Stone, brick and other similar materials are strong in compression. They are fairly strong in shear. But they cannot resist much force in tension. As a result, masonry arch bridges are designed to be constantly under compression. Each arch is constructed over a temporary falsework frame, known as a centring. In the first compression arch bridges, a keystone in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge. This also prevents tension from occurring in the arch ring as loads move across the bridge. Unreinforced concrete was also used. When masonry (cut stone) is used, the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together. The mortar is allowed to set before the falsework is removed.