I have some steel tubes (18 mm OD) which I wish to use as horizontal members in a storage rack (a bit like shelves). However, the tube flex and bend quite a bit when I put heavier boxes on them. The tubes flex about 40 mm over a 1000 mm span. Ideally I would like the flex to be <5 mm.

Is there any cheap(~$0) & readily available material I can put into the tubes which would stiffen them significantly.

I was thinking about filling them with concrete, but that seems a bit messy and I don't have cement/sand to hand. Would concrete work?