Effects due Climate
The lack of durability of concrete on account of freezing and thawing action of frost is not of great importance to Indian conditions. But it is of greatest considerations in most part of the world.
The most severe climatic attack on concrete occurs, when concrete containing moisture is subjected to cycle of freezing and thawing. The capillary pores in the cement paste are of such a size that water in them will freeze, when the ambient temperature is below 0degree C.
The gel pores are so small that water in them does freeze at normal winter temperatures. As water, when freezing expands by 9% of its volume, excess water in the capillaries has to move. Since the cement paste is relatively impermeable high pressures are necessary to move the excess water even over quite small distances. For normal strength concrete, it has been found that movement of the order of 0.2mm is sufficient to require pressures which approach the tensile strength of the paste.
Concrete can be protected from freeze-thaw damage by the entrainment of the appropriate quantities of air distributed through the cement paste, with spacing between bubbles of not more than about 0.4mm. The air bubbles must remain partially empty, so that they can accommodate the excess water moved to them. This will generally be the case, since the bubbles constitute the coarsest pore system, and are therefore the first to, most moisture as the concrete dries. Fully saturated concrete, if permanently submerged, will not need protection against freezing, but concrete which has been saturated and is exposed to freezing as for example in the tidal range, may not be effectively protected by air entrainment.
For effective protection, an air entraining agent must be added to the mix, to entrain the appropriate amount of air, and to induce a bubble system, with an appropriate spacing. When AEA is used, it is only the amount of air entrained which can be measured in the wet concrete. The amount of air required is between 4-8%, depending on the maximum size of aggregate. Air is entrained during the mixing action, even when no AEA is added. The effect of AEA is to stabilize the air bubbles in the form desired.
More air is entrained with a larger dose of AEA but the effect is not linear and with most agents levels off larger doses. For mixes with higher slump, more air is entrained. It is difficult to entrain air is very stiff mixes; the grading and nature of the particles in the fine aggregates have a very marked effect, on the amount of air entrained. It has been shown that the sand is the most important single factor in air entrainment.
It has been suggested that if concrete can be so dense, that there are no inter-connected capillary pores, and then resistances to freeze- thaw deterioration will exist without the need for air entrainment.
The use of high cement content and low w/c ratio will lead in this direction as will the introduction of silica flume, but there is yet firm evidence to show that, it would be wise to dispense with air-entrainment, if freeze-thaw resistance is wanted.