During cold weather, placing warm concrete on a cold subgrade is not ideal. It is best to have the warm concrete temp no more than _____ higher than the temp of the base.

During cold weather, placing warm concrete on a cold subgrade is not ideal. It is best to have the warm concrete temp no more than _____ higher than the temp of the base.

• A. 20 degrees
• B. 10 degrees
• C. 30 degrees
• D. 40 degrees

Answer: (A)

The main idea is controlling the temperature difference between fresh concrete and the subgrade to limit thermal stresses during early curing. When concrete is significantly warmer than a cold subgrade, heat flows quickly into the base, creating a steep temperature gradient in the concrete. This can cause restrained cooling and contraction at the bottom while the top remains warmer, leading to cracking and weaker bonding with the base. Keeping the warm concrete within a modest range of the base helps the temperature throughout the slab stay more uniform, reducing thermal stresses and promoting better curing and strength development. So the maximum allowed difference is 20 degrees Fahrenheit. That makes sense because 20°F is the practical cap that minimizes thermal gradients without being unnecessarily restrictive. A smaller gap, like 10°F, is more conservative but not the standard limit. A larger gap, such as 30 or 40°F, would increase the risk of thermal cracking and other curing problems.

The main idea is controlling the temperature difference between fresh concrete and the subgrade to limit thermal stresses during early curing. When concrete is significantly warmer than a cold subgrade, heat flows quickly into the base, creating a steep temperature gradient in the concrete. This can cause restrained cooling and contraction at the bottom while the top remains warmer, leading to cracking and weaker bonding with the base. Keeping the warm concrete within a modest range of the base helps the temperature throughout the slab stay more uniform, reducing thermal stresses and promoting better curing and strength development.

So the maximum allowed difference is 20 degrees Fahrenheit. That makes sense because 20°F is the practical cap that minimizes thermal gradients without being unnecessarily restrictive. A smaller gap, like 10°F, is more conservative but not the standard limit. A larger gap, such as 30 or 40°F, would increase the risk of thermal cracking and other curing problems.