The gravel in regular concrete is heavy and costs money. I decided to replace it with free plastic bottles and pieces of Styrofoam trash. Sand and cement were the only purchased materials.
Although the air cavities from the bottles don't have the compression strength of solid cement, the spaces between them create interior columns, walls and arches which are load bearing. The top layer was more solid, with pieces of foam and cement to help spread out the weight. The patio only needs to support foot traffic, and it does that just fine.
Step 1: Collecting the Trash
Step 2: Basic Structure
Bottles went down first, with richer amounts of Styrofoam toward the top. I already knew that Styrofoam cement makes a pretty non-compressive layer for walking on. That layer helps spread one's weight out over the foam-like bottle and cement layer below. Foot pressure is not concentrated on a small area.
It supports my weight well now, and I am expecting no problems.