This practice prescribes the design of journal-bearing type rollers to support each end of fiber-reinforced concrete beams tested using Test Method C1399/C1399 M or Test Method 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. 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.
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 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. 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 sub-committee recommends that controlling errors in flexural testing due to 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
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Draft Under Development