placing new characters at the end of string

i have a string say

string crap = "abc:abd:aass";

at the end of my string i want to place new characters like "xx"
so the final string looks like

string crap = "abc:abd:aassxx";

how to resolve this issue...

i could able replace colons (:) with other characters but i am failing
when i wanna place new characters at the end of string.


thanks for any help