What characteristic of aggregates is mainly evaluated to understand their impact on concrete flowability?

The characteristic of aggregates that is primarily evaluated to understand their impact on concrete flowability is particle size distribution. This is crucial because the distribution of different sized particles in the aggregate influences how well they pack together, which directly affects the overall workability or flowability of the concrete mix. A well-graded aggregate, meaning it has a good distribution of various particle sizes, allows for better interlocking and minimizes void spaces, making it easier for the concrete to flow and be worked into place.

When the particle size distribution is balanced, it can lead to a more cohesive mix that flows smoothly, which is particularly important in various applications, whether for placing concrete in forms or for achieving the desired finish. Other factors like specific gravity, moisture level, and aggregate shape do play roles in concrete performance, but it is the particle size distribution that primarily influences the flowability of the concrete.