rebarred

Concrete on its own is strong in compression but weak in tension. When a structural member must span a distance or carry a load that pulls on it, plain concrete will crack and fail. Rebarred concrete solves this by embedding steel reinforcing bars, or rebar, into the concrete matrix before it sets. The steel carries the tensile stresses while the concrete handles compression, creating a composite material far stronger than either alone.

Rebar is typically deformed steel bar, usually Grade 60 or Grade 75 in North America, with a diameter measured in eighths of an inch: #3 (3/8 inch), #4 (1/2 inch), #5 (5/8 inch), and #6 (3/4 inch) are common sizes. The deformations, or ribs, on the bar surface provide mechanical bond with the concrete. Rebar is placed in cages or mats before concrete is poured, held at specified spacing and cover depth to ensure the steel is surrounded by concrete and positioned where stresses demand it most. A typical floor slab might have rebar spaced 12 to 18 inches apart in both directions, while a beam under heavy load may have multiple bars clustered in the tension zone.

Corrosion and cover

Steel rusts when exposed to moisture and oxygen. In rebarred concrete, the bar is protected by the alkaline concrete environment and by concrete cover, the thickness of concrete between the steel and the exposed surface. Specifications typically require 1 to 2 inches of cover in protected environments, more in marine or aggressive exposures. Cracks in the concrete allow water and salt to reach the steel, triggering corrosion that swells and spalls the concrete. Poor cover, inadequate compaction, or inferior concrete quality shortens the life of rebarred structures severely.

Inspection of rebarred members includes visual assessment for cracks and spalling, often combined with cover meter testing to verify actual concrete depth over the bar. Where concrete has deteriorated, the rebar itself is examined for section loss and surface condition. Chloride penetration testing is used in coastal or de-iced environments to predict remaining service life.

Rebarred construction is the dominant concrete practice in modern building, bridge, and infrastructure work. It is distinct from prestressed concrete, where steel is tensioned before or after the concrete sets, and from fiber-reinforced concrete, where distributed fibers provide diffuse reinforcement. The design of rebarred elements follows established codes that specify minimum bar size and spacing, development length (the distance over which bond transfers load), and lap splice requirements where bars must overlap to transfer force.