The strength and durability of cement has made it a staple building material around the world, but engineers from the Royal Melbourne Institute of Technology in Australia have finally come up with a way to manufacture it at a lower cost by swapping in some recycled materials.
Making concrete uses a fairly simple recipe. A binding agent, which is usually a paste-like material called portland cement, is mixed with water and aggregates: a combination of sand, rocks, and gravel. Adjusting the amount of the various ingredients can alter the properties of the concrete, making it stronger or lighter as needed, but some of the items on the ingredient list, like larger stones and gravel, can increase the price, particularly in parts of the world where those materials aren’t always readily available.
One way to help reduce the cost of making concrete is to replace the aggregate with other materials, including used rubber tires that have been ground up into small particles. The idea kills two birds with one stone, as it’s also a smart way to recycle the millions of worn down rubber tires removed from vehicles every year. But to date, engineers have only managed to create concrete that meets required strength standards using a combination of rocks and rubber.
The problem, as the engineers at RMIT University hypothesized in a recently published research paper, is that the rubber alternate has too many pores. During the initial mixing process, the water fills the pores in the rubber particles, but when it eventually dries and that water evaporates, what’s left is countless voids and gaps between the rubber and the cement around it, weakening the bond and reducing the strength and quality of the concrete.
The solution to replacing all of the aggregate material in concrete with recycled tire rubber was to place the wet ingredients into steel molds that compress the mixture with pressure to eliminate all of the rubber’s pores. After drying, the resulting concrete exhibited a much stronger bond between the hardened cement and the rubber particles, giving it a 97% increase in compressive strength, and a 20% boost in tensile strength.
That’s a big increase, but still not quite enough for the rubber tire concrete to be used as a reliable structural element, so the researchers are looking into other ways to reinforce and strengthen it even further. And while the new approach may increase manufacturing costs, in the long run, it should still prove to be a more cost effective alternative to traditional concrete. That’s because, in addition to using cheaper source materials, it results in a lighter material that’s easier and cheaper to ship.