Which of the following is considered a common source of concrete variability?

Variation in material properties, such as aggregates and cement type, is indeed a common source of concrete variability. Different sources of aggregates can have varying gradation, shape, and moisture content, all of which can significantly affect the concrete's workability, strength, and durability. Additionally, the type of cement used can change the chemical composition and hydration process of the concrete, leading to variations in setting times and final strength. Each of these factors can introduce inconsistencies in the concrete mixture, making it a critical point of focus during quality control in concrete production.

While environmental changes, mixing time, and temperature during curing can also influence concrete properties, the inherent variability in the material itself typically has a more direct and significant impact on the concrete's performance and characteristics. Therefore, paying attention to the quality and consistency of the raw materials is essential to ensuring the reliability of concrete mixes.