3.3. Reinforcing concrete

a) Forces: Concrete is very strong when it is subjected to compressive forces (being crushed). E.g.: low house stump of concrete, concrete footings etc..

Figure

Concrete is weak when it is subjected to tensile forces (being stretched). E.g. concrete lintels, concrete beams, concrete stairs etc...

Figure

To illustrate the points made above we suspend a concrete beam between two points. The weight from above causes the beam to bend producing compressive stress along the top edge of the beam and tensile stress along the bottom edge causing the beam to crack.

Figure

If we place a bar of steel, which has very high tensile strength, in the concrete near the bottom edge it would prevent the beam from stretching and cracking, this strengthening of concrete is called reinforcing.

Figure

b) Rules for laying reinforcing steel: It is important that reinforcing steel is placed correctly and where heavy loads are involved (e.g. bridges, concrete buildings etc.) the position and sizes must be calculated by an engineer and his drawings and specification must be strictly followed.

When you place reinforcing steel you must remember the following points:

- To make sure that the concrete sticks to the steel properly. The steel must be clean from rust, grease and dirt. If necessary use a wire brush to clean it

- Steel must be well covered with concrete on all sides to ensure moisture cannot reach it to cause rusting which in turn causes the concrete to crack up.

- When making a reinforced slab lift mesh off the ground with stones or special spacers. In formwork with reinforcing bars use small concrete blocks or special spacers to make sure the steel is the correct distance from the formwork.

- Wire used to tie the steel must not be galvanised. If it is, you must burn off the galvanised coating before use.

steel to close to bottom edge

steel to close to top edge

that is the right way

c) Codes: On as work drawing you find reinforcing fabric and bars referred to with a code. E.g. F62, C12, R10 etc.. As fabric and bars are not labelled you must know the meaning of the codes in order to recognize the various types of reinforcing steel found on a building site or in the store yard.

Reinforcing fabric (mesh):

- There are two types of reinforcing fabric which are made in sheets of 6m x 2.4m or in the case of thinner steel in rolls of 60m x 2.4m.

Square mesh:

Example - F62
"F" means fabric
"6" means that wires are 6 mm dia.
"2" means the spaces are 2 decimetre - 200 mm

Example - F71
"F" means fabric
"7" means that wires are 7 mm dia.
"1" means the spaces are 100 mm

Square mesh

Rectangular mesh:

Example - F818

"F" means fabric

"8" means wire A is 8 mm dia.

"1" means the short side of rectangle is 1 decimetre (100 mm). The long side is normally twice the short side.

"8" means wire B is 8 mm dia.

Example - F1018
"F" means fabric
"10" means wire A is 10 mm dia.
"1" means the short side of rectangle is 100 mm
"8" means wire B is 8 mm dia.

Rectangular mesh

Reinforcing bars:

- There are two types of reinforcing bars supplied in 6 m and 10 m length for plain bars and 5 m or 10 m length for deformed bars.

Plain, round bar:
Example - R10

"R" means round
"10" means 10 mm dia,

Plain, round bar

Deformed bar (contorted)

Example - C12

"C" means contorted
"12" means 12 mm dia.

Deformed bar (contorted)

- Be aware that the code letter for deformed bars is not always a " C ".