Question

1) briefly explain why you would add air to a concrete mix?

2) If one increases the water to cement ratio, what happens to the compressive strength on the concrete?

Answer 1

1) This type of mix is made by mixing Air entertaining agent with OPC clinker at the time of grinding. Some common air entraining agents are:-

i) Alkali salts of wood resin

ii) Synthetic detergents of alkyl-aryl sulphonate type

Iii) Calcium Lignosulohate

On mixing they produce tiny, tough, discrete, non-coalescing air bubbles in the concrete mix. These bubbles modify the plastic properties of plastic concrete. It improves workability i.e less water is required. It's avoids segregation and bleeding of concrete. In cold places due to low temperature water present in concrete freezes, adversely affecting the hydration of concrete; now if air entrained concrete is used it's can easily overcome this problem. Air entrained concrete are also light in weight.

2) One increasing water cement ratio compressive strength of concrete decreases. As higher water content concrete workability improves but on other hand concrete becomes porous which reduces strength. And it's found that if concrete is porous not only carbonation takes at greater depth but corrosion of reinforcement also takes place.