chesney

hello - in process of doing new concrete floor - plans were to do 50mm gravel 50mm builders sand, dpm, 100mm insulation, topped off with 100mm readymix concrete. Someone has suggested I use 'Toutvennant' rather than gravel & sand, what is this? Does anyone have any links or pics of this material. Should I top this 'Toutvennant' with sand also before dpm? thanks in anticipation.