Concrete consists of water, cement and an aggregate such as sand or gravel. The aggregate has to be mined from the ground, and is now in short supply in many parts of the world, while discarded tires can be partially recycled, but are often burned or relegated to landfills.

Attempts to replace some of the aggregate used in concrete with crumbled, used tires has been stymied by a bonding problem because pores in the rubber fill with water when the concrete is first mixed, and become empty holes as the water evaporates and the concrete sets.

As reported in the journal Resources, Conservation & Recycling, scientists at Australia’s RMIT University have produced good-quality concrete in which all of the aggregate has been replaced with tire particles. They started with wet concrete in which all the aggregate is comprised of tire particles, then placed it in special steel molds as it set to place pressure on the concrete, compressing the particles and the pores within.

Once the concrete dried and set, the cement had bonded much better to the tire particles. When compared to  previous 100-percent tire-aggregate concrete produced by conventional means, the preloaded concrete exhibited 97 percent, 59 percent and 20 percent increases in compressive, flexural and tensile strength, respectively.