Unpaid assessments on which bonds have been issued shall bear interest at the rate of 11/2percent per month from the date of the bonds originally issued thereon until the bonds and any refunding bonds issued thereon are fully paid, and the interest due at any time on the unpaid assessment may be called without calling any installment of the assessment. The word installment as used in this section shall be construed as applying to interest as well as the principal as the case may be.