In situ criteria of pull-off test for measuring bond strength of latex-modified concrete overlay

This study evaluated the factors influencing a pull-off test on latex-modified concrete (LMC) overlay by finite element analysis (FEA) and experimental verification and provided field engineers and project managers a guideline for measuring the bond strength in the field. The purpose of FEA was to predict stresses at the interface between the substrate and the overlay and between the disk and the overlay. Good agreement has been obtained between the theoretical and experimental results to confirm the following: the steel disc thickness did not affect the stress concentration factor (SCF) at the concrete interface but significantly influenced that of the steel disc. To prevent failure at the steel disc interface, the normalized steel disc thickness (T/D) should be at least 0.3. The effect of overlay thickness was significant to the stress distribution at the concrete interface. The normalized overlay thickness should be larger than 0.4. SCFs at all diameters decreased rapidly as the substrate coring depth increased from 0.05 to 0.2 of normalized cutting depth relative to core diameter. Those with a 75-mm diameter (D75) and a 100-mm diameter (D100) became stabilized immediately after reaching 0.2 of normalized cutting depth relative to core diameter. Stress distributions at concrete interfaces were not affected by the distance of the core from the edge of the slab (I-e), the distance between cores (I-c), and the typical elastic modulus ratio of LMC to conventional concrete. The experimental results with the major influencing factors (steel disc thickness, core diameter, and partial cutting depth) showed good agreement with FEA results. For theoretical as well as for practical purposes, 100-mm (4-in.) core diameter is optimum; cutting depth should be between 25 mm (1 in.) and 40 mm (1.7 in.).