# Experimental Study on Mechanical Properties of Graded EPS-Steel Fiber-Reinforced Concrete
## 3 Analysis of Experimental Phenomena and Results
### Evaluation of Compressive Strength and Flowability  
Figure 5 presents the relationship between the compressive strength of concrete at different ages and the volume fractions of steel fibers and graded EPS. The uniaxial compressive strength growth patterns of fine and gravel concrete with the incorporation of graded EPS and steel fibers at three different grades under standard curing at 7 and 28 days are generally similar. For example, in Figure 4(a), the linear slope formed by the uniaxial compressive strength values of C50 fine (gravel) concrete at EPS volume fractions of 15%, 25%, and 35% at two different curing ages is demonstrated. In both 7-day and 28-day  
Figure 4: Lateral cracks in the flexural test of graded EPS–steel fiber-reinforced concrete specimens. curing periods, the slopes are negative, with the absolute value of K\({}_{1}\) greater than that of K\({}_{2}\), and the absolute value of K\({}_{3}\) greater than that of K\({}_{4}\). This indicates that, with a constant steel fiber volume fraction, as the graded EPS volume fraction increases, the compressive strength of fine (gravel) concrete decreases at each curing age and grade. The reduction in compressive strength is more significant in the 15-25% EPS volume fraction range compared with the 25-35% range. The reactions observed in C30 and C40 concrete are similar to those in C50, so further discussion is omitted here. The compressive modulus of EPS is much lower than that of concrete, which leads to a reduction in the uniaxial compressive strength of concrete when EPS is added. To explain the slowed rate of compressive strength reduction, the Weymouth particle interference theory [28] is applied. In the experimental concrete, the skeleton formed by coarse aggregates with a nominal particle size greater than 4.75 mm (specifically greater than 5 mm) is filled by finer aggregates and 5 mm EPS particles. The skeleton formed by fine aggregates with a nominal particle size less than 2.36 mm (specifically 2.36 mm to 1.18 mm) is filled by 1 mm EPS particles and mineral powders smaller than 0.075 mm. Additionally, since the designed aggregates are continuously graded, particles of each size are included, ensuring that each skeletal pore is filled with next-level aggregates smaller than the pore size. This process continues until the concrete reaches a dense state. The framework-dense structure, formed by coarse and fine aggregates, graded EPS particles, mineral powder, and cement, is considered an ideal configuration. When the EPS volume fraction increases from 25% to 35%, the filling effect is more pronounced than in the 15-25% range, which slows down the rate of compressive strength reduction. Figure 5: _Cont._  
Figure 5: Compressive strength of EPS–steel fiber-reinforced concrete with different age grades. (**a**) Uniaxial compressive strength of steel fibers with a volume fraction of 1.5% at different ages and EPS volumes. (**b**) Uniaxial compressive strength of steel fibers with a volume fraction of 1.75% at different ages and EPS volumes. (**c**) Uniaxial compressive strength of steel fibers with a volume fraction of 2.25% at different ages and EPS volumes. By comparing the uniaxial compressive strength values of fine (gravel) concrete at 7 and 28 days for the same EPS and steel fiber volume fractions in Figure 5, it is found that the 7-day compressive strength reaches 60-75% of the 28-day value. This indicates that curing during the first 7 days is a crucial factor influencing the uniaxial compressive strength of concrete. Longitudinal comparisons of Figure 5a-c, corresponding to the same EPS volume fraction, show that increasing the steel fiber volume fraction leads to a slight increase in compressive strength, though the effect is not significant. For every 0.25% increase in steel fiber volume fraction, the increase in compressive strength is within 5%. Cracks form in the concrete due to internal stress concentrations, with fine cracks appearing at the mortar-aggregate interface.