I have problem seeing that concrete will give the resistance needed when hammering. Well, it depends if you want to lower your car, or lift it. For me, who would like to lift it, I always hammer on the convex side, or the underside of the spring blades, and when I say underside, I mean what is the underside when mounted on the car. And the hammer to use would be a rather heavy hammer with a peen. And hammer with the peen so it goes across the spring blade. And again, the idea is to stretch the steel, to stretch the blade on the side you want to get longer so it curves away from the hammering.
The piece of steel you have under the blade needs to be more rounded than the maximum curve of the spring blade. But, not over a sharp edge so the edge goes into the spring blade and stretch it on that side too.
I don't know if I explained it in an English that is possible to understand. :)
