The practice will describe the design of suitable supporting rollers to be used in beam tests conducted to assess the flexural performance of a FRC mixture. The design will be generic and will not involve proprietary equipment or a patented design. The practice will include drawings and dimensions of the rollers, the type of material the rollers are to be fabricated from, and operation of the rollers during a test. The rollers exhibit a degree of friction in operation, and a method will be described for correcting the test result to account for this friction.
Evidence presented to the sub-committee over the last three years, plus papers published in ACI Journal of Materials, the Journal of ASTM International, and the Journal of Testing and Evaluation, support the proposition that substantial errors can arise in the apparent flexural performance of fiber reinforced concrete beams as a result of friction in the supporting rollers. The errors can be particularly severe if the rollers are fixed against rotation, or lock up after a small amount of rotation, thereby greatly increasing the amount of friction imparted at the lower surface of the beam as the support points on the bottom surface of the beam move away from the center during crack widening. The magnitude of the increase in apparent performance for a FRC beam tested on fixed or locked-up rollers compared with a nominally identical FRC beam tested on rollers free from substantial restraint is of the order of 100-200%. This represents a severe and unconservative bias in the apparent post-crack performance of an FRC mixture. In order to prevent such large errors in performance assessment from occurring, it is necessary to use supporting rollers designed not to lock up during testing, thereby ensuring that a low amount of frictional restraint acts at all times. Moreover, the frictional restraint against rotation exhibited by the roller should be consistent under a wide range of applied loads. The design should also be stiff so that the load train rigidity remains high, and robust enough to withstand the rigors of long-term use. The design proposed in this new practice satisfies these requirements. In addition, the practice includes equations that may be used for correcting the measured load during flexural testing to account for roller friction.
Keywordspost-crack; modulus of rupture; toughness; residual strength; bias
The title and scope are in draft form and are under development within this ASTM Committee.Back to Top
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