All materials furnished to and all labor for an authority in excess of the probable cost of $500, except materials and labor used in the maintenance and operation of housing projects and employees directly under the authority, shall be supplied or done under contract made after competitive bids have been requested and submitted.

39 Del. Laws, c. 16, § ?18; Code 1935, § ?5470; 31 Del. C. 1953, § ?4317;