Contracts for improvements made under the Combined Improvement Act shall be let and the improvements made in the same manner as required for street improvements. The city council or village board of trustees may direct the improvements to be made under a single contract or that separate bids be taken for the street improvement, installation of water mains, and installation of sewers, but the aggregate of such contracts shall not exceed the estimate as shown in the ordinance creating the district. For the purpose of making partial payment as the work progresses, warrants may be issued by the mayor and city council or the village board of trustees upon certificate of the engineer in charge showing the amount of the work completed and materials necessarily purchased and delivered for the orderly and proper continuance of the project in an amount not exceeding ninety-five percent of the cost thereof, which warrants shall be redeemed and paid from the amounts received on the special assessments or from the sale of bonds issued to pay the cost of the project as provided in section 19-2414. The city or village shall pay to the contractor interest, at the rate of eight percent per annum on the amounts due on partial and final payments, beginning forty-five days after the certification of the amounts due by the engineer in charge and approval by the city council or village board of trustees and running until the date that the warrant is tendered to the contractor.