Reinforced Concrete

Concrete is strong in compression but relatively weak in tension. The underside of a loaded beam, such as a lintel over a door, is in tension.

Concrete subject to tension loading must be reinforced with steel bars or mesh. The amount and type of reinforcement should be carefully calculated or alternatively, a standard design obtained from a reliable source should be followed without variation.

Important factors relative to reinforced concrete:

• 1 The steel bars should be cleaned of rust and dirt before they are placed.
• 2 In order to obtain good adhesion between the concrete and the steel bars, the bars should be overlapped where they join by at least forty times the diameter. When plain bars are used the ends of the bars must be hooked.
• 3 The reinforcement bars should be tied together well and supported so they won't move when concrete is placed and compacted.
• 4 The steel bars must be in the tensile zone and covered with concrete to a thickness of three times the diameter or by at least 25mm to protect them from water and air which causes rusting.
• 5 The concrete must be well compacted around the bars. 6 Concrete should be at least C20 or 1:2:4 nominal mix and have a maximum aggregate size of 20mm.

Concrete floors are sometimes reinforced with welded steel mesh or chicken wire, placed 25mm from the upper surface of the concrete, to limit the size of any cracking. However, such load-distributing reinforcement is necessary only when loadings are heavy, the underlying soil is not dependable, or when cracking must be minimized as in water tanks.