Why is air entrainment added to concrete and what is a typical air content range?

Why is air entrainment added to concrete and what is a typical air content range?

• A. To improve freeze-thaw durability; typical air content 4-8% by volume.
• B. To increase slump; typical air content 0-2%.
• C. To reduce drying shrinkage; typical air content 12-16%.
• D. To accelerate curing; typical air content 8-12%.

Answer: (A)

Air entrainment adds tiny, well-distributed air voids in concrete so there is space for water to expand when it freezes. This reliefs internal pressures during freeze–thaw cycles, preventing cracking and damage and thereby improving durability in cold environments. The typical air content for freeze-thaw–resistant concrete is about 4 to 8 percent by volume. Keeping within this range balances durability with strength and avoids excessive porosity. Other options either imply a different purpose (like increasing slump or accelerating curing) or suggest air contents that are too low to provide protection or too high, which would weaken the concrete.

Air entrainment adds tiny, well-distributed air voids in concrete so there is space for water to expand when it freezes. This reliefs internal pressures during freeze–thaw cycles, preventing cracking and damage and thereby improving durability in cold environments. The typical air content for freeze-thaw–resistant concrete is about 4 to 8 percent by volume. Keeping within this range balances durability with strength and avoids excessive porosity. Other options either imply a different purpose (like increasing slump or accelerating curing) or suggest air contents that are too low to provide protection or too high, which would weaken the concrete.