Which of the following increases when vibration aids consolidation?

When vibration aids in the consolidation of concrete, the effectiveness of this process primarily leads to an increase in concrete density. During the mixing and pouring of concrete, there often exists a certain amount of air trapped within the mix, which can create voids and reduce the overall density of the final product.

By applying vibration, these trapped air bubbles are encouraged to rise to the surface, allowing the solid particles within the concrete to pack more closely together. This densification process results in a more compact and robust material by effectively reducing the volume of air voids within the mixture. Consequently, as the air voids are reduced and the solid components are consolidated, the density of the concrete increases.

This enhancement in density is crucial for improving the structural integrity and durability of the concrete once it cures, making it more capable of bearing loads and resisting external factors. The other options do not directly relate to the effects of vibration during consolidation; for instance, concrete stiffness is influenced by the material composition and temperature rather than consolidation alone, and while water content can affect workability, it does not inherently increase due to vibration.