During cold weather, placing warm concrete on a cold subgrade may result in a rubbery, partially set top surface while the underlying concrete still hasn't set and is bleeding. It's best to have the warm concrete temperature no more than _____ higher than the temperature of the base.

During cold weather, placing warm concrete on a cold subgrade may result in a rubbery, partially set top surface while the underlying concrete still hasn't set and is bleeding. It's best to have the warm concrete temperature no more than _____ higher than the temperature of the base.

• A. 10 degrees
• B. 15 degrees
• C. 20 degrees
• D. 25 degrees

Answer: (C)

Controlling how much warmer the fresh concrete is than the base is crucial to avoid an uneven cure. If the concrete is too warm relative to the subgrade, the top layer can stiffen and set quickly while the interior remains plastic and continues to bleed. Keeping the warm concrete within about 20 degrees Fahrenheit of the base helps the curing process stay more uniform, reducing the risk of a rubbery, partially set top surface while the bottom is still bleeding. This 20-degree limit represents a practical balance between promoting early strength and avoiding rapid surface set that compromises the interior and moisture movement. Choices with far smaller differences can be impractical, while a larger difference increases the likelihood of the surface crusting and the described issue.

Controlling how much warmer the fresh concrete is than the base is crucial to avoid an uneven cure. If the concrete is too warm relative to the subgrade, the top layer can stiffen and set quickly while the interior remains plastic and continues to bleed. Keeping the warm concrete within about 20 degrees Fahrenheit of the base helps the curing process stay more uniform, reducing the risk of a rubbery, partially set top surface while the bottom is still bleeding. This 20-degree limit represents a practical balance between promoting early strength and avoiding rapid surface set that compromises the interior and moisture movement. Choices with far smaller differences can be impractical, while a larger difference increases the likelihood of the surface crusting and the described issue.