Concrete Cover Blocks for Concrete reinforcement structure
Purpose of provision of concrete cover
The concrete cover must have a minimum thickness
for three main reasons:
to protect the steel reinforcement bars (rebars)
from environmental effects to prevent their corrosion;
to protect the reinforcement bars from fire, and;
to give reinforcing bars sufficient embedding to
enable them to be stressed without slipping.
The premature failure of corroded steel
reinforcements and the expansion of the iron corrosion products around the
rebars are amongst the main causes of the concrete degradation. The carbon steel of
rebars is protected from oxidation by atmospheric oxygen by
the high pH of
concrete interstitial water. Iron bar surface is passivated as long as the pH
value is higher than 10.5. Fresh cement water has a pH of about 13.5 while
evolved cement water pH ~ 12.5
is controlled by the dissolution of calcium hydroxide (portlandite). Carbon dioxide present
in the air slowly diffuses through the concrete cover over the rebar and
progressively reacts with the alkaline hydroxides (KOH, NaOH) and with calcium
hydroxide leading to the carbonatation of
the hydrated cement paste. As a result, the pH of the cement drops and when its
value is below 10.5 –
9.5, steel surface is no longer passivated and starts to corrode. A sufficient
thickness of concrete cover is thus required in order to slow down the
carbonatation process towards the rebar. The minimum concrete cover will depend
on the environmental conditions encountered and must be thicker when the
concrete is also exposed to moisture and chloride (proximity
to the sea, use of de-icing salt for bridges or roads, ...). A high quality
concrete made with a low water-to-cement (w/c) ratio will have a lower porosity and
will be less permeable to water and to the ingress of corrosive species
(dissolved oxygen, chloride, ...). A thicker cover or a more compact concrete
will also reduce the diffusion of CO2 in the concrete, protecting it better from
carbonatation and maintaining a higher pH for a longer time period, increasing
so the rebar service life.
 |
| Astra Concrete Cover Blocks in Kerala |
Concrete
Cover Blocks Use is Must but why?
Designing
a reinforced concrete structure there are only two main considerations, the
amount of reinforcement and the strength of the concrete. To achieve required
strength, proper cover (rebar spacing) is specified in design code as in IS
Code-IS 456-2000 but how this cover is obtained is ignored. Too large or too
small cover causes problems. To use any available materials like broken marble
or tile pieces or piece of aggregate is crude and amateurish method and at the
cost of strength and durability.
Polypropylene
and PVC covers are also available but these are foreign body materials to
concrete, fails to essentially form a permanent bond with concrete. Thermal
coefficient of expansion is different with concrete. Water vapours entering in
the structure, hair cracks are developed which may leads to structure failure.
Thus use of Polypropylene and PVC covers is not preferred throughout the world.
For More details Contact Daiba
Call to 8089081528, email to astracoverblocks@rediffmail.com
