Modern Diorama??

You mean a modern aircraft-ramp?

Why would you need balsa?

An aircraft ramp is basically just a big area covered in concrete...   I'd use a piece of plywood and then score the area. Then I'd spread it with sheetrock mud, cut the expansion-joints to scale, and add tar-caulking.  Then paint and add markings..

A sheet of pink or blue foam insulation would work too. Very easy to cut.

Depends what scale. In 72nd i used card. The rough surface of kit boxes is perfect.

I'd use cardboard- many off-white colors of cardboard work well as concrete.  Pen in the lines with a dark gray or black pen.  Plaster also makes good concrete.

If you were given the balsa free, then okay, but I think it is too expensive and valuable to use as a base that you would cover with other materials. I use particle board for bases.  Save the balsa for when you really need a soft wood- that stuff is expensive!

When I use cardboard for concrete I glue it down to particle board.

Plaster also makes good concrete.

Trouble with plaster is that the working time is short... I use it for water though...  The "poured Plaster covered in wrinkled tinfoil" method..

Sheetrock-mud on the other hand, is spreadable and dries slow enough to smooth with a spreader, but fast enough to  be workable in an hour, and sandable..  It also comes in small tubs, enough for sevearal dioramas-worth of ramps, and no mixing is required.. AKA "Joint Compound"... It looks like cement or asphalt in about any scale as well..  I then paint it with tempera paint (It's too porous for "real" paints as it absorbs a lot it, making you use more, and therefire not very economical...

Cutting the expansion-joints is done with any scoring-tool, or the back-edge of an X-acto and a straight-edge... Then I put dark grey or brown oil-paint in a syringe to make the tar-caulking.. This allows the caulking to stand a little proud as well..

Getting some static-grass growing up through cracks in the "concrete" can go a long way in making a ramp look a bit more "real" too..