Whenever the city council shall provide in any ordinance ordering any local improvement that such work shall be done by contract, the city council may order the issuance of local improvement warrants against the local improvement fund established in the ordinance ordering such improvement to pay the cost and expense of such local improvement; that in the event the city council intends to provide for the issuance of local improvement warrants when the work is to be done by contract, it shall so provide in the ordinance ordering such improvement, and such warrants shall be issued to such contractors on monthly estimates in amounts not exceeding seventy-five (75) percent of such monthly estimates as made by the City Engineer; and the remaining twenty-five (25) percent to be paid in such warrants upon acceptance of work as completed, by the city council.