By comparing plastics and metals, we choose plastics to make rebar chairbetter because plastics are more resistant to corrosion.
Metal is often considered to be sturdier than plastic; however the energy created by the reaction of metal pieces with the different compounds of concrete will cause cracking within the concrete. Plastic does not interact with the concrete in the same way as metal, and plastic will not cause concrete to crack.
Plastic rebar supports are not only economically friendly, but will ultimately extend the life of any concrete product that is manufactured.