What is the chief factor controlling the strength of concrete?

The chief factor controlling the strength of concrete is the water-cement ratio. This ratio is crucial because it directly influences the hydration process of the cement and the overall properties of the concrete once it has cured. A lower water-cement ratio typically results in higher strength and durability, as it produces a denser concrete with fewer voids. Conversely, if too much water is used, it can weaken the final product, leading to increased porosity and reduced strength.

While each of the other factors listed can impact the concrete's performance to varying degrees, they do not have as profound an effect on strength as the water-cement ratio. For instance, aggregate size can affect the workability and density of the mix, but it does not directly control the strength. Similarly, mixing time and temperature can influence the curing process and overall quality but are secondary to the foundational importance of the water-cement ratio in achieving desired strength properties in concrete.