Illegal concurrent statement?

Hi

I wanna evaluate for each bit in the std_logic_vector I(14 downto 0) the 
following espression:


      for I in 0 to 14 loop
          RanIB_tmp(I) <= '1' when
          ((DST(I)(conv_integer(std_logic_vector(OLD_Dest)))='1')
          and
   	 (CDT(conv_integer(std_logic_vector(OLD_Dest)))='1'))
          else '0';
     end loop;

However, when I try to run the code it tells me that I am using an 
illegal concurrent statement? Anyone an idea how I could write this in
an loop so that I can save some copy & paste?

many thanks
Philipp