Principles of Reinforced Concrete

Tensile and Shear Strength

• Concrete has very little tensile and shear strength. When tension is applied lengthwise, concrete pulls apart and crumbles more easily than other construction materials, such as steel or polymers. The lack of ductility -- the ability to stretch -- contributes to the lack of tensile strength. "Shear" strength refers to the capacity of a material to withstand breaking under pressure. Reinforcement improves the tensile and shear strength of concrete and holds it together if cracks or breaks occur.

Compressive Strength

• The compressive strength of concrete is the ability of the slab to withstand downward pressure. It is measured in pounds per square inch (psi). Driveways, sidewalks, patios and other residential applications require compressive strength in the neighborhood of 2,500 psi, whereas a building, bridge or parking garage requires significantly more. Increasing compressive strength requires specialty concrete mixes, including reinforcement with strands and fibers of steel. Ultra-high-strength reinforced concrete can reach strengths up to 120,000 psi.

Fiber Reinforcement

• Fibers for reinforcement of concrete are specified according to project requirements. Fibers may be steel, synthetic or organic. The length, strength and bonding capacity of different types of fibers are considered when making concrete. Each component, including reinforcement fiber, is carefully considered when formulating concrete mixtures. Consistency and measurable attributes of reinforcement fibers are critical for architectural and civil engineering projects.

Reinforcement Bar (Rebar)

• Rebar is steel bar with ridges that provides better bonding and anchoring with concrete. It comes in a variety of sizes, from 3/8-inch diameter to 2 1/4-inch diameter. Rebar is specified by grade for different applications. The grade indicates the tensile strength and yield strength. The yield strength indicates the amount that rebar flexes and bounces back into shape. Common grades are 40, 60 and 75, with 75 being the strongest. Since rebar rusts, it is sometimes covered with a polymer coating for certain applications, such as seawalls where corrosive salts are present.