Here is a picture of one of the cores as it's being built, Royston. Show me the concrete...

http://www.sharpprintinginc.com/911/images/pagemaster/wtc2small_1.jpg


Here is a cross section photograph of one of the columns. Show me the concrete and rebar...

http://911research.wtc7.net/wtc/arch/docs/corebase1.jpg


Here is a thorough explanation of how the cores were built...

http://911research.wtc7.net/wtc/arch/core.html