Study on development and verification method of rebound strength measurement curve of Class IV grout for structural reinforcement by finite element analysis

In this study, the expressions of linear function, power function, logarithmic function, exponential function, and quadratic function were adopted to carry out regression analysis on the test data obtained from a HT225‐A standard rebound hammer with nominal impact energy of 2.207 J and a HT450‐A high‐strength rebound hammer with nominal impact energy of 4.500 J. The aim of the study is to develop a dedicated rebound strength measurement curve for Class IV cementitious grout, which is commonly used in structural reinforcement applications. Four strength grades of grout (A40, A50, A60, and A70) were tested across six curing ages (1, 3, 7, 14, 28, and 60 days), with six cubic specimens prepared for each age. Results showed that the power function using data from the standard rebound hammer achieved the highest accuracy, with a correlation coefficient of R 2 = 0.97, and met the required error limits. The resulting curve is applicable for compressive strength estimation of external grout brands as well. Furthermore, finite element analysis in ABAQUS was used to evaluate the effects of key boundary conditions. Simulations revealed that the impact spacing should be no less than 20 mm to avoid interference, grout thickness should be greater than 30 mm to ensure accuracy, and error reduced from 30.67% at 20 mm to 3.27% at 30 mm. The distance from the specimen edge should be greater than 20 mm, with the error decreased from 71.91% at 0 mm to negligible levels at 20 mm. Corresponding conversion factors for non‐ideal conditions were also proposed. These findings provide practical guidance for the reliable field application of the rebound method in grout strength assessment.