above physical properties, the most important in the design and construction of common reinforced concrete structures are compressive, flexural or tensile, and shear strength.
Compressive strength is important in the design of load bearing members (e.g. columns), where the load is parallel and concentric with the structural member’s axis. As mentioned above, concrete is strong in compression, so it is an excellent material for designing and building load bearing structures such as walls, roads, runways, foundations, and footings. Wherever a large load must be supported axially (i.e. forming an axis), concrete is an excellent structural material.
On the other hand, as described above, concrete is extremely weak in tension or flexure (bending). Therefore, it is not ideal to use concrete in applications where it will be subjected to these stresses as it can easily fail. Concrete is never primarily considered for use in a tensile stress environment.
In some circumstances, however, and depending on the structure, concrete is often subjected to dual loading. This occurs when one side is subjected to compressive loads, for which concrete is ideally suited, and the other side to tensile stresses, which is the exact opposite to compressive stresses. This is due to the concrete’s modulus of elasticity and Poisson’s Ratio. Dual loading is very common in structures; therefore, concrete must be coupled with structural steel, which behaves favourably in tensile stress environments.
Permeability is also important in the design and construction of concrete structures—especially swimming pools. Since concrete has a relatively low permeability, it is an ideal material for building structures meant to retain liquids (i.e. water). Concrete’s low permeability slows down or restricts the flow of liquids/water through the concrete wall, thus contributing to a watertight pool and making it an ideal construction material.
Reinforced concrete
If it were not for concrete’s weakness in tension and flexure, it would be the ideal building material for all structures. However, by integrating structural steel into a concrete element, it makes it extremely strong in tension and flexure (bending).
By proper structural analysis, engineers can calculate where concrete structural members will undergo tensile or flexural stresses when loaded as well as the magnitude of these stresses. By using the high-tensile strength of structural steel (i.e. rebar), a hybrid material known as reinforced concrete is created, which is strong in compression, tension, and shear.
Editor’s note: In the next issue, Petrocelli will discuss the ins and outs of structural steel and how it is used to create reinforced concrete. He will also look at various applications where reinforced concrete is used in swimming pool construction.
John Petrocelli, P.Eng., is the president of Spider Tie Canada Inc., a Canadian distributor of Spider Tie products. He holds a degree in civil engineering from the University of Toronto, specializing in concrete construction, structural engineering, soil mechanics, and project management. Petrocelli is also a licensed professional engineer in Ontario and a member of the Professional Engineers Ontario (PEO) and the Ontario Society of Professional Engineers (OSPE). He can be reached via e-mail at jpetrocelli@spidertie.ca or by calling (416) 655-8171.
