Volume 41, Issue 1 (January 2013)
Analysis of Support Apparatus for Flexural Load-Deflection Testing: Minimizing Bias Caused by Arching Forces
It is in the best interests of standards development that bias and inter-laboratory variability be minimized in the reporting of test results. However, the apparatus sections of flexural test standards, such as ASTM C78, C1399, C1550, and C1609, may not adequately address important issues raised in this study that could adversely affect test results. Specifically, current standards specifications fail to address arching forces and effective span control. The mechanics of commonly applied journal-bearing-type support designs are analyzed and compared to an alternative ball or needle-bearing-type support design. The analysis demonstrates that significant bias can be introduced as a direct consequence of the mechanics that is the nature of journal-bearing-type supports. The analysis applies equally well to flexural tests involving small deformations of portland cement concrete or similar brittle materials, as it does to tests that are carried out involving relatively large deformations, such as with fiber-reinforced concrete.