If concrete is impermeable, corrosive agents cannot penetrate and attack it. Concrete basically has two types of pores, which determine permeability. These are capillary pores (with a diameter varying between 0.01 to 10 micron) in the cement, paste which coats the aggregates and larger micro voids, between 1 mm to 10 mm, which are caused by faulty compaction of fresh concrete.
When voids are interconnected because of their larger number and size a continuous link is formed, which makes the concrete permeable.
FACTOR’S CONTROLLING PERMEABILITY
There are three major factors which determine the permeability in concrete.
(a) Water cement ration,
(b) Compaction and curing.
fig.1. Each factor is equally important, if one of these factors is not controlled, the result will be increase, in permeability.
It can be shown that permeability increases as water cement ration increase. Typically, at a water cement ratio of around 0.4, permeability is practically nill.
PERMEABILITY REDUCED BY SUPERPLASTICISERS
As per tests were carried out to determine the effect of superplasticisers on water permeability 150 mm size cubes water stored for 28 day’s in lime – saturated water at 230C and subjected to water pressure on one face.