Which property is characterized by concrete's ability to improve with age?

The property characterized by concrete's ability to improve with age is compressive strength. As concrete cures over time, it undergoes a chemical reaction called hydration, where water molecules interact with cement particles to form stronger bonds. This process continues even after the initial setting, allowing the concrete to gain strength over weeks, months, and even years.

As the hydration process completes, the internal structure of the concrete becomes denser and more robust, leading to increased resistive capabilities against compression forces. Therefore, the compressive strength of concrete is typically measured at various intervals, often at 7 days and then again at 28 days, to track its progression.

The other options do not represent properties that improve with time in the same manner. Porosity, for example, is more about the volume of voids in the material and may not decrease with age unless specific treatments are applied. Color stability also does not relate to concrete's aging process; it is influenced by factors such as UV exposure and the type of pigments used. Flexibility, which is not a strong suit of concrete, refers to the ability of a material to bend without breaking, and this characteristic does not improve as concrete ages. Thus, compressive strength is the only property that distinctly improves with