Supports the following standards: ASTM D6927, ASTM D5581 Marshall breaking heads consist of an upper and lower cylindrical segment having 3" inside radius of curvature for 6" samples and 2" for a 4" sample. The lower segment is mounted on a base with two perpendicular guide rods extending vertically from the base. One guide rod is larger than the other, with a correspondingly larger guide sleeve in the upper segment to ensure correct assembly. Guide sleeves in the upper segment bring the two sections together without appreciable binding or loose motion on the guide rods.