What is the primary cause of D cracking in concrete?

D cracking in concrete primarily occurs due to the characteristics of coarse aggregates used in the concrete mix. This phenomenon is related to the ability of aggregates to withstand freeze-thaw cycles. When the aggregates contain moisture and are subjected to freezing conditions, they can expand, leading to internal stress. If the aggregates are not properly selected or processed, this can result in cracking at the surface of the concrete, known as D cracking.

High-quality aggregates with lower porosity and proper gradation are essential in preventing this issue. When aggregates are susceptible to freezing and thawing, particularly if they are porous or contain certain minerals, the concrete is likely to experience premature degradation.

Other factors, such as the water-cement ratio and cement type, do indeed influence the overall durability and performance of concrete, but they are not the primary factors responsible for D cracking. Pore water pressure plays a role in other forms of cracking but is not central to the formation of D cracks specifically linked to coarse aggregate behavior. Understanding the nature of the aggregates is critical for predicting and preventing D cracking in concrete structures.