Public improvements of all kinds may be made by the appropriate department, either by direct employment of the necessary labor and the purchase of the necessary supplies and materials, with separate accounting as to each improvement so made, or by contract duly let after competitive bidding, either for a gross price, or upon a unit basis for the improvement, or by contract containing a guaranteed maximum and stipulating that the City shall pay within such maximum the cost of labor and materials, plus a fixed percentage of profit to the contractor. Contracts may provide a bonus per day for completion of the contract prior to a specified date, and liquidated damages to the City to be exacted in a like sum for every day of delay beyond a specified date.