Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 155
mild steel of diameter 16–40 mm, deformed during manufacture to form
surface protrusions, which can aid the interfacial bonding between steel and
concrete. As the concrete sets, it shrinks and grips the steel, so that the
tensile strength of the composite can be made equal to that of the reinforcement
in the direction of the load. Steel and concrete happen to possess very similar
coefficients of thermal expansion, so that changes in temperature do not
generate interfacial stresses. The alkaline environment provided by the concrete
surrounding the steel acts as a corrosion inhibitor for the steel.
Under tensile loading, assuming ‘no slip’ at the interface, the concrete and
the steel undergo the same strain. Since the fracture strain of concrete is less
than that of steel, extensive cracking occurs in the vicinity of the concrete/
steel interface on the side of the beam in tension. It is important to limit the
stress in the steel so that these cracks do not extend to the outside surface of
the beam.
The effectiveness of the reinforcement can be increased by elastically
prestressing the high-tensile steel bars in tension. When the concrete has
hardened, becoming anchored to the steel, the load on the steel is released
bringing the concrete into a state of compression. The magnitude of the
precompression should be similar in magnitude to that of the anticipated
tensile stress in the beam. When the reinforced beam is subjected to tension
in service, the effect is to unload the precompression and the concrete should
not undergo a tensile stress.
In ‘post-tensioning’, ducts are left when the structure is cast and wires are
threaded loosely in the ducts. When the concrete sets the wires are tensioned
and anchored to the ends of the ducts. Liquid grout is then injected to fill the
ducts. This method of stressing may be used in the construction of bridges,
since a long beam can be manufactured from short segments and then tensioned
in this manner after assembly.
4.4.6 Durability of concrete
Lack of durability due to chemical causes (as opposed to mechanical causes
such as abrasion) arises from attack by sulphates, acids, seawater and other
chlorides. Since this attack takes places within the concrete, the degree of
permeability of the concrete is critical. The permeability is normally governed
by the porosity of the cement paste, which in turn is determined by the water/
cement ratio employed and by the degree of hydration.
If the concrete is sufficiently permeable that aggressive salts can penetrate
right up to the reinforcement and if water and oxygen are also available, then
corrosion of the reinforcement will take place, leading to possible cracking
of the structure. The development of cracks in concrete may also arise from
volume changes due to shrinkage and temperature variations. In practice,
these movements are restrained and, therefore, they induce stress. If these