AASHTO T 280:14(2018) Standard Method of Test for Concrete Pipe, Manhole Sections, or Tile.
4.3.5. The lower bearing strips shall be fastened to a wooden or steel beam or directly to a concrete base. any of which shall provide sufficient rigidity so that deflection is not greater than 1/720 of the specimen length when the maximum load is applied. The rigid base shall be at least 150 mm [6 in.] wide. The interior vertical sides of the strips shall be parallel and spaced a distance apart of not more than 90 mm/m [1 in./ft] of specimen diameter, but in no case less than 25 mm [1 in.]. The bearing faces of the lower strips shall not vary from a straight line vertically or horizontally by more than 2.5 mm/m [1/32 in./fI] of length under no load.
4.3.6. The upper bearing shall be a rigid wood beam with or without an attached hard rubber strip. The wood block shall be sound, free of knots, and straight and true from end to end. It shall be fastened to a steel or wood-faced steel beam of such dimensions that deflections under maximum load will not be greater than 1/720 of the specimen length. The bearing face of the upper bearing block shall not deviate from a straight line by more than 2.5 mm/m [1/32 in./ft] of length. When a hard rubber strip is used on the bearing face, it shall have a durorneter hardness of not less than 45 nor more than 60, and shall have a width of not less than 50 mm [2 in.] and a thickness of not less than 25 mm [1 in.] nor more than 38 mm [11/2 in.] and shall be secured to a wood beam meeting the above requirements.
4.3.7. If mutually agreed upon by the manufacturer and the owner prior to the test, before the specimen is placed, a fillet of plaster of paris not exceeding 25 mm [1 in.] in thickness shall be cast on the surface of the upper and lower bearings. The width of the fillet cap, upper or lower, shall not be more than 80 mm/m [1 injft] of the specimen diameter, but in no case less than 25 mm [1 in.].
4.3.8. The equipment shall be so designed that the load will be distributed about the center of the overall length (L,) of the specimen (Figures 1. 2, 3, and 4). At the option of the manufacturer, the center of the load shall be applied at any point of the overall length (L,) of the specimen. The load shall be applied either at a single point or at multiple points dependent on the length of the specimen being tested and the rigidity of the test frame.
Note 1 —The user of these test methods is advised that multiple points of load applications to the upper bearing will permit use of lighter beams without excess deflection.
4.4. calibration—The loading device shall be one that shall provide an accuracy of ±2 percent at the specified test loads. A calibration curve shall be used. The machines used for performing the three-edge-bearing tests shall be verified in accordance with ASTM E4.AASHTO T 280 pdf download.