g PVS- Repeated freezing and thawing.
- Use of de-icing salts.
2e
X denotes an increase in the volume of cement paste due to the chemical action
between the products of hydration of cement and solutions containing sulphates. The sulphate
solutions react with C 3 A forming a chemical enttringite which expands and causes disruption
in concrete. Sulphates occur in three forms—calcium, sodium and magnesium sulphates.
Calcium has low solubility so that it does not constitute high risk. Of the remaining two
magnesium sulphate has the most severe disruptive action because:
(a) the reaction product is insoluble, precipitating out of solution and leaving the way clear
for further attack, and (b) magnesium sulphate reacts with the C 3 S hydrate in cement.
A common source of sulphate salts is the soil around the concrete foundation, from the
water used in making concrete, and by using unwashed aggregates.
The remedy is to use blast furnance slag cement, sulphate resisting cement, supersulphated
cement, and by reducing the permeability.
g
X Concrete is alkaline in nature and has an initial pH value of 12-13. So long as the
steel reinforcement is in alkaline environment, its corrosion cannot start. However, carbon
dioxide present in the atmosphere reacts with concrete in presence of water (the moist
conditions). Infect, in the presence of moisture carbon dioxide changes to carbonic acid and
attacks the concrete; Ca(oH) 2 present in concrete is converted to CaCo 3 causing reduction in pH
of concrete. The concrete thus turns acidic. Once pH value in the concrete-cover drops below
10, the corrosion of steel reinforcement begins due to the reduction in the alkaline environment.
Because of the corrosion of steel reinforcement, its volume increases and consequently the
concrete cracks and spelling takes place.
w 2y X Petroleum and its products do not directly affect hardened concrete. To overcome
the deteriorating affects of mineral oils on fresh concrete permeability is reduced by surface
treatment by applying at least four coats of sodium silicate.
y 2e X Acetic acid, lactic acid, and butyric acid severely attack concrete. Formic acid
is corrosive to concrete.
22e 2y 22p
X Vegetable oils contain small amount of free fatty
essence and deteriorate concrete slowly. Fish and cotton seed oils are found to be the most
corrosive.
X is a retarding agent and gradually corrodes concrete. The remedy is to coat the concrete
surface with sodium silicate solution, tar or asphalt.
& X H 2 S gas evolved from septic sewage may promote formation of H 2 SO 4 affecting the
concrete. However, sewers running full are not affected.
2i
22g
X Concrete is a heterogeneous material. The ingredients of concrete
have dissimilar thermal coefficients which affect durability.
gX Cracks are inherent in concrete and cannot be completely prevented but can be
minimized. Use of unsound materials, high w/c ratio, bad jointing techniques, freezing, thermal
effects, etc. lead to cracks and make the concrete less durable.