What are shrinkage cracks a result of?

Shrinkage cracks occur due to surface tension caused by rapid evaporation of moisture from the surface of concrete. When the moisture content in freshly placed concrete evaporates quickly, the surface can dry out and shrink faster than the underlying concrete. This differential shrinkage leads to tension stress within the material, ultimately resulting in cracks.

Understanding how environmental factors, such as temperature and humidity, influence the evaporation rate is crucial for managing concrete placement and curing processes. In contrast, the other options relate to different issues in concrete performance, such as aggregate ratios or curing techniques, but do not directly cause shrinkage cracks in the same manner.