We ran a cost comparison of a precast wall built with 40 mm thick slabs against a 9 inch thick brick wall. At current costs, the precast wall is almost 30% cheaper than a brick wall (Note: the cost comparison is before prevailing taxes)
In addition to the above one time costs, a precast concrete boundary wall also has lower life time costs in terms of:
- Longer Life span (precast concrete has lasted for more than 100 years)
- No need for plastering on an on going basis
The IS code for Heavy Duty covers provides for 2mm thick mild steel sheet around the periphery of the cover and two mild steel lifting hooks placed about 2 to 3 inches away from the extreme edge of the cover. It also provides for a gap of 5mm around the cover between the cover and the frame besides a test load of 20 MT for failure.
All the above specifications except the test load have miserably failed to provide a cover which can be opened and closed as and when required, due to the following reasons explained. First, rusting and subsequent swelling of the mild steel rim compresses and hardens the dust collected in the 5mm gap between the cover and frame. This makes it practically impossible to remove the cover from the frame. Second, the hooks are so positioned that the workman who is trying to remove the cover by usually available tool, i.e pickaxe, gets no leverage. In this process, sometimes the workman pulls out the hook from the cover. Common sense would dictate that the fulcrum should be obtained from an object away from the object the workman is trying to lift. Third, the m.s. hooks corrode with time and ultimately fail.
Recognizing these issues, we have modified the design of the manhole cover in three areas:
- The metal strip around the cover has been deleted, since it was not to remain for a long time anyway
- Position of the lifting hooks has been shifted to the extreme edges of the cover so that when using a pickaxe to move the cover, the frame edge automatically becomes the fulcrum
- The mild steel hooks have been HOT DIP GALVANISED to prevent rusting of hooks for a long time to come