Crack Width Estimation Using Measurements of Central Deflection in ASTM C1609/C1609M

The ASTM C1609/C1609M beam test, Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading), has been used for assessing the flexural performance of fiber-reinforced concrete (FRC) for many years. The procedure involves the application of third-point loading to a relatively short, simply supported beam that consequently cracks and experiences extension and pull-out of fibers across the crack. The flexural performance of FRC tested in this way is normally expressed as a function of vertical deflection at the center of the beam. This is limiting because central deflection is specific to this test and is difficult to relate to structural applications. However, the two halves of the cracked beam experience essentially rigid-body rotation about the crack, raising the possibility that the maximum crack width can be calculated as a function of central deflection. Crack width is a more useful assessment parameter because it can be related to many structural applications. In the current investigation, seven FRC mixtures were produced and tested in accordance with ASTM C1609/C1609M, with both central deflection and crack width measured in all tests. The maximum crack width was then calculated as a function of central deflection and compared with the measured values. The results indicate that crack width can be calculated with sufficient accuracy using only the central deflection and crack offset from the center, thereby permitting flexural performance to be expressed as a function of crack width without any change to the current test procedure. The coefficient of variation in postcrack flexural strength was found to be slightly smaller for the same set of specimens when expressed as a function of maximum crack width rather than central deflection.