Predicting the Mechanical Thresholds of Recycled HDPE-Modified Concrete Using a Novel Non-Destructive Structural Quality Index

Abstract

This study examines the mechanical and physical properties of sustainable concrete containing recycled high-density polyethylene (HDPE) waste. Coarse aggregate was replaced with crushed HDPE granules at 0%, 10%, 20%, and 30%, with a constant 1% proportion of hand-produced HDPE fibers remaining in all modified mixtures. The apparent density, compressive strength, flexural strength, and ultrasonic pulse velocity (V) of the composite matrix were measured after 28 days of in vitro testing. The experimental results demonstrated a proportional drop in density, mechanical strength, and sound velocity as polymer content increased, which was attributed to microstructure discontinuity and interfacial transition zone weakness. Unlike common percentage-based evaluations of decrease, this research proposes a new approach based on physical attributes and linear regression modelling. The non-destructive structural quality index (SQI) reliably forecasts mechanical stress thresholds based on density and sound wave velocity measurements, with an error margin of no more than ±6% and a coefficient of determination (R²), ranging between 0.82 and 0.97. The findings demonstrate that the created mathematical models provide a highly reliable non-destructive quality assessment tool for optimizing the application of environmentally friendly, lightweight, low-strength concrete in a circular economy framework.