So I've measured and dug out the area today, bought the paving slabs, bought a tarp and covered the area, and I've stopped there. My plan is concrete, set slabs on top, but I've never worked with concrete before. Do I just buy bags of it from the garden centre? Or do I mix it with some sort of ballast? All that will be going on it is a table, 4 chairs and a couple of plant pots, if weight comes into it which I presume it does.