Reinforced Concrete

The steel found in many concrete structures is called REINFORCEMENT.
Reinforcement helps concrete resist TENSILE and SHEAR forces, and helps control
CRACKING in concrete.
CONCRETE PROPERTIES
Normal Concrete:
HIGH compressive strength
VERY LOW tensile strength
VERY LOW shear strength
Reinforced Concrete:
VERY HIGH compressive strength
VERY HIGH tensile strength
VERY HIGH shear strength
WHY USE REINFORCEMENT?
As a force is applied to concrete there will be
compressive, tensile and shear forces acting
on the concrete. Concrete naturally resists
compression (squashing), very well, but is
relatively weak in tension (stretching).
Horizontal and/or vertical reinforcement is
used in all types of concrete structures where
tensile or shear forces may crack or break
the concrete. HORIZONTAL reinforcement
helps resist tension forces. VERTICAL
reinforcement helps resist shear forces.




Below are some examples of reinforcement use:
In a SUSPENDED (off-the-ground) concrete slab,
horizontal reinforcement resists tension and vertical
reinforcement (in say supporting beams) resists
shear forces.
In a SLAB-ON-GROUND, reinforcement increases
the tensile strength and helps control the width of
shrinkage cracks.
See CHAPTER 16 Cracking in Concrete
It does not prevent cracks but controls the width that cracks can open.
Uses of reinforcement include:
Increasing the spacing of control joints
Odd shaped slabs
Slabs with re-entrant corners.
REINFORCEMENT POSITION
The position of reinforcement will be shown in the plans. Reinforcement must be fixed in
the right position to best resist compressive, tensile and shear forces and help control
cracking.
The reinforcement in trenches and slabs rests on
BAR CHAIRS and must be securely fixed to the
bar chairs so it won’t move when concrete is
placed around it.
Concrete Cover The reinforcement must be placed so there is enough concrete covering
it to protect it from rusting.
Typical covers are shown in the diagram. To ensure durability, both the concrete cover and
strength should be shown in the plans.



Cracking and Reinforcement Reinforcement alone WILL NOT STOP cracking, but helps
control cracking. It is used to control the width of shrinkage cracks.
See CHAPTER 16 Cracking in Concrete
Concrete Reinforcement Bond To help control the width of cracks, or their location (at
joints), there must be a strong bond between concrete and reinforcement. This allows the
tensile forces (which concrete has a very low ability to resist) to be transferred to the
reinforcement.
To help achieve a strong bond:
The reinforcement should be CLEAN (free from flakey rust, dirt or grease).
The concrete should be PROPERLY COMPACTED around the reinforcement bars.
Reinforcing bars and mesh should be located so that there is enough room between
the bars to place and compact the concrete.
To improve the transfer of tensile forces to the steel,
the reinforcement is often anchored by:
BENDING,
HOOKING, or
LAPPING the bars.
Types of Reinforcement Two types of steel
reinforcement used are mesh sheets or loose bars.
Loose bars are normally deformed, while mesh may
be made from either smooth or deformed bars.
Typical bar diameters are 12, 16, 20 and 24 mm.
Typical mesh sizes are SL42, 52, 62, 72 and 82. The
SL stands for Square mesh Low Ductility and the
numbers represent meanings as well. For example
for SL42 the 4 is the nominal bar size and the 2
refers to the wire spacing (200 mm).
Fibre Reinforcement Synthetic fibres can be added to concrete to aid in minimising early
age plastic shrinkage and can reduce the presence of excessive bleedwater. However,
synthetic fibres are not a replacement for fabric or steel reinforcement. In slab on ground
construction the control joint spacing is the same as plain concrete.
Steel fibres are used for the above and to improve the toughness of concrete. However
they can be used to control drying shrinkage cracking over limited spacings and for oddshaped
slabs. They also increase the flexural, or bending, strength of concrete.