It is unlawful to place or pile or to cause or permit to be placed or piled any sand, gravel, lime, cement mortar, plaster, concrete or any other substance or mixture, or to allow the same to remain on any portion of any paved street or sidewalk in the city; or to make or mix, or to cause or permit to be made or mixed, any mortar, plaster, concrete or any other like substance or mixture on any portion of any paved street or sidewalk in the city; provided, that in cases where work is being done on buildings or pavements, the mayor may grant the person or contractor doing such work permission to mix cement, concrete or other building materials, in tight boxes or on tightly joined boards, on such pavements or walk under such restrictions as the mayor may deem proper. (Prior code § 15-2-5)