Modeling the Compressive Strength of Fiber–Reinforced Alkali-Activated Concrete Using Non-Destructive Methods in Short Curing Periods

The indirect assessment of concrete compressive strength using non-destructive techniques, including the Schmidt rebound hammer and ultrasonic pulse velocity methods, is crucial for evaluating and tracking the strength of structural concrete used in construction and rehabilitation projects. However, studies focusing on alkali-activated concrete (AAC) remain insufficient. This study investigates the mechanical properties of fiber-reinforced alkali-activated concretes (FAACs) produced with short curing times, addressing this gap in the literature. Ground blast furnace slag (GBFS) was used as the binder, and waste volcanic rock dust (RD), produced in a natural stone processing plant, served as a fine filler. These materials were activated with sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) to produce alkali-activated concrete, both without fibers (AAC) and with fibers (FAAC). Steel fiber (SF) and polyester fiber (PYF) were used separately and in hybrid forms at 0.25%; 0.50%; 0.75%, and 1.0% volume fractions, resulting in a total of 117 cubic specimens (15×15×15 cm). The specimens’ mechanical properties were evaluated using regression analyses based on data from Schmidt rebound number (RN), ultrasonic pulse velocity (UPV) non-destructive methods and uniaxial compressive strength (UCS) destructive tests. Optimized functions predicting compressive strength based on RN and UPV measurements were developed, highlighting the significant effects of fiber type, ratio, and mode of use. Additionally, multivariate prediction models were statistically developed using data from the three test methods.