A few remarks about wood vs. concrete: BMX guys tend to prefer plywood as it is softer to ride – and fall. The ramps are usually quite high so you would need truckloads of stone/concrete. Concrete ramps are more suitable for skateboarding because the surface is smoother and the wheels smaller. Plywood is fine for skateboarding too, as long as the ramps are tightly built. Obviously, concrete structures last a lifetime, whereas wooden ramps have to be repaired after 5 years, after which they can be used another 5 years or so.