I am making a small lightweight ramp for a wheel chair. I would like it to be Styrofoam covered with a hard coating. It needs to be outside waterproof, lightweight, and able to take a beating in and out of the car.
Why not check out the commercially available options, see what they use, then check youtube for DIY (do it yourself) options. Stryofoam will fracture. Balsa wood is light but not necessarily durable. Fiberglass might hold up (depending on the thickness), but the stress capacity is something already tested by those who make ramps (even small ones). Resin may be suitable for coating, but it's not inexpensive and needs special handling (is potentially toxic).
Styrofoam won't hold up. What you need is the coating that they spray truck linings with. It is thin, hard and very durable. They can also put nonskid groves in it as well.
Styrofoam will break under the weight. Around here i see them made of wood or metal.
There is some stuff you can buy for a truck bed that dries so hard it is unbelievable. Not sure what it is called but may work on Styrofoam. I saw it at Walmart.
Truck bed liner spray may EAT the styro..