Permeable formwork has special advantage of least susceptible to unfriendly exposure around concrete surface. It is noticed that with the application of such special formwork water-cement ratio of concrete near surface is considerably lowered.
Now question is-what is the extent of reduction in water-cement ratio?
-The depth up to which water-cement is changed is 20 mm from face of member near formwork.
-The degree of reduction is about is about 0.15. The reduction in water-cement ratio is not uniform. This is 0.15 at zone next to formwork and gradually decreases to reach no reduction at 20 mm from the surface of formwork.
Now why is water-cement ratio important? Water-cement ratio is the most important parameter that has influence in almost all properties of concrete. This great reduction in water-cement ratio results reduction in permeability of water and surface absorption as well near outer surface of concrete where they are in contact with hostile environment.
But is 20 mm enough for clear cover of concrete from reinforcing steel? When a concrete mass is cast in direct contact with soil, the expected clear cover is 75 mm (maximum value is provided here). But outer 20 mm of less permeable concrete greatly increase durability.
The another important change is surface hardness. This property is also increased enforcing resistance against erosion and cavitations.