The subdivider shall furnish the city a maintenance bond in the amount of 15% of the final contract price to insure the maintenance of the improvements furnished by the subdivider. The bond shall be valid for a period of 15 months commencing with the date of acceptance of the improvement by the city, and shall be in a form, and underwritten by a surety company, approved by the city attorney and the city council.
('69 Code, App. B, § 10.14)