Grains are live, they breathe in oxygen and breathe out Carbon Dioxide. This makes atmosphere inside grain silo rich in Carbon Dioxide. Carbon Dioxide is acidic and when it reacts with Concrete, which is alkaline. This process is called carbonation of concrete and it destroys the ability of concrete to protect the reinforcing steel from corrosion.
It is seen that 200 mm thick concrete is carbonated in 50 years time in such carbon dioxide rich atmosphere. Minor cracks is concrete further aggravates this carbonation problem in concrete. Pure carbon dioxide carbonates concrete to a depth of 20-30 mm in only 4 weeks time.
Never use Concrete with pulverised fuel ash (A power plant byproduct) for grain silo storage or foundations. Concrete having pulverised fuel ash has very low resistance to carbonation, hence should never be used for grain silos.
Good quality Cement based paints must be used to improve carbonation resistance of concrete. Paint resists carbonation of concrete by providing a sacrificial layer against carbon dioxide attack.