Section 24-352. Execution and filing of contracts; supervision and inspection of work and materials; employer's bond

The contracts, when so awarded,  shall be executed in triplicate by the contractor or contractors on  the
  one  part  and  the commissioner acting for the city, on the other part.
  One of such originals shall be delivered  to  the  contractor,  and  the
  other  two  shall  be filed, one in the finance department and the other
  with the commissioner.  The  work  and  materials  called  for  by  such
  contract   shall   be   done  and  furnished  under  the  direction  and
  supervision, and subject to the  inspection  of  the  commissioner,  the
  commissioner's  engineers, supervisors and inspectors. No contract shall
  take effect until the commissioner shall certify thereon in writing that
  the commissioner's acceptance will, in the commissioner's judgment, best
  secure the public interest and the efficient  performance  of  the  work
  therein mentioned.