how to use the decoded raw value to send IR signal in arduino
If the signal is reported as "unknown" the library couldn't identify the protocol (RC5, RC6, NEC, SONY, etc).  If the library doesn't know the protocol,  the decoded number is meaningless. Simply put, an IR remote transmission has a frequency; a header; some data (the actual remote command); and a footer.  Without knowing the protocol the library doesn't know the length and shape of each section, so the purpose of the decoded number is only to show quickly if the decoding is receiving the same code at each keypress. You can verify this yourself.  Every time something makes an already known remote code be reported as "unknown", the code received is different from a real code, so simply guessing (or knowing beforehand) the protocol is not going to help anyway.