1.1 This practice prescribes the design of journal-bearing-type rollers to support each end of fiber-reinforced concrete beams tested using Test Methods C1399/C1399M or C1609/C1609M. The roller design is intended to provide a consistent and relatively low value of effective coefficient of friction at the beam supports. The bearing design incorporates metal-on-metal sliding surfaces lubricated with grease. NOTE 1-During the progress of a test, a crack opens between the loaded third points causing the underside of each half of the beam to move away from the center. The design is intended to provide for unlimited rotation of the roller at the point of contact with the test beam in response to this motion. 1.2 The design of the supporting rollers is a significant factor in determining the magnitude of the arching forces that cause error in flexural test results. Improperly designed supporting rollers can influence the apparent flexural behavior of fiber-reinforced concrete beams. A method is included for correcting the measured residual strength of a third-point loaded fiber-reinforced concrete beam to account for the effective coefficient of friction in the supporting rollers. NOTE 2-The effective coefficient of friction can be determined using a method similar to that described by Bernard. 1.3 Units-The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. 1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Evidence presented to the subcommittee 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 concrete beams in general and fiber-reinforced concrete beams in particular 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. 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 subcommittee recommends that controlling errors in flexural testing as a result of friction in the supporting rollers can be accomplished in either of two ways: using roller bearing supports or establishing a single design for journal-bearing supports for which the effective coefficient of friction is relatively low and measurable. The design proposed in this new practice satisfies the latter requirements. In addition, the practice includes equations that may be used for correcting the measured load during flexural testing to account for roller friction.
Keywordsfriction; fiber-reinforced concrete; post-crack; flexural performance; residual strength; roller supports::
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