Variables and Constants - Presentation

The constants are the amount of time for the curing process, the same location (temperature, humidity), the Thickness (height), length, and width.
The independent variables are the amount of sand, cement, water, Fly Ash and Sheetrock in the bricks.
The dependent variable is the strength of the concrete mixture.
A plain concrete brick (made without Fly Ash or Sheetrock) is going to be my control for testing the affect of Fly Ashes.