Effect of misalignment on pulloff test results: numerical and experimental assessments

The successful application of a concrete repair system is often evaluated through pull-off testing. For such in-situ quality control (QC) testing, the inherent risk of misalignment might affect the recorded value and eventually make a difference in the acceptance of the work. So far, the issue of eccentricity in pull-off testing has been ignored in field practice, because it is seen as an academic issue. This paper presents the results of a project intended to quantify the effect of misalignment on pull-off tensile strength evaluation and provide a basis for improving QC specifications if necessary. The test program consisted first in an analytical evaluation of the problem through 2-D FEM simulations and, in a second phase, in laboratory experiments in which the test variables were the misalignment angle (0°, 2° and 4°) and the coring depth (15 mm [1.18 in.], 30 mm [2.36 in.]). It was found that calculations provide a conservative but realistic lower bound limit for evaluation the influence of misalignment upon pull-off test results: a 2° misalignment can be expect to yield a pull-off strength reduction of 7 to 9 % respectively for 15-mm [1.18-in.] and 30-mm [2.36-in.] coring depths, and the corresponding decrease resulting from a 4° misalignment reach between 13 and 16%;
From a practical standpoint, the results generated in this study indicate that when specifying a pull-off strength limit in the field, the value should be increased (probable order of magnitude: 15%) to take into account the potential reduction due to testing misalignment.