annual rate contract for supply of isi marked pre-cast concrete manhole covers , supply of isi marked pre-cast concrete manhole covers conforming to is: 12592:2002 or its latest amendments of suitable size, quality and load bearing, for placing of m.h. covers. , heavy duty-20 rectangular with m.s. frame size - 900 x 450mm , heavy duty-20 square with m.s. frame size - 560 x 560mm , heavy duty-20 circular with precast concrete frame size - 450mm , heavy duty-20 circular with precast concrete frame size - 500mm , heavy duty-20 circular with precast concrete frame size - 560mm , heavy duty-20 circular with precast concrete frame size - 600mm , extra heavy duty-35 rectangular with m.s. frame size - 900 x 560 mm , extra heavy duty-35 square with m.s. frame size - 560 x 560 mm , extra heavy duty-35 circular with precast concrete frame size - 450 mm , extra heavy duty-35 circular with precast concrete frame size - 500 mm , extra heavy duty-35 circular with precast concrete frame size - 560 mm , extra heavy duty-35 circular with precast concrete frame size - 600 mm