Is it possible for bitcoins to manifest itself as concrete currency so that it can be used as paper money ?
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.)
Without the internet there is no "global" bitcoin network. How would you know if a transaction is valid??
The only thing that could work, totally seperate from bitcoin is local network alto currencies.
IF internet is down for a limited time, developers could deal with some local network alto currency in exchange for bitcoin, the risk of course would be on the bitcoin accepting entity.