Concrete by itself has significant compressive strength but very little tensile strength. This property limits the usefulness of concrete in structural applications. Since ancient times natural fibers have been added to concrete to prevent cracks in masonry structures. However, it was not until the 1800s when it was understood that steel and concrete had similar chemical and thermal expansion characteristics that the practice of reinforcing concrete with steel was adopted. This has widely expanded the types of projects for which concrete can be used.