The Effect of Precompression on Adhesion between Concrete and Fiber-Reinforced Mortars and Assessment of Compressive Strength of Mortars Using In-Situ Tests

The interface area between substrate concrete and repair mortars is of great importance. Shrinkage and incorrect compaction of the mortar causes fine cavities, and consequently, the bond strength decreases at the interface area. Therefore, in this paper, the influence of different precompressions on the shear and tensile bond strength between polypropylene fiber-reinforced mortar and substrate concrete has been investigated. Friction-transfer and pull-off tests were applied to measure shear and tensile bond strength, respectively. Furthermore, the effect of polypropylene fibers on shrinkage and bond strength is presented. Subsequently, using photography and analysis by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) pattern, the effect of polypropylene fibers and precompression on the bond strength between fiber-reinforced mortars and the concrete substrate has been studied. Moreover, the compressive strength of fiber-reinforced mortars was evaluated by using semi-destructive in-situ tests. In addition to determining the correlation coefficient between in-situ tests and laboratory tests, calibration diagrams, and transformation equations of records obtained from semi-destructive tests to the mortar compressive strength, are presented. Finite element software ABAQUS has also been used to model the tests mentioned above and compare the outputs with laboratory results. The gained results reveal a positive effect of precompression on the bond strength between concrete and the repair layer. Polypropylene fibers also reduce the shrinkage of mortars and hence increase adhesion. In the XRD test, it was observed that adding polypropylene fibers to the mortar diminishes calcium hydroxide and thus increases calcium silicate hydrate, which has a positive effect on the properties of the mortar. Besides, the SEM photos reveal that the process of hydration and formation of calcium silicate hydrate (C-S-H) gel in the fibers' vicinity is carried out well and has made a better uniformity to the mortar composition. The results of numerical modeling are highly consistent with laboratory results. Due to the high correlation coefficient between in-situ and laboratory tests, it is possible to evaluate fiber-reinforced mortars' compressive strength by using friction-transfer and pull-off tests.