No person shall place or pile or cause or permit to be placed or piled any sand, gravel, lime, cement, plaster, concrete or any other substance or mixture or allow the same to remain on any portion of any paved street or sidewalk in the City; or make, 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 City Council may grant the person or contractor doing such work permission to mix cement, concrete or building materials in tight boxes or tightly joined boards on such pavements or walks under such restrictions as the City Council may deem proper. (Prior code § 21-18)