Q:

Can bitcoin system be modded to work without internet

Is it possible for bitcoins to manifest itself as  concrete currency so that it can be used as paper money ?

A:

It depends on what you mean by "work without the internet".
There is already at least one vendor who is making physical bitcoins. That is accomplished by exposing the public key on the face of the coin and hiding the private key under a tamper proof sticker. Such a scheme still relies on the internet, however, because the value of the coins is dependent upon the internet based Bitcoin blockchain.
The actual Bitcoin protocol itself could not be modified to exist without the Internet (or some equivalent wide area network). This is because to prevent the double spend problem without a central authority you must have communication between the network to determine if a coin has already been spent. (The physical coins above essentially rely on trusting the "central authority" of the coin maker. For example, that he will not mint duplicate coins, mint coins without valid private keys, and that his tamper proof stickers are truly tamper proof.)

