ACI Cold & Hot Weather Concreting 3

Question 1

What does ACI 306 define as the minimum duration for protecting concrete from freezing after placement in cold weather?

Accepted Answer

Until compressive strength reaches at least 500 psi, typically not less than 2 days

Answer

ACI 306 requires concrete to be protected until it achieves at least 500 psi compressive strength, generally a minimum of 2 days, to resist freeze damage.

Question 2

During hot weather finishing operations, plastic shrinkage cracks are most likely to form when the evaporation rate exceeds:

Accepted Answer

0.20 lb/ft²/hr

Answer

ACI 305 identifies an evaporation rate exceeding 0.20 lb/ft²/hr as the threshold above which plastic shrinkage cracking becomes a significant risk.

Question 3

When heating aggregates or water to raise concrete temperature in cold weather, which approach is generally preferred to avoid localized overheating?

Accepted Answer

Heat the mixing water rather than aggregates when possible

Answer

Heating mixing water is preferred because it is easier to control and avoids the risk of flash set that can occur when hot water contacts cement directly.

Question 4

Fogging or misting above a fresh concrete slab in hot weather is used primarily to:

Accepted Answer

Reduce the ambient temperature and evaporation rate above the surface

Answer

Fogging cools the air above the slab and raises humidity, reducing the evaporation rate from the concrete surface without adding water to the mix.

Question 5

In cold weather concreting, the use of Type III (high-early-strength) cement is advantageous primarily because it:

Accepted Answer

Reaches protective strength faster, reducing required heating duration

Answer

Type III cement's faster strength gain allows concrete to reach the 500 psi freeze-resistance threshold sooner, shortening the required protection period.

(ACI) American Concrete Institute Concrete Finisher Practice Test

(ACI) American Concrete Institute Concrete Finisher Practice Test

ACI Cold & Hot Weather Concreting 3