Answer
There is no reason to be concerned with multicollinearity.
Work Step by Step
In MINITAB, enter the Slump values in C1, the 7-Day psi values in C2 and the 28-Day psi values in C3.
Select Stat -> Basic Statistics -> Correlation
In variables enter C1 C2 C3. You may deselect "Display p-values"
Click OK.
Since the correlation between Slump and 7-Day psi is not greater than 0.7 or less than -0.7, there is no evidence that multicollinearity exists.
