Using "twist-off" and "pull-off" tests to investigate the effect of polypropylene fibers on the bond of mortar/concrete and to evaluate their in-situ compressive strength

In the concrete repair industry, the adhesion between the repair layer and the concrete plays a decisive role in the successful composite performance of the repair layers. Due to the shrinkage and its effect on adhesion loss, in this paper the effect of polypropylene fibers on the adhesion between mortar/concrete has been investigated. A new "twist-off" test has been used to perform the experiments. In all experiments, the results of the "twist-off" test were compared with the results of the "pull-off" test. X-ray diffraction pattern and scanning electron microscopy tests were used to further analyze the results. Also, the effect of fibers on shrinkage and mechanical properties of mortars and its relationship on adhesion between mortar and concrete were investigated. In this regard, the tests of "twist-off" and "pull-off" were used in the laboratory and compared with the outputs of computer modeling. Also, by examining the correlation coefficient between the results of in-situ tests and laboratory tests, calibration diagrams were presented to convert the readings obtained from the "twist-off" and "pull-off" tests into the compressive strength of mortars. The results indicate that for the fibrous sample, the peak intensity of Ca (OH)2 or calcium hydroxide is reduced, resulting in the production of more hydrated calcium silicate or C-S-H gel, which results in improved final properties of the mortar and increased adhesion. Also, on average, the shear and tensile bond strength of 90 days obtained from "twist-off" and "pull-off" tests with the addition of fibers increased by 49.5% and 43.1%, respectively.