Every ordinance providing for an improvement to be paid for in whole or in part by special assessments shall contain an estimate of the life of the proposed improvement, prepared as may be directed by the city manager. Any assessment thereafter made for replacing such improvement within such estimated period of life shall be limited to a sum not in excess of fifty per cent (50%) of the cost of such replacement. Assessments for replacements at or after the expiration of such estimated period of life shall be subject to no limitation except as provided for assessments for original improvements.