What environmental condition could potentially harm newly placed concrete?

Air temperatures above 90 degrees with low humidity represent a significant environmental condition that can be detrimental to newly placed concrete. High temperatures can accelerate the hydration process of cement, which is essential for concrete to gain strength. However, when coupled with low humidity, this can lead to rapid evaporation of moisture from the surface of the concrete before it has properly set. This evaporation can cause issues such as surface crusting, which prevents proper curing, and can lead to cracking and weaker concrete overall.

Proper curing of concrete is crucial to ensure that it develops its intended strength and durability. High temperatures can also increase the risk of what is known as 'plastic shrinkage cracking,' which occurs while the concrete is still in its plastic (unset) stage due to shrinkage as it dries too quickly. Therefore, managing these conditions through shading, misting, or using curing compounds is essential to protect new concrete from these potential harms.

While the other environmental conditions listed may also pose challenges, they do not have the same immediate impact on the curing process as high temperatures combined with low humidity do.