I am building a garden bed, making use of some clay pavers that had previously been used as flooring for a tin shed. I have dug out the 3m x 1.2m wide area, and now have to do some bricklaying. I intend to have two and a half courses of pavers below ground level and half a paver above ground level. A man at Bunnings told me that cement should do the job but another man said that I should have a first layer of concrete for strength. Who is right?