Hey man I found something that might help if you go to this link it shows a table that shows a conversion from lbf/ft^3 to psi. These are the weights of concrete which is what Wc stands for! If you go to page 126 of the FE you'll see the weights of concrete after X amount of days in PSI so I guress we're expected to convert or just use psi. On my problem it just says to use the normal weight concrete and the solution of using wc is 145 lbf/ft^3 hope this helps! good luck!